,transcriber's ,note3 ,references to page numbers in table of contents and index removed1 as well as the numbers themselves4 ,illustration3 ,,alexander ,,graham ,,bell ,the ,inventor of the ,telephone4 ,cyclopedia of ,telephony and ,telegraphy ,a ,general ,reference ,work on ,,telephony1 ,,substations1 ,,party-,,line ,,systems1 ,,protection1 ,,manual ,,switchboards1 ,,automatic ,,systems1 ,,power ,,plants1 ,,special ,,service ,,features1 ,,construction1 ,,engineering1 ,,operation1 ,,maintenance1 ,,telegraphy1 ,,wireless ,,telegraphy ,,and ,,telephony1 ,,etc4 ,prepared by a ,corps of ,,telephone ,,and ,,telegraph ,,experts1 ,,and ,,electrical ,,engineers ,,of ,,the ,,highest ,,professional ,,standing ,illustrated with over ,two ,thousand ,engravings ,,four ,,volumes ,,chicago ,,american ,,school ,,of ,,correspondence #aiai ,authors and ,collaborators 99 99 99 99 99 ,,kempster ,b4 ,,miller4 ,m4,e4 ,consulting ,engineer and ,telephone ,expert ,of the ,firm of ,mc,meen and ,miller1 ,electrical ,engineers and ,patent ,experts1 ,chicago ,american ,institute of ,electrical ,engineers ,western ,society of ,engineers 99 99 99 99 99 ,,george ,w4 ,,patterson1 ,s4,b41 ,ph4,d4 ,head1 ,department of ,electrical ,engineering1 ,university of ,michigan 99 99 99 99 99 ,,charles ,,thom ,chief of ,quadruplex ,department1 ,western ,union ,main ,office1 ,new ,york ,city 99 99 99 99 99 ,,robert ,,andrews ,,millikan1 ,ph4,d4 ,associate ,professor of ,physics1 ,university of ,chicago ,member1 ,executive ,council1 ,american ,physical ,society 99 99 99 99 99 ,,samuel ,g4 ,mc,m,e,e,n ,consulting ,engineer and ,telephone ,expert ,of the ,firm of ,mc,meen and ,miller1 ,electrical ,engineers and ,patent ,experts1 ,chicago ,american ,institute of ,electrical ,engineers ,western ,society of ,engineers 99 99 99 99 99 ,,lawrence ,k4 ,,sager1 ,s4,b41 ,m4,p4,l4 ,patent ,attorney and ,electrical ,expert ,formerly ,assistant ,examiner1 ,u4,s4 ,patent ,office 99 99 99 99 99 ,,glenn ,m4 ,,hobbs1 ,ph4,d4 ,secretary1 ,american ,school of ,correspondence ,formerly ,instructor in ,physics1 ,university of ,chicago ,american ,physical ,society 99 99 99 99 99 ,,charles ,g4 ,,ashley ,electrical ,engineer and ,expert in ,wireless ,telegraphy and ,telephony 99 99 99 99 99 ,a4 ,,frederick ,,collins ,editor1 ,collins ,wireless ,bulletin ,author of ,wireless ,telegraphy1 ,its ,history1 ,theory1 and ,practice 99 99 99 99 99 ,,francis ,b4 ,,crocker1 ,e4,m41 ,ph4,d4 ,head1 ,department of ,electrical ,engineering1 ,columbia ,university ,past-,president1 ,american ,institute of ,electrical ,engineers 99 99 99 99 99 ,,morton ,,arendt1 ,e4,e4 ,instructor in ,electrical ,engineering1 ,columbia ,university1 ,new ,york 99 99 99 99 99 ,,edward ,b4 ,,waite ,head1 ,instruction ,department1 ,american ,school of ,correspondence ,american ,society of ,mechanical ,engineers ,western ,society of ,engineers 99 99 99 99 99 ,,david ,p4 ,,moreton1 ,b4,s41 ,e4,e4 ,associate ,professor of ,electrical ,engineering1 ,armour ,institute of ,technology ,american ,institute of ,electrical ,engineers1 99 99 99 99 99 ,,leigh ,s4 ,,keith1 ,b4,s4 ,managing ,engineer with ,mc,meen and ,miller1 ,electrical ,engineers and ,patent ,experts ,chicago ,associate ,member1 ,american ,institute of ,electrical ,engineers 99 99 99 99 99 ,,jessie ,m4 ,,shepherd1 ,a4,b4 ,associate ,editor1 ,textbook ,department1 ,american ,school of ,correspondence 99 99 99 99 99 ,,ernest ,l4 ,,wallace1 ,b4,s4 ,assistant ,examiner1 ,united ,states ,patent ,office1 ,washington1 ,d4 ,c4 99 99 99 99 99 ,,george ,r4 ,,metcalfe1 ,m4,e4 ,editor1 ,american ,institute of ,electrical ,engineers ,formerly ,head of ,publication ,department1 ,westinghouse ,elec4 ,mfg4 ,co4 99 99 99 99 99 ,j4,p4 ,,schroeter ,graduate1 ,munich ,technical ,school ,instructor in ,electrical ,engineering1 ,american ,school of ,correspondence 99 99 99 99 99 ,,james ,,dixon1 ,e4,e4 ,american ,institute of ,electrical ,engineers 99 99 99 99 99 ,,harris ,c4 ,,trow1 ,s4,b41 ,managing ,editor ,editor-in-,chief1 ,textbook ,department1 ,american ,school of ,correspondence ,authorities ,consulted ,the editors have freely consulted the standard technical literature of ,america and ,europe in the preparation of these volumes4 ,they desire to express their indebtedness particularly to the following eminent authorities1 whose well-known works should be in the library of every telephone and telegraph engineer4 ,grateful acknowledgment is here made also for the invaluable co-operation of the foremost engineering firms and manufacturers in making these volumes thoroughly representative of the very best and latest practice in the transmission of intelligence1 also for the valuable drawings1 data1 suggestions1 criticisms1 and other courtesies4 99 99 99 99 99 ,,arthur ,e4 ,,kennely1 ,d4,sc4 ,professor of ,electrical ,engineering1 ,harvard ,university4 ,joint ,author of ,the ,electric ,telephone4 ,the ,electric ,telegraph1 ,alternating ,currents1 ,arc ,lighting1 ,electric ,heating1 ,electric ,motors1 ,electric ,railways1 ,incandescent ,lighting1 etc4 99 99 99 99 99 ,,henry ,,smith ,,carhart1 ,a4,m41 ,,ll4,d4 ,professor of ,physics and ,director of the ,physical ,laboratory1 ,university of ,michigan4 ,author of ,primary ,batteries1 ,elements of ,physics1 ,university ,physics1 ,electrical ,measurements1 ,high ,school ,physics1 etc4 99 99 99 99 99 ,,francis ,b4 ,,crocker1 ,m4,e41 ,ph4,d4 ,head of ,department of ,electrical ,engineering1 ,columbia ,university1 ,new ,york2 ,past-,president1 ,american ,institute of ,electrical ,engineers4 ,author of ,electric ,lighting2 ,joint ,author of ,management of ,electrical ,machinery4 99 99 99 99 99 ,,horatio ,a4 ,,foster ,consulting ,engineer2 ,member of ,american ,institute of ,electrical ,engineers2 ,member of ,american ,society of ,mechanical ,engineers4 ,author of ,electrical ,engineer's ,pocket-,book4 99 99 99 99 99 ,,william ,s4 ,,franklin1 ,m4,s41 ,d4,sc4 ,professor of ,physics1 ,lehigh ,university4 ,joint ,author of ,the ,elements of ,electrical ,engineering1 ,the ,elements of ,alternating ,currents4 99 99 99 99 99 ,,lamar ,,lyndon1 ,b4,e41 ,m4,e4 ,consulting ,electrical ,engineer2 ,associate ,member of ,american ,institute of ,electrical ,engineers2 ,member1 ,american ,electro-,chemical ,society4 ,author of ,storage ,battery ,engineering4 99 99 99 99 99 ,,robert ,,andrews ,,millikan1 ,ph4,d4 ,professor of ,physics1 ,university of ,chicago4 ,joint ,author of ,a ,first ,course in ,physics1 ,electricity1 ,sound and ,light1 etc4 99 99 99 99 99 ,,kempster ,b4 ,,miller1 ,m4,e4 ,consulting ,engineer and ,telephone ,expert2 of the ,firm of ,mc,meen and ,miller1 ,electrical ,engineers and ,patent ,experts1 ,chicago4 ,author of ,american ,telephone ,practice4 99 99 99 99 99 ,,william ,h4 ,,preece ,chief of the ,british ,postal ,telegraph4 ,joint ,author of ,telegraphy1 ,a ,manual of ,telephony1 etc4-- 99 99 99 99 99 ,,louis ,,bell1 ,ph4,d4 ,consulting ,electrical ,engineer2 ,lecturer on ,power ,transmission1 ,massachusetts ,institute of ,technology4 ,author of ,electric ,power ,transmission1 ,power ,distribution for ,electric ,railways1 ,the ,art of ,illumination1 ,wireless ,telephony1 etc4 99 99 99 99 99 ,,oliver ,,heaviside1 ,f4,r4,s4 ,author of ,electro-,magnetic ,theory1 ,electrical ,papers1 etc4 99 99 99 99 99 ,,silvanus ,p4 ,,thompson1 ,d4,sc1 ,b4,a41 ,f4,r4,s41 ,f4,r4,a4,s4 ,principal and ,professor of ,physics in the ,city and ,guilds of ,london ,technical ,college4 ,author of ,electricity and ,magnetism1 ,dynamo-,electric ,machinery1 ,polyphase ,electric ,currents and ,alternate-,current ,motors1 ,the ,electromagnet1 etc4 99 99 99 99 99 ,,andrew ,,gray1 ,m4,a41 ,f4,r4,s4,e4 ,author of ,absolute ,measurements in ,electricity and ,magnetism4 99 99 99 99 99 ,,albert ,,cushing ,,crehore1 ,a4,b41 ,ph4,d4 ,electrical ,engineer2 ,assistant ,professor of ,physics1 ,dartmouth ,college2 ,formerly instructor in ,physics1 ,cornell ,university4 ,author of ,synchronous and ,other ,multiple ,telegraphs2 ,joint ,author of ,alternating ,currents4 99 99 99 99 99 ,j4 ,j4 ,,thomson1 ,d4,sc1 ,,ll4,d41 ,ph4,d41 ,f4,r4,s4 ,fellow of ,trinity ,college1 ,cambridge ,university2 ,cavendish ,professor of ,experimental ,physics1 ,cambridge ,university4 ,author of ,the ,conduction of ,electricity through ,gases1 ,electricity and ,matter4 99 99 99 99 99 ,,frederick ,,bedell1 ,ph4 ,d4 ,professor of ,applied ,electricity1 ,cornell ,university4 ,author of ,the ,principles of the ,transformer2 ,joint ,author of ,alternating ,currents4 99 99 99 99 99 ,,dugald ,c4 ,,jackson1 ,c4,e4 ,head of ,department of ,electrical ,engineering1 ,massachusetts ,institute of ,technology2 ,member1 ,american ,institute of ,electrical ,engineers1 etc4 ,author of ,a ,textbook on ,electromagnetism and the ,construction of ,dynamos2 ,joint ,author of ,alternating ,currents and ,alternating-,current ,machinery4 99 99 99 99 99 ,,michael ,,idvorsky ,,pupin1 ,a4,b41 ,sc4,d41 ,ph4,d4 ,professor of ,electro-,mechanics1 ,columbia ,university1 ,new ,york4 ,author of ,propagation of ,long ,electric ,waves1 and ,wave-,transmission over ,non-,uniform ,cables and ,long-,distance ,air ,lines4 99 99 99 99 99 ,,frank ,,baldwin ,,jewett1 ,a4,b41 ,ph4,d4 ,transmission and ,protection ,engineer1 with ,american ,telephone ,telegraph ,co4 ,author of ,modern ,telephone ,cable1 ,effect of ,pressure on ,insulation ,resistance4 99 99 99 99 99 ,,arthur ,,crotch ,formerly ,lecturer on ,telegraphy and ,telephony at the ,municipal ,technical ,schools1 ,norwich1 ,eng4 ,author of ,telegraphy and ,telephony4 99 99 99 99 99 ,,james ,,erskine-,,murray1 ,d4,sc4 ,fellow of the ,royal ,society of ,edinburgh2 ,member of the ,institution of ,electrical ,engineers4 ,author of ,a ,handbook of ,wireless ,telegraphy4 99 99 99 99 99 ,a4,h4 ,,mcmillan1 ,a4,b41 ,,ll4,b4 ,author of ,telephone ,law1 ,a ,manual on the ,organization and ,operation of ,telephone ,companies4 99 99 99 99 99 ,,william ,,esty1 ,s4,b41 ,m4,a4 ,head of ,department of ,electrical ,engineering1 ,lehigh ,university4 ,joint ,author of ,the ,elements of ,electrical ,engineering4 99 99 99 99 99 ,,george ,w4 ,,wilder1 ,ph4,d4 ,formerly ,professor of ,telephone ,engineering1 ,armour ,institute of ,technology4 ,author of ,telephone ,principles and ,practice1 ,simultaneous ,telegraphy and ,telephony1 etc4 99 99 99 99 99 ,,william ,l4 ,,hooper1 ,ph4,d4 ,head of ,department of ,electrical ,engineering1 ,tufts ,college4 ,joint ,author of ,electrical ,problems for ,engineering ,students4 99 99 99 99 99 ,,david ,s4 ,,hulfish ,technical ,editor1 ,the ,nickelodeon2 ,telephone and ,motion-,picture ,expert2 ,solicitor of ,patents4 ,author of ,how to ,read ,telephone ,circuit ,diagrams4 99 99 99 99 99 ,j4,a4 ,,fleming1 ,m4,a41 ,d4,sc4 7,lond471 ,f4,r4,s4 ,professor of ,electrical ,engineering in ,university ,college1 ,london2 ,late ,fellow and ,scholar of ,st4 ,john's ,college1 ,cambridge2 ,fellow of ,university ,college1 ,london4 ,author of ,the ,alternate-,current ,transformer1 ,radiotelegraphy and ,radiotelephony1 ,principles of ,electric ,wave ,telegraphy1 ,cantor ,lectures on ,electrical ,oscillations and ,electric ,waves1 ,hertzian ,wave ,wireless ,telegraphy1 etc4 99 99 99 99 99 ,f4,a4,c4 ,,perrine1 ,a4,m41 ,d4,sc4 ,consulting ,engineer3 ,formerly ,president1 ,stanley ,electric ,manufacturing ,company2 ,formerly ,professor of ,electrical ,engineering1 ,leland ,stanford1 ,jr4 ,university4 ,author of ,conductors for ,electrical ,distribution4 99 99 99 99 99 ,a4 ,,frederick ,,collins ,editor1 ,collins ,wireless ,bulletin4 ,author of ,wireless ,telegraphy1 ,its ,history1 ,theory and ,practice1 ,manual of ,wireless ,telegraphy1 ,design and ,construction of ,induction ,coils1 etc4 99 99 99 99 99 ,,schuyler ,s4 ,,wheeler1 ,d4,sc4 ,president1 ,crocker-,wheeler ,co42 ,past-,president1 ,american ,institute of ,electrical ,engineers4 ,joint ,author of ,management of ,electrical ,machinery4 99 99 99 99 99 ,,charles ,,proteus ,,steinmetz ,consulting ,engineer1 with the ,general ,electric ,co42 ,professor of ,electrical ,engineering1 ,union ,college4 ,author of ,the ,theory and ,calculation of ,alternating-,current ,phenomena1 ,theoretical ,elements of ,electrical ,engineering1 etc4 99 99 99 99 99 ,,george ,w4 ,,patterson1 ,s4,b41 ,ph4,d4 ,head of ,department of ,electrical ,engineering1 ,university of ,michigan4 ,joint ,author of ,electrical ,measurements4 99 99 99 99 99 ,,william ,,maver1 ,,jr4 ,ex-,electrician ,baltimore and ,ohio ,telegraph ,company2 ,member of the ,american ,institute of ,electrical ,engineers4 ,author of ,american ,telegraphy and ,encyclopedia of the ,telegraph1 ,wireless ,telegraphy4 99 99 99 99 99 ,,john ,,price ,,jackson1 ,m4,e4 ,professor of ,electrical ,engineering1 ,pennsylvania ,state ,college4 ,joint ,author of ,alternating ,currents and ,alternating-,current ,machinery4 99 99 99 99 99 ,,augustus ,,treadwell1 ,,jr41 ,e4,e4 ,associate ,member1 ,american ,institute of ,electrical ,engineers4 ,author of ,the ,storage ,battery1 ,a ,practical ,treatise on ,secondary ,batteries4 99 99 99 99 99 ,,edwin ,j4 ,,houston1 ,ph4,d4 ,professor of ,physics1 ,franklin ,institute1 ,pennsylvania2 ,joint ,inventor of ,thomson-,houston ,system of ,arc ,lighting2 ,electrical ,expert and ,consulting ,engineer4 ,joint ,author of ,the ,electric ,telephone1 ,the ,electric ,telegraph1 ,alternating ,currents1 ,arc ,lighting1 ,electric ,heating1 ,electric ,motors1 ,electric ,railways1 ,incandescent ,lighting1 etc4 99 99 99 99 99 ,,william ,j4 ,,hopkins ,professor of ,physics in the ,drexel ,institute of ,art1 ,science1 and ,industry1 ,philadelphia4 ,author of ,telephone ,lines and their ,properties4 ,illustration3 ,a ,,typical ,,small ,,magneto ,,switchboard ,,installation ,illustration3 ,a ,,typical ,,central ,,office ,,for ,,rural ,,exchange ,line ,protectors on ,wall at ,left4 ,foreword ,the present day development of the talking wire has annihilated both time and space1 and has enabled men thousands of miles apart to get into almost instant communication4 ,the user of the telephone and the telegraph forgets the tremendousness of the feat in the simplicity of its accomplishment2 but the man who has made the feat possible knows that its very simplicity is due to the complexity of the principles and appliances involved2 and he realizes his need of a practical1 working understanding of each principle and its application4 ,the ,cyclopedia of ,telephony and ,telegraphy presents a comprehensive and authoritative treatment of the whole art of the electrical transmission of intelligence4 ,the communication engineer--if so he may be called--requires a knowledge both of the mechanism of his instruments and of the vagaries of the current that makes them talk4 ,he requires as well a knowledge of plants and buildings1 of office equipment1 of poles and wires and conduits1 of office system and time-saving methods1 for the transmission of intelligence is a business as well as an art4 ,and to each of these subjects1 and to all others pertinent1 the ,cyclopedia gives proper space and treatment4 ,the sections on ,telephony cover the installation1 maintenance1 and operation of all standard types of telephone systems2 they present without prejudice the respective merits of manual and automatic exchanges2 and they give special attention to the prevention and handling of operating troubles4 ,the sections on ,telegraphy cover both commercial service and train dispatching4 ,practical methods of wireless communication--both by telephone and by telegraph--are thoroughly treated4 ,the drawings1 diagrams1 and photographs incorporated into the ,cyclopedia have been prepared especially for this work2 and their instructive value is as great as that of the text itself4 ,they have been used to illustrate and illuminate the text1 and not as a medium around which to build the text4 ,both drawings and diagrams have been simplified so far as is compatible with their correctness1 with the result that they tell their own story and always in the same language4 ,the ,cyclopedia is a compilation of many of the most valuable ,instruction ,papers of the ,american ,school of ,correspondence1 and the method adopted in its preparation is that which this ,school has developed and employed so successfully for many years4 ,this method is not an experiment1 but has stood the severest of all tests--that of practical use--which has demonstrated it to be the best yet devised for the education of the busy1 practical man4 ,in conclusion1 grateful acknowledgment is due to the staff of authors and collaborators1 without whose hearty co-operation this work would have been impossible4 ,table of ,contents ,,volume ,i ,,fundamental ,,principles ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen,a ,acoustics--,characteristics of ,sound--,loudness--,pitch--,vibration of ,diaphragms--,timbre--,human ,voice--,human ,ear--,speech--,magneto ,telephones--,loose-,contact ,principle--,induction ,coils--,simple ,telephone ,circuit--,capacity--,telephone ,currents--,audible and ,visible ,signals--,telephone ,lines--,conductors--,inductance--,insul ation ,,substation ,,equipment ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen ,transmitters--,variable ,resistance--,materials--,single and ,multiple ,electrodes--,solid-,back ,transmitter--,types of ,transmitters--,electrodes--,packing--,a cousticon ,transmitter--,switchboard ,transmitter--,receivers--,types of ,receivers--,operator's ,receiver--,primary ,cells--,series and ,multiple ,connections--,types of ,primary ,cells--,magneto ,signaling ,apparatus--,battery ,bell--,magneto ,bell--,magneto ,generator--,armature--,automatic ,shunt--,polarized ,ringer--,hook ,switch--,electromagnets--,impedance1 ,induction1 and ,repeating ,coils--,non-,inductive ,resistance ,devices--,differentially-,wound ,unit--,condensers--,materials--,current ,supply to ,transmitters--,local ,battery--,common ,battery--,diagrams of ,common-,battery ,systems--,telephone ,sets3 ,magneto1 ,series and ,bridging1 ,common-,battery ,,party-,,line ,,systems ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen ,non-,selective ,party-,line ,systems--,series and ,bridging--,signal ,code--,selective ,party-,line ,systems3 ,polarity1 ,harmonic1 ,step-by-,step1 and ,broken-,line--,lock-,out ,party-,line ,systems3 ,poole1 ,step-by-,step1 and ,broken-,line ,,protection ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen ,electrical ,hazards--,high ,potentials--,air-,gap ,arrester--,discharge across ,gaps--,types of ,arrester--,vacuum ,arrester--,strong ,currents--,fuses--,sneak ,currents--,line ,protection--,central-,office and ,subscribers' ,station ,protectors--,city ,exchange ,requirements--,electrolysis ,,manual ,,switchboards ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen ,the ,telephone ,exchange--,subscribers'1 ,trunk1 and ,toll ,lines--,districts--,switchboards--,simp le ,magneto ,switchboard--,operation--,commercial ,types of ,drops and ,jacks--,manual vs4 ,automatic ,restoration--,switchboard ,plugs and ,cords--,ringing and ,listening ,keys--,operator's ,telephone ,equipment--,circuits of ,complete ,switchboard--,night-,alarm ,circuits--,grounded and ,metallic ,circuit ,line--,cord ,circuit--,switchboard ,assembly ,,review ,,questions ,,index ,footnote ,a3 ,for professional standing of authors1 see list of ,authors and ,collaborators at front of volume4 ,illustration3 ,,old ,,branch-,,terminal ,,multiple ,,board1 ,,paris1 ,,france ,,telephony ,,introduction ,the telephone was invented in #ahge by ,alexander ,graham ,bell1 a resident of the ,united ,states1 a native of ,scotland1 and by profession a teacher of deaf mutes in the art of vocal speech4 ,in that year1 ,professor ,bell was engaged in the experimental development of a system of multiplex telegraphy1 based on the use of rapidly varying currents4 ,during those experiments1 he observed an iron reed to vibrate before an electromagnet as a result of another iron reed vibrating before a distant electromagnet connected to the nearer one by wires4 ,the telephone resulted from this observation with great promptness4 ,in the instrument first made1 sound vibrated a membrane diaphragm supporting a bit of iron near an electromagnet2 a line joined this simple device of three elements to another like it2 a battery in the line magnetized both electromagnet cores2 the vibration of the iron in the sending device caused the current in the line to undulate and to vary the magnetism of the receiving device4 ,the diaphragm of the latter was vibrated in consequence of the varying pull upon its bit of iron1 and these vibrations reproduced the sound that vibrated the sending diaphragm4 ,the first public use of the electric telephone was at the ,centennial ,exposition in ,philadelphia in #ahgf4 ,it was there tested by many interested observers1 among them ,sir ,william ,thomson1 later ,lord ,kelvin1 the eminent ,scotch authority on matters of electrical communication4 ,it was he who contributed so largely to the success of the early telegraph cable system between ,england and ,america4 ,two of his comments which are characteristic are as follows3 ,to-day ,i have seen that which yesterday ,i should have deemed impossible4 ,soon lovers will whisper their secrets over an electric wire4 99 99 99 99 99 ,who can but admire the hardihood of invention which devised such slight means to realize the mathematical conception that if electricity is to convey all the delicacies of sound which distinguish articulate speech1 the strength of its current must vary continuously as nearly as may be in simple proportion to the velocity of a particle of the air engaged in constituting the sound4 ,contrary to usual methods of improving a new art1 the earliest improvement of the telephone simplified it4 ,the diaphragms became thin iron disks1 instead of membranes carrying iron2 the electromagnet cores were made of permanently magnetized steel instead of temporarily magnetized soft iron1 and the battery was omitted from the line4 ,the undulatory current in a system of two such telephones joined by a line is produced in the sending telephone by the vibration of the iron diaphragm4 ,the vibration of the diaphragm in the receiving telephone is produced by the undulatory current4 ,sound is produced by the vibration of the diaphragm of the receiving telephone4 ,such a telephone is at once the simplest known form of electric generator or motor for alternating currents4 ,it is capable of translating motion into current or current into motion through a wide range of frequencies4 ,it is not known that there is any frequency of alternating current which it is not capable of producing and translating4 ,it can produce and translate currents of greater complexity than any other existing electrical machine4 ,though possessing these admirable qualities as an electrical machine1 the simple electromagnetic telephone had not the ability to transmit speech loudly enough for all practical uses4 ,transmitters producing stronger telephonic currents were developed soon after the fundamental invention4 ,some forms of these were invented by ,professor ,bell himself4 ,other inventors contributed devices embodying the use of carbon as a resistance to be varied by the motions of the diaphragm4 ,this general form of transmitting telephone has prevailed and at present is the standard type4 ,it is interesting to note that the earliest incandescent lamps1 as invented by ,mr4 ,edison1 had a resistance material composed of carbon1 and that such a lamp retained its position as the most efficient small electric illuminant until the recent introduction of metal filament lamps4 ,it is possible that some form of metal may be introduced as the resistance medium for telephone transmitters1 and that such a change as has taken place in incandescent lamps may increase the efficiency of telephone transmitting devices4 ,at the time of the invention of the telephone1 there were in existence two distinct types of telegraph1 working in regular commercial service4 ,in the more general type1 many telegraph stations were connected to a line and whatever was telegraphed between two stations could be read by all the stations of that line4 ,in the other and less general type1 many lines1 each having a single telegraph station1 were centered in an office or exchange1 and at the desire of a user his line could be connected to another and later disconnected from it4 ,both of these types of telegraph service were imitated at once in telephone practice4 ,lines carrying many telephones each1 were established with great rapidity4 ,telephones actually displaced telegraphic apparatus in the exchange method of working in ,america4 ,the fundamental principle on which telegraph or telephone exchanges operate1 being that of placing any line in communication with any other in the system1 gave to each line an ultimate scope so great as to make this form of communication more popular than any arrangement of telephones on a single line4 ,beginning in #ahgg1 telephone exchanges were developed with great rapidity in all of the larger communities of the ,united ,states4 ,telegraph switching devices were utilized at the outset or were modified in such minor particulars as were necessary to fit them to the new task4 ,in its simplest form1 a telephone system is1 of course1 a single line permanently joining two telephones4 ,in its next simplest form1 it is a line permanently joining more than two telephones4 ,in its most useful form1 it is a line joining a telephone to some means of connecting it at will to another4 ,a telephone exchange central office contains means for connecting lines at will in that useful way4 ,the least complicated machine for that purpose is a switchboard to be operated by hand1 having some way of letting the operator know that a connection is wished and a way of making it4 ,the customary way of connecting the lines always has been by means of flexible conductors fitted with plugs to be inserted in sockets4 ,if the switchboard be small enough so that all the lines are within arm's reach of the operator1 the whole process is individual1 and may be said to be at its best and simplest4 ,there are but few communities1 however1 in which the number of lines to be served and calls to be answered is small enough so that the entire traffic of the exchange can be handled by a single person4 ,an obvious way1 therefore1 is to provide as many operators in a central office as may be required by the number of calls to be answered1 and to terminate before each of the operators enough of the lines to bring enough work to keep that operator economically occupied4 ,this presents the additional problem1 how to connect a line terminating before one operator to a line normally terminating before another operator4 ,the obvious answer is to provide lines from each operator's place of work to each other operator's place1 connecting a calling line to some one of these lines which are local within the central office1 and1 in turn1 connecting that chosen local line to the line which is called4 ,such lines between operators have come to be known as trunk lines1 because of the obvious analogy to trunk lines of railways between common centers1 and such a system of telephone lines may be called a trunking system4 ,very good service has been given and can be given by such an arrangement of local trunks1 but the growth in lines and in traffic has developed in most instances certain weaknesses which make it advisable to find speedier1 more accurate1 and more reliable means4 ,for the serving of a large traffic from a large number of lines1 as is required in practically every city of the world1 a very great contribution to the practical art was made by the development of the multiple switchboard4 ,such a switchboard is merely such a device as has been described for the simpler cases1 with the further refinement that within reach of each operator in the central office appears every line which enters that office1 and this without regard to what point in the switchboard the lines may terminate for the answering of calls4 ,in other words1 while each operator answers a certain subordinate group of the total number of lines1 each operator may reach1 for calling purposes1 every line which enters that office4 ,it is probable that the invention and development of the multiple switchboard was the first great impetus toward the wide-spread use of telephone service4 ,coincident with the development of the multiple switchboard for manually operated1 central-office mechanisms was the beginning of the development of automatic apparatus under the control of the calling subscriber for finding and connecting with a called line4 ,it is interesting to note the general trend of the early development of automatic apparatus in comparison with the development1 to that time1 of manual telephone apparatus4 ,while the manual apparatus on the one hand attempted to meet its problem by providing local trunks between the various operators of a central office1 and failing of success in that1 finally developed a means which placed all the lines of a central office within connecting reach of each operator1 automatic telephony1 beginning at that point1 failed of success in attempting to bring each line in the central office within connecting reach of each connecting mechanism4 ,in other terms1 the first automatic switching equipment consisted of a machine for each line1 which machine was so organized as to be able to find and connect its calling line with any called line of the entire central-office group4 ,it may be said that an attempt to develop this plan was the fundamental reason for the repeated failure of automatic apparatus to solve the problem it attacked4 ,all that the earlier automatic system did was to prove more or less successfully that automatic apparatus had a right to exist1 and that to demand of the subscriber that he manipulate from his station a distant machine to make the connection without human aid was not fallacious4 ,when it had been recognized that the entire multiple switchboard idea could not be carried into automatic telephony with success1 the first dawn of hope in that art may be said to have come4 ,success in automatic telephony did come by the re-adoption of the trunking method4 ,as adopted for automatic telephony1 the method contemplates that the calling line shall be extended1 link by link1 until it finds itself lengthened and directed so as to be able to seize the called line in a very much smaller multiple than the total group of one office of the exchange4 ,a similar curious reversion has taken place in the development of telephone lines4 ,the earliest telephone lines were merely telegraph lines equipped with telephone instruments1 and the earliest telegraph lines were planned by ,professor ,morse to be insulated wires laid in the earth4 ,a lack of skill in preparing the wires for putting in the earth caused these early underground lines to be failures4 ,at the urging of one of his associates1 ,professor ,morse consented to place his earliest telegraph lines on poles in the air4 ,each such line originally consisted of two wires1 one for the going and one for the returning current1 as was then considered the action4 ,upon its being discovered that a single wire1 using the earth as a return1 would serve as a satisfactory telegraph line1 such practice became universal4 ,upon the arrival of the telephone1 all lines obviously were built in the same way1 and until force of newer circumstances compelled it1 the present metallic circuit without an earth connection did not come into general use4 ,the extraordinary growth of the number of telephone lines in a community and the development of other methods of electrical utilization1 as well as the growth in the knowledge of telephony itself1 ultimately forced the wires underground again4 ,at the same time and for the same causes1 a telephone line became one of two wires1 so that it becomes again the counterpart of the earliest telegraph line of ,professor ,morse4 ,another curious and interesting example of this reversion to type exists in the simple telephone receiver4 ,an early improvement in telephone receivers after ,professor ,bell's original invention was to provide the necessary magnetism of the receiver core by making it of steel and permanently magnetizing it1 whereas ,professor ,bell's instrument provided its magnetism by means of direct current flowing in the line4 ,in later days the telephone receiver has returned almost to the original form in which ,professor ,bell produced it and this change has simplified other elements of telephone-exchange apparatus in a very interesting and gratifying way4 ,by reason of improvements in methods of line construction and apparatus arrangement1 the radius of communication steadily has increased4 ,commercial speech now is possible between points several thousand miles apart1 and there is no theoretical reason why communication might not be established between any two points on the earth's surface4 ,the practical reasons of demand and cost may prevent so great an accomplishment as talking half around the earth4 ,so far as science is concerned there would seem to be no reason why this might not be done today1 by the careful application of what already is known4 ,in the ,united ,states1 telephone service from its beginning has been supplied to users by private enterprise4 ,in other countries1 it is supplied by means of governmentally-owned equipment4 ,in general1 it may be said that the adequacy and the amount1 as well as the quality of telephone service1 is best in countries where the service is provided by private enterprise4 ,telephone systems in the ,united ,states were under the control of the ,bell ,telephone ,company from the invention of the device in #ahgf until #ahic4 ,the fundamental telephone patent expired in #ahic4 ,this opened the telephone art to the general public1 because it no longer was necessary to secure telephones solely from the patent-holding company nor to pay royalty for the right to use them1 if secured at all4 ,manufacturers of electrical apparatus generally then began to make and sell telephones and telephone apparatus1 and operating companies1 also independent of the ,bell organization1 began to install and use telephones4 ,at the end of seventeen years of patent monopoly in the ,united ,states1 there were in operation a little over #bej1jjj telephones4 ,in the seventeen years since the expiration of the fundamental patent1 independent telephone companies throughout the ,united ,states have installed and now have in daily successful use over #c1iaa1djj telephones4 ,in other words1 since its first beginnings1 independent telephony has brought into continuous daily use nearly sixteen times as many telephones as were brought into use in the equal time of the complete monopoly of the ,bell organization4 ,at the beginning of #aiaj1 there were in service by the ,bell organization about #c1fcc1ijj telephones4 ,these with the #c1iaa1djj independent telephones1 make a total of #g1ede1cjj1 or about one-twelfth as many telephones as there are inhabitants of the ,united ,states4 ,the influence of this development upon the lives of the people has been profound4 ,whether the influence has been wholly for good may not be so conclusively apparent4 ,lord ,bacon has declared that1 excepting only the alphabet and the art of printing1 those inventions abridging distance are of the greatest service to mankind4 ,if this be true1 it may be said that the invention of telephony deserves high place among the civilizing influences4 ,there is no industrial art in which the advancement of the times has been followed more closely by practical application than in telephony4 ,commercial speech by telephone is possible by means of currents which so far are practically unmeasurable4 ,in other words1 it is possible to speak clearly and satisfactorily over a line by means of currents which cannot be read1 with certainty as to their amount1 by any electrical measuring device so far known4 ,in this regard1 telephony is less well fortified than are any of the arts utilizing electrical power in larger quantities4 ,the real wonder is that with so little knowledge of what takes place1 particularly as to amount1 those working in the art have been able to do as well as they have4 ,when an exact knowledge of quantity is easily obtainable1 very striking advances may be looked for4 ,the student of these phases of physical science and industrial art will do well to combine three processes3 study of the words of others2 personal experimentation2 and digestive thought4 ,the last mentioned is the process of profoundest value4 ,on it finally depends mastery4 ,it is not of so much importance how soon the concept shall finally be gained as that it is gained4 ,a statement by another may seem lifeless and inert and the meaning of an observation may be obscure4 ,digestive thought is the only assimilative process4 ,the whole art of telephony hangs on taking thought of things4 ,judge ,r4,f4 ,taylor of ,indiana said of ,professor ,bell1 ,it has been said that no man by taking thought may add a cubit to his stature1 yet here is a man who1 by taking thought1 has added not cubits but miles to the lengths of men's tongues and ears4 ,in observations of many students1 it is found that the notion of each must pass through a certain period of incubation before his private and personal knowledge of ,ohm's law is hatched4 ,once hatched1 however1 it is his4 ,by just such a process must come each principal addition to his stock of concepts4 ,the periods may vary and practice in the uses of the mind may train it in alertness in its work4 ,if time is required1 time should be given1 the object always being to keep thinking or re-reading or re-trying until the thought is wholly encompassed and possessed4 ,,chapter ,i ,,acoustics ,telephony is the art of reproducing at a distant point1 usually by the agency of electricity1 sounds produced at a sending point4 ,in this art the elements of two general divisions of physical science are concerned1 sound and electricity4 ,sound is the effect of vibrations of matter upon the ear4 ,the vibrations may be those of air or other matter4 ,various forms of matter transmit sound vibrations in varying degrees1 at different specific speeds1 and with different effects upon the vibrations4 ,any form of matter may serve as a transmitting medium for sound vibrations4 ,sound itself is an effect of sound vibrations upon the ear4 ,propagation of ,sound4 ,since human beings communicate with each other by means of speech and hearing through the air1 it is with air that the acoustics of telephony principally is concerned4 ,in air1 sound vibrations consist of successive condensations and rarefactions tending to proceed outwardly from the source in all directions4 ,the source is the center of a sphere of sound vibrations4 ,whatever may be the nature of the sounds or of the medium transmitting them1 they consist of waves emitted by the source and observed by the ear4 ,a sound wave is one complete condensation and rarefaction of the transmitting medium4 ,it is produced by one complete vibration of the sound-producing thing4 ,sound waves in air travel at a rate of about #a1jij feet per second4 ,the rate of propagation of sound waves in other materials varies with the density of the material4 ,for example1 the speed of transmission is much greater in water than in air1 and is much less in highly rarefied air than in air at ordinary density4 ,the propagation of sound waves in a vacuum may be said not to take place at all4 ,characteristics of ,sound4 ,three qualities distinguish sound3 loudness1 pitch1 and timbre4 ,loudness4 ,loudness depends upon the violence of the effect upon the ear2 sounds may be alike in their other qualities and differ in loudness1 the louder sounds being produced by the stronger vibrations of the air or other medium at the ear4 ,other things being equal1 the louder sound is produced by the source radiating the greater energy and so producing the greater degree of condensation and rarefaction of the medium4 ,pitch4 ,pitch depends upon the frequency at which the sound waves strike the ear4 ,pitches are referred to as high or low as the frequency of waves reaching the ear are greater or fewer4 ,familiar low pitches are the left-hand strings of a piano2 the larger ones of stringed instruments generally2 bass voices2 and large bells4 ,familiar high pitches are right-hand piano strings2 smaller ones of other stringed instruments2 soprano voices2 small bells2 and the voices of most birds and insects4 ,doppler's ,principle3--,as pitch depends upon the frequency at which sound waves strike the ear1 an object may emit sound waves at a constant frequency1 yet may produce different pitches in ears differently situated4 ,such a case is not usual1 but an example of it will serve a useful purpose in fixing certain facts as to pitch4 ,conceive two railroad trains to pass each other1 running in opposite directions1 the engine bells of both trains ringing4 ,passengers on each train will hear the bell of the other1 first as a rising pitch1 then as a falling one4 ,passengers on each train will hear the bell of their own train at a constant pitch4 ,the difference in the observations in such a case is due to relative positions between the ear and the source of the sound4 ,as to the bell of their own train1 the passengers are a fixed distance from it1 whether the train moves or stands2 as to the bell of the other train1 the passengers first rapidly approach it1 then pass it1 then recede from it4 ,the distances at which it is heard vary as the secants of a circle1 the radius in this case being a length which is the closest approach of the ear to the bell4 ,if the bell have a constant intrinsic fundamental pitch of #bjj waves per second 7a wave-length of about #e4e feet71 it first will be heard at a pitch of about #bjj waves per second4 ,but this pitch rises rapidly1 as if the bell were changing its own pitch1 which bells do not do4 ,the rising pitch is heard because the ear is rushing down the wave-train1 every instant nearer to the source4 ,at a speed of #de miles an hour1 the pitch rises rapidly1 about #ab vibrations per second4 ,if the rate of approach between the ear and the bell were constant1 the pitch of the bell would be heard at #bab waves per second4 ,but suddenly the ear passes the bell1 hears the pitch stop rising and begin to fall2 and the tone drops #ab waves per second as it had risen4 ,such a circumflex is an excellent example of the bearing of wavelengths and frequencies upon pitch4 ,vibration of ,diaphragms3--,sound waves in air have the power to move other diaphragms than that of the ear4 ,sound waves constantly vibrate such diaphragms as panes of windows and the walls of houses4 ,the recording diaphragm of a phonograph is a window pane bearing a stylus adapted to engrave a groove in a record blank4 ,in the cylinder form of record1 the groove varies in depth with the vibrations of the diaphragm4 ,in the disk type of phonograph1 the groove varies sidewise from its normal true spiral4 ,if the disk record be dusted with talcum powder1 wiped1 and examined with a magnifying glass1 the waving spiral line may be seen4 ,its variations are the result of the blows struck upon the diaphragm by a train of sound waves4 ,in reproducing a phonograph record1 increasing the speed of the record rotation causes the pitch to rise1 because the blows upon the air are increased in frequency and the wave-lengths shortened4 ,a transitory decrease in speed in recording will cause a transitory rise in pitch when that record is reproduced at uniform speed4 ,timbre4 ,character of sound denotes that difference of effect produced upon the ear by sounds otherwise alike in pitch and loudness4 ,this characteristic is called timbre4 ,it is extraordinarily useful in human affairs1 human voices being distinguished from each other by it1 and a great part of the joy of music lying in it4 ,a bell1 a stretched string1 a reed1 or other sound-producing body1 emits a certain lowest possible tone when vibrated4 ,this is called its fundamental tone4 ,the pitch1 loudness1 and timbre of this tone depend upon various controlling causes4 ,usually this fundamental tone is accompanied by a number of others of higher pitch1 blending with it to form the general tone of that object4 ,these higher tones are called harmonics4 ,the ,germans call them overtones4 ,they are always of a frequency which is some multiple of the fundamental frequency4 ,that is1 the rate of vibration of a harmonic is #b1 #c1 #d1 #e1 or some other integral number1 times as great as the fundamental itself4 ,a tone having no harmonics is rare in nature and is not an attractive one4 ,the tones of the human voice are rich in harmonics4 ,in any tone having a fundamental and harmonics 7multiples71 the wave-train consists of a complex series of condensations and rarefactions of the air or other transmitting medium4 ,in the case of mere noises the train of vibrations is irregular and follows no definite order4 ,this is the difference between vowel sounds and other musical tones on the one hand and all unmusical sounds 7or noises7 on the other4 ,human ,voice4 ,human beings communicate with each other in various ways4 ,the chief method is by speech4 ,voice is sound vibration produced by the vocal cords1 these being two ligaments in the larynx4 ,the vocal cords in man are actuated by the air from the lungs4 ,the size and tension of the vocal cords and the volume and the velocity of the air from the lungs control the tones of the voice4 ,the more tightly the vocal cords be drawn1 other things being equal1 the higher will be the pitch of the sound2 that is1 the higher the frequency of vibration produced by the voice4 ,the pitches of the human voice lie1 in general1 between the frequencies of #hg and #gfh per second4 ,these are the extremes of pitch1 and it is not to be understood that any such range of pitch is utilized in ordinary speech4 ,an average man speaks mostly between the fundamental frequencies of #he and #afj per second4 ,many female speaking voices use fundamental frequencies between #aej and #cbj vibrations per second4 ,it is obvious from what has been said that in all cases these speaking fundamentals are accompanied by their multiples1 giving complexity to the resulting wave-trains and character to the speaking voice4 ,speech-sounds result from shocks given to the air by the organs of speech2 these organs are principally the mouth cavity1 the tongue1 and the teeth4 ,the vocal cords are voice-organs2 that is1 man only truly speaks1 yet the lower animals have voice4 ,speech may be whispered1 using no voice4 ,note the distinction between speech and voice1 and the organs of both4 ,the speech of adults has a mean pitch lower than that of children2 of adult males1 lower than that of females4 ,there is no close analogue for the voice-organ in artificial mechanism1 but the use of the lips in playing a bugle1 trumpet1 cornet1 or trombone is a fairly close one4 ,here the lips1 in contact with each other1 are stretched across one end of a tube 7the mouthpiece7 while the air is blown between the lips by the lungs4 ,a musical tone results2 if the instrument be a bugle or a trumpet of fixed tube length1 the pitch will be some one of several certain tones1 depending on the tension on the lips4 ,the loudness depends on the force of the blast of air2 the character depends largely on the bugle4 ,human ,ear4 ,the human ear1 the organ of hearing in man1 is a complex mechanism of three general parts1 relative to sound waves3 a wave-collecting part2 a wave-observing part1 and a wave-interpreting part4 ,the outer ear collects and reflects the waves inwardly to beat upon the tympanum1 or ear drum1 a membrane diaphragm4 ,the uses of the rolls or convolutions of the outer ear are not conclusively known1 but it is observed that when they are filled up evenly with a wax or its equivalent1 the sense of direction of sound is impaired1 and usually of loudness also4 ,the diaphragm of the ear vibrates when struck by sound waves1 as does any other diaphragm4 ,by means of bone and nerve mechanism1 the vibration of the diaphragm finally is made known to the brain and is interpretable therein4 ,the human ear can appreciate and interpret sound waves at frequencies from #cb to about #cb1jjj vibrations per second4 ,below the lesser-number1 the tendency is to appreciate the separate vibrations as separate sounds4 ,above the higher number1 the vibrations are inaudible to the human ear4 ,the most acute perception of sound differences lies at about #c1jjj vibrations per second4 ,it may be that the range of hearing of organisms other than man lies far above the range with which human beings are familiar4 ,some trained musicians are able to discriminate between two sounds as differing one from the other when the difference in frequency is less than one-thousandth of either number4 ,other ears are unable to detect a difference in two sounds when they differ by as much as one full step of the chromatic scale4 ,whatever faculty an individual may possess as to tone discrimination1 it can be improved by training and practice4 ,,chapter ,,ii ,,electrical ,,reproduction ,,of ,,speech ,the art of telephony in its present form has for its problem so to relate two diaphragms and an electrical system that one diaphragm will respond to all the fundamental and harmonic vibrations beating upon it and cause the other to vibrate in exact consonance1 producing just such vibrations1 which beat upon an ear4 ,the art does not do all this today2 it falls short of it in every phase4 ,many of the harmonics are lost in one or another stage of the process2 new harmonics are inserted by the operations of the system itself and much of the volume originally available fails to reappear4 ,the art1 however1 has been able to change commercial and social affairs in a profound degree4 ,conversion from ,sound ,waves to ,vibration of ,diaphragm4 ,however produced1 by the voice or otherwise1 sounds to be transmitted by telephone consist of vibrations of the air4 ,these vibrations1 upon reaching a diaphragm1 cause it to move4 ,the greatest amplitude of motion of a diaphragm is1 or is wished to be1 at its center1 and its edge ordinarily is fixed4 ,the diaphragm thus serves as a translating device1 changing the energy carried by the molecules of the air into localized oscillations of the matter of the diaphragm4 ,the waves of sound in the air advance2 the vibrations of the molecules are localized4 ,the agency of the air as a medium for sound transmission should be understood to be one in which its general volume has no need to move from place to place4 ,what occurs is that the vibrations of the sound-producer cause alternate condensations and rarefactions of the air4 ,each molecule of the air concerned merely oscillates through a small amplitude1 producing1 by joint action1 shells of waves1 each traveling outward from the sound-producing center like rapidly growing coverings of a ball4 ,conversion from ,vibration to ,voice ,currents4 ,fig4 #a illustrates a simple machine adapted to translate motion of a diaphragm into an alternating electrical current4 ,the device is merely one form of magneto telephone chosen to illustrate the point of immediate conversion4 #a is a diaphragm adapted to vibrate in response to the sounds reaching it4 #b is a permanent magnet and #c is its armature4 ,the armature is in contact with one pole of the permanent magnet and nearly in contact with the other4 ,the effort of the armature to touch the pole it nearly touches places the diaphragm under tension4 ,the free arm of the magnet is surrounded by a coil #d1 whose ends extend to form the line4 ,illustration3 ,fig4 #a4 ,type of ,magneto ,telephone ,when sound vibrates the diaphragm1 it vibrates the armature also1 increasing and decreasing the distance from the free pole of the magnet4 ,the lines of force threading the coil #d are varied as the gap between the magnet and the armature is varied4 ,the result of varying the lines of force through the turns of the coil is to produce an electromotive force in them1 and if a closed path is provided by the line1 a current will flow4 ,this current is an alternating one having a frequency the same as the sound causing it4 ,as in speech the frequencies vary constantly1 many pitches constituting even a single spoken word1 so the alternating voice currents are of great varying complexity1 and every fundamental frequency has its harmonics superposed4 ,conversion from ,voice ,currents to ,vibration4 ,the best knowledge of the action of such a telephone as is shown in ,fig4 #a leads to the conclusion that a half-cycle of alternating current is produced by an inward stroke of the diaphragm and a second half-cycle of alternating current by the succeeding outward stroke1 these half-cycles flowing in opposite directions4 ,assume one complete cycle of current to pass through the line and also through another such device as in ,fig4 #a and that the first half-cycle is of such direction as to increase the permanent magnetism of the core4 ,the effort of this increase is to narrow the gap between the armature and pole piece4 ,the diaphragm will throb inward during the half-cycle of current4 ,the succeeding half-cycle being of opposite direction will tend to oppose the magnetism of the core4 ,in practice1 the flow of opposing current never would be great enough wholly to nullify and reverse the magnetism of the core1 so that the opposition results in a mere decrease1 causing the armature's gap to increase and the diaphragm to respond by an outward blow4 ,complete ,cycle of ,conversion4 ,the cycle of actions thus is complete2 one complete sound-wave in air has produced a cycle of motion in a diaphragm1 a cycle of current in a line1 a cycle of magnetic change in a core1 a cycle of motion in another diaphragm1 and a resulting wave of sound4 ,it is to be observed that the chain of operation involves the expenditure of energy only by the speaker1 the only function of any of the parts being that of translating this energy from one form to another4 ,in every stage of these translations1 there are losses2 the devising of means of limiting these losses as greatly as possible is a problem of telephone engineering4 ,illustration3 ,fig4 #b4 ,magneto ,telephones and ,line ,magneto ,telephones4 ,the device in ,fig4 #a is a practical magneto receiver and transmitter4 ,it is chosen as best picturing the idea to be proposed4 ,fig4 #b illustrates a pair of magneto telephones of the early ,bell type2 #a-a are diaphragms2 #b-b are permanent magnets with a free end of each brought as near as possible1 without touching1 to the diaphragm4 ,each magnet bears on its end nearest the diaphragm a winding of fine wire1 the two ends of each of these windings being joined by means of a two-wire line4 ,all that has been said concerning ,fig4 #a is true also of the electrical and magnetic actions of the devices of ,fig4 #b4 ,in the latter1 the flux which threads the fine wire winding is disturbed by motions of the transmitting diaphragm4 ,this disturbance of the flux creates electromotive forces in those windings4 ,similarly1 a variation of the electromotive forces in the windings varies the pull of the permanent magnet of the receiving instrument upon its diaphragm4 ,illustration3 ,no4 #aj ,,series ,,multiple ,,switchboard ,monarch ,telephone ,mfg4 ,co4 ,illustration3 ,fig4 #c4 ,magneto ,telephones without ,permanent ,magnets ,fig4 #c illustrates a similar arrangement1 but it is to be understood that the cores about which the windings are carried in this case are of soft iron and not of hard magnetized steel4 ,the necessary magnetism which constantly enables the cores to exert a pull upon the diaphragm is provided by the battery which is inserted serially in the line4 ,such an arrangement in action differs in no particular from that of ,fig4 #b1 for the reason that it matters not at all whether the magnetism of the core be produced by electromagnetic or by permanently magnetic conditions4 ,the arrangement of ,fig4 #c is a fundamental counterpart of the original telephone of ,professor ,bell1 and it is of particular interest in the present stage of the art for the reason that a tendency lately is shown to revert to the early type1 abandoning the use of the permanent magnet4 ,the modifications which have been made in the original magneto telephone1 practically as shown in ,fig4 #b1 have been many4 ,thirty-five years' experimentation upon and daily use of the instrument has resulted in its refinement to a point where it is a most successful receiver and a most unsuccessful transmitter4 ,its use for the latter purpose may be said to be nothing4 ,as a receiver1 it is not only wholly satisfactory for commercial use in its regular function1 but it is1 in addition1 one of the most sensitive electrical detecting devices known to the art4 ,loose ,contact ,principle4 ,early experimenters upon ,bell's device1 all using in their first work the arrangement utilizing current from a battery in series with the line1 noticed that sound was given out by disturbing loose contacts in the line circuit4 ,this observation led to the arrangement of circuits in such a way that some imperfect contacts could be shaken by means of the diaphragm1 and the resistance of the line circuit varied in this manner4 ,an early and interesting form of such imperfect contact transmitter device consisted merely of metal conductors laid loosely in contact4 ,a simple example is that of three wire nails1 the third lying across the other two1 the two loose contacts thus formed being arranged in series with a battery1 the line1 and the receiving instrument4 ,such a device when slightly jarred1 by the voice or other means1 causes abrupt variation in the resistance of the line1 and will transmit speech4 ,early ,conceptions4 ,the conception of the possibility and desirability of transmitting speech by electricity may have occurred to many1 long prior to its accomplishment4 ,it is certain that one person1 at least1 had a clear idea of the general problem4 ,in #ahed1 ,charles ,bourseul1 a ,frenchman1 wrote3 ,i have asked myself1 for example1 if the spoken word itself could not be transmitted by electricity2 in a word1 if what was spoken in ,vienna might not be heard in ,paris8 ,the thing is practicable in this way3 ,illustration3 ,fig4 #d4 ,reis ,transmitter ,suppose that a man speaks near a movable disk sufficiently flexible to lose none of the vibrations of the voice2 that this disk alternately makes and breaks the connection from a battery2 you may have at a distance another disk which will simultaneously execute the same vibrations4 ,the idea so expressed is weak in only one particular4 ,this particular is shown by the words italicized by ourselves4 ,it is impossible to transmit a complex series of waves by any simple series of makes and breaks4 ,philipp ,reis1 a ,german1 devised the arrangement shown in ,fig4 #d for the transmission of sound1 letting the make and break of the contact between the diaphragm #a and the point #b interrupt the line circuit4 ,his receiver took several forms1 all electromagnetic4 ,his success was limited to the transmission of musical sounds1 and it is not believed that articulate speech ever was transmitted by such an arrangement4 ,it must be remembered that the art of telegraphy1 particularly in ,america1 was well established long before the invention of the telephone1 and that an arrangement of keys1 relays1 and a battery1 as shown in ,fig4 #e1 was well known to a great many persons4 ,attaching the armatures of the relays of such a line to diaphragms1 as in ,fig4 #f1 at any time after #ahch1 would have produced the telephone4 ,the hardihood of invention to conceive such a change was the quality required4 ,illustration3 ,fig4 #e4 ,typical ,telegraph ,line ,limitations of ,magneto ,transmitter4 ,for reasons not finally established1 the ability of the magneto telephone to produce large currents from large sounds is not equal to its ability to produce large sounds from large currents4 ,as a receiving device1 it is unexcelled1 and but slight improvement has been made since its first invention4 ,it is inadequate as a transmitter1 and as early as #ahgf1 ,professor ,bell exhibited other means than electromagnetic action for producing the varying currents as a consequence of diaphragm motion4 ,much other inventive effort was addressed to this problem1 the aim of all being to send out more robust voice currents4 ,illustration3 ,fig4 #f4 ,telegraph ,equipment ,converted into ,telephone ,equipment ,other ,methods of ,producing ,voice ,currents4 ,some of these means are the variation of resistance in the path of direct current1 variation in the pressure of the source of that current1 and variation in the electrostatic capacity of some part of the circuit4 ,electrostatic ,telephone4 ,the latter method is principally that of ,dolbear and ,edison4 ,dolbear's thought is illustrated in ,fig4 #g4 ,two conducting plates are brought close together4 ,one is free to vibrate as a diaphragm1 while the other is fixed4 ,the element #a in ,fig4 #g is merely a stud to hold rigid the plate it bears against4 ,each of two instruments connected by a line contains such a pair of plates1 and a battery in the line keeps them charged to its potential4 ,the two diaphragms of each instrument are kept drawn towards each other because their unlike charges attract each other4 ,the vibration of one of the diaphragms changes the potential of the other pair2 the degree of attraction thus is varied1 so that vibration of the diaphragm and sound waves result4 ,examples of this method of telephone transmission are more familiar to later practice in the form of condenser receivers4 ,a condenser1 in usual present practice1 being a pair of closely adjacent conductors of considerable surface insulated from each other1 a rapidly varying current actually may move one or both of the conductors4 ,ordinarily these are of thin sheet metal 7foil7 interleaved with an insulating material1 such as paper or mica4 ,voice currents can vibrate the metal sheets in a degree to cause the condenser to speak4 ,these condenser methods of telephony have not become commercial4 ,illustration3 ,fig4 #g4 ,electrostatic ,telephone ,variation of ,electrical ,pressure4 ,variation of the pressure of the source is a conceivable way of transmitting speech4 ,to utilize it1 would require that the vibrations of the diaphragm cause the electromotive force of a battery or machine to vary in harmony with the sound waves4 ,so far as we are informed this method never has come into practical use4 ,variation of ,resistance4 ,variation of resistance proportional to the vibrations of the diaphragm is the method which has produced the present prevailing form of transmission4 ,professor ,bell's ,centennial exhibit contained a water-resistance transmitter4 ,dr4 ,elisha ,gray also devised one4 ,in both1 the diaphragm acted to increase and diminish the distance between two conductors immersed in water1 lowering and raising the resistance of the line4 ,it later was discovered by ,edison that carbon possesses a peculiarly great property of varying its resistance under pressure4 ,professor ,david ,e4 ,hughes discovered that two conducting bodies1 preferably of rather poor conductivity1 when laid together so as to form a loose contact between them1 possessed1 in remarkable degree1 the ability to vary the resistance of the path through them when subject to such vibrations as would alter the intimacy of contact4 ,he thus discovered and formulated the principles of loose contact upon which the operation of all modern transmitters rests4 ,hughes' device was named by him a microphone1 indicating a magnification of sound or an ability to respond to and make audible minute sounds4 ,it is shown in ,fig4 #h4 ,firmly attached to a board are two carbon blocks1 shown in section in the figure4 ,a rod of carbon with cone-shaped ends is supported loosely between the two blocks1 conical depressions in the blocks receiving the ends of the rod4 ,a battery and magneto receiver are connected in series with the device4 ,under certain conditions of contact1 the arrangement is extraordinarily sensitive to small sounds and approaches an ability indicated by its name4 ,its practical usefulness has been not as a serviceable speech transmitter1 but as a stimulus to the devising of transmitters using carbon in other ways4 ,variation of the resistance of metal conductors and of contact between metals has served to transmit voice currents1 but no material approaches carbon in this property4 ,illustration3 ,fig4 #h4 ,hughes' ,microphone ,carbon4 ,adaptability4 ,the application of carbon to use in transmitters has taken many forms4 ,they may be classified as those having a single contact and those having a plurality of contacts2 in all cases1 the intimacy of contact is varied by the diaphragm excursions4 ,an example of the single-contact type is the ,blake transmitter1 long familiar in ,america4 ,an example of the multiple-contact type is the loose-carbon type universal now4 ,other types popular at other times and in particular places use solid rods or blocks of carbon having many points of contact1 though not in a powdered or granular form4 ,fig4 #i shows an example of each of the general forms of transmitters4 ,the use of granular carbon as a transmitter material has extended greatly the radius of speech1 and has been a principal contributing cause for the great spread of the telephone industry4 ,illustration3 ,fig4 #i4 ,general ,types of ,transmitters ,superiority4 ,the superiority of carbon over other resistance-varying materials for transmitters is well recognized1 but the reason for it is not well known4 ,various theories have been proposed to explain why1 for example1 the resistance of a mass of carbon granules varies with the vibrations or compressions to which they are subjected4 ,four principal theories respectively allege3 ,first1 that change in pressure actually changes the specific resistance of carbon4 ,second1 that upon the surface of carbon bodies exists some gas in some form of attachment or combination1 variations of pressure causing variations of resistance merely by reducing the thickness of this intervening gas4 ,third1 that the change of resistance is caused by variations in the length of electrical arcs between the particles4 ,fourth1 that change of pressure changes the area of contact1 as is true of solids generally4 ,one may take his choice4 ,a solid carbon block or rod is not found to decrease its resistance by being subjected to pressure4 ,the gas theory lacks experimental proof also4 ,the existence of arcs between the granules never has been seen or otherwise observed under normal working conditions of a transmitter2 when arcs surely are experimentally established between the granules the usefulness of the transmitter ceases4 ,the final theory1 that change of pressure changes area of surface contact1 does not explain why other conductors than carbon are not good materials for transmitters4 ,this1 it may be noticed1 is just what the theories set out to make clear4 ,there are many who feel that more experimental data is required before a conclusive and satisfactory theory can be set up4 ,there is need of one1 for a proper theory often points the way for effective advance in practice4 ,carbon and magneto transmitters differ wholly in their methods of action4 ,the magneto transmitter produces current2 the carbon transmitter controls current4 ,the former is an alternating-current generator2 the latter is a rheostat4 ,the magneto transmitter produces alternating current without input of any electricity at all2 the carbon transmitter merely controls a direct current1 supplied by an external source1 and varies its amount without changing its direction4 ,the carbon transmitter1 however1 may be associated with other devices in a circuit in such a way as to transform direct currents into alternating ones1 or it may be used merely to change constant direct currents into undulating ones1 which never reverse direction1 as alternating currents always do4 ,these distinctions are important4 ,illustration3 ,fig4 #aj4 ,battery in ,line ,circuit ,limitations4 ,a carbon transmitter being merely a resistance-varying device1 its usefulness depends on how much its resistance can vary in response to motions of air molecules4 ,a granular-carbon transmitter may vary between resistances of #e to #ej ohms while transmitting a particular tone1 having the lower resistance when its diaphragm is driven inward4 ,conceive this transmitter to be in a line as shown in ,fig4 #aj1 the line1 distant receiver1 and battery together having a resistance of #a1jjj ohms4 ,the minimum resistance then is #a1jje ohms and the maximum #a1jej ohms4 ,the variation is limited to about #d4e per cent4 ,the greater the resistance of the line and other elements than the transmitter1 the less relative change the transmitter can produce1 and the less loudly the distant receiver can speak4 ,illustration3 ,fig #aa4 ,battery in ,local ,circuit ,induction ,coil4 ,mr4 ,edison realized this limitation to the use of the carbon transmitter direct in the line1 and contributed the means of removing it4 ,his method is to introduce an induction coil between the line and the transmitter1 its function being to translate the variation of the direct current controlled by the transmitter into true alternating currents4 ,an induction coil is merely a transformer1 and for the use under discussion consists of two insulated wires wound around an iron core4 ,change in the current carried by one of the windings produces a current in the other4 ,if direct current be flowing in one of the windings1 and remains constant1 no current whatever is produced in the other4 ,it is important to note that it is change1 and change only1 which produces that alternating current4 ,fig4 #aa shows an induction coil related to a carbon transmitter1 a battery1 and a receiver4 ,fig4 #ab shows exactly the same arrangement1 using conventional signs4 ,the winding of the induction coil which is in series with the transmitter and the battery is called the primary winding2 the other is called the secondary winding4 ,in the arrangement of ,figs4 #aa and #ab the battery has no metallic connection with the line1 so that it is called a local battery4 ,the circuit containing the battery1 transmitter1 and primary winding of the induction coil is called the local circuit4 ,let us observe what is the advantage of this arrangement over the case of ,fig4 #aj4 ,using the same values of resistance in the transmitter and line1 assume the local circuit apart from the transmitter to have a fixed resistance of #e ohms4 ,the limits of variations in the local circuit1 therefore1 are #aj and #ee ohms1 thus making the maximum #e4e times the minimum1 or an increase of #dej per cent as against #d4e per cent in the case of ,fig4 #aj4 ,the changes1 therefore1 are #ajj times as great4 ,illustration3 ,fig4 #ab4 ,conventional ,diagram of ,talking ,circuit ,the relation between the windings of the induction coil in this practice are such that the secondary winding contains many more turns than the primary winding4 ,changes in the circuit of the primary winding produce potentials in the secondary winding correspondingly higher than the potentials producing them4 ,these secondary potentials depend upon the ratio of turns in the two windings and therefore1 within close limits1 may be chosen as wished4 ,high potentials in the secondary winding are admirably adapted to transmit currents in a high-resistance line1 for exactly the same reason that long-distance power transmission meets with but one-quarter of one kind of loss when the sending potential is doubled1 one-hundredth of that loss when it is raised tenfold1 and similarly4 ,the induction coil1 therefore1 serves the double purpose of a step-up transformer to limit line losses and a device for vastly increasing the range of change in the transmitter circuit4 ,fig4 #ac is offered to remind the student of the action of an induction coil or transformer in whose primary circuit a direct current is increased and decreased4 ,an increase of current in the local winding produces an impulse of opposite direction in the turns of the secondary winding2 a decrease of current in the local winding produces an impulse of the same direction in the turns of the secondary winding4 ,the key of ,fig4 #ac being closed1 current flows upward in the primary winding as drawn in the figure1 inducing a downward impulse of current in the secondary winding and its circuit as noted at the right of the figure4 ,on the key being opened1 current ceases in the primary circuit1 inducing an upward impulse of current in the secondary winding and circuit as shown4 ,during other than instants of opening and closing 7changing7 the local circuit1 no current whatever flows in the secondary circuit4 ,illustration3 ,fig4 #ac4 ,induction-,coil ,action ,it is by these means that telephone transmitters draw direct current from primary batteries and send high-potential alternating currents over lines2 the same process produces what in ,therapeutics are called ,faradic currents1 and enables also a simple vibrating contact-maker to produce alternating currents for operating polarized ringers of telephone sets4 ,detrimental ,effects of ,capacity4 ,electrostatic capacity plays an important part in the transmission of speech4 ,its presence between the wires of a line and between them and the earth causes one of the losses from which long-distance telephony suffers4 ,its presence in condensers assists in the solution of many circuit and apparatus problems4 ,a condenser is a device composed of two or more conductors insulated from each other by a medium called the dielectric4 ,a pair of metal plates separated by glass1 a pair of wires separated by air1 or a pair of sheets of foil separated by paper or mica may constitute a condenser4 ,the use of condensers as pieces of apparatus and the problems presented by electrostatic capacity in lines are discussed in other chapters4 ,measurements of ,telephone ,currents4 ,it has been recognized in all branches of engineering that a definite advance is possible only when quantitative data exists4 ,the lack of reliable means of measuring telephone currents has been a principal cause of the difficulty in solving many of its problems4 ,it is only in very recent times that accurate and reliable means have been worked out for measuring the small currents which flow in telephone lines4 ,these ways are of two general kinds3 by thermal and by electromagnetic means4 ,thermal ,method4 ,the thermal methods simply measure1 in some way1 the amount of heat which is produced by a received telephone current4 ,when this current is allowed to pass through a conductor the effect of the heat generated in that conductor1 is observed in one of three ways3 by the expansion of the conductor1 by its change in resistance1 or by the production of an electromotive force in a thermo-electric couple heated by the conductor4 ,any one of these three ways can be used to get some idea of the amount of current which is received4 ,none of them gives an accurate knowledge of the forms of the waves which cause the reproduction of speech in the telephone receiver4 ,illustration3 ,fig4 #ad4 ,oscillogram of ,telephone ,currents ,electromagnetic ,method4 ,an electromagnetic device adapted to tell something of the magnitude of the telephone current and also something of its form1 i4e41 something of its various increases and decreases and also of its changes in direction is the oscillograph4 ,an oscillograph is composed of a magnetic field formed by direct currents or by a permanent magnet1 a turn of wire under mechanical tension in that field1 and a mirror borne by the turn of wire1 adapted to reflect a beam of light to a photographic film or to a rotating mirror4 ,when a current is to be measured by the oscillograph1 it is passed through the turn of wire in the magnetic field4 ,while no current is passing1 the wire does not move in the magnetic field and its mirror reflects a stationary beam of light4 ,a photographic film moved in a direction normal to the axis of the turn of wire will have drawn upon it a straight line by the beam of light4 ,if the beam of light1 however1 is moved by a current1 from side to side at right angles to this axis1 it will draw a wavy line on the photographic film and this wavy line will picture the alternations of that current and the oscillations of the molecules of air which carried the originating sound4 ,fig4 #ad is a photograph of nine different vowel sounds which have caused the oscillograph to take their pictures4 ,they are copies of records made by ,mr4 ,bela ,gati1 assisted by ,mr4 ,tolnai4 ,the measuring instrument consisted of an oscillograph of the type described1 the transmitter being of the carbon type actuated by a #b-volt battery4 ,the primary current was transformed by an induction coil of the ordinary type and the transformed current was sent through a non-inductive resistance of #c1jjj ohms4 ,no condensers were placed in the circuit4 ,it will be seen that the integral values of the curves1 starting from zero1 are variable4 ,the positive and the negative portions of the curves are not equal1 so that the solution of the individual harmonic motion is difficult and laborious4 ,these photographs point out several facts very clearly4 ,one is that the alternations of currents in the telephone line1 like the motions of the molecules of air of the original sound1 are highly complex and are not1 as musical tones are1 regular recurrences of equal vibrations4 ,they show also that any vowel sound may be considered to be a regular recurrence of certain groups of vibrations of different amplitudes and of different frequencies4 ,,chapter ,,iii ,,electrical ,,signals ,electric calls or signals are of two kinds3 audible and visible4 ,illustration3 ,fig4 #ae4 ,telegraph ,sounder and ,key ,illustration3 ,fig4 #af4 ,vibrating ,bell ,audible ,signals4 ,telegraph ,sounder4 ,the earliest electric signal was an audible one1 being the telegraph sounder1 or the ,morse register considered apart from its registering function4 ,each telegraph sounder serves as an audible electric signal and is capable of signifying more than that the call is being made4 ,such a signal is operated by the making and breaking of current from a battery4 ,an arrangement of this kind is shown in ,fig4 #ae1 in which pressure upon the key causes the current from the battery to energize the sounder and give one sharp audible rap of the lever upon the striking post4 ,vibrating ,bell4 ,the vibrating bell1 so widely used as a door bell1 is a device consequent to the telegraph4 ,its action is to give a series of blows on its gong when its key or push button closes the battery circuit4 ,at the risk of describing a trite though not trivial thing1 it may be said that when the contact #a of ,fig4 #af is closed1 current from the battery energizes the armature #b1 causing the latter to strike a blow on the gong and to break the line circuit as well1 by opening the contact back of the armature4 ,so de-energized1 the armature falls back and the cycle is repeated until the button contact is released4 ,a comparison of this action with that of the polarized ringer 7to be described later7 will be found of interest4 ,illustration3 ,fig4 #ag4 ,elemental ,magneto-,generator ,magneto-,bell4 ,the magneto-bell came into wide use with the spread of telephone service4 ,its two fundamental parts are an alternating-current generator and a polarized bell-ringing device4 ,each had its counterpart long before the invention of the telephone1 though made familiar by the latter4 ,the alternating-current generator of the magneto-bell consists of a rotatable armature composed of a coil of insulated wire and usually a core of soft iron1 its rotation taking place in a magnetic field4 ,this field is usually provided by a permanent magnet1 hence the name magneto-generator4 ,the purist in terms may well say1 however1 that every form whatever of the dynamo-electric generator is a magneto-generator1 as magnetism is one link in every such conversion of mechanical power into electricity4 ,the terms magneto-electric1 magneto-generator1 etc41 involving the term magneto1 have come to imply the presence of permanently magnetized steel as an element of the construction4 ,in its early form1 the magneto-generator consisted of the arrangement shown in ,fig4 #ag1 wherein a permanent magnet can rotate on an axis before an electromagnet having soft iron cores and a winding4 ,reversals of magnetism produce current in alternately reversing half-cycles1 one complete rotation of the magnet producing one such cycle4 ,obviously the result would be the same if the magnet were stationary and the coils should rotate1 which is the construction of more modern devices4 ,the turning of the crank of a magneto-bell rotates the armature in the magnetic field by some form of gearing at a rate usually of the order of twenty turns per second1 producing an alternating current of that frequency4 ,this current is caused by an effective electromotive force which may be as great as #ajj volts1 produced immediately by the energy of the user4 ,in an equipment using a magneto-telephone as both receiver and transmitter and a magneto-bell as its signal-sending machine1 as was usual in #ahgg1 it is interesting to note that the entire motive power for signals and speech transmission was supplied by the muscular tissues of the user--a case of working one's passage4 ,illustration3 ,fig4 #ah4 ,extension of a ,permanent ,magnet ,the alternating current from the generator is received and converted into sound by means of the polarized ringer1 a device which is interesting as depending upon several of the electrical1 mechanical1 and magnetic actions which are the foundations of telephone engineering4 ,illustration3 ,fig4 #ai4 ,extension of a ,permanent ,magnet ,why the ringer rings may be gathered from a study of ,figs4 #ah to #ba4 ,a permanent magnet will impart temporary magnetism to pieces of iron near it4 ,in ,fig4 #ah two pieces of iron are so energized4 ,the ends of these pieces which are nearest to the permanent magnet #a are of the opposite polarity to the end they approach1 the free ends being of opposite polarity4 ,in the figure1 these free ends are marked ,n1 meaning they are of a polarity to point north if free to point at all4 ,english-speaking persons call this north polarity4 ,similarly1 as in ,fig4 #ai1 any arrangement of iron near a permanent magnet always will have free poles of the same polarity as the end of the permanent magnet nearest them4 ,a permanent magnet so related to iron forms part of a polarized ringer4 ,so does an electromagnet composed of windings and iron cores4 ,fig4 #bj reminds us of the law of electromagnets4 ,if current flows from the plus towards the minus side1 with the windings as drawn1 polarities will be induced as marked4 ,illustration3 ,fig4 #bj4 ,electromagnet ,illustration3 ,fig4 #ba4 ,polarized ,ringer ,if1 now1 such an electromagnet1 a permanent magnet1 and a pivoted armature be related to a pair of gongs as shown in ,fig4 #ba1 a polarized ringer results4 ,it should be noted that a permanent magnet has both its poles presented 7though one of the poles is not actually attached7 to two parts of the iron of the electro-magnet4 ,the result is that the ends of the armature are of south polarity and those of the core are of north polarity4 ,all the markings of ,fig4 #ba relate to the polarity produced by the permanent magnet4 ,if1 now1 a current flow in the ringer winding from plus to minus1 obviously the right-hand pole will be additively magnetized1 the current tending to produce north magnetism there2 also the left-hand pole will be subtractively magnetized1 the current tending to produce south magnetism there4 ,if the current be of a certain strength1 relative to the certain ringer under study1 magnetism in the left pole will be neutralized and that in the right pole doubled4 ,hence the armature will be attracted more by the right pole than by the left and will strike the right-hand gong4 ,a reversal of current produces an opposite action1 the left-hand gong being struck4 ,the current ceasing1 the armature remains where last thrown4 ,illustration3 ,,operator',s ,,equipment ,clement ,automanual ,system4 ,it is important to note that the strength of action depends upon the strength of the current up to a certain point only4 ,that depends upon the strength of the permanent magnet4 ,whenever the current is great enough just to neutralize the normal magnetism of one pole and to double that of the other1 no increase in current will cause the device to ring any louder4 ,this makes obvious the importance of a proper permanent magnetism and displays the fallacy of some effort to increase the output of various devices depending upon these principles4 ,this discussion of magneto-electric signaling is introduced here because of a belief in its being fundamental4 ,chapter ,,viii treats of such a signaling in further detail4 ,telephone ,receiver4 ,the telephone receiver itself serves a useful purpose as an audible signal4 ,an interrupted or alternating current of proper frequency and amount will produce in it a musical tone which can be heard throughout a large room4 ,this fact enables a telephone central office to signal a subscriber who has left his receiver off the switch hook1 so that normal conditions may be restored4 ,visible ,signals4 ,electromagnetic ,signal4 ,practical visual signals are of two general kinds3 electromagnetic devices for moving a target or pointer1 and incandescent lamps4 ,the earliest and most widely used visible signal in telephone practice was the annunciator1 having a shutter adapted to fall when the magnet is energized4 ,fig4 #bb is such a signal4 ,shutter #a is held by the catch #b from dropping to the right by its own gravity4 ,the name gravity-drop is thus obvious4 ,current energizing the core attracts the armature #c1 lifts the catch #b1 and the shutter falls4 ,a simple modification of the gravity-drop produces the visible signal shown in ,fig4 #bc4 ,energizing the core lifts a target so as to render it visible through an opening in the plate #a4 ,a contrast of color between the plate and the target heightens the effect4 ,illustration3 ,fig4 #bb4 ,gravity-,drop ,the gravity-drop is principally adapted to the magneto-bell system of signaling1 where an alternating current is sent over the line to a central office by the operation of a bell crank at the subscriber's station1 this current1 lasting only as long as the crank is turned1 energizes the drop1 which may be restored by hand or otherwise and will remain latched4 ,the visible signal is better adapted to lines in which the signaling is done by means of direct current1 as1 for example1 in systems where the removal of the receiver from the hook at the subscriber's station closes the line circuit1 causing current to flow through the winding of the visible signal and so displaying it until the receiver has been hung upon the hook or the circuit opened by some operation at the central office4 ,visible signals of the magnetic type of ,fig4 #bc have been widely used in connection with common-battery systems1 both for line signals and for supervisory purposes1 indicating the state and the progress of the connection and conversation4 ,illustration3 ,fig4 #bc4 ,electromagnetic ,visible ,signal ,illustration3 ,fig4 #bd4 ,lamp ,signal and ,lens ,electric-,lamp ,signal4 ,incandescent electric lamps appeared in telephony as a considerable element about #ahij4 ,they are better than either form of mechanical visible signals because of three principal qualities3 simplicity and ease of restoring them to normal as compared with drops2 their compactness2 and their greater prominence when displayed4 ,of the latter quality1 one may say that they are more insistent1 as they give out light instead of reflecting it1 as do all other visible signals4 ,in its best form1 the lamp signal is mounted behind a hemispherical lens1 either slightly clouded or cut in facets4 ,this lens serves to distribute the rays of light from the lamp1 with the result that the signal may be seen from a wide angle with the axis of the lens1 as shown in ,fig4 #bd4 ,this is of particular advantage in connection with manual-switchboard connecting cords1 as it enables the signals to be mounted close to and even among the cords1 their great visible prominence when shining saving them from being hidden4 ,the influence of the lamp signal was one of the potent ones in the development of the type of multiple switchboard which is now universal as the mechanism of large manual exchanges4 ,the first large trial of such an equipment was in #ahif in ,worcester1 ,mass4 ,no large and successful multiple switchboard with any other type of signal has been built since that time4 ,any electric signal has upper and lower limits of current between which it is to be actuated4 ,it must receive current enough to operate but not enough to become damaged by overheating4 ,the magnetic types of visible signals have a wider range between these limits than have lamp signals4 ,if current in a lamp is too little1 its filament either will not glow at all or merely at a dull red1 insufficient for a proper signal4 ,if the current is too great1 the filament is heated beyond its strength and parts at the weakest place4 ,this range between current limits in magnetic visible signals is great enough to enable them to be used direct in telephone lines1 the operating current through the line and signal in series with a fixed voltage at the central office being not harmfully great when the entire line resistance is shunted out at or near the central office4 ,the increase of current may be as great as ten times without damage to the winding of such a signal4 ,in lamps1 the safe margin is much less4 ,the current which just gives a sufficient lighting of the signal may be about doubled with safety to the filament of the lamp4 ,consequently it is not feasible to place the lamp directly in series with long exposed lines4 ,a short circuit of such a line near the central office will burn it out4 ,illustration3 ,fig4 #be4 ,lamp ,signal ,controlled by ,relay ,the qualities of electromagnets and lamps in these respects are used to advantage by the lamp signal arrangement shown in ,fig4 #be4 ,a relay is in series with the line and provides a large range of sensibility4 ,it is able to carry any current the central-office current source can pass through it4 ,the local circuit of the relay includes the lamp4 ,energizing the relay lights the lamp1 and the reverse2 the lamp is thus isolated from danger and receives the current best adapted to its needs4 ,all lines are not long and when enclosed in cable or in well-insulated interior wire1 may be only remotely in danger of being short-circuited4 ,such conditions exist in private-branch exchanges1 which are groups of telephones1 usually local to limited premises1 connected to a switchboard on those premises4 ,such a situation permits the omission of the line relay1 the lamp being directly in the line4 ,fig4 #bf shows the extreme simplicity of the arrangement1 containing no moving parts or costly elements4 ,lamps for such service have improved greatly since the demand began to grow4 ,the small bulk permitted by the need of compactness1 the high filament resistance required for simplicity of the general power scheme of the system1 and the need of considerable sturdiness in the completed thing have made the task a hard one4 ,the practical result1 however1 is a signal lamp which is highly satisfactory4 ,illustration3 ,fig4 #bf4 ,lamp ,signal ,directly in ,line ,illustration3 ,fig4 #bg4 ,lamp ,signal and ,ballast ,the nature of carbon and certain earths being that their conductivity rises with the temperature and that of metals being that their conductivity falls with the temperature1 has enabled the ,nernst lamp to be successful4 ,the same relation of properties has enabled incandescent-lamp signals to be connected direct to lines without relays1 but compensated against too great a current by causing the resistance in series with the lamp to be increased inversely as the resistance of the filament4 ,employment of a ballast resistance in this way is referred to in ,chapter ,,xi4 ,in ,fig4 #bg is shown its relation to a signal lamp directly in the line4 #a is the carbon-filament lamp2 #b is the ballast4 ,the latter's conductor is fine iron wire in a vacuum4 ,the resistance of the lamp falls as that of the ballast rises4 ,within certain limits1 these changes balance each other1 widening the range of allowable change in the total resistance of the line4 ,,chapter ,,iv ,,telephone ,,lines ,the line is a path over which the telephone current passes from telephone to telephone4 ,the term telephone line circuit is equivalent4 ,line and line circuit mean slightly different things to some persons1 line meaning the out-of-doors portion of the line and line circuit meaning the indoor portion1 composed of apparatus and associated wiring4 ,such shades of meaning are inevitable and serve useful purposes4 ,the opening definition hereof is accurate4 ,a telephone line consists of two conductors4 ,one of these conductors may be the earth2 the other always is some conducting material other than the earth--almost universally it is of metal and in the form of a wire4 ,a line using one wire and the earth as its pair of conductors has several defects1 to be discussed later herein4 ,both conductors of a line may be wires1 the earth serving as no part of the circuit1 and this is the best practice4 ,a line composed of one wire and the earth is called a grounded line2 a line composed of two wires not needing the earth as a conductor is called a metallic circuit4 ,in the earliest telephone practice1 all lines were grounded ones4 ,the wires were of iron1 supported by poles and insulated from them by glass1 earthenware1 or rubber insulators4 ,for certain uses1 such lines still represent good practice4 ,for telegraph service1 they represent the present standard practice4 ,copper is a better conductor than iron1 does not rust1 and when drawn into wire in such a way as to have a sufficient tensile strength to support itself is the best available conductor for telephone lines4 ,only one metal surpasses it in any quality for the purpose3 silver is a better conductor by #a or #b per cent4 ,copper is better than silver in strength and price4 ,in the open country1 telephone lines consist of bare wires of copper1 of iron1 of steel1 or of copper-covered steel supported on insulators borne by poles4 ,if the wires on the poles be many1 cross-arms carry four to ten wires each and the insulators are mounted on pins in the cross-arms4 ,if the wires on the poles be few1 the insulators are mounted on brackets nailed to the poles4 ,wires so carried are called open wires4 ,in towns and cities where many wires are to be carried along the same route1 the wires are reduced in size1 insulated by a covering over each1 and assembled into a group4 ,such a bundle of insulated wires is called a cable4 ,it may be drawn into a duct in the earth and be called an underground cable2 it may be laid on the bottom of the sea or other water and be called a submarine cable2 or it may be suspended on poles and be called an arial cable4 ,in the most general practice each wire is insulated from all others by a wrapping of paper ribbon1 which covering is only adequate when very dry4 ,cables formed of paper-insulated wires1 therefore1 are covered by a seamless1 continuous lead sheath1 no part of the paper insulation of the wires being exposed to the atmosphere during the cable's entire life in service4 ,telephone cables for certain uses are formed of wires insulated with such materials as soft rubber1 gutta-percha1 and cotton or jute saturated with mineral compounds4 ,when insulated with rubber or gutta-percha1 no continuous lead sheath is essential for insulation1 as those materials1 if continuous upon the wire1 insulate even when the cable is immersed in water4 ,sheaths and other armors can assist in protecting these insulating materials from mechanical injury1 and often are used for that purpose4 ,the uses to which such cables are suitable in telephony are not many1 as will be shown4 ,a wire supported on poles requires that it be large enough to support its own weight4 ,the smaller the wire1 the weaker it is1 and with poles a given distance apart1 the strength of the wire must be above a certain minimum4 ,in regions where freezing occurs1 wires in the open air can collect ice in winter and everywhere open wires are subject to wind pressure2 for these reasons additional strength is required4 ,speaking generally1 the practical and economical spacing of poles requires that wires1 to be strong enough to meet the above conditions1 shall have a diameter not less than 4#jh inch1 if of hard-drawn copper1 and 4#jfd inch1 if of iron or steel4 ,the honor of developing ways of drawing copper wire with sufficient tensile strength for open-air uses belongs to ,mr4 ,thomas ,b4 ,doolittle of ,massachusetts4 ,lines whose lengths are limited to a few miles do not require a conductivity as great as that of copper wire of 4#jh-;inch diameter4 ,a wire of that size weighs approximately #ajj pounds per mile4 ,less than #ajj pounds of copper per mile of wire will not give strength enough for use on poles2 but as little as #aj pounds per mile of wire gives the necessary conductivity for the lines of the thousands of telephone stations in towns and cities4 ,open wires1 being exposed to the elements1 suffer damage from storms2 their insulation is injured by contact with trees2 they may make contact with electric power circuits1 perhaps injuring apparatus1 themselves1 and persons2 they endanger life and property by the possibility of falling2 they and their cross-arm supports are less sightly than a more compact arrangement4 ,grouping small wires of telephone lines into cables has1 therefore1 the advantage of allowing less copper to be used1 of reducing the space required1 of improving appearance1 and of increasing safety4 ,on the other hand1 this same grouping introduces negative advantages as well as the foregoing positive ones4 ,it is not possible to talk as far or as well over a line in an ordinary cable as over a line of two open wires4 ,long-distance telephone circuits1 therefore1 have not yet been placed in cables for lengths greater than #bjj or #cjj miles1 and special treatment of cable circuits is required to talk through them for even #ajj miles4 ,one may talk #b1jjj miles over open wires4 ,the reasons for the superiority of the open wires have to do with position rather than material4 ,obviously it is possible to insulate and bury any wire which can be carried in the air4 ,the differences in the properties of lines whose wires are differently situated with reference to each other and surrounding things are interesting and important4 ,a telephone line composed of two conductors always possesses four principal properties in some amount3 7#a7 conductivity of the conductors2 7#b7 electrostatic capacity between the conductors2 7#c7 inductance of the circuit2 7#d7 insulation of each conductor from other things4 ,conductivity of ,conductors4 ,the conductivity of a wire depends upon its material1 its cross-section1 its length1 and its temperature4 ,conductivity of a copper wire1 for example1 increases in direct ratio to its weight1 in inverse ratio to its length1 and its conductivity falls as the temperature rises4 ,resistance is the reciprocal of conductivity and the properties1 conductivity and resistance1 are more often expressed in terms of resistance4 ,the unit of the latter is the ohm2 of the former the mho4 ,a conductor having a resistance of #ajj ohms has a conductivity of 4#ja mho4 ,the exact correlative terms are resistance and conductance1 resistivity and conductivity4 ,the use of the terms as in the foregoing is in accordance with colloquial practice4 ,current in a circuit having resistance only1 varies inversely as the resistance4 ,electromotive force being a cause1 and resistance a state1 current is the result4 ,the formula of this relation1 ,ohm's law1 is ,c ,e_/,r ,c being the current which results from ,e1 the electromotive force1 acting upon ,r1 the resistance4 ,the units are3 of current1 the ampere2 of electromotive force1 the volt2 of resistance1 the ohm4 ,as the conductivity or resistance of a line is the property of controlling importance in telegraphy1 a similar relation was expected in early telephony4 ,as the current in the telephone line varies rapidly1 certain other properties of the line assume an importance they do not have in telegraphy in any such degree4 ,the importance that these properties assume is1 that if they did not act and the resistance of the conductors alone limited speech1 transmission would be possible direct from ,europe to ,america over a pair of wires weighing #bjj pounds per mile of wire1 which is less than half the weight of the wire of the best long-distance land lines now in service4 ,the distance from ,europe to ,america is about twice as great as the present commercial radius by land lines of #dce-pound wire4 ,in other words1 good speech is possible through a mere resistance twenty times greater than the resistance of the longest actual open-wire line it is possible to talk through4 ,the talking ratio between a mere resistance and the resistance of a regular telephone cable is still greater4 ,electrostatic ,capacity4 ,it is the possession of electrostatic capacity which enables the condenser1 of which the ,leyden jar is a good example1 to be useful in a telephone line4 ,the simplest form of a condenser is illustrated in ,fig4 #bh1 in which two conducting surfaces are separated by an insulating material4 ,the larger the surfaces1 the closer they are together2 and the higher the specific inductive capacity of the insulator1 the greater the capacity of the device4 ,an insulator used in this relation to two conducting surfaces is called the dielectric4 ,illustration3 ,fig4 #bh4 ,simple ,condenser ,illustration3 ,fig4 #bi4 ,condenser ,symbols ,two conventional signs are used to illustrate condensers1 the upper one of ,fig4 #bi growing out of the original condenser of two metal plates1 the lower one suggesting the thought of interleaved conductors of tin foil1 as for many years was the practice in condenser construction4 ,with relation to this property1 a telephone line is just as truly a condenser as is any other arrangement of conductors and insulators4 ,assume such a line to be open at the distant end and its wires to be well insulated from each other and the earth4 ,telegraphy through such a line by ordinary means would be impossible4 ,all that the battery or other source could do would be to cause current to flow into the line for an infinitesimal time1 raising the wires to its potential1 after which no current would flow4 ,but1 by virtue of electrostatic capacity1 the condition is much as shown in ,fig4 #cj4 ,the condensers which that figure shows bridged across the line from wire to wire are intended merely to fix in the mind that there is a path for the transfer of electrical energy from wire to wire4 ,illustration3 ,fig4 #cj4 ,line with ,shunt ,capacity ,a simple test will enable two of the results of a short-circuiting capacity to be appreciated4 ,conceive a very short line of two wires to connect two local battery telephones4 ,such a line possesses negligible resistance1 inductance1 and shunt capacity4 ,its insulation is practically infinite4 ,let condensers be bridged across the line1 one by one1 while conversation goes on4 ,the listening observer will notice that the sounds reaching his ear steadily grow less loud as the capacity across the line increases4 ,the speaking observer will notice that the sounds he hears through the receiver in series with the line steadily grow louder as the capacity across the line increases4 ,fig4 #ca illustrates the test4 ,the speaker's observation in this test shows that increasing the capacity across the line increased the amount of current entering it4 ,the hearer's observation in this test shows that increasing the capacity across the line decreased the amount of energy turned into sound at his receiver4 ,illustration3 ,fig4 #ca4 ,test of ,line with ,varying ,shunt ,capacity ,the unit of electrostatic capacity is the farad4 ,as this unit is inconveniently large1 for practical applications the unit microfarad--millionth of a farad--is employed4 ,if quantities are known in microfarads and are to be used in calculations in which the values of the capacity require to be farads1 care should be taken to introduce the proper corrective factor4 ,the electrostatic capacity between the conductors of a telephone line depends upon their surface area1 their length1 their position1 and the nature of the materials separating them from each other and from other things4 ,for instance1 in an open wire line of two wires1 the electrostatic capacity depends upon the diameter of the wires1 upon the length of the line1 upon their distance apart1 upon their distance above the earth1 and upon the specific inductive capacity of the air4 ,air being so common an insulating medium1 it is taken as a convenient material whose specific inductive capacity may be used as a basis of reference4 ,therefore1 the specific inductive capacity of air is taken as unity4 ,all solid matter has higher specific inductive capacity than air4 ,the electrostatic capacity of two open wires 4#afe inch diameter1 #a ft4 apart1 and #cj ft4 above the earth1 is of the order of 4#jji microfarads per mile4 ,this quantity would be higher if the wires were closer together2 or nearer the earth2 or if they were surrounded by a gas other than the air or hydrogen2 or if the wires were insulated not by a gas but by any solid covering4 ,as another example1 a line composed of two wires of a diameter of 4#jcf inch1 if wrapped with paper and twisted into a pair as a part of a telephone-cable1 has a mutual electrostatic capacity of approximately 4#jh microfarads per mile1 this quantity being greater if the cable be more tightly compressed4 ,the use of paper as an insulator for wires in telephone cables is due to its low specific inductive capacity4 ,this is because the insulation of the wires is so largely dry air4 ,rubber and similar insulating materials give capacities as great as twice that of dry paper4 ,the condenser or other capacity acts as an effective barrier to the steady flow of direct currents4 ,applying a fixed potential causes a mere rush of current to charge its surface to a definite degree1 dependent upon the particular conditions4 ,the condenser does not act as such a barrier to alternating currents1 for it is possible to talk through a condenser by means of the alternating voice currents of telephony1 or to pass through it alternating currents of much lower frequency4 ,a condenser is used in series with a polarized ringer for the purpose of letting through alternating current for ringing the bell1 and of preventing the flow of direct current4 ,the degree to which the condenser allows alternating currents to pass while stopping direct currents1 depends on the capacity of the condenser and on the frequencies of alternating current4 ,the larger the condenser capacity or the higher the frequency of the alternations1 the greater will be the current passing through the circuit4 ,the degree to which the current is opposed by the capacity is the reactance of that capacity for that frequency4 ,the formula is ,capacity reactance #a _/,comega wherein ,c is the capacity in farads and omega is #bpin1 or twice #c4adaf times the frequency4 ,all the foregoing leads to the generalization that the higher the frequency1 the less the opposition of a capacity to an alternating current4 ,if the frequency be zero1 the reactance is infinite1 i4e41 the circuit is open to direct current4 ,if the frequency be infinite1 the reactance is zero1 i4e41 the circuit is as if the condenser were replaced by a solid conductor of no resistance4 ,compare this statement with the correlative generalization which follows the next thought upon inductance4 ,inductance of the ,circuit4 ,inductance is the property of a circuit by which change of current in it tends to produce in itself and other conductors an electromotive force other than that which causes the current4 ,its unit is the henry4 ,the inductance of a circuit is one henry when a change of one ampere per second produces an electromotive force of one volt4 ,induction between circuits occurs because the circuits possess inductance2 it is called mutual induction4 ,induction within a circuit occurs because the circuit possesses inductance2 it is called self-induction4 ,lenz' law says3 ,in all cases of electromagnetic induction1 the induced currents have such a direction that their reaction tends to stop the motion which produced them4 ,illustration3 ,fig4 #cb4 ,spiral of ,wire ,illustration3 ,fig4 #cc4 ,spiral of ,wire ,around ,iron ,core ,all conductors possess inductance1 but straight wires used in lines have negligible inductance in most actual cases4 ,all wires which are wound into coils1 such as electromagnets1 possess inductance in a greatly increased degree4 ,a wire wound into a spiral1 as indicated in ,fig4 #cb1 possesses much greater inductance than when drawn out straight4 ,if iron be inserted into the spiral1 as shown in ,fig4 #cc1 the inductance is still further increased4 ,it is for the purpose of eliminating inductance that resistance coils are wound with double wires1 so that current passing through such coils turns in one direction half the way and in the other direction the other half4 ,a simple test will enable the results of a series inductance in a line to be appreciated4 ,conceive a very short line of two wires to connect two local battery telephones4 ,such a line possesses negligible resistance1 inductance1 and shunt capacity4 ,its insulation is practically infinite4 ,let inductive coils such as electromagnets be inserted serially in the wires of the line one by one1 while conversation goes on4 ,the listening observer will notice that the sounds reaching his ear steadily grow faint as the inductance in the line increases and the speaking observer will notice the same thing through the receiver in series with the line4 ,both observations in this test show that the amount of current entering and emerging from the line decreased as the inductance increased4 ,compare this with the test with bridged capacity and the loading of lines described later herein1 observing the curious beneficial result when both hurtful properties are present in a line4 ,the test is illustrated in ,fig4 #cd4 ,the degree in which any current is opposed by inductance is termed the reactance of that inductance4 ,its formula is ,inductive reactance ,lomega wherein ,l is the inductance in henrys and omega is #bpin1 or twice #c4adaf times the frequency4 ,to distinguish the two kinds of reactance1 that due to the capacity is called capacity reactance and that due to inductance is called inductive reactance4 ,all the foregoing leads to the generalization that the higher the frequency1 the greater the opposition of an inductance to an alternating current4 ,if the frequency be zero1 the reactance is zero1 i4e41 the circuit conducts direct current as mere resistance4 ,if the frequency be infinite1 the reactance is infinite1 i4e41 the circuit is open to the alternating current and that current cannot pass through it4 ,compare this with the correlative generalization following the preceding thought upon capacity4 ,illustration3 ,fig4 #cd4 ,test of ,line with ,varying ,serial ,inductance ,capacity and inductance depend only on states of matter4 ,their reactances depend on states of matter and actions of energy4 ,in circuits having both resistance and capacity or resistance and inductance1 both properties affect the passage of current4 ,the joint reaction is expressed in ohms and is called impedance4 ,its value is the square root of the sum of the squares of the resistance and reactance1 or1 ,z being impedance1 ------------------------- _/ #a ,z _/ ,r#b ---------------- _/ ,c#bomega#b and -------------------------- ,z _/ ,r#b ,l#bomega#b _/ the symbols meaning as before4 ,in words1 these formulas mean that1 knowing the frequency of the current and the capacity of a condenser1 or the frequency of the current and the inductance of a circuit 7a line or piece of apparatus71 and in either case the resistance of the circuit1 one may learn the impedance by calculation4 ,insulation of ,conductors4 ,the fourth property of telephone lines1 insulation of the conductors1 usually is expressed in ohms as an insulation resistance4 ,in practice1 this property needs to be intrinsically high1 and usually is measured by millions of ohms resistance from the wire of a line to its mate or to the earth4 ,it is a convenience to employ a large unit4 ,a million ohms1 therefore1 is called a megohm4 ,in telephone cables1 an insulation resistance of #ejj megohms per mile at #fj ,fahrenheit is the usual specification4 ,so high an insulation resistance in a paper-insulated conductor is only attained by applying the lead sheath to the cable when its core is made practically anhydrous and kept so during the splicing and terminating of the cable4 ,insulation resistance varies inversely as the length of the conductor4 ,if a piece of cable #ebh feet long has an insulation resistance of #f1gej megohms1 a mile 7ten times as much7 of such cable1 will have an insulation resistance of #fge megohms1 or one-tenth as great4 ,inductance vs4 ,capacity4 ,the mutual capacity of a telephone line is greater as its wires are closer together4 ,the self-induction of a telephone line is smaller as its wires are closer together4 ,the electromotive force induced by the capacity of a line leads the impressed electromotive force by #ij degrees4 ,the inductive electromotive force lags #ij degrees behind the impressed electromotive force4 ,and so1 in general1 the natures of these two properties are opposite4 ,in a cable1 the wires are so close together that their induction is negligible1 while their capacity is so great as to limit commercial transmission through a cable having 4#jf microfarads per mile capacity and #id ohms loop resistance per mile1 to a distance of about #cj miles4 ,in the case of open wires spaced #ab inches apart1 the limit of commercial transmission is greater1 not only because the wires are larger1 but because the capacity is lower and the inductance higher4 ,table ,i shows-the practical limiting conversation distance over uniform lines with present standard telephone apparatus4 ,,table ,i ,limiting ,transmission ,distances ---------------------------------------- ----------- ,,size ,,and ,,gauge ,,of ,,wire ,,limiting ,,distance ---------------------------------------- ----------- ,no4 #h ,b4 ,w4 ,g4 copper #ijj miles #aj ,b4 ,w4 ,g4 copper #gjj miles #aj ,b4 ,s4 copper #djj miles #ab ,n4 ,b4 ,s4 copper #djj miles #ab ,b4 ,s4 copper #bdj miles #ad ,n4 ,b4 ,s4 copper #bdj miles #h ,b4 ,w4 ,g4 iron #ace miles #aj ,b4 ,w4 ,g4 iron #abj miles #ab ,b4 ,w4 ,g4 iron #ij miles #af ,b4 ,s4 cable1 copper #dj miles #ai ,b4 ,s4 cable1 copper #cj miles #bb ,b4 ,s4 cable1 copper #bj miles ---------------------------------------- ----------- ,in #ahic1 ,oliver ,heaviside proposed that the inductance of telephone lines be increased above the amount natural for the inter-axial spacing1 with a view to counteracting the hurtful effects of the capacity4 ,his meaning was that the increased inductance--a harmful quality in a circuit not having also a harmfully great capacity--would act oppositely to the capacity1 and if properly chosen and applied1 should decrease or eliminate distortion by making the line's effect on fundamentals and harmonics more nearly uniform1 and as well should reduce the attenuation by neutralizing the action of the capacity in dissipating energy4 ,there are two ways in which inductance might be introduced into a telephone line4 ,as the capacity whose effects are to be neutralized is distributed uniformly throughout the line1 the counteracting inductance must also be distributed throughout the line4 ,mere increase of distance between two wires of the line very happily acts both to increase the inductance and to lower the capacity2 unhappily for practical results1 the increase of separation to bring the qualities into useful neutralizing relation is beyond practical limits4 ,the wires would need to be so far above the earth and so far apart as to make the arrangement commercially impossible4 ,practical results have been secured in increasing the distributed inductance by wrapping fine iron wire over each conductor of the line4 ,such a treatment increases the inductance and improves transmission4 ,the most marked success has come as a result of the studies of ,professor ,michael ,idvorsky ,pupin4 ,he inserts inductances in series with the wires of the line1 so adapting them to the constants of the circuit that attenuation and distortion are diminished in a gratifying degree4 ,this method of counteracting the effects of a distributed capacity by the insertion of localized inductance requires not only that the requisite total amount of inductance be known1 but that the proper subdivision and spacing of the local portions of that inductance be known4 ,professor ,pupin's method is described in a paper entitled ,wave ,transmission ,over ,non-uniform ,cables and ,long-,distance ,air ,lines1 read by him at a meeting of the ,american ,institute of ,electrical ,engineers in ,philadelphia1 ,may #ai1 #aijj4 ,,note4 ,united ,states ,letters ,patent were issued to ,professor ,pupin on ,june #ai1 #aijj1 upon his practical method of reducing attenuation of electrical waves4 ,a paper upon ,propagation of ,long ,electric ,waves was read by ,professor ,pupin before the ,american ,institute of ,electrical ,engineers on ,march #bb1 #ahii1 and appears in ,vol4 #ae of the ,transactions of that society4 ,the student will find these documents useful in his studies on the subject4 ,he is referred also to ,electrical ,papers and ,electromagnetic ,theory of ,oliver ,heaviside4 ,professor ,pupin likens the transmission of electric waves over long-distance circuits to the transmission of mechanical waves over a string4 ,conceive an ordinary light string to be fixed at one end and shaken by the hand at the other2 waves will pass over the string from the shaken to the fixed end4 ,certain reflections will occur from the fixed end4 ,the amount of energy which can be sent in this case from the shaken to the fixed point is small1 but if the string be loaded by attaching bullets to it1 uniformly throughout its length1 it now may transmit much more energy to the fixed end4 ,illustration3 ,,main ,,entrance ,,and ,,public ,,office1 ,,san ,,francisco ,,home ,,telephone ,,company ,contract ,department on ,left4 ,accounting ,department on ,right4 ,the addition of inductance to a telephone line is analogous to the addition of bullets to the string1 so that a telephone line is said to be loaded when inductances are inserted in it1 and the inductances themselves are known as loading coils4 ,fig4 #ce shows the general relation of ,pupin loading coils to the capacity of the line4 ,the condensers of the figure are merely conventionals to represent the condenser which the line itself forms4 ,the inductances of the figure are the actual loading coils4 ,illustration3 ,fig4 #ce4 ,loaded ,line ,the loading of open wires is not as successful in practice as is that of cables4 ,the fundamental reason lies in the fact that two of the properties of open wires--insulation and capacity--vary with atmospheric change4 ,the inserted inductance remaining constant1 its benefits may become detriments when the other two constants change4 ,the loading of cable circuits is not subject to these defects4 ,such loading improves transmission2 saves copper2 permits the use of longer underground cables than are usable when not loaded2 lowers maintenance costs by placing interurban cables underground2 and permits submarine telephone cables to join places not otherwise able to speak with each other4 ,underground long-distance lines now join or are joining ,boston and ,new ,york1 ,philadelphia and ,new ,york1 ,milwaukee and ,chicago4 ,england and ,france are connected by a loaded submarine cable4 ,there is no theoretical reason why ,europe and ,america should not speak to each other4 ,the student wishing to determine for himself what are the effects of the properties of lines upon open or cable circuits will find most of the subject in the following equation4 ,it tells the value of a in terms of the four properties1 a being the attenuation constant of the line4 ,that is1 the larger a is1 the more the voice current is reduced in passing over the line4 ,the equation is ---------------------------------------- ------------------------------- _/ ---------------------------------------- ------- a _/ _/7,r#b,l#bomega#b77,s#b;,c#bomega#b 7,,rs-,,lcomega#b _/ _/ ,the quantities are ,r ,resistance in ohms ,l ,inductance in henrys ,c ,mutual 7shunt7 capacity in farads omega #bpin #f4bhcb times the frequency ,s ,shunt leakage in mhos ,the quantity ,s is a measure of the combined direct-current conductance 7reciprocal of insulation resistance7 and the apparent conductance due to dielectric hysteresis4 ,,note4 ,an excellent paper1 assisting such study1 and of immediate practical value as helping the understanding of cables and their reasons1 is that of ,mr4 ,frank ,b4 ,jewett1 presented at the ,thousand ,islands ,convention of the ,american ,institute of ,electrical ,engineers1 ,july #a1 #aiji4 ,chapter #dc treats cables in further detail4 ,they form a most important part of telephone wire-plant practice1 and their uses are becoming wider and more valuable4 ,possible ,ways of ,improving ,transmission4 ,practical ways of improving telephone transmission are of two kinds3 to improve the lines and to improve the apparatus4 ,the foregoing shows what are the qualities of lines and the ways they require to be treated4 ,apparatus treatment1 in the present state of the art1 is addressed largely to the reduction of losses4 ,theoretical considerations seem to show1 however1 that great advance in apparatus effectiveness still is possible4 ,more powerful transmitters--and more faithful ones--more sensitive and accurate receivers1 and more efficient translating devices surely are possible4 ,discovery may need to intervene1 to enable invention to restimulate4 ,in both telegraphy and telephony1 the longer the line the weaker the current which is received at the distant end4 ,in both telegraphy and telephony1 there is a length of line with a given kind and size of wire and method of construction over which it is just possible to send intelligible speech or intelligible signals4 ,a repeater1 in telegraphy1 is a device in the form of a relay which is adapted to receive these highly attenuated signal impulses and to re-transmit them with fresh power over a new length of line4 ,an arrangement of two such relays makes it possible to telegraph both ways over a pair of lines united by such a repeater4 ,it is practically possible to join up several such links of lines to repeating devices and1 if need be1 even submarine cables can be joined to land lines within practical limits4 ,if it were necessary1 it probably would be possible to telegraph around the world in this way4 ,if it were possible to imitate the telegraph repeater in telephony1 attenuated voice currents might be caused to actuate it so as to send on those voice currents with renewed power over a length of line1 section by section4 ,such a device has been sought for many years1 and it once was quoted in the public press that a reward of one million dollars had been offered by ,charles ,j4 ,glidden for a successful device of that kind4 ,the records of the patent offices of the world show what effort has been made in that direction and many more devices have been invented than have been patented in all the countries together4 ,like some other problems in telephony1 this one seems simpler at first sight than it proves to be after more exhaustive study4 ,it is possible for any amateur to produce at once a repeating device which will relay telephone circuits in one direction4 ,it is required1 however1 that in practice the voice currents be relayed in both directions1 and further1 that the relay actually augment the energy which passes through it2 that is1 that it will send on a more powerful current than it receives4 ,most of the devices so far invented fail in one or the other of these particulars4 ,several ways have been shown of assembling repeating devices which will talk both ways1 but not many assembling repeating devices have been shown that will talk both ways and augment in both directions4 ,illustration3 ,fig4 #cf4 ,shreeve ,repeater and ,circuit ,practical repeaters have been produced1 however1 and at least one type is in daily successful use4 ,it is not conclusively shown even of it that it augments in the same degree all of the voice waves which reach it1 or even that it augments some of them at all4 ,its action1 however1 is distinctly an improvement in commercial practice4 ,it is the invention of ,mr4 ,herbert ,e4 ,shreeve and is shown in ,fig4 #ci4 ,primarily it consists of a telephone receiver1 of a particular type devised by ,gundlach1 associated with a granular carbon transmitter button4 ,it is further associated with an arrangement of induction coils or repeating coils1 the object of these being to accomplish the two-way action1 that is1 of speaking in both directions and of preventing reactive interference between the receiving and transmitting elements4 ,the battery #a energizes the field of the receiving element2 the received line current varies that field2 the resulting motion varies the resistance of the carbon button and transforms current from battery #b into a new alternating line current4 ,by reactive interference is meant action whereby the transmitter element1 in emitting a wave1 affects its own controlling receiver element1 thus setting up an action similar to that which occurs when the receiver of a telephone is held close to its transmitter and humming or singing ensues4 ,no repeater is successful unless it is free from this reactive interference4 ,illustration3 ,fig4 #cg4 ,mercury-,arc ,telephone ,relay ,enough has been accomplished by practical tests of the ,shreeve device and others like it to show that the search for a method of relaying telephone voice currents is not looking for a pot of gold at the end of the rainbow4 ,the most remarkable truth established by the success of repeaters of the ,shreeve type is that a device embodying so large inertia of moving parts can succeed at all4 ,if this mean anything1 it is that a device in which inertia is absolutely eliminated might do very much better4 ,many of the methods already proposed by inventors attack the problem in this way and one of the most recent and most promising ways is that of ,mr4 ,j4,b4 ,taylor1 the circuit of whose telephone-relay patent is shown in ,fig4 #cg4 ,in it1 #a is an electromagnet energized by voice currents2 its varying field varies an arc between the electrodes #b-b and #c in a vacuum tube4 ,these fluctuations are transformed into line currents by the coil #d4 ,,chapter ,v ,,transmitters ,variable ,resistance4 ,as already pointed out in ,chapter ,,ii1 the variable-resistance method of producing current waves1 corresponding to sound waves for telephonic transmission1 is the one that lends itself most readily to practical purposes4 ,practically all telephone transmitters of today employ this variable-resistance principle4 ,the reason for the adoption of this method instead of the other possible ones is that the devices acting on this principle are capable1 with great simplicity of construction1 of producing much more powerful results than the others4 ,their simplicity is such as to make them capable of being manufactured at low cost and of being used successfully by unskilled persons4 ,materials4 ,of all the materials available for the variable-resistance element in telephone transmitters1 carbon is by far the most suitable1 and its use is well nigh universal4 ,sometimes one of the rarer metals1 such as platinum or gold1 is to be found in commercial transmitters as part of the resistance-varying device1 but1 even when this is so1 it is always used in combination with carbon in some form or other4 ,most of the transmitters in use1 however1 depend solely upon carbon as the conductive material of the variable-resistance element4 ,arrangement of ,electrodes4 ,following the principles pointed out by ,hughes1 the transmitters of today always employ as their variable-resistance elements one or more loose contacts between one or more pairs of electrodes1 which electrodes1 as just stated1 are usually of carbon4 ,always the arrangement is such that the sound waves will vary the intimacy of contact between the electrodes and1 therefore1 the resistance of the path through the electrodes4 ,a multitude of arrangements have been proposed and tried4 ,sometimes a single pair of electrodes has been employed having a single point of loose contact between them4 ,these may be termed single-contact transmitters4 ,sometimes the variable-resistance element has included a greater number of electrodes arranged in multiple1 or in series1 or in series-multiple1 and these have been termed multiple-electrode transmitters1 signifying a plurality of electrodes4 ,a later development1 an outgrowth of the multiple-electrode transmitter1 makes use of a pair of principal electrodes1 between which is included a mass of finely divided carbon in the form of granules or small spheres or pellets4 ,these1 regardless of the exact form of the carbon particles1 are called granular-carbon transmitters4 ,illustration3 ,fig4 #ch4 ,blake ,transmitter ,single ,electrode4 ,blake4 ,the most notable example of the single-contact transmitter is the once familiar ,blake instrument4 ,at one time this formed a part of the standard equipment of almost every telephone in the ,united ,states1 and it was also largely used abroad4 ,probably no transmitter has ever exceeded it in clearness of articulation1 but it was decidedly deficient in power in comparison with the modern transmitters4 ,in this instrument1 which is shown in ,fig4 #ch1 the variable-resistance contact was that between a carbon and a platinum electrode4 ,the diaphragm #a was of sheet iron mounted1 as usual in later transmitters1 in a soft rubber gasket #b4 ,the whole diaphragm was mounted in a cast-iron ring #c1 supported on the inside of the box containing the entire instrument4 ,the front electrode #d was mounted on a light spring #e1 the upper end of which was supported by a movable bar or lever #f1 flexibly supported on a spring #g secured to the casting which supported the diaphragm4 ,the tension of this spring #e was such as to cause the platinum point to press lightly away from the center of the diaphragm4 ,the rear electrode was of carbon in the form of a small block #i1 secured in a heavy brass button #aj4 ,the entire rear electrode structure was supported on a heavier spring #aa carried on the same lever as the spring #e4 ,the tension of this latter spring was such as to press against the front electrode and1 by its greater strength1 press this against the center of the diaphragm4 ,the adjustment of the instrument was secured by means of the screw #ab1 carried in a lug extending rearwardly from the diaphragm supporting casting1 this screw1 by its position1 determining the strength with which the rear electrode pressed against the front electrode and that against the diaphragm4 ,this instrument was ordinarily mounted in a wooden box together with the induction coil1 which is shown in the upper portion of the figure4 ,the ,blake transmitter has passed almost entirely out of use in this country1 being superseded by the various forms of granular instruments1 which1 while much more powerful1 are not perhaps capable of producing quite such clear and distinct articulation4 ,the great trouble with the single-contact transmitters1 such as the ,blake1 was that it was impossible to pass enough current through the single point of contact to secure the desired power of transmission without overheating the contact4 ,if too much current is sent through such transmitters1 an undue amount of heat is generated at the point of contact and a vibration is set up which causes a peculiar humming or squealing sound which interferes with the transmission of other sounds4 ,multiple ,electrode4 ,to remedy this difficulty the so-called multiple-electrode transmitter was brought out4 ,this took a very great number of forms1 of which the one shown in ,fig4 #ci is typical4 ,the diaphragm shown at #a1 in this particular form1 was made of thin pine wood4 ,on the rear side of this1 suspended from a rod #c carried in a bracket #d1 were a number of carbon rods or pendants #e1 loosely resting against a rod #b1 carried on a bracket #f also mounted on the rear of the diaphragm4 ,the pivotal rod #c and the rod #b1 against which the pendants rested1 were sometimes1 like the pendant rods1 made of carbon and sometimes of metal1 such as brass4 ,when the diaphragm vibrated1 the intimacy of contact between the pendant rod #e and the rod #b was altered1 and thus the resistance of the path through all of the pendant rods in multiple was changed4 ,illustration3 ,fig4 #ci4 ,multiple-,electrode ,transmitter ,a multitude of forms of such transmitters came into use in the early eighties1 and while they in some measure remedied the difficulty encountered with the ,blake transmitter1 i4e41 of not being able to carry a sufficiently large current1 they were all subject to the effects of extreme sensitiveness1 and would rattle or break when called upon to transmit sounds of more than ordinary loudness4 ,furthermore1 the presence of such large masses of material1 which it was necessary to throw into vibration by the sound waves1 was distinctly against this form of transmitter4 ,the inertia of the moving parts was so great that clearness of articulation was interfered with4 ,granular ,carbon4 ,the idea of employing a mass of granular carbon1 supported between two electrodes1 one of which vibrated with the sound waves and the other was stationary1 was proposed by ,henry ,hunnings in the early eighties4 ,while this idea forms the basis of all modern telephone transmitters1 yet it did not prevent the almost universal adoption of the single-contact form of instrument during the next decade4 ,western ,electric ,solid-,back ,transmitter4 ,in the early nineties1 however1 the granular-carbon transmitter came into its own with the advent and wide adoption of the transmitter designed by ,anthony ,c4 ,white1 known as the ,white1 or solid-back1 transmitter4 ,this has for many years been the standard instrument of the ,bell companies operating throughout the ,united ,states1 and has found large use abroad4 ,a horizontal cross-section of this instrument is shown in ,fig4 #dj1 and a rear view of the working parts in ,fig4 #da4 ,the working parts are all mounted on the front casting #a4 ,this is supported in a cup #b1 in turn supported on the lug #c1 which is pivoted on the transmitter arm or other support4 ,the front and rear electrodes of this instrument are formed of thin carbon disks shown in solid black4 ,the rear electrode1 the larger one of these disks1 is securely attached by solder to the face of a brass disk having a rearwardly projecting screw-threaded shank1 which serves to hold it and the rear electrode in place in the bottom of a heavy brass cup #d4 ,the front electrode is mounted on the rear face of a stud4 ,clamped against the head of this stud1 by a screw-threaded clamping ring #g1 is a mica washer1 or disk #f4 ,the center portion of this mica washer is therefore rigid with respect to the front electrode and partakes of its movements4 ,the outer edge of this mica washer is similarly clamped against the front edge of the cup #d1 a screw-threaded ring #i serving to hold the edge of the mica rigidly against the front of the cup4 ,the outer edge of this washer is1 therefore1 rigid with respect to the rear electrode1 which is fixed4 ,whatever relative movement there is between the two electrodes must1 therefore1 be permitted by the flexing of the mica washer4 ,this mica washer not only serves to maintain the electrodes in their normal relative positions1 but also serves to close the chamber which contains the electrodes1 and1 therefore1 to prevent the granular carbon1 with which the space between the electrodes is filled1 from falling out4 ,illustration3 ,fig4 #dj4 ,white ,solid-,back ,transmitter ,the cup #d1 containing the electrode chamber1 is rigidly fastened with respect to the body of the transmitter by a rearwardly projecting shank held in a bridge piece #h which is secured at its ends to the front block4 ,the needed rigidity of the rear electrode is thus obtained and this is probably the reason for calling the instrument the solid-back4 ,the front electrode1 on the other hand1 is fastened to the center of the diaphragm by means of a shank on the stud1 which passes through a hole in the diaphragm and is clamped thereto by two small nuts4 ,against the rear face of the diaphragm of this transmitter there rest two damping springs4 ,these are not shown in ,fig4 #dj but are in ,fig4 #da4 ,they are secured at one end to the rear flange of the front casting #a1 and bear with their other or free ends against the rear face of the diaphragm4 ,the damping springs are prevented from coming into actual contact with the diaphragm by small insulating pads4 ,the purpose of the damping springs is to reduce the sensitiveness of the diaphragm to extraneous sounds4 ,as a result1 the ,white transmitter does not pick up all of the sounds in its vicinity as readily as do the more sensitive transmitters1 and thus the transmission is not interfered with by extraneous noises4 ,on the other hand1 the provision of these heavy damping springs makes it necessary that this transmitter shall be spoken into directly by the user4 ,illustration3 ,fig4 #da4 ,white ,solid-,back ,transmitter ,the action of this transmitter is as follows3 ,sound waves are concentrated against the center of the diaphragm by the mouth-piece1 which is of the familiar form4 ,these waves impinge against the diaphragm1 causing it to vibrate1 and this1 in turn1 produces similar vibrations in the front electrode4 ,the vibrations of the front electrode are permitted by the elasticity of the mica washer #f4 ,the rear electrode is1 however1 held stationary within the heavy chambered block #d and which in turn is held immovable by its rigid mounting4 ,as a result1 the front electrode approaches and recedes from the rear electrode1 thus compressing and decompressing the mass of granular carbon between them4 ,as a result1 the intimacy of contact between the electrode plates and the granules and also between the granules themselves is altered1 and the resistance of the path from one electrode to the other through the mass of granules is varied4 ,new ,western ,electric ,transmitter4 ,the ,white transmitter was the prototype of a large number of others embodying the same features of having the rear electrode mounted in a stationary cup or chamber and the front electrode movable with the diaphragm1 a washer of mica or other flexible insulating material serving to close the front of the electrode chamber and at the same time to permit the necessary vibration of the front electrode with the diaphragm4 ,illustration3 ,fig4 #db4 ,new ,western ,electric ,transmitter ,one of these transmitters1 embodying these same features but with modified details1 is shown in ,fig4 #db1 this being the new transmitter manufactured by the ,western ,electric ,company4 ,in this the bridge of the original ,white transmitter is dispensed with1 the electrode chamber being supported by a pressed metal cup #a1 which supports the chamber as a whole4 ,the electrode cup1 instead of being made of a solid block as in the ,white instrument1 is composed of two portions1 a cylindrical or tubular portion #b and a back #c4 ,the cylindrical portion is externally screw-threaded so as to engage an internal screw thread in a flanged opening in the center of the cup #a4 ,by this means the electrode chamber is held in place in the cup #a1 and by the same means the mica washer #d is clamped between the flange in this opening and the tubular portion #b of the electrode chamber4 ,the front electrode is carried1 as in the ,white transmitter1 on the mica washer and is rigidly attached to the center of the diaphragm so as to partake of the movement thereof4 ,it will be seen1 therefore1 that this is essentially a ,white transmitter1 but with a modified mounting for the electrode chamber4 ,a feature in this transmitter that is not found in the ,white transmitter is that both the front and the rear electrodes1 in fact1 the entire working portions of the transmitter1 are insulated from the exposed metal parts of the instrument4 ,this is accomplished by insulating the diaphragm and the supporting cup #a from the transmitter front4 ,the terminal #e on the cup #a forms the electrical connection for the rear electrode1 while the terminal #f1 which is mounted on but insulated from the cup #a and is connected with the front electrode by a thin flexible connecting strip1 forms the electrical connection for the front electrode4 ,kellogg ,transmitter4 ,the transmitter of the ,kellogg ,switchboard and ,supply ,company1 originally developed by ,mr4 ,w4,w4 ,dean and modified by his successors in the ,kellogg ,company1 is shown in ,fig4 #dc4 ,in this1 the electrode chamber1 instead of being mounted in a stationary and rigid position1 as in the case of the ,white instrument1 is mounted on1 and1 in fact1 forms a part of the diaphragm4 ,the electrode which is associated with the mica washer instead of moving with the diaphragm1 as in the ,white instrument1 is rigidly connected to a bridge so as to be as free as possible from all vibrations4 ,referring to ,fig4 #dc1 which is a horizontal cross-section of the instrument1 #a indicates the diaphragm4 ,this is of aluminum and it has in its center a forwardly deflected portion forming a chamber for the electrodes4 ,the front electrode #b of carbon is backed by a disk of brass and rigidly secured in the front of this chamber1 as clearly indicated4 ,the rear electrode #c1 also of carbon1 is backed by a disk of brass1 and is clamped against the central portion of a mica disk by means of the enlarged head of stud #f4 ,a nut #g1 engaging the end of a screw-threaded shank from the back of the rear electrode1 serves to bind these two parts together securely1 clamping the mica washer between them4 ,the outer edge of the mica washer is clamped to the main diaphragm #a by an aluminum ring and rivets1 as clearly indicated4 ,it is seen1 therefore1 that the diaphragm itself contains the electrode chamber as an integral part thereof4 ,the entire structure of the diaphragm1 the front and back electrodes1 and the granular carbon within are permanently assembled in the factory and cannot be dissociated without destroying some of the parts4 ,the rear electrode is held rigidly in place by the bridge #e and the stud #f1 this stud passing through a block #i mounted on the bridge but insulated from it4 ,the stud #f is clamped in the block #i by means of the set screw #h1 so as to hold the rear electrode in proper position after this position has been determined4 ,illustration3 ,fig4 #dc4 ,kellogg ,transmitter ,in this transmitter1 as in the transmitter shown in ,fig4 #db1 all of the working parts are insulated from the exposed metal casing4 ,the diaphragm is insulated from the front of the instrument by means of a washer #d of impregnated cloth1 as indicated4 ,the rear electrode is insulated from the other portions of the instrument by means of the mica washer and by means of the insulation between the block #i and the bridge #e4 ,the terminal for the rear electrode is mounted on the block #i1 while the terminal for the front electrode1 shown at #aj1 is mounted on1 but insulated from1 the bridge4 ,this terminal #aj is connected with the diaphragm and therefore with the front electrode by means of a thin1 flexible metallic connection4 ,this transmitter is provided with damping springs similar to those of the ,white instrument4 ,it is claimed by advocates of this type of instrument that1 in addition to the ordinary action due to the compression and decompression of the granular carbon between the electrodes1 there exists another action due to the agitation of the granules as the chamber is caused to vibrate by the sound waves4 ,in other words1 in addition to the ordinary action1 which may be termed the piston action between the electrodes1 it is claimed that the general shaking-up effect of the granules when the chamber vibrates produces an added effect4 ,certain it is1 however1 that transmitters of this general type are very efficient and have proven their capability of giving satisfactory service through long periods of time4 ,another interesting feature of this instrument as it is now manufactured is the use of a transmitter front that is struck up from sheet metal rather than the employment of a casting as has ordinarily been the practice4 ,the formation of the supporting lug for the transmitter from the sheet metal which forms the rear casing or shell of the instrument is also an interesting feature4 ,automatic ,electric ,company ,transmitter4 ,the transmitter of the ,automatic ,electric ,company1 of ,chicago1 shown in ,fig4 #dd1 is of the same general type as the one just discussed1 in that the electrode chamber is mounted on and vibrates with the diaphragm instead of being rigidly supported on the bridge as in the case of the ,white or solid-back type of instrument4 ,in this instrument the transmitter front #a is struck up from sheet metal and contains a rearwardly projecting flange1 carrying an internal screw thread4 ,a heavy inner cup #b1 together with the diaphragm #c1 form an enclosure containing the electrode chamber4 ,the diaphragm is1 in this case1 permanently secured at its edge to the periphery of the inner cup #b by a band of metal #d so formed as to embrace the edges of both the cup and the diaphragm and permanently lock them together4 ,this inner chamber is held in place in the transmitter front #a by means of a lock ring #e externally screw-threaded to engage the internal screw-thread on the flange on the front4 ,the electrode chamber proper is made in the form of a cup1 rigidly secured to the diaphragm so as to move therewith1 as clearly indicated4 ,the rear electrode is mounted on a screw-threaded stud carried in a block which is fitted to a close central opening in the cup #b4 ,this transmitter does not make use of a mica washer or diaphragm1 but employs a felt washer which surrounds the shank of the rear electrode and serves to close and seal the carbon containing cup4 ,by this means the granular carbon is retained in the chamber and the necessary flexibility or freedom of motion is permitted between the front and the rear electrodes4 ,as in the ,kellogg and the later ,bell instruments1 the entire working parts of this transmitter are insulated from the metal containing case1 the inner chamber1 formed by the cup #b and the diaphragm #c1 being insulated from the transmitter front and its locking ring by means of insulating washers1 as shown4 ,fig4 #dd4 ,automatic ,electric ,company ,transmitter ,monarch ,transmitter4 ,the transmitter of the ,monarch ,telephone ,manufacturing ,company1 shown in ,fig4 #de1 differs from both the stationary-cup and the vibrating-cup types1 although it has the characteristics of both4 ,it might be said that it differs from each of these two types of transmitters in that it has the characteristics of both4 ,this transmitter1 it will be seen1 has two flexible mica washers between the electrodes and the walls of the electrode cup4 ,the front and the back electrodes are attached to the diaphragm and the bridge1 respectively1 by a method similar to that employed in the solid-back transmitters1 while the carbon chamber itself is free to vibrate with the diaphragm as is characteristic of the ,kellogg transmitter4 ,illustration3 ,fig4 #de4 ,monarch ,transmitter ,an aluminum diaphragm is employed1 the circumferential edge of which is forwardly deflected to form a seat4 ,the edge of the diaphragm rests against and is separated from the brass front by means of a one-piece gasket of specially treated linen4 ,this forms an insulator which is not affected by heat or moisture4 ,as in the transmitters previously described1 the electrodes are firmly soldered to brass disks which have solid studs extending from their centers4 ,in the case of both the front and the rear electrodes1 a mica disk is placed over the supporting stud and held in place by a brass hub which has a base of the same size as the electrode4 ,the carbon-chamber wall consists of a brass ring to which are fastened the mica disks of the front and the back electrodes by means of brass collars clamped over the edge of the mica and around the rim of the brass ring forming the chamber4 ,illustration3 ,,main ,,office ,,building1 ,,berkeley1 ,,california ,containing ,automatic ,equipment1 ,forming ,part of ,larger ,system ,operating in ,san ,francisco and ,vicinity4 ,bay ,cities ,home ,telephone ,company4 ,electrodes4 ,the electrode plates of nearly all modern transmitters are of specially treated carbon4 ,these are first copper-plated and soldered to their brass supporting disks4 ,after this they are turned and ground so as to be truly circular in form and to present absolutely flat faces toward each other4 ,these faces are then highly polished and the utmost effort is made to keep them absolutely clean4 ,great pains are taken to remove from the pores of the carbon1 as well as from the surface1 all of the acids or other chemicals that may have entered them during the process of electroplating them or of soldering them to the brass supporting disk4 ,that the two electrodes1 when mounted in a transmitter1 should be parallel with each other1 is an item of great importance as will be pointed out later4 ,in a few cases1 as previously stated1 gold or platinum has been substituted for the carbon electrodes in transmitters4 ,these are capable of giving good results when used in connection with the proper form of granular carbon1 but1 on the whole1 the tendency has been to abandon all forms of electrode material except carbon1 and its use is now well nigh universal4 ,preparation of ,carbon4 ,the granular carbon is prepared from carefully selected anthracite coal1 which is specially treated by roasting or re-carbonizing and is then crushed to approximately the proper fineness4 ,the crushed carbon is then screened with extreme care to eliminate all dust and to retain only granules of uniform size4 ,packing4 ,in the earlier forms of granular-carbon transmitters a great deal of trouble was experienced due to the so-called packing of the instrument4 ,this1 as the term indicates1 was a trouble due to the tendency of the carbon granules to settle into a compact mass and thus not respond to the variable pressure4 ,this was sometimes due to the presence of moisture in the electrode chamber2 sometimes to the employment of granules of varying sizes1 so that they would finally arrange themselves under the vibration of the diaphragm into a fairly compact mass2 or sometimes1 and more frequently1 to the granules in some way wedging the two electrodes apart and holding them at a greater distance from each other than their normal distance4 ,the trouble due to moisture has been entirely eliminated by so sealing the granule chambers as to prevent the entrance of moisture4 ,the trouble due to the lack of uniformity in size of the granules has been entirely eliminated by making them all of one size and by making them of sufficient hardness so that they would not break up into granules of smaller size4 ,the trouble due to the settling of the granules and wedging the electrodes apart has been practically eliminated in well-designed instruments1 by great mechanical nicety in manufacture4 ,almost any transmitter may be packed by drawing the diaphragm forward so as to widely separate the electrodes4 ,this allows the granules to settle to a lower level than they normally occupy and when the diaphragm is released and attempts to resume its normal position it is prevented from doing so by the mass of granules between4 ,transmitters of the early types could be packed by placing the lips against the mouthpiece and drawing in the breath4 ,the slots now provided at the base of standard mouthpieces effectually prevent this4 ,in general it may be said that the packing difficulty has been almost entirely eliminated1 not by the employment of remedial devices1 such as those often proposed for stirring up the carbon1 but by preventing the trouble by the design and manufacture of the instruments in such forms that they will not be subject to the evil4 ,carrying ,capacity4 ,obviously1 the power of a transmitter is dependent on the amount of current that it may carry1 as well as on the amount of variation that it may make in the resistance of the path through it4 ,granular carbon transmitters are capable of carrying much heavier current than the old ,blake or other single or multiple electrode types4 ,if forced to carry too much current1 however1 the same frying or sizzling sound is noticeable as in the earlier types4 ,this is due to the heating of the electrodes and to small arcs that occur between the electrodes and the granules4 ,one way to increase the current-carrying capacity of a transmitter is to increase the area of its electrodes1 but a limit is soon reached in this direction owing to the increased inertia of the moving electrode1 which necessarily comes with its larger size4 ,the carrying capacity of transmitters may also be increased by providing special means for carrying away the heat generated in the variable-resistance medium4 ,several schemes have been proposed for this4 ,one is to employ unusually heavy metal for the electrode chamber1 and this practice is best exemplified in the ,white solid-back instrument4 ,it has also been proposed by others to water-jacket the electrode chamber1 and also to keep it cool by placing it in close proximity to the relatively cool joints of a thermopile4 ,neither of these two latter schemes seems to be warranted in ordinary commercial practice4 ,sensitiveness4 ,in all the transmitters so far discussed damping springs of one form or another have been employed to reduce the sensitiveness of the instrument4 ,for ordinary commercial use too great a degree of sensitiveness is a fault1 as has already been pointed out4 ,there are1 however1 certain adaptations of the telephone transmitter which make a maximum degree of sensitiveness desirable4 ,one of these adaptations is found in the telephone equipments for assisting partially deaf people to hear4 ,in these the transmitter is carried on some portion of the body of the deaf person1 the receiver is strapped or otherwise held at his ear1 and a battery for furnishing the current is carried in his pocket4 ,it is not feasible1 for this sort of use1 that the sound which this transmitter is to reproduce shall always occur immediately in front of the transmitter4 ,it more often occurs at a distance of several feet4 ,for this reason the transmitter is made as sensitive as possible1 and yet is so constructed that it will not be caused to produce too loud or unduly harsh sounds in response to a loud sound taking place immediately in front of it4 ,another adaptation of such highly sensitive transmitters is found in the special intercommunicating telephone systems for use between the various departments or desks in business offices4 ,in these it is desirable that the transmitter shall be able to respond adequately to sounds occurring anywhere in a small-sized room1 for instance4 ,acousticon ,transmitter4 ,in ,fig4 #df is shown a transmitter adapted for such use4 ,this has been termed by its makers the acousticon transmitter4 ,like all the transmitters previously discussed1 this is of the variable-resistance type1 but it differs from them all in that it has no damping springs2 in that carbon balls are substituted for carbon granules2 and in that the diaphragm itself serves as the front electrode4 ,this transmitter consists of a cup #a1 into which is set a cylindrical block #b1 in one face of which are a number of hemispherical recesses4 ,the diaphragm #c is made of thin carbon and is so placed in the transmitter as to cover the openings of the recesses in the carbon block1 and lie close enough to the carbon block1 without engaging it1 to prevent the carbon particles from falling out4 ,the diaphragm thus serves as the front electrode and the carbon block as the rear electrode4 ,the recesses in the carbon block are about two-thirds filled with small carbon balls1 which are about the size of fine sand4 ,the front piece #d of the transmitter is of sheet metal and serves to hold the diaphragm in place4 ,to admit the sound waves it is provided with a circular opening opposite to and about the size of the rear electrode block4 ,on this front piece are mounted the two terminals of the transmitter1 connected respectively to the two electrodes1 terminal #e being insulated from the front piece and connected by a thin metal strip with the diaphragm1 while terminal #f is mounted directly on the front piece and connected through the cup #a with the carbon block #b1 or back electrode of the transmitter4 ,illustration3 ,fig #df4 ,acousticon ,transmitter ,when this transmitter is used in connection with outfits for the deaf1 it is placed in a hard rubber containing case1 consisting of a hollow cylindrical piece #g1 which has fastened to it a cover #h4 ,this cover has a circular row of openings or holes near its outer edge1 as shown at #i1 through which the sound waves may pass to the chamber within1 and thence find their way through the round hole in the center of the front plate #d to the diaphragm #c4 ,it is probable also that the front face of the cover #h of the outer case vibrates1 and in this way also causes sound waves to impinge against the diaphragm4 ,this arrangement provides a large receiving surface for the sound waves1 but1 owing to the fact that the openings in the containing case are not opposite the opening in the transmitter proper1 the sound waves do not impinge directly against the diaphragm4 ,this peculiar arrangement is probably the result of an endeavor to prevent the transmitter from being too strongly actuated by violent sounds close to it4 ,instruments of this kind are very sensitive and under proper conditions are readily responsive to words spoken in an ordinary tone ten feet away4 ,illustration3 ,fig4 #dg4 ,switchboard ,transmitter ,switchboard ,transmitter4 ,another special adaptation of the telephone transmitter is that for use of telephone operators at central-office switchboards4 ,the requirements in this case are such that the operator must always be able to speak into the transmitter while seated before the switchboard1 and yet allow both of her hands to be free for use4 ,this was formerly accomplished by suspending an ordinary granular-carbon transmitter in front of the operator1 but a later development has resulted in the adoption of the so-called breast transmitter1 shown in ,fig4 #dg4 ,this is merely an ordinary granular-carbon transmitter mounted on a plate which is strapped on the breast of the operator1 the transmitter being provided with a long curved mouthpiece which projects in such a manner as to lie just in front of the operator's lips4 ,this device has the advantage of automatically following the operator in her movements4 ,the breast transmitter shown in ,fig4 #dg1 is that of the ,dean ,electric ,company4 ,illustration3 ,fig4 #dh4 ,transmitter ,symbols ,conventional ,diagram4 ,there are several common ways of illustrating transmitters in diagrams of circuits in which they are employed4 ,the three most common ways are shown in ,fig4 #dh4 ,the one at the left is supposed to be a side view of an ordinary instrument1 the one in the center a front view1 and the one at the right to be merely a suggestive arrangement of the diaphragm and the rear electrode4 ,the one at the right is best and perhaps most common2 the center one is the poorest and least used4 ,,chapter ,,vi ,,receivers ,the telephone receiver is the device which translates the energy of the voice currents into the energy of corresponding sound waves4 ,all telephone receivers today are of the electromagnetic type1 the voice currents causing a varying magnetic pull on an armature or diaphragm1 which in turn produces the sound waves corresponding to the undulations of the voice currents4 ,early ,receivers4 ,the early forms of telephone receivers were of the single-pole type2 that is1 the type wherein but one pole of the electromagnet was presented to the diaphragm4 ,the single-pole receiver that formed the companion piece to the old ,blake transmitter and that was the standard of the ,bell companies for many years1 is shown in ,fig4 #di4 ,while this has almost completely passed out of use1 it may be profitably studied in order that a comparison may be made between certain features of its construction and those of the later forms of receivers4 ,the coil of this receiver was wound on a round iron core #b1 flattened at one end to afford means for attaching the permanent magnet4 ,the permanent magnet was of laminated construction1 consisting of four hard steel bars #a1 extending nearly the entire length of the receiver shell4 ,these steel bars were all magnetized separately and placed with like poles together so as to form a single bar magnet4 ,they were laid together in pairs so as to include between the pairs the flattened end of the pole piece #b at one end and the flattened portion of the tail piece #c at the other end4 ,this whole magnet structure1 including the core1 the tail piece1 and the permanently magnetized steel bars1 was clamped together by screws as shown4 ,the containing shell was of hard rubber consisting of three pieces1 the barrel #d1 the ear-piece #e1 and the tail cap #f4 ,the barrel and the ear piece engaged each other by means of a screw thread and served to clamp the diaphragm between them4 ,the compound bar magnet was held in place within the shell by means of a screw #g passing through the hard rubber tail cap #f and into the tail block #c of the magnet4 ,external binding posts mounted on the tail cap1 as shown1 were connected by heavy leading-in wires to the terminals of the electromagnet4 ,a casual consideration of the magnetic circuit of this instrument will show that it was inefficient1 since the return path for the lines of force set up by the bar magnet was necessarily through a very long air path4 ,notwithstanding this1 these receivers were capable of giving excellent articulation and were of marvelous delicacy of action4 ,a very grave fault was that the magnet was supported in the shell at the end farthest removed from the diaphragm4 ,as a result it was difficult to maintain a permanent adjustment between the pole piece and the diaphragm4 ,one reason for this was that hard rubber and steel contract and expand under changes of temperature at very different rates1 and therefore the distance between the pole piece and the diaphragm changed with changes of temperature4 ,another grave defect1 brought about by this tying together of the permanent magnet and the shell which supported the diaphragm at the end farthest from the diaphragm1 was that any mechanical shocks were thus given a good chance to alter the adjustment4 ,illustration3 ,fig4 #di4 ,single-,pole ,receiver ,modern ,receivers4 ,receivers of today differ from this old single-pole receiver in two radical respects4 ,in the first place1 the modern receiver is of the bi-polar type1 consisting essentially of a horseshoe magnet presenting both of its poles to the diaphragm4 ,in the second place1 the modern practice is to either support all of the working parts of the receiver1 i4e41 the magnet1 the coils1 and the diaphragm1 by an inner metallic frame entirely independent of the shell2 or1 if the shell is used as a part of the structure1 to rigidly fasten the several parts close to the diaphragm rather than at the end farthest removed from the diaphragm4 ,western ,electric ,receiver4 ,the standard bi-polar receiver of the ,western ,electric ,company1 in use by practically all of the ,bell operating companies throughout this country and in large use abroad1 is shown in ,fig4 #ej4 ,in this the shell is of three pieces1 consisting of the barrel #a1 the ear cap #b1 and the tail cap #c4 ,the tail cap and the barrel are permanently fastened together to form substantially a single piece4 ,two permanently magnetized bar magnets #d-d are employed1 these being clamped together at their upper ends1 as shown1 so as to include the soft iron block #e between them4 ,the north pole of one of these magnets is clamped to the south pole of the other1 so that in reality a horseshoe magnet is formed4 ,at their lower ends1 these two permanent magnets are clamped against the soft iron pole pieces #f-f1 a threaded block #g also being clamped rigidly between these pole pieces at this point4 ,on the ends of the pole pieces the bobbins are wound4 ,the whole magnet structure is secured within the shell #a by means of a screw thread on the block #g which engages a corresponding internal screw thread in the shell #a4 ,as a result of this construction the whole magnet structure is bound rigidly to the shell structure at a point close to the diaphragm1 comparatively speaking1 and as a result of this close coupling1 the relation between the diaphragm and the pole piece is very much more rigid and substantial than in the case where the magnet structure and the shell were secured together at the end farthest removed from the diaphragm4 ,illustration3 ,fig4 #ej4 ,western ,electric ,receiver ,although this receiver shown in ,fig4 #ej is the standard in use by the ,bell companies throughout this country1 its numbers running well into the millions1 it cannot be said to be a strictly modern receiver1 because of at least one rather antiquated feature4 ,the binding posts1 by which the circuit conductors are led to the coils of this instrument1 are mounted on the outside of the receiver shell1 as indicated1 and are thus subject to danger of mechanical injury and they are also exposed to the touch of the user1 so that he may1 in case of the wires being charged to an abnormal potential1 receive a shock4 ,probably a more serious feature than either one of these is that the terminals of the flexible cords which attach to these binding posts are attached outside of the receiver shell1 and are therefore exposed to the wear and tear of use1 rather than being protected as they should be within the shell4 ,notwithstanding this undesirable feature1 this receiver is a very efficient one and is excellently constructed4 ,illustration3 ,fig4 #ea4 ,kellogg ,receiver ,kellogg ,receiver4 ,in ,fig4 #ea is shown a bi-polar receiver with internal or concealed binding posts4 ,this particular receiver is typical of a large number of similar kinds and is manufactured by the ,kellogg ,switchboard and ,supply ,company4 ,two straight permanently magnetized bar magnets #a-a are clamped together at their opposite ends so as to form a horseshoe magnet4 ,at the end opposite the diaphragm these bars clamp between them a cylindrical piece of iron #b1 so as to complete the magnetic circuit at the end4 ,at the end nearest the diaphragm they clamp between them the ends of the soft iron pole pieces #c-c1 and also a block of composite metal #d having a large circular flange #d' which serves as a means for supporting the magnet structure within the shell4 ,the screws by means of which the disk #d' is clamped to the shouldered seat in the shell do not enter the shell directly1 but rather enter screw-threaded brass blocks which are moulded into the structure of the shell4 ,it is seen from this construction that the diaphragm and the pole pieces and the magnet structure itself are all rigidly secured together through the medium of the shell at a point as close as possible to the diaphragm4 ,between the magnets #a-a there is clamped an insulating block #e1 to which are fastened the terminal plates #f1 one on each side of the receiver4 ,these terminal plates are thoroughly insulated from the magnets themselves and from all other metallic parts by means of sheets of fiber1 as indicated by the heavy black lines4 ,on these plates #f are carried the binding posts for the receiver cord terminals4 ,a long tongue extends from each of the plates #f through a hole in the disk #d'1 into the coil chamber of the receiver1 at which point the terminal of the magnet winding is secured to it4 ,this tongue is insulated from the disk #d'1 where it passes through it1 by means of insulating bushing1 as shown4 ,the other terminal of the magnet coils is brought out to the other plate #f by means of a similar tongue on the other side4 ,in order that the receiver terminals proper may not be subjected to any strain in case the receiver is dropped and its weight caught on the receiver cord1 a strain loop is formed as a continuation of the braided covering of the receiver cord1 and this is tied to the permanent magnet structure1 as shown4 ,by making this strain loop short1 it is obvious that whatever pull the cord receives will not be taken by the cord conductors leading to the binding posts or by the binding posts or the cord terminals themselves4 ,a number of other manufacturers have gone even a step further than this in securing permanency of adjustment between the receiver diaphragm and pole pieces4 ,they have done this by not depending at all on the hard rubber shell as a part of the structure1 but by enclosing the magnet coil in a cup of metal upon which the diaphragm is mounted1 so that the permanency of relation between the diaphragm and the pole pieces is dependent only upon the metallic structure and not at all upon the less durable shell4 ,direct-,current ,receiver4 ,until about the middle of the year #aiji1 it was the universal practice to employ permanent magnets for giving the initial polarization to the magnet cores of telephone receivers4 ,this is still done1 and necessarily so1 in receivers employed in connection with magneto telephones4 ,in common-battery systems1 however1 where the direct transmitter current is fed from the central office to the local stations1 it has been found that this current which must flow at any rate through the line may be made to serve the additional purpose of energizing the receiver magnets so as to give them the necessary initial polarity4 ,a type of receiver has come into wide use as a result1 which is commonly called the direct-current receiver1 deriving its name from the fact that it employs the direct current that is flowing in the common-battery line to magnetize the receiver cores4 ,the ,automatic ,electric ,company1 of ,chicago1 was probably the first company to adopt this form of receiver as its standard type4 ,their receiver is shown in cross-section in ,fig4 #eb1 and a photograph of the same instrument partially disassembled is given in ,fig4 #ec4 ,the most noticeable thing about the construction of this receiver is the absence of permanent magnets4 ,the entire working parts are contained within the brass cup #a1 which serves not only as a container for the magnet1 but also as a seat for the diaphragm4 ,this receiver is therefore illustrative of the type mentioned above1 wherein the relation between the diaphragm and the pole pieces is not dependent upon any connection through the shell4 ,illustration3 ,fig4 #eb4 ,automatic ,electric ,company ,direct-,current ,receiver ,illustration3 ,fig4 #ec4 ,automatic ,electric ,company ,direct-,current ,receiver ,the coil of this instrument consists of a single cylindrical spool #b1 mounted on a cylindrical core4 ,this bobbin lies within a soft iron-punching #c1 the form of which is most clearly shown in ,fig4 #ec1 and this punching affords a return path to the diaphragm for the lines of force set up in the magnet core4 ,obviously a magnetizing current passing through the winding of the coil will cause the end of the core toward the diaphragm to be polarized1 say positively1 while the end of the enclosing shell will be polarized in the other polarity1 negatively4 ,both poles of the magnet are therefore presented to the diaphragm and the only air gap in the magnetic circuit is that between the diaphragm and these poles4 ,the magnetic circuit is therefore one of great efficiency1 since it consists almost entirely of iron1 the only air gap being that across which the attraction of the diaphragm is to take place4 ,the action of this receiver will be understood when it is stated that in common-battery practice1 as will be shown in later chapters1 a steady current flows over the line for energizing the transmitter4 ,on this current is superposed the incoming voice currents from a distant station4 ,the steady current flowing in the line will1 in the case of this receiver1 pass through the magnet winding and establish a normal magnetic field in the same way as if a permanent magnet were employed4 ,the superposed incoming voice currents will then be able to vary this magnetic field in exactly the same way as in the ordinary receiver4 ,an astonishing feature of this recent development of the so-called direct-current receiver is that it did not come into use until after about twenty years of common-battery practice4 ,there is nothing new in the principles involved1 as all of them were already understood and some of them were employed by ,bell in his original telephone2 in fact1 the idea had been advanced time and again1 and thrown aside as not being worth consideration4 ,this is an illustration of a frequent occurrence in the development of almost any rapidly growing art4 ,ideas that are discarded as worthless in the early stages of the art are finally picked up and made use of4 ,the reason for this is that in some cases the ideas come in advance of the art1 or they are proposed before the art is ready to use them4 ,in other cases the idea as originally proposed lacked some small but essential detail1 or1 as is more often the case1 the experimenter in the early days did not have sufficient skill or knowledge to make it fit the requirements as he saw them4 ,monarch ,receiver4 ,the receiver of the ,automatic ,electric ,company just discussed employs but a single electromagnet by which the initial magnetization of the cores and also the variable magnetization necessary for speech reproduction is secured4 ,the problem of the direct-current receiver has been attacked in another way by ,ernest ,e4 ,yaxley1 of the ,monarch ,telephone ,manufacturing ,company1 with the result shown in ,fig4 #ed4 ,the construction in this case is not unlike that of an ordinary permanent-magnet receiver1 except that in the place of the permanent magnets two soft iron cores #a-a are employed4 ,on these are wound two long bobbins of insulated wire so that the direct current flowing over the telephone line will pass through these and magnetize the cores to the same degree and for the same purpose as in the case of permanent magnets4 ,these soft iron magnet cores #a-a continue to a point near the coil chamber1 where they join the two soft iron pole pieces #b-b1 upon which the ordinary voice-current coils are wound4 ,the two long coils #d-d1 which may be termed the direct-current coils1 are of somewhat lower resistance than the two voice-current coils #c-c4 ,they are1 however1 by virtue of their greater number of turns and the greater amount of iron that is included in their cores1 of much higher impedance than the voice-current coils #c-c4 ,these two sets of coils #d-d and #c-c are connected in multiple4 ,as a result of their lower ohmic resistance the coils #d-d will take a greater amount of the steady current which comes over the line1 and therefore the greater proportion of the steady current will be employed in magnetizing the bar magnets4 ,on account of their higher impedance to alternating currents1 however1 nearly all of the voice currents which are superposed on the steady currents1 flowing in the line will pass through the voice-current coils #c-c1 and1 being near the diaphragm1 these currents will so vary the steady magnetism in the cores #b-b as to produce the necessary vibration of the diaphragm4 ,illustration3 ,fig4 #ed4 ,monarch ,direct-,current ,receiver ,this receiver1 like the one of the ,automatic ,electric ,company1 does not rely on the shell in any respect to maintain the permanency of relation between the pole pieces and the diaphragm4 ,the cup #e1 which is of pressed brass1 contains the voice-current coils and also acts as a seat for the diaphragm4 ,the entire working parts of this receiver may be removed by merely unscrewing the ear piece from the hard rubber shell1 thus permitting the whole works to be withdrawn in an obvious manner4 ,dean ,receiver4 ,of such decided novelty as to be almost revolutionary in character is the receiver recently put on the market by the ,dean ,electric ,company and shown in ,fig4 #ee4 ,this receiver is of the direct-current type and employs but a single cylindrical bobbin of wire4 ,the core of this bobbin and the return path for the magnetic lines of force set up in it are composed of soft iron punchings of substantially ,e shape4 ,these punchings are laid together so as to form a laminated soft-iron field1 the limbs of which are about square in cross-section4 ,the coil is wound on the center portion of this ,e as a core1 the core being1 as stated1 approximately square in cross-section4 ,the general form of magnetic circuit in this instrument is therefore similar to that of the ,automatic ,electric ,company's receiver1 shown in ,figs4 #eb and #ec1 but the core is laminated instead of being solid as in that instrument4 ,illustration3 ,fig4 #ee4 ,dean ,steel ,shell ,receiver ,the most unusual feature of this ,dean receiver is that the use of hard rubber or composition does not enter into the formation of the shell1 but instead a shell composed entirely of steel stampings has been substituted therefor4 ,the main portion of this shell is the barrel #a4 ,great skill has evidently been exercised in the forming of this by the cold-drawn process1 it presenting neither seams nor welds4 ,the ear piece #b is also formed of steel of about the same gauge as the barrel #a4 ,instead of screw-threading the steel parts1 so that they would directly engage each other1 the ingenious device has been employed of swaging a brass ring #c in the barrel portion and a similar brass ring #d in the ear cap portion1 these two being slotted and keyed1 as shown at #h1 so as to prevent their turning in their respective seats4 ,the ring #c is provided with an external screw thread and the ring #d with an internal screw thread1 so that the receiver cap is screwed on to the barrel in the same way as in the ordinary rubber shell4 ,by the employment of these brass screw-threaded rings1 the rusting together of the parts so that they could not be separated when required--a difficulty heretofore encountered in steel construction of similar parts--has been remedied4 ,illustration3 ,fig4 #ef4 ,working ,parts of ,dean ,receiver ,the entire working parts of this receiver are contained within the cup #e1 the edge of which is flanged outwardly to afford a seat for the diaphragm4 ,the diaphragm is locked in place on the shell by a screw-threaded ring #f1 as is clearly indicated4 ,a ring #g of insulating material is seated within the enlarged portion of the barrel #a1 and against this the flange of the cup #e rests and is held in place by the cap #b when it is screwed home4 ,the working parts of this receiver partially disassembled are shown in ,fig4 #ef1 which gives a clear idea of some of the features not clearly illustrated in ,fig4 #ee4 ,it cannot be denied that one of the principal items of maintenance of subscribers' station equipment has been due to the breakage of receiver shells4 ,the users frequently allow their receiver to fall and strike heavily against the wall or floor1 thus not only subjecting the cords to great strain1 but sometimes cracking or entirely breaking the receiver shell4 ,the innovation thus proposed by the ,dean ,company of making the entire receiver shell of steel is of great interest4 ,the shell1 as will be seen1 is entirely insulated from the circuit of the receiver so that no contact exists by which a user could receive a shock4 ,the shell is enameled inside and out with a heavy black insulating enamel baked on1 and said to be of great durability4 ,how this enamel will wear remains to be seen4 ,the insulation of the interior portions of the receiver is further guarded by providing a lining of fiber within the shell at all points where it seems possible that a cross could occur between some of the working parts and the metal of the shell4 ,this type of receiver has not been on the market long enough to draw definite conclusions1 based on experience in use1 as to what its permanent performance will be4 ,thus far in this chapter only those receivers which are commonly called hand receivers have been discussed4 ,these are the receivers that are ordinarily employed by the general public4 ,illustration3 ,fig4 #eg4 ,operator's ,receiver ,operator's ,receiver4 ,at the central office in telephone exchanges the operators are provided with receivers in order that they may communicate with the subscribers or with other operators4 ,in order that they may have both of their hands free to set up and take down the connections and to perform all of the switching operations required1 a special form of receiver is employed for this purpose1 which is worn as a part of a head-gear and is commonly termed a head receiver4 ,these are necessarily of very light construction1 in order not to be burdensome to the operators1 and obviously they must be efficient4 ,they are ordinarily held in place at the ear by a metallic head band fitting over the head of the operator4 ,illustration3 ,,grant ,,avenue ,,office ,,of ,,home ,,telephone ,,company1 ,,san ,,francisco1 ,,cal4 ,a ,type of ,central-,office ,buildings in ,down-,town ,districts of ,large ,cities4 ,such a receiver is shown in cross-section in ,fig4 #eg1 and completely assembled with its head band in ,fig4 #eh4 ,referring to ,fig4 #eg the shell #a of the receiver is of aluminum and the magnets are formed of steel rings #b1 cross-magnetized so as to present a north pole on one side of the ring and a south pole on the other4 ,the two ,l-shaped pole pieces #c are secured by screws to the poles of these ring magnets1 and these pole pieces carry the magnet coils1 as is clearly indicated4 ,these poles are presented to a soft iron diaphragm in exactly the same way as in the larger hand receivers1 the diaphragm being clamped in place by a hard rubber ear piece1 as shown4 ,the head bands are frequently of steel covered with leather4 ,they have assumed numerous forms1 but the general form shown in ,fig4 #eh is the one commonly adopted4 ,illustration3 ,fig4 #eh4 ,operator's ,receiver and ,cord ,illustration3 ,fig4 #ei4 ,receiver ,symbols ,conventional ,symbols4 ,the usual diagrammatic symbols for hand and head receivers are shown in ,fig4 #ei4 ,they are self-explanatory4 ,the symbol at the left in this figure1 showing the general outline of the receiver1 is the one most commonly used where any sort of a receiver is to be indicated in a circuit diagram1 but where it becomes desirable to indicate in the diagram the actual connections with the coil or coils of the receiver1 the symbol shown at the right is to be preferred1 and obviously it may be modified as to number of windings and form of core as desired4 ,,chapter ,,vii ,,primary ,,cells ,galvani1 an ,italian physician1 discovered1 in #aghf1 that a current of electricity could be produced by chemical action4 ,in #ahjj1 ,volta1 a physicist1 also an ,italian1 threw further light on ,galvani's discovery and produced what we know as the voltaic1 or galvanic1 cell4 ,in honor of these two discoverers we have the words volt1 galvanic1 and the various words and terms derived therefrom4 ,simple ,voltaic ,cell4 ,a very simple voltaic cell may be made by placing two plates1 one of copper and one of zinc1 in a glass vessel partly filled with dilute sulphuric acid1 as shown in ,fig4 #fj4 ,when the two plates are not connected by a wire or other conductor1 experiment shows that the copper plate bears a positive charge with respect to the zinc plate1 and the zinc plate bears a negative charge with respect to the copper4 ,when the two plates are connected by a wire1 a current flows from the copper to the zinc plate through the metallic path of the wire1 just as is to be expected when any conductor of relatively high electrical potential is joined to one of relatively low electrical potential4 ,ordinarily1 when one charged body is connected to another of different potential1 the resulting current is of but momentary duration1 due to the redistribution of the charges and consequent equalization of potential4 ,in the case of the simple cell1 however1 the current is continuous1 showing that some action is maintaining the charges on the two plates and therefore maintaining the difference of potential between them4 ,the energy of this current is derived from the chemical action of the acid on the zinc4 ,the cell is in reality a sort of a zinc-burning furnace4 ,in the action of the cell1 when the two plates are joined by a wire1 it may be noticed that the zinc plate is consumed and that bubbles of hydrogen gas are formed on the surface of the copper plate4 ,theory4 ,just why or how chemical action in a voltaic cell results in the production of a negative charge on the consumed plate is not known4 ,modern theory has it that when an acid is diluted in water the molecules of the acid are split up or dissociated into two oppositely charged atoms1 or groups of atoms1 one bearing a positive charge and the other a negative charge of electricity4 ,such charged atoms or groups of atoms are called ions4 ,this separation of the molecules of a chemical compound into positively and negatively charged ions is called dissociation4 ,thus1 in the simple cell under consideration the sulphuric acid1 by dissociation1 splits up into hydrogen ions bearing positive charges1 and ,,so#d ions bearing negative charges4 ,the solution as a whole is neutral in potential1 having an equal number of equal and opposite charges4 ,illustration3 ,fig4 #fj4 ,simple ,voltaic ,cell ,it is known that when a metal is being dissolved by an acid1 each atom of the metal which is torn off by the solution leaves the metal as a positively charged ion4 ,the carrying away of positive charges from a hitherto neutral body leaves that body with a negative charge4 ,hence the zinc1 or consumed plate1 becomes negatively charged4 ,in the chemical attack of the sulphuric acid on the zinc1 the positive hydrogen ions are liberated1 due to the affinity of the negative ,,so#d ions for the positive zinc ions1 this resulting in the formation of zinc sulphate in the solution4 ,now the solution itself becomes positively charged1 due to the positive charges leaving the zinc plate with the zinc ions1 and the free positively charged hydrogen ions liberated in the solution as just described are repelled to the copper plate1 carrying their positive charges thereto4 ,hence the copper plate1 or the unconsumed plate1 becomes positively charged and also coated with hydrogen bubbles4 ,the plates or electrodes of a voltaic cell need not consist of zinc and copper1 nor need the fluid1 called the electrolyte1 be of sulphuric acid2 any two dissimilar elements immersed in an electrolyte that attacks one of them more readily than the other will form a voltaic cell4 ,in every such cell it will be found that one of the plates is consumed1 and that on the other plate some element is deposited1 this element being sometimes a gas and sometimes a solid4 ,the plate which is consumed is always the negative plate1 and the one on which the element is deposited is always the positive1 the current through the connecting wire always being1 therefore1 from the unconsumed to the consumed plate4 ,thus1 in the simple copper-zinc cell just considered1 the zinc is consumed1 the element hydrogen is deposited on the copper1 and the current flow through the external circuit is from the copper to the zinc4 ,the positive charges1 leaving the zinc1 or consumed1 plate1 and passing through the electrolyte to the copper1 or unconsumed1 plate1 constitute in effect a current of electricity flowing within the electrolyte4 ,the current within the cell passes1 therefore1 from the zinc plate to the copper plate4 ,the zinc is1 therefore1 said to be positive with respect to the copper4 ,difference of ,potential4 ,the amount of electromotive force1 that is generated between two dissimilar elements immersed in an electrolyte is different for different pairs of elements and for different electrolytes4 ,for a given electrolyte each element bears a certain relation to another2 i4e41 they are either electro-positive or electro-negative relative to each other4 ,in the following list a group of elements are arranged with respect to the potentials which they assume with respect to each other with dilute sulphuric acid as the electrolyte4 ,the most electro-positive elements are at the top and the most electro-negative at the bottom4 ,sodium ,lead ,copper ,magnesium ,iron ,silver ,zinc ,nickel ,gold ,cadmium ,bismuth ,platinum ,tin ,antimony -,graphite 7,carbon7 ,any two elements selected from this list and immersed in dilute sulphuric acid will form a voltaic cell1 the amount of difference of potential1 or electromotive force1 depending on the distance apart in this series of the two elements chosen4 ,the current within the cell will always flow from the one nearest the top of the list to the one nearest the bottom1 i4e41 from the most electro-positive to the most electro-negative2 and1 therefore1 the current in the wire joining the two plates will flow from the one lowest down in the list to the one highest up4 ,from this series it is easy to see why zinc and copper1 and also zinc and carbon1 are often chosen as elements of voltaic cells4 ,they are widely separated in the series and comparatively cheap4 ,this series may not be taken as correct for all electrolytes1 for different electrolytes alter somewhat the order of the elements in the series4 ,thus1 if two plates1 one of iron and the other of copper1 are immersed in dilute sulphuric acid1 a current is set up which proceeds through the liquid from the iron to the copper2 but1 if the plates after being carefully washed are placed in a solution of potassium sulphide1 a current is produced in the opposite direction4 ,the copper is now the positive element4 ,table ,,ii shows the electrical deportment of the principal metals in three different liquids4 ,it is arranged like the preceding one1 each metal being electro-positive to any one lower in the list4 ,,table ,,ii ,behavior of ,metals in ,different ,electrolytes ---------------------------------------- ----------------- ,,caustic ,,potash ,,hydrochloric ,,acid ,,potassium ,,sulphide ---------------------------------------- ----------------- ,zinc ,zinc ,zinc ,tin ,cadmium ,copper ,cadmium ,tin ,cadmium ,antimony ,lead ,tin ,lead ,iron ,silver ,bismuth ,copper ,antimony ,iron ,bismuth ,lead ,copper ,nickel ,bismuth ,nickel ,silver ,nickel - ,silver - ,antimony - ,iron ---------------------------------------- ----------------- ,it is important to remember that in all cells1 no matter what elements or what electrolyte are used1 the electrode which is consumed is the one that becomes negatively charged and its terminal1 therefore1 becomes the negative terminal or pole1 while the electrode which is not consumed is the one that becomes positively charged1 and its terminal is1 therefore1 the positive terminal or pole of the cell4 ,however1 because the current in the electrolyte flows from the consumed plate to the unconsumed plate1 the consumed plate is called the positive plate and the unconsumed1 the negative4 ,this is likely to become confusing1 but if one remembers that the active plate is the positive plate1 because it sends forth positive ions in the electrolyte1 and1 therefore1 itself becomes negatively charged1 one will have the proper basis always to determine the direction of the current flow1 which is the important thing4 ,polarization4 ,if the simple cell already described have its terminals connected by a wire for some time1 it will be found that the current rapidly weakens until it ceases to be manifest4 ,this weakening results from two causes3 first1 the hydrogen gas which is liberated in the action of the cell is deposited in a layer on the copper plate1 thereby covering the plate and reducing the area of contact with the liquid4 ,this increases the internal resistance of the cell1 since hydrogen is a non-conductor4 ,second1 the plate so covered becomes in effect a hydrogen electrode1 and hydrogen stands high as an electro-positive element4 ,there is1 therefore1 actual reduction in the electromotive force of the cell1 as well as an increase in internal resistance4 ,this phenomenon is known as polarization1 and in commercial cells means must be taken to prevent such action as far as possible4 ,the means by which polarization of cells is prevented or reduced in practice may be divided into three general classes3 ,first--mechanical means4 ,if the hydrogen bubbles be simply brushed away from the surface of the electrode the resistance and the counter polarity which they cause will be diminished4 ,the same result may be secured if air be blown into the solution through a tube1 or if the liquid be kept agitated4 ,if the surface of the electrode be roughened or covered with points1 the bubbles collect more freely at the points and are more quickly carried away to the surface of the liquid4 ,these means are1 however1 hardly practical except in cells for laboratory use4 ,second--chemical means4 ,if a highly oxidizing substance be added to the electrolyte1 it will destroy the hydrogen bubbles by combining with them while they are in a nascent state1 and this will prevent the increase in internal resistance and the opposing electromotive force4 ,such substances are bichromate of potash1 nitric acid1 and chlorine1 and are largely used4 ,third--electro-chemical means4 ,double cells1 arranged to separate the elements and liquids by means of porous partitions or by gravity1 may be so arranged that solid copper is liberated instead of hydrogen at a point where the current leaves the liquid1 thereby entirely obviating polarization4 ,this method also is largely used4 ,local ,action4 ,when a simple cell stands idle1 i4e41 with its circuit open1 small hydrogen bubbles may be noticed rising from the zinc electrode instead of from copper1 as is the case where the circuit is closed4 ,this is due to impurities in the zinc plate1 such as particles of iron1 tin1 arsenic1 carbon1 etc4 ,each of these particles acts with the surrounding zinc just as might be expected of any pair of dissimilar elements opposed to each other in an electrolyte2 in other words1 they constitute small voltaic cells4 ,local currents1 therefore1 are generated1 circulating between the two adjacent metals1 and1 as a result1 the zinc plate and the electrolyte are needlessly wasted and the general condition of the cell is impaired4 ,this is called local action4 ,amalgamated ,zincs4 ,local action might be prevented by the use of chemically pure zinc1 but this1 on account of its expense1 cannot be employed commercially4 ,local action1 however1 may be overcome to a great extent by amalgamating the zinc1 i4e41 coating it with mercury4 ,the iron particles or other impurities do not dissolve in the mercury1 as does the zinc1 but they float to the surface1 whence the hydrogen bubbles which may form speedily carry them off1 and1 in other cases1 the impurities fall to the bottom of the cell4 ,as the zinc in the pasty amalgam dissolves in the acid1 the film of mercury unites with fresh zinc1 and so always presents a clear1 bright1 homogeneous surface to the action of the electrolyte4 ,the process of amalgamating the zinc may be performed by dipping it in a solution composed of ,nitric ,acid #a lb4 ,muriatic ,acid #b lbs4 ,mercury #h oz4 ,the acids should be first mixed and then the mercury slowly added until dissolved4 ,clean the zinc with lye and then dip it in the solution for a second or two4 ,rinse in clean water and rub with a brush4 ,another method of amalgamating zincs is to clean them by dipping them in dilute sulphuric acid and then in mercury1 allowing the surplus to drain off4 ,commercial zincs1 for use in voltaic cells as now manufactured1 usually have about #d per cent of mercury added to the molten zinc before casting into the form of plates or rods4 ,series and ,multiple ,connections4 ,when a number of voltaic cells are joined in series1 the positive pole of one being connected to the negative pole of the next one1 and so on throughout the series1 the electromotive forces of all the cells are added1 and the electromotive force of the group1 therefore1 becomes the sum of the electromotive forces of the component cells4 ,the currents through all the cells in this case will be equal to that of one cell4 ,if the cells be joined in multiple1 the positive poles all being connected by one wire and the negative poles by another1 then the currents of all the cells will be added while the electromotive force of the combination remains the same as that of a single cell1 assuming all the cells to be alike in electromotive force4 ,obviously combinations of these two arrangements may be made1 as by forming strings of cells connected in series1 and connecting the strings in multiple or parallel4 ,the term battery is frequently applied to a single voltaic cell1 but this term is more properly used to designate a plurality of cells joined together in series1 or in multiple1 or in series multiple so as to combine their actions in causing current to flow through an external circuit4 ,we may therefore refer to a battery of so many cells4 ,it has1 however1 become common1 though technically improper1 to refer to a single cell as a battery1 so that the term battery1 as indicating necessarily more than one cell1 has largely lost its significance4 ,cells may be of two types1 primary and secondary4 ,primary cells are those consisting of electrodes of dissimilar elements which1 when placed in an electrolyte1 become immediately ready for action4 ,secondary cells1 commonly called storage cells and accumulators1 consist always of two inert plates of metal1 or metallic oxide1 immersed in an electrolyte which is incapable of acting on either of them until a current has first been passed through the electrolyte from one plate to the other4 ,on the passage of a current in this way1 the decomposition of the electrolyte is effected and the composition of the plates is so changed that one of them becomes electro-positive and the other electro-negative4 ,the cell is then1 when the charging current ceases1 capable of acting as a voltaic cell4 ,this chapter is devoted to the primary cell or battery alone4 ,types of ,primary ,cells4 ,primary cells may be divided into two general classes3 first1 those adapted to furnish constant current2 and second1 those adapted to furnish only intermittent currents4 ,the difference between cells in this respect rests largely in the means employed for preventing or lessening polarization4 ,obviously in a cell in which polarization is entirely prevented the current may be allowed to flow constantly until the cell is completely exhausted2 that is1 until the zinc is all eaten up or until the hydrogen is exhausted from the electrolyte or both4 ,on the other hand some cells are so constituted that polarization takes place faster than the means intended to prevent it can act4 ,in other words1 the polarization gradually gains on the preventive means and so gradually reduces the current by increasing the resistance of the cell and lowering its electromotive force4 ,in cells of this kind1 however1 the arrangement is such that if the cell is allowed to rest1 that is1 if the external circuit is opened1 the depolarizing agency will gradually act to remove the hydrogen from the unattacked electrode and thus place the cell in good condition for use again4 ,of these two types of primary cells the intermittent-current cell is of far greater use in telephony than the constant-current cell4 ,this is because the use of primary batteries in telephony is1 in the great majority of cases1 intermittent1 and for that reason a cell which will give a strong current for a few minutes and which after such use will regain practically all of its initial strength and be ready for use again1 is more desirable than one which will give a weaker current continuously throughout a long period of time4 ,since the cells which are adapted to give constant current are commonly used in connection with circuits that are continuously closed1 they are called closed-circuit cells4 ,the other cells1 which are better adapted for intermittent current1 are commonly used on circuits which stand open most of the time and are closed only occasionally when their current is desired4 ,for this reason these are termed open-circuit cells4 ,open-,circuit ,cells4 ,le,clanch ,cell3--,by far the most important primary cell for telephone work is the so-called ,le,clanch cell4 ,this assumes a large variety of forms1 but always employs zinc as the negatively charged element1 carbon as the positively charged element1 and a solution of sal ammoniac as the electrolyte4 ,this cell employs a chemical method of taking care of polarization1 the depolarizing agent being peroxide of manganese1 which is closely associated with the carbon element4 ,the original form of the ,le,clanch cell1 a form in which it was very largely used up to within a short time ago1 is shown in ,fig4 #fa4 ,in this the carbon element is placed within a cylindrical jar of porous clay1 the walls of this jar being of such consistency as to allow moisture slowly to permeate through it4 ,within this porous cup1 as it is called1 a plate or disk of carbon is placed1 and around this the depolarizing agent1 consisting of black oxide of manganese4 ,this is usually mixed with1 broken carbon1 so as to increase the effective area of the carbon element in contact with the depolarizing agent1 and also to reduce the total internal resistance of the cell4 ,the zinc electrode usually consisted merely in a rod of zinc1 as shown1 with a suitable terminal at its upper end4 ,illustration3 ,fig4 #fa4 ,le,clanch ,cell ,the chemical action taking place within the ,le,clanch cell is1 briefly1 as follows3 ,sal ammoniac is chemically known as chloride of ammonium and is a combination of chlorine and ammonia4 ,in the action which is assumed to accompany the passage of current in this cell1 the sal ammoniac is decomposed1 the chlorine leaving the ammonia to unite with an atom of the zinc plate1 forming chloride of zinc and setting free ammonia and hydrogen4 ,the ammonia is immediately dissolved in the water of the cell1 and the hydrogen enters the porous cup and would speedily polarize the cell by adhering to the carbon plate but for the fact that it encounters the peroxide of manganese4 ,this material is exceedingly rich in oxygen and it therefore readily gives up a part of its oxygen1 which forms water by combination with the already liberated hydrogen and leaves what is termed a sesquioxide of manganese4 ,this absorption or combination of the hydrogen prevents immediate polarization1 but hydrogen is evolved during the operation of the cell more rapidly than it can combine withtypo was 'wth' the oxygen of the manganese1 thereby leading to polarization more rapidly than the depolarizer can prevent it when the cell is heavily worked4 ,when1 however1 the cell is left with its external circuit open for a time1 depolarization ensues by the gradual combination of the hydrogen with the oxygen of the peroxide of manganese1 and as a result the cell recuperates and in a short time attains its normal electromotive force4 ,the electromotive force of this cell when new is about #a4dg volts4 ,the internal resistance of the cell of the type shown in ,fig4 #fa is approximately #a ohm1 ordinarily less rather than more4 ,a more recent form of ,le,clanch cell is shown in cross-section in ,fig4 #fb4 ,this uses practically the same materials and has the same chemical action as the old disk ,le,clanch cell shown in ,fig4 #fa4 ,it dispenses1 however1 with the porous cup and instead employs a carbon electrode1 which in itself forms a cup for the depolarizing agent4 ,illustration3 ,fig4 #fb4 ,carbon ,cylinder ,le,clanch ,cell ,the carbon electrode is in the form of a corrugated hollow cylinder which engages by means of an internal screw thread a corresponding screw thread on the outer side of the carbon cover4 ,within this cylinder is contained a mixture of broken carbon and peroxide of manganese4 ,the zinc electrode is in the form of a hollow cylinder almost surrounding the carbon electrode and separated therefrom by means of heavy rubber bands stretched around the carbon4 ,the rod1 forming the terminal of the zinc1 passes through a porcelain bushing on the cover plate to obviate short circuits4 ,this type of cell has an electromotive force of about #a4ee volts and recuperates very quickly after severe use4 ,it also has considerably lower internal resistance than the type of ,le,clanch cell employing a porous cup1 and1 therefore1 is capable of generating a considerably larger current4 ,cells of this general type have assumed a variety of forms4 ,in some the carbon electrode1 together with the broken carbon and peroxide of manganese1 were packed into a canvas bag which was suspended in the electrolyte and usually surrounded by the zinc electrode4 ,in other forms the carbon electrode has moulded with it the manganese depolarizer4 ,in order to prevent the salts within the cell from creeping over the edge of the containing glass jar and also over the upper portion of the carbon electrode1 it is common practice to immerse the upper end of the carbon element and also the upper edge of the glass jar in hot paraffin4 ,in setting up the ,le,clanch cell1 place not more than four ounces of white sal ammoniac in the jar1 fill the jar one-third full of water1 and stir until the sal ammoniac is all dissolved4 ,then put the carbon and zinc elements in place4 ,a little water poured in the vent hole of the porous jar or carbon cylinder will tend to hasten the action4 ,an excess of sal ammoniac should not be used1 as a saturated solution tends to deposit crystals on the zinc2 on the other hand1 the solution should not be allowed to become too weak1 as in that case the chloride of zinc will form on the zinc4 ,both of these causes materially increase the resistance of the cell4 ,a great advantage of the ,le,clanch cell is that when not in use there is but little material waste4 ,it contains no highly corrosive chemicals4 ,such cells require little attention1 and the addition of water now and then to replace the loss due to evaporation is about all that is required until the elements become exhausted4 ,they give a relatively high electromotive force and have a moderately low internal resistance1 so that they are capable of giving rather large currents for short intervals of time4 ,if properly made1 they recuperate quickly after polarization due to heavy use4 ,dry ,cell4 ,all the forms of cells so far considered may be quite properly termed wet cells because of the fact that a free liquid electrolyte is used4 ,this term is employed in contradistinction to the later developed cell1 commonly termed the dry cell4 ,this term dry cell is in some respects a misnomer1 since it is not dry and if it were dry it would not work4 ,it is essential to the operation of these cells that they shall be moist within1 and when such moisture is dissipated the cell is no longer usable1 as there is no further useful chemical action4 ,the dry cells are all of the ,le,clanch type1 the liquid electrolyte of that type being replaced by a semi-solid substance that is capable of retaining moisture for a considerable period4 ,as in the ordinary wet ,le,clanch cell1 the electrodes are of carbon and zinc1 the zinc element being in the form of a cylindrical cup and forming the retaining vessel of the cell1 while the carbon element is in the form of a rod or plate and occupies a central position with regard to the zinc1 being held out of contact with the zinc1 however1 at all points4 ,a cross-section of an excellent form of dry cell is shown in ,fig4 #fc4 ,the outer casing is of zinc1 formed in the shape of a cylindrical cup1 and serves not only as the retaining vessel1 but as the negatively charged electrode4 ,the outer surface of the zinc is completely covered on its sides and bottom with heavy pasteboard so as to insulate it from bodies with which it may come in contact1 and particularly from the zinc cups of other cells used in the same battery4 ,the positively charged electrode is a carbon rod corrugated longitudinally1 as shown1 in order to obtain greater surface4 ,this rod is held in the center of the zinc cup out of contact therewith1 and the intervening space is filled with a mixture of peroxide of manganese1 powdered carbon1 and sal ammoniac4 ,several thicknesses of blotting paper constitute a lining for the inner portion of the zinc electrode and serve to prevent the manganese mixture from coming directly into contact therewith4 ,the cell is sealed with pitch1 which is placed on a layer of sand and sawdust mixed in about equal parts4 ,illustration3 ,fig4 #fc4 ,dry ,cell ,the electrolyte in such cells varies largely as to quantities and proportions of the materials employed in various types of cells1 and also varies in the method in which the elements are introduced into the container4 ,the following list and approximate proportions of material will serve as a fair example of the filling mixture in well-known types of cells4 ,manganese dioxide #de per cent ,carbon or graphite1 or both #de per cent ,sal ammoniac #g per cent ,zinc chloride #c per cent ,water is added to the above and a sufficient amount of mixture is taken for each cell to fill the zinc cup about seven-eighths full when the carbon is in place4 ,the most suitable quantity of water depends upon the original dryness and fineness of material and upon the quality of the paper lining4 ,in some forms of dry batteries1 starch or other paste is added to improve the contact of the electrolyte with the zinc and promote a more even distribution of action throughout the electrolyte4 ,mercury1 too1 is often added to effect amalgamation of the zinc4 ,as in the ordinary wet type of ,le,clanch cell1 the purpose of the manganese is to act as a depolarizer2 the carbon or graphite being added to give conductivity to the manganese and to form a large electrode surface4 ,it is important that the sal ammoniac1 which is the active agent of the cell1 should be free from lumps in order to mix properly with the manganese and carbon4 ,a small local action takes place in the dry cell1 caused by the dissimilar metals necessarily employed in soldering up the zinc cup and in soldering the terminal rod of zinc to the zinc cup proper4 ,this action1 however1 is slight in the better grades of cells4 ,as a result of this1 and also of the gradual drying out of the moisture within the cell1 these cells gradually deteriorate even when not in use--this is commonly called shelf-wear4 ,shelf-wear is much more serious in the very small sizes of dry cells than in the larger ones4 ,dry cells are made in a large number of shapes and sizes4 ,the most useful form1 however1 is the ordinary cylindrical type4 ,these are made in sizes varying from one and one-half inches high and three-quarters inch in diameter to eight inches high and three and three-quarters inches in diameter4 ,the most used and standard size of dry cell is of cylindrical form six inches high and two and three-quarters inches in diameter4 ,the dry cell when new and in good condition has an open-circuit voltage of from #a4e to #a4f volts4 ,perhaps #a4ee represents the usual average4 ,a cell of the two and three-quarters by six-inch size will give throughout its useful life probably thirty ampere hours as a maximum1 but this varies greatly with the condition of use and the make of cell4 ,its effective voltage during its useful life averages about one volt1 and if during this life it gives a total discharge of thirty ampere hours1 the fair energy rating of the cell will be thirty watt-hours4 ,this may not be taken as an accurate figure1 however1 as the watt-hour capacity of a cell depends very largely1 not only on the make of the cell1 but on the rate of its discharge4 ,an examination of ,fig4 #fc shows that the dry cell has all of the essential elements of the ,le,clanch cell4 ,the materials of which the electrodes are made are the same and the porous cup of the disk ,le,clanch cell is represented in the dry cell by the blotting-paper cylinder1 which separates the zinc from the carbon electrode4 ,the positively charged electrode must not be considered as merely the carbon plate or rod alone1 but rather the carbon rod with its surrounding mixture of peroxide of manganese and broken carbon4 ,such being the case1 it is obvious that the separation between the electrodes is very small1 while the surface presented by both electrodes is very large4 ,as a result1 the internal resistance of the cell is small and the current which it will give on a short circuit is correspondingly large4 ,a good cell of the two and three-quarters by six-inch size will give eighteen or twenty amperes on short-circuit1 when new4 ,as the action of the cell proceeds1 zinc chloride and ammonia are formed1 and there being insufficient water to dissolve the ammonia1 there results the formation of double chlorides of zinc and ammonium4 ,these double chlorides are less soluble than the chlorides and finally occupy the pores of the paper lining between the electrolyte and the zinc and greatly increase the internal resistance of the cell4 ,this increase of resistance is further contributed to by the gradual drying out of the cell as its age increases4 ,within the last few years dry batteries have been so perfected mechanically1 chemically1 and electrically that they have far greater outputs and better recuperative power than any of the other types of ,le,clanch batteries1 while in point of convenience and economy1 resulting from their small size and non-breakable1 non-spillable features and low cost1 they leave no room for comparison4 ,closed-,circuit ,cells4 ,gravity-,cell3--,coming now to the consideration of closed-circuit or constant-current cells1 the most important is the well-known gravity1 or blue-stone1 cell1 devised by ,daniell4 ,it is largely used in telegraphy1 and often in telephony in such cases as require a constantly flowing current of small quantity4 ,such a cell is shown in ,fig4 #fd4 ,the elements of the gravity cell are electrodes of copper and zinc4 ,the solution in which the copper plate is immersed is primarily a solution of copper sulphate1 commonly known as blue-stone1 in water4 ,the zinc plate after the cell is in action is immersed in a solution of sulphate of zinc which is formed around it4 ,the glass jar is usually cylindrical1 the standard sizes being #e inches diameter and #g inches deep2 and also #f inches diameter and #h inches deep4 ,the copper electrode is of sheet copper of the form shown1 and it is partly covered with crystals of blue-stone or copper sulphate4 ,frequently1 in later forms of cells1 the copper electrode consists merely of a straight1 thick1 rectangular bar of copper laid horizontally1 directly on top of the blue-stone crystals4 ,in all cases a rubber-insulated wire is attached by riveting to the copper electrode1 and passes up through the electrolyte to form the positive terminal4 ,illustration3 ,fig4 #fd4 ,gravity ,cell ,the zinc is1 as a rule1 of crowfoot form1 as shown1 whence this cell derives the commonly applied name of crowfoot cell4 ,this is essentially a two-fluid cell1 for in its action zinc sulphate is formed1 and this being lighter than copper sulphate rises to the top of the jar and surrounds the zinc4 ,gravity1 therefore1 serves to keep the two fluids separate4 ,illustration3 ,,interior ,,of ,,warehouse ,,for ,,telephone ,,construction ,,material ,in the action of the cell1 when the external circuit is closed1 sulphuric acid is formed which attacks the zinc to form sulphate of zinc and to liberate hydrogen1 which follows its tendency to attach itself to the copper plate4 ,but in so doing the hydrogen necessarily passes through the solution of sulphate of copper surrounding the copper plate4 ,the hydrogen immediately combines with the ,,so#d radical1 forming therewith sulphuric acid1 and liberating metallic copper4 ,this sulphuric acid1 being lighter than the copper sulphate1 rises to the surface of the zinc and attacks the zinc1 thus forming more sulphate of zinc4 ,the metallic copper so formed is deposited on the copper plate1 thereby keeping the surface bright and clean4 ,since hydrogen is thus diverted from the copper plate1 polarization does not ensue4 ,the zinc sulphate being colorless1 while the copper sulphate is of a dark blue color1 the separating line of the two liquids is easily distinguishable4 ,this line is called the blue line and care should be taken that it does not reach the zinc and cause a deposit of copper to be placed thereon4 ,as has been stated1 these two liquids do not mix readily1 but they will eventually mingle unless the action of the cell is sufficient to use up the copper sulphate as speedily as it is dissolved4 ,thus it will be seen that while the cell is free from polarization and local action1 there is1 nevertheless1 a deteriorating effect if the cell is allowed to remain long on open circuit4 ,therefore1 it should be used when a constant current is required4 ,prevention of ,creeping3--,much trouble has been experienced in gravity cells due to the creeping of the salts over the edge of the jar4 ,frequently the upper edges of the jars are coated by dipping in hot paraffin wax in the hope of preventing this4 ,sometimes oil is poured on top of the fluid in the jar to prevent the creeping of the salts and the evaporation of the electrolyte4 ,the following account of experiments performed by ,mr4 ,william ,reid1 of ,chicago1 throws light on the relative advantages of these and other methods of preventing creeping4 ,the experiment was made with gravity cells having #e-;inch by #g-;inch glass jars4 ,four cells were made up and operated in a rather dry1 warm place1 although perhaps under no more severe local conditions than would be found in most telephone exchanges4 ,cell ,no4 #a was a plain cell as ordinarily used4 ,cell ,no4 #b had the top of the rim of the jar treated with paraffin wax by dipping the rim to about one inch in depth in melted paraffin wax4 ,cell ,no4 #c had melted paraffin wax poured over the surface of the liquid forming a seal about #c_/af inch in thickness4 ,after cooling1 a few small holes were bored through the seal to let gases escape4 ,cell ,no4 #d had a layer of heavy paraffin oil nearly #a_/b inch in thickness 7about #f oz4 being used7 on top of the solutions4 ,these cells were all run on a load of 4#bb to 4#bi amperes for #ae-a_/b hours per day for thirty days1 after which the following results were noted3 7a7 ,the plain cell1 or cell ,no4 #a1 had to have #bf ounces of water added to it to replace that which had evaporated4 ,the creeping of zinc sulphate salts was very bad4 7b7 ,the waxed rim cell1 or cell ,no4 #b1 evaporated #bf ounces of water and the creeping of zinc sulphate salts was not prevented by the waxed rim4 ,the wax proved of no value4 7c7 ,the wax sealed cell1 or cell ,no4 #c1 showed practically no evaporation and only very slight creeping of zinc sulphate salts4 ,the creeping of salts that took place was only around spots where the edges of the seal were loose from the jar4 7d7 ,the paraffin oil sealed cell1 or cell ,no4 #d1 showed no evaporation and no creeping of salts4 ,it was concluded by ,mr4 ,reid from the above experiments that the wax applied to the rim of the jar is totally ineffective and has no merits4 ,the wax seal loosens around the edges and does not totally prevent creeping of the zinc sulphate salts1 although nearly so4 ,the wax-sealed jar must have holes drilled in it to allow the gases to escape4 ,the method is hardly commercial1 as it is difficult to make a neat appearing cell1 besides making it almost impossible to manipulate its contents4 ,a coat of paraffin oil approximately #a_/b inch in thickness 7about #f ounces7 gives perfect protection against evaporation and creeping of the zinc sulphate salts4 ,the cell1 having the paraffin-oil seal1 had a very neat1 clean appearance as compared with cells ,no4 #a and ,no4 #b4 ,it was found that the zinc could be drawn out through the oil1 cleaned1 and replaced with no appreciable effect on voltage or current4 ,setting ,up3--,in setting up the battery the copper electrode is first unfolded to form a cross and placed in the bottom of the jar4 ,enough copper sulphate1 or blue-stone crystals1 is then dropped into the jar to almost cover the copper4 ,the zinc crowfoot is then hung in place1 occupying a position about #d inches above the top of the copper4 ,clear water is then poured in sufficient to fill the jar within about an inch of the top4 ,if it is not required to use the cell at once1 it may be placed on short circuit for a time and allowed to form its own zinc sulphate4 ,the cell may1 however1 be made immediately available for use by drawing about one-half pint of a solution of zinc sulphate from a cell already in use and pouring it into the jar1 or1 when this is not convenient1 by putting into the liquid four or five ounces of pulverized sulphate of zinc1 or by adding about ten drops of sulphuric acid4 ,when the cell is in proper working condition1 one-half inch in thickness of heavy paraffin oil of good quality may be added4 ,if the blue line gets too low1 and if there is in the bottom of the cell a sufficient quantity of sulphate of copper1 it may be raised by drawing off a portion of the zinc sulphate with a battery syringe and replacing this with water4 ,if the blue line gets too high1 it may be lowered by short-circuiting the cell for a time1 or by the addition of more sulphate of zinc solution from another battery4 ,if the copper sulphate becomes exhausted1 it should be replenished by dropping in more crystals4 ,care should be taken in cold weather to maintain the temperature of the battery above #fe or #gj ,fahrenheit4 ,if below this temperature1 the internal resistance of a cell increases very rapidly1 so much so that even at #ej ,fahrenheit the action becomes very much impaired4 ,this follows from the facts that the resistance of a liquid decreases as its temperature rises1 and that chemical action is much slower at lower temperatures4 ,the gravity cell has a practically constant voltage of #a4jh volts4 ,its internal resistance is comparatively high1 seldom falling below #a ohm and often rising to #f ohms4 ,at best1 therefore1 it is only capable of producing about #a ampere4 ,the gravity cell is perhaps the most common type of cell wherein depolarization is affected by electro-chemical means4 ,fuller ,cell3--,a form of cell that is adapted to very heavy open-circuit work and also closed-circuit work where heavier currents are required than can be supplied by the gravity battery is the ,fuller4 ,in this the electrodes are of zinc and carbon1 respectively1 the zinc usually being in the form of a heavy cone and placed within a porous cup4 ,the electrolyte of the ,fuller cell is known as electropoion fluid1 and consists of a mixture of sodium or potassium bichromate1 sulphuric acid1 and water4 ,the various parts of the standard ,fuller cell1 as once largely employed by the various ,bell operating companies1 are shown in ,fig4 #fe4 ,in this the jar was made of flint glass1 cylindrical in form1 six inches in diameter and eight inches deep4 ,it is important that a good grade of glass be used for the jar in this cell1 because1 on account of the nature of the electrolyte1 breakage is disastrous in the effects it may produce on adjacent property4 ,the carbon plate is rectangular in form1 about four inches wide1 eight and three-quarters inches long1 and one-quarter inch thick4 ,the metal terminal at the top of the carbon block is of bronze1 both it and the lock nuts and bolts being nickel-plated to minimize corrosion4 ,the upper end of the carbon block is soaked in paraffin so hot as to drive all of the moisture out of the paraffin and out of the pores of the block itself4 ,the zinc1 as is noted from the cut1 is in the form of a truncated cone4 ,it is about two and one-eighth inches in diameter at the base and two and one-half inches high4 ,cast into the zinc is a soft copper wire about ,no4 #ab ,b4 ,s4 gauge4 ,this wire extends above the top of the jar so as to form a convenient terminal for the cell4 ,the porous cup is cylindrical in form1 about three inches in diameter and seven inches deep4 ,the wooden cover is of kiln-dried white wood thoroughly coated with two coats of asphalt paint4 ,it is provided with a slot for the carbon and a hole for the copper wire extending to the zinc4 ,the electrolyte for this cell is made as follows3 ,sodium bichromate #f oz4 ,sulphuric acid #ag oz4 ,soft water #ef oz4 ,this solution is mixed by dissolving the bichromate of sodium in the water and then adding slowly the sulphuric acid4 ,potassium bichromate may be substituted for the sodium bichromate4 ,in setting up this cell1 the amalgamated zinc is placed within the porous cup1 in the bottom of which are about two teaspoonfuls of mercury1 the latter serving to keep the zinc well amalgamated4 ,the porous cup is then placed in the glass jar and a sufficient quantity of the electrolyte is placed in the outer jar to come within about one and one-half inches of the top of the porous cup4 ,about two teaspoonfuls of salt are then placed in the porous cup and sufficient soft water added to bring the level of the liquid within the porous cup even with the level of the electrolyte in the jar surrounding the cup4 ,the carbon is then placed through the slot in the cover1 and the wire from the zinc is passed through the hole in the cover provided for it1 and the cover is allowed to fall in place4 ,the cell is now ready for immediate use4 ,the action of this cell is as follows3 ,the sulphuric acid attacks the zinc and forms zinc sulphate1 liberating hydrogen4 ,the hydrogen attempts to pass to the carbon plate as usual1 but in so doing it meets with the oxygen of the chromic acid and forms water therewith4 ,the remainder of the chromic acid combines with the sulphuric acid to form chromium sulphate4 ,illustration3 ,fig #fe4 ,fuller ,cell ,the mercury placed in the bottom of the porous cup with the zinc keeps the zinc in a state of perpetual amalgamation4 ,this it does by capillary action1 as the mercury spreads over the entire surface of the zinc4 ,the initial amalgamation1 while not absolutely essential1 helps in a measure this capillary action4 ,in another well-known type of the ,fuller battery the carbon is a hollow cylinder1 surrounding the porous cup4 ,in this type the zinc usually took the form of a long bar having a cross-shaped section1 the length of this bar being sufficient to extend the entire depth of the porous cup4 ,this type of cell has the advantage of a somewhat lower internal resistance than the standard form just described4 ,should the electrolyte become supersaturated by virtue of the battery being neglected or too heavily overworked1 a set of secondary reactions will occur in the cell1 resulting in the formation of the yellow crystals upon the carbon4 ,this seriously affects the e4m4f4 of the cell and also its internal resistance4 ,should this occur1 some of the solution should be withdrawn and dilute sulphuric acid inserted in its place and the crystals which have formed on the carbon should be carefully washed off4 ,should the solution lose its orange tint and turn blue1 it indicates that more bichromate of potash or bichromate of sodium is needed4 ,this cell gives an electromotive force of #b4a volts and a very large current when it is in good condition1 since its internal resistance is low4 ,the ,fuller cell was once largely used for supplying current to telephone transmitters at subscribers' stations1 where very heavy service was demanded1 but the advent of the so-called common-battery systems1 in some cases1 and of the high-resistance transmitter1 in other cases1 has caused a great lessening in its use4 ,this is fortunate as the cell is a dirty one to handle and is expensive to maintain4 ,the ,fuller cell still warrants attention1 however1 as an available source of current1 which may be found useful in certain cases of emergency work1 and in supplying special but temporary needs for heavier current than the ,le,clanch or gravity cell can furnish4 ,lalande ,cell3--,a type of cell1 specially adapted to constant-current work1 and sometimes used as a central source of current in very small common-battery exchanges is the so-called copper oxide1 or ,lalande cell1 of which the ,edison and the ,gordon are types4 ,in all of these the negatively charged element is of zinc1 the positively charged element a mass of copper oxide1 and the electrolyte a solution of caustic potash in water4 ,in the ,edison cell the copper oxide is in the form of a compressed slab which with its connecting copper support forms the electrode4 ,in the ,gordon and other cells of this type the copper oxide is contained loosely in a perforated cylinder of sheet copper4 ,the copper oxide serves not only as an electrode1 but also as a depolarizing agent1 the liberated hydrogen in the electrolyte uniting with the oxygen of the copper oxide to form water1 and leaving free metallic copper4 ,on open circuit the elements are not attacked1 therefore there is no waste of material while the cell is not in use4 ,this important feature1 and the fact that the internal resistance is low1 make this cell well adapted for all forms of heavy open-circuit work4 ,the fact that there is no polarizing action within the cell makes it further adaptable to heavy closed-circuit service4 ,these cells are intended to be so proportioned that all of their parts become exhausted at once so that when the cell fails1 complete renewals are necessary4 ,therefore1 there is never a question as to which of the elements should be renewed4 ,after the elements and solution are in place about one-fourth of an inch of heavy paraffin oil is poured upon the surface of the solution in order to prevent evaporation4 ,this cell requires little attention and will maintain a constant e4m4f4 of about two-thirds of a volt until completely exhausted4 ,it is non-freezable at all ordinary temperatures4 ,its low voltage is its principal disadvantage4 ,standard ,cell4 ,chloride of ,silver ,cell3--,the chloride of silver cell is largely used as a standard for testing purposes4 ,its compactness and portability and its freedom from local action make it particularly adaptable to use in portable testing outfits where constant electromotive force and very small currents are required4 ,illustration3 ,fig4 #ff4 ,chloride of ,silver ,cell ,a cross-section of one form of the cell is shown in ,fig4 #ff4 ,its elements are a rod of chemically-pure zinc and a rod of chloride of silver immersed in a water solution of sal ammoniac4 ,as ordinarily constructed1 the glass jar or tube is usually about #b-a_/b inches long by #a inch in diameter4 ,after the solution is poured in and the elements are in place the glass tube is hermetically sealed with a plug of paraffin wax4 ,the e4m4f4 of a cell of this type is #a4jc volts and the external resistance varies with the age of the cell1 being about #d ohms at first4 ,care should be taken not to short-circuit these cells1 or use them in any but high-resistance circuits1 as they have but little energy and become quickly exhausted if compelled to work in low-resistance circuits4 ,conventional ,symbol4 ,the conventional symbol for a cell1 either of the primary or the secondary type1 consists of a long thin line and a short heavy line side by side and parallel4 ,a battery is represented by a number of pairs of such lines1 as in ,fig4 #fg4 ,the two lines of each pair are supposed to represent the two electrodes of a cell4 ,where any significance is to be placed on the polarity of the cell or battery the long thin line is supposed to represent the positively charged plate and the short thick line the negatively charged plate4 ,the number of pairs may indicate the number of cells in the battery4 ,frequently1 however1 a few pairs of such lines are employed merely for the purpose of indicating a battery without regard to its polarity or its number of cells4 ,illustration3 ,fig4 #fg4 ,battery ,symbols ,in ,fig4 #fg the representation at ,a is that of a battery of a number of cells connected in parallel2 that at ,b of a battery with the cells connected in series2 and that at ,c of a battery with one of its poles grounded4 ,,chapter ,,viii ,,magneto ,,signaling ,,apparatus ,method of ,signaling4 ,the ordinary apparatus1 by which speech is received telephonically1 is not capable of making sufficiently loud sounds to attract the attention of people at a distance from the instrument4 ,for this reason it is necessary to employ auxiliary apparatus for the purpose of signaling between stations4 ,in central offices where an attendant is always on hand1 the sense of sight is usually appealed to by the use of signals which give a visual indication1 but in the case of telephone instruments for use by the public1 the sense of hearing is appealed to by employing an audible rather than a visual signal4 ,battery ,bell4 ,the ordinary vibrating or battery bell1 such as is employed for door bells1 is sometimes1 though not often1 employed in telephony4 ,it derives its current from primary batteries or from any direct-current source4 ,the reason why they are not employed to a greater extent in telephony is that telephone signals usually have to be sent over lines of considerable length and the voltage that would be required to furnish current to operate such bells over such lengths of line is higher than would ordinarily be found in the batteries commonly employed in telephone work4 ,besides this the make-and-break contacts on which the1 ordinary battery bell depends for its operation are an objectionable feature from the standpoint of maintenance4 ,magneto ,bell4 ,fortunately1 however1 there has been developed a simpler type of electric bell1 which operates on smaller currents1 and which requires no make-and-break contacts whatever4 ,this simpler form of bell is commonly known as the polarized1 or magneto1 bell or ringer4 ,it requires for its operation1 in its ordinary form1 an alternating current1 though in its modified forms it may be used with pulsating currents1 that is1 with periodically recurring impulses of current always in the same direction4 ,magneto ,generator4 ,in the early days of telephony there was nearly always associated with each polarized bell a magneto generator for furnishing the proper kind of current to ring such bells4 ,each telephone was therefore equipped1 in addition to the transmitter and receiver1 with a signal-receiving device in the form of a polarized bell1 and with a current generator by which the user was enabled to develop his own currents of suitable kind and voltage for ringing the bells of other stations4 ,considering the signaling apparatus of the telephones alone1 therefore1 each telephone was equipped with a power plant for generating currents used by that station in signaling other stations1 the prime mover being the muscles of the user applied to the turning of a crank on the side of the instrument2 and also with a current-consuming device in the form of a polarized electromagnetic bell adapted to receive the currents generated at other stations and to convert a portion of their energy into audible signals4 ,the magneto generator is about the simplest type of dynamo-electric machine1 and it depends upon the same principles of operation as the much larger generators1 employed in electric-lighting and street-railway power plants1 for instance4 ,instead of developing the necessary magnetic field by means of electromagnets1 as in the case of the ordinary dynamo1 the field of the magneto generator is developed by permanent magnets1 usually of the horseshoe form4 ,hence the name magneto4 ,illustration3 ,fig4 #fh4 ,principles of ,magneto ,generator ,in order to concentrate the magnetic field within the space in which the armature revolves1 pole pieces of iron are so arranged in connection with the poles of the permanent magnet as to afford a substantially cylindrical space in which the armature conductors may revolve and through which practically all the magnetic lines of force set up by the permanent magnets will pass4 ,in ,fig4 #fh there is shown1 diagrammatically1 a horseshoe magnet with such a pair of pole pieces1 between which a loop of wire is adapted to rotate4 ,the magnet #a is of hardened steel and permanently magnetized4 ,the pole pieces are shown at #b and #c1 each being of soft iron adapted to make good magnetic contact on its flat side with the inner flat surface of the bar magnet1 and being bored out so as to form a cylindrical recess between them as indicated4 ,the direction of the magnetic lines of force set up by the bar magnet through the interpolar space is indicated by the long horizontal arrows1 this flow being from the north pole 7,n7 to the south pole 7,s7 of the magnet4 ,at #d there is shown a loop of wire supposed to revolve in the magnetic field of force on the axis #e-e4 ,theory4 ,in order to understand how currents will be generated in this loop of wire #d1 it is only necessary to remember that if a conductor is so moved as to cut across magnetic lines of force1 an electromotive force will be set up in the conductor which will tend to make the current flow through it4 ,the magnitude of the electromotive force will depend on the rate at which the conductor cuts through the lines of force1 or1 in other words1 on the number of lines of force that are cut through by the conductor in a given unit of time4 ,again1 the direction of the electromotive force depends on the direction of the cutting1 so that if the conductor be moved in one direction across the lines of force1 the electromotive force and the current will be in one direction2 while if it moves in the opposite direction across the lines of force1 the electromotive force and the current will be in the reverse direction4 ,it is1 evident that as the loop of wire #d revolves in the field of force about the axis #e-e1 the portions of the conductor parallel to the axis will cut through the lines of force1 first in one direction and then in the other1 thus producing electromotive forces therein1 first in one direction and then in the other4 ,referring now to ,fig4 #fh1 and supposing that the loop #d is revolving in the direction of the curved arrow shown between the upper edges of the pole pieces1 it will be evident that just as the loop stands in the vertical position1 its horizontal members will be moving in a horizontal direction1 parallel with the lines of force and1 therefore1 not cutting them at all4 ,the electromotive force and the current will1 therefore1 be zero at this time4 ,as the loop advances toward the position shown in dotted lines1 the upper portion of the loop that is parallel with the axis will begin to cut downwardly through the lines of force1 and likewise the lower portion of the loop that is parallel with the axis will begin to cut upwardly through the lines of force4 ,this will cause electromotive forces in opposite directions to be generated in these portions of the loop1 and these will tend to aid each other in causing a current to circulate in the loop in the direction shown by the arrows associated with the dotted representation of the loop4 ,it is evident that as the motion of the loop progresses1 the rate of cutting the lines of force will increase and will be a maximum when the loop reaches a horizontal position1 or at that time the two portions of the loop that are parallel with the axis will be traveling at right angles to the lines of force4 ,at this point1 therefore1 the electromotive force and the current will be a maximum4 ,from this point until the loop again assumes a vertical position1 the cutting of the lines of force will still be in the same direction1 but at a constantly decreasing rate1 until1 finally1 when the loop is vertical the movement of the parts of the loop that are parallel with the axis will be in the direction of the lines of force and1 therefore1 no cutting will take place4 ,at this point1 therefore1 the electromotive force and the current in the loop again will be zero4 ,we have seen1 therefore1 that in this half revolution of the loop from the time when it was in a vertical position to a time when it was again in a vertical position but upside down1 the electromotive force varied from zero to a maximum and back to zero1 and the current did the same4 ,it is easy to see that1 as the loop moves through the next half revolution1 an exactly similar rise and fall of electromotive force and current will take place2 but this will be in the opposite direction1 since that portion of the loop which was going down through the lines of force is now going up1 and the portion which was previously going up is now going down4 ,the law concerning the generation of electromotive force and current in a conductor that is cutting through lines of magnetic force1 may be stated in another way1 when the conductor is bent into the form of a loop1 as in the case under consideration3 ,thus1 if the number of lines of force which pass through a conducting loop be varied1 electromotive forces will be generated in the loop4 ,this will be true whether the number of lines passing through the loop be varied by moving the loop within the field of force or by varying the field of force itself4 ,in any case1 if the number of lines of force be increased1 the current will flow in one way1 and if it be diminished the current will flow in the other way4 ,the amount of the current will depend1 other things being equal1 on the rate at which the lines of force through the loop are being varied1 regardless of the method by which the variation is made to take place4 ,one revolution of the loop1 therefore1 results in a complete cycle of alternating current consisting of one positive followed by one negative impulse4 ,the diagram of ,fig4 #fh is merely intended to illustrate the principle involved4 ,in the practical construction of magneto generators more than one bar magnet is used1 and1 in addition1 the conductors in the armature are so arranged as to include a great many loops of wire4 ,furthermore1 the conductors in the armature are wound around an iron core so that the path through the armature loops or turns1 may present such low reluctance to the passage of lines of force as to greatly increase the number of such lines and also to cause practically all of them to go through the loops in the armature conductor4 ,armature4 ,the iron upon which the armature conductors are wound is called the core4 ,the core of an ordinary armature is shown in ,fig4 #fi4 ,this is usually made of soft gray cast iron1 turned so as to form bearing surfaces at #a and #b1 upon which the entire armature may rotate1 and also turned so that the surfaces #c will be truly cylindrical with respect to the axis through the center of the shaft4 ,the armature conductors are put on by winding the space between the two parallel faces #d as full of insulated wire as space will admit4 ,one end of the armature winding is soldered to the pin #e and1 therefore1 makes contact with the frame of the generator1 while the other end of the winding is soldered to the pin #f1 which engages the stud #g1 carried in an insulating bushing in a longitudinal hole in the end of the armature shaft4 ,it is thus seen that the frame of the machine will form one terminal of the armature winding1 while the insulated stud #g will form the other terminal4 ,illustration3 ,fig4 #fi4 ,generator ,armature ,another form of armature largely employed in recent magneto generators is illustrated in ,fig4 #gj4 ,in this the shaft on which the armature revolves does not form an integral part of the armature core but consists of two cylindrical studs #b and #c projecting from the centers of disks #d and #e1 which are screwed to the ends of the core #a4 ,this ,h type of armature core1 as it is called1 while containing somewhat more parts than the simpler type shown in ,fig4 #fi1 possesses distinct advantages in the matter of winding4 ,by virtue of its simpler form of winding space1 it is easier to insulate and easier to wind1 and furthermore1 since the shaft does not run through the winding space1 it is capable of holding a considerably greater number of turns of wire4 ,the ends of the armature winding are connected1 one directly to the frame and the other to an insulated pin1 as is shown in the illustration4 ,illustration3 ,fig4 #gj4 ,generator ,armature ,illustration3 ,fig4 #ga4 ,generator ,field and ,armature ,the method commonly employed of associating the pole pieces with each other and with the permanent magnets is shown in ,fig4 #ga4 ,it is very important that the space in which the armature revolves shall be truly cylindrical1 and that the bearings for the armature shall be so aligned as to make the axis of rotation of the armature coincide with the axis of the cylindrical surface of the pole pieces4 ,a rigid structure is1 therefore1 required and this is frequently secured1 as shown in ,fig4 #ga1 by joining the two pole pieces #a and #b together by means of heavy brass rods #c and #d1 the rods being shouldered and their reduced ends passed through holes in flanges extending from the pole pieces1 and riveted4 ,the bearing plates in which the armature is journaled are then secured to the ends of these pole pieces1 as will be shown in subsequent illustrations4 ,this assures proper rigidity between the pole pieces and also between the pole pieces and the armature bearings4 ,the reason why this degree of rigidity is required is that it is necessary to work with very small air gaps between the armature core and its pole pieces and unless these generators are mechanically well made they are likely to alter their adjustment and thus allow the armature faces to scrape or rub against the pole pieces4 ,in ,fig4 #ga one of the permanent horseshoe magnets is shown1 its ends resting in grooves on the outer faces of the pole pieces and usually clamped thereto by means of heavy iron machine screws4 ,with this structure in mind1 the theory of the magneto generator developed in connection with ,fig4 #fh may be carried a little further4 ,when the armature lies in the position shown at the left of ,fig4 #ga1 so that the center position of the core is horizontal1 a good path is afforded for the lines of force passing from one pole to the other4 ,practically all of these lines will pass through the iron of the core rather than through the air1 and1 therefore1 practically all of them will pass through the convolutions of the armature winding4 ,when the armature has advanced1 say #de degrees1 in its rotation in the direction of the curved arrow1 the lower right-hand portion of the armature flange will still lie opposite the lower face of the right-hand pole piece and the upper left-hand portion of the armature flange will still lie opposite the upper face of the left-hand pole piece4 ,as a result there will still be a good path for the lines of force through the iron of the core and comparatively little change in the number of lines passing through the armature winding4 ,as the corners of the armature flange pass away from the corners of the pole pieces1 however1 there is a sudden change in condition which may be best understood by reference to the right-hand portion of ,fig4 #ga4 ,the lines of force now no longer find path through the center portion of the armature core--that lying at right angles to their direction of flow4 ,two other paths are at this time provided through the now horizontal armature flanges which serve almost to connect the two pole pieces4 ,the lines of force are thus shunted out of the path through the armature coils and there is a sudden decrease from a large number of lines through the turns of the winding to almost none4 ,as the armature continues in its rotation the two paths through the flanges are broken1 and the path through the center of the armature core and1 therefore1 through the coils themselves1 is restablished4 ,as a result of this consideration it will be seen that in actual practice the change in the number of lines passing through the armature winding is not of the gradual nature that would be indicated by a consideration of ,fig4 #fh alone1 but rather1 is abrupt1 as the corners of the armature flanges leave the corners of the pole pieces4 ,this abrupt change produces a sudden rise in electromotive force just at these points in the rotation1 and1 therefore1 the electromotive force and the current curves of these magneto generators is not usually of the smooth sine-wave type but rather of a form resembling the sine wave with distinct humps added to each half cycle4 ,illustration3 ,fig4 #gb4 ,generator with ,magnets ,removed ,as is to be expected from any two-pole alternating generator1 there is one cycle of current for each revolution of the armature4 ,under ordinary conditions a person is able to turn the generator handle at the rate of about two hundred revolutions a minute1 and as the ratio of gearing is about five to one1 this results in about one thousand revolutions per minute of the generator1 and1 therefore1 in a current of about one thousand cycles per minute1 this varying widely according to the person who is doing the turning4 ,illustration3 ,,howard ,,office ,,of ,,home ,,telephone ,,company1 ,,san ,,francisco ,an ,all-,concrete ,building ,serving the ,district ,south of ,market ,street4 ,the end plates which support the bearings for the armature are usually extended upwardly1 as shown in ,fig4 #gb1 so as to afford bearings for the crank shaft4 ,the crank shaft carries a large spur gear which meshes with a pinion in the end of the armature shaft1 so that the user may cause the armature to revolve rapidly4 ,the construction shown in ,fig4 #gb is typical of that of a modern magneto generator1 it being understood that the permanent magnets are removed for clearness of illustration4 ,fig4 #gc is a view of a completely assembled generator such as is used for service requiring a comparatively heavy output4 ,other types of generators having two1 three1 or four permanent magnets instead of five1 as shown in this figure1 are also standard4 ,illustration3 ,fig4 #gc4 ,five-,bar ,generator ,referring again to ,fig4 #fi1 it will be remembered that one end of the armature winding shown diagrammatically in that figure1 is terminated in the pin #e1 while the other terminates in the pin #g4 ,when the armature is assembled in the frame of the generator it is evident that the frame itself is in metallic connection with one end of the armature winding1 since the pin #e is in metallic contact with the armature casting and this is in contact with the frame of the generator through the bearings4 ,the frame of the machine is1 therefore1 one terminal of the generator4 ,when the generator is assembled a spring of one form or another always rests against the terminal pin #g of the armature so as to form a terminal for the armature winding of such a nature as to permit the armature to rotate freely4 ,such spring1 therefore1 forms the other terminal of the generator4 ,automatic ,shunt4 ,under nearly all conditions of practice it is desirable to have the generator automatically perform some switching function when it is operated4 ,as an example1 when the generator is connected so that its armature is in series in a telephone line1 it is quite obvious that the presence of the resistance and the impedance of the armature winding would be objectionable if left in the circuit through which the voice currents had to pass4 ,for this reason1 what is termed an automatic shunt is employed on generators designed for series work2 this shunt is so arranged that it will automatically shunt or short-circuit the armature winding when it is at rest and also break this shunt when the generator is operated1 so as to allow the current to pass to line4 ,illustration3 ,fig #gd4 ,generator ,shunt ,switch ,a simple and much-used arrangement for this purpose is shown in ,fig4 #gd1 where #a is the armature2 #b is a wire leading from the frame of the generator and forming one terminal of the generator circuit2 and #c is a wire forming the other terminal of the generator circuit1 this wire being attached to the spring #d1 which rests against the center pin of the armature so as to make contact with the opposite end of the armature winding to that which is connected with the frame4 ,the circuit through the armature may be traced from the terminal wire #b through the frame2 thence through the bearings to the armature #a and through the pin to the right-hand side of the armature winding4 ,continuing the circuit through the winding itself1 it passes to the center pin projecting from the left-hand end of the armature shaft2 thence to the spring #d which rests against this pin2 and thence to the terminal wire #c4 ,normally1 this path is shunted by what is practically a short circuit1 which may be traced from the terminal #b through the frame of the generator to the crank shaft #e2 thence to the upper end of the spring #d and out by the terminal wire #c4 ,this is the condition which ordinarily exists and which results in the removal of the resistance and the impedance on the armature winding from any circuit in which the generator is placed1 as long as the generator is not operated4 ,an arrangement is provided1 however1 whereby the crank shaft #e will be withdrawn automatically from engaging with the upper end of the spring #d1 thus breaking the shunt around the armature circuit1 whenever the generator crank is turned4 ,in order to accomplish this the crank shaft #e is capable of partial rotation and of slight longitudinal movement within the hub of the large gear wheel4 ,a spring #g usually presses the crank shaft toward the left and into engagement with the spring #d4 ,a pin #h carried by the crank shaft1 rests in a ,v-shaped notch in the end of the hub #f and as a result1 when the crank is turned the pin rides on the surface of this notch before the large gear wheel starts to turn1 and thus moves the crank shaft #e to the right and breaks the contact between it and the spring #d4 ,thus1 as long as the generator is being operated1 its armature is connected in the circuit of the line1 but as soon as it becomes idle the armature is automatically short-circuited4 ,such devices as this are termed automatic shunts4 ,in still other cases it is desirable to have the generator circuit normally open so that it will not affect in any way the electrical characteristics of the line while the line is being used for talking4 ,in this case the arrangement is made so that the generator will automatically be placed in proper circuit relation with the line when it is operated4 ,illustration3 ,fig4 #ge4 ,generator ,cut-in ,switch ,a common arrangement for doing this is shown in ,fig4 #ge1 wherein the spring #a normally rests against the contact pin of the armature and forms one terminal of the armature circuit4 ,the spring #b is adapted to form the other terminal of the armature circuit but it is normally insulated from everything4 ,the circuit of the generator is1 therefore1 open between the spring #b and the shaft #c1 but as soon as the generator is operated the crank shaft is bodily moved to the left by means of the ,v-shaped notch in the driving collar #d and is thus made to engage the spring #b4 ,the circuit of the generator is then completed from the spring #a through the armature pin to the armature winding2 thence to the frame of the machine and through shaft #c to the spring #b4 ,such devices as this are largely used in connection with so-called bridging telephones in which the generators and bells are adapted to be connected in multiple across the line4 ,a better arrangement for accomplishing the automatic switching on the part of the generator is to make no use of the crank shaft as a part of the conducting path as is the case in both ,figs4 #gd and #ge1 but to make the crank shaft1 by its longitudinal movement1 impart the necessary motion to a switch spring which1 in turn1 is made to engage or disengage a corresponding contact spring4 ,an arrangement of this kind that is in common use is shown in ,fig4 #gf4 ,this needs no further explanation than to say that the crank shaft is provided on its end with an insulating stud #a1 against which a switching spring #b bears4 ,this spring normally rests against another switch spring #c1 but when the generator crank shaft moves to the right upon the turning of the crank1 the spring #b disengages spring #c and engages spring #d1 thus completing the circuit of the generator armature4 ,it is seen that this operation accomplishes the breaking of one circuit and the making of another1 a function that will be referred to later on in this work4 ,illustration3 ,fig4 #gf4 ,generator ,cut-in ,switch ,pulsating ,current4 ,sometimes it is desirable to have a generator capable of developing a pulsating current instead of an alternating current2 that is1 a current which will consist of impulses all in one direction rather than of impulses alternating in direction4 ,it is obvious that this may be accomplished if the circuit of the generator be broken during each half revolution so that its circuit is completed only when current is being generated in one direction4 ,such an arrangement is indicated diagrammatically in ,fig4 #gg4 ,instead of having one terminal of the armature winding brought out through the frame of the generator as is ordinarily done1 both terminals are brought out to a commuting device carried on the end of the armature shaft4 ,thus1 one end of the loop representing the armature winding is shown connected directly to the armature pin #a1 against which bears a spring #b1 in the usual manner4 ,the other end of the armature winding is carried directly to a disk #c1 mounted on but insulated from the shaft and revolving therewith4 ,one-half of the circumferential surface of this disk is of insulating material #d and a spring #e rests against this disk and bears alternately upon the conducting portion #c or the insulating portion #d1 according to the position of the armature in its revolution4 ,it is obvious that when the generator armature is in the position shown the circuit through it is from the spring #b to the pin #a2 thence to one terminal of the armature loop2 thence through the loop and back to the disk #c and out by the spring #e4 ,if1 however1 the armature were turned slightly1 the spring #e would rest on the insulating portion #d and the circuit would be broken4 ,illustration3 ,fig4 #gg4 ,pulsating-,current ,commutator ,illustration3 ,fig4 #gh4 ,generator ,symbols ,it is obvious that if the brush #e is so disposed as to make contact with the disk #c only during that portion of the revolution while positive current is being generated1 the generator will produce positive pulsations of current1 all the negative ones being cut out4 ,if1 on the other hand1 the spring #e may be made to bear on the opposite side of the disk1 then it is evident that the positive impulses would all be cut out and the generator would develop only negative impulses4 ,such a generator is termed a direct-current generator or a pulsating-current generator4 ,the symbols for magneto or hand generators usually embody a simplified side view1 showing the crank and the gears on one side and the shunting or other switching device on the other4 ,thus in ,fig4 #gh are shown three such symbols1 differing from each other only in the details of the switching device4 ,the one at the left shows the simple shunt1 adapted to short-circuit the generator at all times save when it is in operation4 ,the one in the center shows the cut-in1 of which another form is described in connection with ,fig4 #ge2 while the symbol at the right of ,fig4 #gh is of the make-and-break device1 discussed in connection with ,fig4 #gf4 ,in such diagrammatic representations of generators it is usual to somewhat exaggerate the size of the switching springs1 in order to make clear their action in respect to the circuit connections in which the generator is used4 ,polarized ,ringer4 ,the polarized bell or ringer is1 as has been stated1 the device which is adapted to respond to the currents sent out by the magneto generator4 ,in order that the alternately opposite currents may cause the armature to move alternately in opposite directions1 these bells are polarized1 i4e41 given a definite magnetic set1 so to speak2 so the effect of the currents in the coils is not to create magnetism in normally neutral iron1 but rather to alter the magnetism in iron already magnetized4 ,western ,electric ,ringer4 ,a typical form of polarized bell is shown in ,fig4 #gi1 this being the standard bell or ringer of the ,western ,electric ,company4 ,the two electromagnets are mounted side by side1 as shown1 by attaching their cores to a yoke piece #a of soft iron4 ,this yoke piece also carries the standards #b upon which the gongs are mounted4 ,the method of mounting is such that the standards may be adjusted slightly so as to bring the gongs closer to or farther from1 the tapper4 ,the soft iron yoke piece #a also carries two brass posts #c which1 in turn1 carry another yoke #d of brass4 ,in this yoke #d is pivoted1 by means of trunnion screws1 the armature #e1 this extending on each side of the pivot so that its ends lie opposite the free poles of the electromagnets4 ,from the center of the armature projects the tapper rod carrying the ball or striker which plays between the two gongs4 ,in order that the armature and cores may be normally polarized1 a permanent magnet #f is secured to the center of the yoke piece #a4 ,this bends around back of the electromagnets and comes into close proximity to the armature #e4 ,by this means one end of each of the electromagnet cores is given one polarity--say north--while the armature is given the other polarity--say south4 ,the two coils of the electromagnet are connected together in series in such a way that current in a given direction will act to produce a north pole in one of the free poles and a south pole in the other4 ,if it be assumed that the permanent magnet maintains the armature normally of south polarity and that the current through the coils is of such direction as to make the left-hand core north and the right-hand core south1 then it is evident that the left-hand end of the armature will be attracted and the right-hand end repelled4 ,this will throw the tapper rod to the right and sound the right-hand bell4 ,a reversal in current will obviously produce the opposite effect and cause the tapper to strike the left-hand bell4 ,an important feature in polarized bells is the adjustment between the armature and the pole pieces4 ,this is secured in the ,western ,electric bell by means of the nuts #g1 by which the yoke #d is secured to the standards #c4 ,by moving these nuts up or down on the standards the armature may be brought closer to or farther from the poles1 and the device affords ready means for clamping the parts into any position to which they may have been adjusted4 ,illustration3 ,fig4 #gi4 ,polarized ,bell ,kellogg ,ringer4 ,another typical ringer is that of the ,kellogg ,switchboard and ,supply ,company1 shown in ,fig4 #hj4 ,this differs from that of the ,western ,electric ,company mainly in the details by which the armature adjustment is obtained4 ,the armature supporting yoke #a is attached directly to the cores of the magnets1 no supporting side rods being employed4 ,instead of providing means whereby the armature may be adjusted toward or from the poles1 the reverse practice is employed1 that is1 of making the poles themselves extensible4 ,this is done by means of the iron screws #b which form extensions of the cores and which may be made to approach or recede from the armature by turning them in such direction as to screw them in or out of the core ends4 ,illustration3 ,fig4 #hj4 ,polarized ,bell ,illustration3 ,fig4 #ha4 ,biased ,bell ,biased ,bell4 ,the pulsating-current generator has already been discussed and its principle of operation pointed out in connection with ,fig4 #gg4 ,the companion piece to this generator is the so-called biased ringer4 ,this is really nothing but a common alternating-current polarized ringer with a light spring so arranged as to hold the armature normally in one of its extreme positions so that the tapper will rest against one of the gongs4 ,such a ringer is shown in ,fig4 #ha and needs no further explanation4 ,it is obvious that if a current flows in the coils of such a ringer in a direction tending to move the tapper toward the left1 then no sound will result because the tapper is already moved as far as it can be in that direction4 ,if1 however1 currents in the opposite direction are caused to flow through the windings1 then the electromagnetic attraction on the armature will overcome the pull of the spring and the tapper will move over and strike the right-hand gong4 ,a cessation of the current will allow the spring to exert itself and throw the tapper back into engagement with the left-hand gong4 ,a series of such pulsations in the proper direction will1 therefore1 cause the tapper to play between the two gongs and ring the bell as usual4 ,a series of currents in a wrong direction will1 however1 produce no effect4 ,conventional ,symbols4 ,in ,fig4 #hb are shown six conventional symbols of polarized bells4 ,the three at the top1 consisting merely of two circles representing the magnets in plan view1 are perhaps to be preferred as they are well standardized1 easy to draw1 and rather suggestive4 ,the three at the bottom1 showing the ringer as a whole in side elevation1 are somewhat more specific1 but are objectionable in that they take more space and are not so easily drawn4 ,illustration3 ,fig4 #hb4 ,ringer ,symbols ,symbols ,a or ,b may be used for designating any ordinary polarized ringer4 ,symbols ,c and ,d are interchangeably used to indicate a biased ringer4 ,if the bell is designed to operate only on positive impulses1 then the plus sign is placed opposite the symbol1 while a minus sign so placed indicates that the bell is to be operated only by negative impulses4 ,some specific types of ringers are designed to operate only on a given frequency of current4 ,that is1 they are so designed as to be responsive to currents having a frequency of sixty cycles per second1 for instance1 and to be unresponsive to currents of any other frequency4 ,either symbols ,e or ,f may be used to designate such ringers1 and if it is desired to indicate the particular frequency of the ringer this is done by adding the proper numeral followed by a short reversed curve sign indicating frequency4 ,thus #ej would indicate a frequency of fifty cycles per second4 ,,chapter ,,ix ,,the ,,hook ,,switch ,purpose4 ,in complete telephone instruments1 comprising both talking and signaling apparatus1 it is obviously desirable that the two sets of apparatus1 for talking and signaling respectively1 shall not be connected with the line at the same time4 ,a certain switching device is1 therefore1 necessary in order that the signaling apparatus alone may be left operatively connected with the line while the instrument is not being used in the transmission of speech1 and in order that the signaling apparatus may be cut out when the talking apparatus is brought into play4 ,in instruments employing batteries for the supply of transmitter current1 another switching function is the closing of the battery circuit through the transmitter and the induction coil when the instrument is in use for talking1 since to leave the battery circuit closed all the time would be an obvious waste of battery energy4 ,in the early forms of telephones these switching operations were performed by a manually operated switch1 the position of which the user was obliged to change before and after each use of the telephone4 ,the objection to this was not so much in the manual labor imposed on the user as in the tax on his memory4 ,it was found to be practically a necessity to make this switching function automatic1 principally because of the liability of the user to forget to move the switch to the proper position after using the telephone1 resulting not only in the rapid waste of the battery elements but also in the inoperative condition of the signal-receiving bell4 ,the solution of this problem1 a vexing one at first1 was found in the so-called automatic hook switch or switch hook1 by which the circuits of the instrument were made automatically to assume their proper conditions by the mere act1 on the part of the user1 of removing the receiver from1 or placing it upon1 a conveniently arranged hook or fork projecting from the side of the telephone casing4 ,automatic ,operation4 ,it may be taken as a fundamental principle in the design of any piece of telephone apparatus that is to be generally used by the public1 that the necessary acts which a person must perform in order to use the device must1 as far as possible1 follow as a natural result from some other act which it is perfectly obvious to the user that he must perform4 ,so in the case of the switch hook1 the user of a telephone knows that he must take the receiver from its normal support and hold it to his ear2 and likewise1 when he is through with it1 that he must dispose of it by hanging it upon a support obviously provided for that purpose4 ,in its usual form a forked hook is provided for supporting the receiver in a convenient place4 ,this hook is at the free end of a pivoted lever1 which is normally pressed upward by a spring when the receiver is not supported on it4 ,when1 however1 the receiver is supported on it1 the lever is depressed by its weight4 ,the motion of the lever is mechanically imparted to the members of the switch proper1 the contacts of which are usually enclosed so as to be out of reach of the user4 ,this switch is so arranged that when the hook is depressed the circuits are held in such condition that the talking apparatus will be cut out1 the battery circuit opened1 and the signaling apparatus connected with the line4 ,on the other hand1 when the hook is in its raised position1 the signaling apparatus is cut out1 the talking apparatus switched into proper working relation with the line1 and the battery circuit closed through the transmitter4 ,in the so-called common-battery telephones1 where no magneto generator or local battery is included in the equipment at the subscriber's station1 the mere raising of the hook serves another important function4 ,it acts1 not only to complete the circuit through the substation talking apparatus1 but1 by virtue of the closure of the line circuit1 permits a current to flow over the line from the central-office battery which energizes a signal associated with the line at the central office4 ,this use of the hook switch in the case of the common-battery telephone is a good illustration of the principle just laid down as to making all the functions which the subscriber has to perform depend1 as far as possible1 on acts which his common sense alone tells him he must do4 ,thus1 in the common-battery telephone the subscriber has only to place the receiver at his ear and ask for what he wants4 ,this operation automatically displays a signal at the central office and he does nothing further until the operator inquires for the number that he wants4 ,he has then nothing to do but wait until the called-for party responds1 and after the conversation his own personal convenience demands that he shall dispose of the receiver in some way1 so he hangs it up on the most convenient object1 the hook switch1 and thereby not only places the apparatus at his telephone in proper condition to receive another call1 but also conveys to the central office the signal for disconnection4 ,likewise in the case of telephones operating in connection with automatic exchanges1 the hook switch performs a number of functions automatically1 of which the subscriber has no conception2 and while1 in automatic telephones1 there are more acts required of the user than in the manual1 yet a study of these acts will show that they all follow in a way naturally suggested to the user1 so that he need have but the barest fundamental knowledge in order to properly make use of the instrument4 ,in all cases1 in properly designed apparatus1 the arrangement is such that the failure of the subscriber to do a certain required act will do no damage to the apparatus or to the system1 and1 therefore1 will inconvenience only himself4 ,design4 ,the hook switch is in reality a two-position switch1 and while at present it is a simple affair1 yet its development to its high state of perfection has been slow1 and its imperfections in the past have been the cause of much annoyance4 ,several important points must be borne in mind in the design of the hook switch4 ,the spring provided to lift the hook must be sufficiently strong to accomplish this purpose and yet must not be strong enough to prevent the weight of the receiver from moving the switch to its other position4 ,the movement of this spring must be somewhat limited in order that it will not break when used a great many times1 and also it must be of such material and shape that it will not lose its elasticity with use4 ,the shape and material of the restoring spring are1 of course1 determined to a considerable extent by the length of the lever arm which acts on the spring1 and on the space which is available for the spring4 ,the various contacts by which the circuit changes are brought about upon the movement of the hook-switch lever usually take the form of springs of ,german silver or phosphor-bronze1 hard rolled so as to have the necessary resiliency1 and these are usually tipped with platinum at the points of contact so as to assure the necessary character of surface at the points where the electric circuits are made or broken4 ,a slight sliding movement between each pair of contacts as they are brought together is considered desirable1 in that it tends to rub off any dirt that may have accumulated1 yet this sliding movement should not be great1 as the surfaces will then cut each other and1 therefore1 reduce the life of the switch4 ,contact ,material4 ,on account of the high cost of platinum1 much experimental work has been done to find a substitute metal suitable for the contact points in hook switches and similar uses in the manufacture of telephone apparatus4 ,platinum is unquestionably the best known material1 on account of its non-corrosive and heat-resisting qualities4 ,hard silver is the next best and is found in some first-class apparatus4 ,the various cheap alloys intended as substitutes for platinum or silver in contact points may be dismissed as worthless1 so far as the writers' somewhat extensive investigations have shown4 ,in the more recent forms of hook switches1 the switch lever itself does not form a part of the electrical circuit1 but serves merely as the means by which the springs that are concerned in the switching functions are moved into their alternate cooperative relations4 ,one advantage in thus insulating the switch lever from the current-carrying portions of the apparatus and circuits is that1 since it necessarily projects from the box or cabinet1 it is thus liable to come in contact with the person of the user4 ,by insulating it1 all liability of the user receiving shocks by contact with it is eliminated4 ,wall ,telephone ,hooks4 ,kellogg4 ,a typical form of hook switch1 as employed in the ordinary wall telephone sets1 is shown in ,fig4 #hc1 this being the standard hook of the ,kellogg ,switchboard and ,supply ,company4 ,in this the lever #a is pivoted at the point #c in a bracket #e that forms the base of all the working parts and the means of securing the entire hook switch to the box or framework of the telephone4 ,this switch lever is normally pressed upward by a spring #b1 mounted on the bracket #e1 and engaging the under side of the hook lever at the point #d4 ,attached to the lever arm #a is an insulated pin #f4 ,the contact springs by which the various electrical circuits are made and broken are shown at #g1 #h1 #i1 #aj1 and #aa1 these being mounted in one group with insulated bushings between them2 the entire group is secured by machine screws to a lug projecting horizontally from the bracket #e4 ,the center spring #i is provided with a forked extension which embraces the pin #f on the hook lever4 ,it is obvious that an up-and-down motion of the hook lever will move the long spring #i in such manner as to cause electrical contact either between it and the two upper springs #g and #h1 or between it and the two lower springs #aj and #aa4 ,the hook is shown in its raised position1 which is the position required for talking4 ,when lowered the two springs #g and #h are disengaged from the long spring #i and from each other1 and the three springs #i1 #aj1 and #aa are brought into electrical engagement1 thus establishing the necessary signaling conditions4 ,illustration3 ,fig4 #hc4 ,long ,lever ,hook ,switch ,the right-hand ends of the contact springs are shown projecting beyond the insulating supports4 ,this is for the purpose of facilitating making electrical joints between these springs and the various wires which lead from them4 ,these projecting ends are commonly referred to as ears1 and are usually provided with holes or notches into which the connecting wire is fastened by soldering4 ,western ,electric4 ,fig4 #hd shows the type of hook switch quite extensively employed by the ,western ,electric ,company in wall telephone sets where the space is somewhat limited and a compact arrangement is desired4 ,it will readily be seen that the principle on which this hook switch operates is similar to that employed in ,fig4 #hc1 although the mechanical arrangement of the parts differs radically4 ,the hook lever #a is pivoted at #c on a bracket #b1 which serves to support all the other parts of the switch4 ,the contact springs are shown at #d1 #e1 and #f1 and this latter spring #f is so designed as to make it serve as an actuating spring for the hook4 ,this is accomplished by having the curved end of this spring press against the lug #g of the hook and thus tend to raise the hook when it is relieved of the weight of the receiver4 ,the two shorter springs #h and #i have no electrical function but merely serve as supports against which the springs #d and #e may rest1 when the receiver is on the hook1 these springs #d and #e being given a light normal tension toward the stop springs #h and #i4 ,it is obvious that in the particular arrangement of the springs in this switch no contacts are closed when the receiver is on the hook4 ,illustration3 ,fig4 #hd4 ,short ,lever ,hook ,switch ,concerning this latter feature1 it will be noted that the particular form of ,kellogg hook switch1 shown in ,fig4 #hc1 makes two contacts and breaks two when it is raised4 ,similarly the ,western ,electric ,company's makes two contacts but does not break any when raised4 ,from such considerations it is customary to speak of a hook such as that shown in ,fig4 #hc as having two make and two break contacts1 and such a hook as that shown in ,fig4 #hd as having two make contacts4 ,it will be seen from either of these switches that the modification of the spring arrangement1 so as to make them include a varying number of make-and-break contacts1 is a simple matter1 and switches of almost any type are readily modified in this respect4 ,illustration3 ,fig4 #he4 ,removable ,lever ,hook ,switch ,dean4 ,in ,fig4 #he is shown a decidedly unique hook switch for wall telephone sets which forms the standard equipment of the ,dean ,electric ,company4 ,the hook lever #a is pivoted at #b1 an auxiliary lever #c also being pivoted at the same point4 ,the auxiliary lever #c carries at its rear end a slotted lug #d1 which engages the long contact spring #e1 and serves to move it up and down so as to engage and disengage the spring #f1 these two springs being mounted on a base lug extending from the base plate #g1 upon which the entire hook-switch mechanism is mounted4 ,the curved spring #h1 also mounted on this same base1 engages the auxiliary lever #c at the point #i and normally serves to press this up so as to maintain the contact springs #e in engagement with contact spring #f4 ,the switch springs are moved entirely by the auxiliary lever #c1 but in order that this lever #c may be moved as required by the hook lever #a1 this lever is provided with a notched lug #aj on its lower side1 which notch is engaged by a forwardly projecting lug #aa that is integral with the auxiliary lever #c4 ,the switch lever may be bodily removed from the remaining parts of the hook switch by depressing the lug #aa with the finger1 so that it disengages the notch in lug #aj1 and then drawing the hook lever out of engagement with the pivot stud #b1 as shown in the lower portion of the figure4 ,it will be noted that the pivotal end of the hook lever is made with a slot instead of a hole as is the customary practice4 ,the advantage of being able to remove the hook switch bodily from the other portions arises mainly in connection with the shipment or transportation of instruments4 ,the projecting hooks cause the instruments to take up more room and thus make larger packing boxes necessary than would otherwise be used4 ,moreover1 in handling the telephones in store houses or transporting them to the places where they are to be used1 the projecting hook switch is particularly liable to become damaged4 ,it is for convenience under such conditions that the ,dean hook switch is made so that the switch lever may be removed bodily and placed1 for instance1 inside the telephone box for transportation4 ,desk-,stand ,hooks4 ,the problem of hook-switch design for portable desk telephones1 while presenting the same general characteristics1 differs in the details of construction on account of the necessarily restricted space available for the switch contacts in the desk telephone4 ,illustration3 ,,west ,,office ,,of ,,home ,,telephone ,,company1 ,,san ,,francisco ,serving the ,general ,western ,business and ,residence ,districts4 ,western ,electric4 ,in ,fig4 #hf is shown an excellent example of hook-switch design as applied to the requirements of the ordinary portable desk set4 ,this figure is a cross-sectional view of the base and standard of a familiar type of desk telephone4 ,the base itself is of stamped metal construction1 as indicated1 and the standard which supports the transmitter and the switch hook for the receiver is composed of a black enameled or nickel-plated brass tube #a1 attached to the base by a screw-threaded joint1 as shown4 ,the switch lever #b is pivoted at #c in a brass plug #d1 closing the upper end of the tube forming the standard4 ,this brass plug supports also the transmitter1 which is not shown in this figure4 ,attached to the plug #d by the screw #e is a heavy strip #f1 which reaches down through the tube to the base plate of the standard and is held therein by a screw #g4 ,the plug #d1 carrying with it the switch-hook lever #b and the brass strip #f1 may be lifted bodily out of the standard #a by taking out the screw #g which holds the strip #f in place1 as is clearly indicated4 ,on the strip #f there is mounted the group of switch springs by which the circuit changes of the instrument are brought about when the hook is raised or lowered4 ,the spring #h is longer than the others1 and projects upwardly far enough to engage the lug on the switch-hook lever #b4 ,this spring1 which is so bent as to close the contacts at the right when not prevented by the switch lever1 also serves as an actuating spring to raise the lever #b when the receiver is removed from it4 ,this spring1 when the receiver is removed from the hook1 engages the two springs at the right1 as shown1 or when the receiver is placed on the hook1 breaks contact with the two right-hand springs and makes contact respectively with the left-hand spring and also with the contact #i which forms the transmitter terminal4 ,illustration3 ,fig4 #hf4 ,desk-,stand ,hook ,switch ,it is seen from an inspection of this switch hook that it has two make and two break contacts4 ,the various contact springs are connected with the several binding posts shown1 these forming the connectors for the flexible cord conductors leading into the base and up through the standard of the desk stand4 ,by means of the conductors in this cord the circuits are led to the other parts of the instrument1 such as the induction coil1 call bell1 and generator1 if there is one1 which1 in the case of the ,western ,electric ,company's desk set1 are all mounted separately from the portable desk stand proper4 ,this hook switch is accessible in an easy manner and yet not subject to the tampering of idle or mischievous persons4 ,by taking out the screw #g the entire hook switch may be lifted out of the tube forming the standard1 the cords leading to the various binding posts being slid along through the tube4 ,by this means the connections to the hook switch1 as well as the contact of the switch itself1 are readily inspected or repaired by those whose duty it is to perform such operations4 ,kellogg4 ,in ,fig4 #hg is shown a sectional view of the desk-stand hook switch of the ,kellogg ,switchboard and ,supply ,company4 ,in this it will be seen that instead of placing the switch-hook springs within the standard or tube1 as in the case of the ,western ,electric ,company1 they are mounted in the base where they are readily accessible by merely taking off the base plate from the bottom of the stand4 ,the hook lever operates on the long spring of the group of switch springs by means of a toggle joint in an obvious manner4 ,this switch spring itself serves by its own strength to raise the hook lever when released from the weight of the receiver4 ,illustration3 ,fig4 #hg4 ,desk-,stand ,hook ,switch ,in this switch1 the hook lever1 and in fact the entire exposed metal portions of the instrument1 are insulated from all of the contact springs and1 therefore1 there is little liability of shocks on the part of the person using the instrument4 ,conventional ,symbols4 ,the hook switch plays a very important part in the operation of telephone circuits2 for this reason readily understood conventional symbols1 by which they may be conveniently represented in drawings of circuits1 are desirable4 ,in ,fig4 #hh are shown several symbols such as would apply to almost any circuit1 regardless of the actual mechanical details of the particular hook switch which happened to be employed4 ,thus diagram ,a in ,fig4 #hh shows a hook switch having a single make contact and this diagram might be used to refer to the hook switch of the ,dean ,electric ,company shown in ,fig4 #he1 in which only a single contact is made when the receiver is removed1 and none is made when it is on the hook4 ,similarly1 diagram ,b might be used to represent the hook switch of the ,kellogg ,company1 shown in ,fig4 #hc1 the arrangement being for two make and two break contacts4 ,likewise diagram ,c might be used to represent the hook switch of the ,western ,electric ,company1 shown in ,fig4 #hd1 which1 as before stated1 has two make contacts only4 ,diagram ,d shows another modification in which contacts made by the hook switch1 when the receiver is removed1 control two separate circuits4 ,assuming that the solid black portion represents insulation1 it is obvious that the contacts are divided into two groups1 one insulated from the other4 ,illustration3 ,fig4 #hh4 ,hook ,switch ,symbols ,illustration3 ,,compressed ,,air ,,wagon ,,for ,,pneumatic ,,drilling ,,and ,,chipping ,,in ,,manholes ,,chapter ,x ,,electromagnets ,,and ,,inductive ,,coils ,electromagnet4 ,the physical thing which we call an electromagnet1 consisting of a coil or helix of wire1 the turns of which are insulated from each other1 and within which is usually included an iron core1 is by far the most useful of all the so-called translating devices employed in telephony4 ,in performing the ordinary functions of an electromagnet it translates the energy of an electrical current into the energy of mechanical motion4 ,an almost equally important function is the converse of this1 that is1 the translation of the energy of mechanical motion into that of an electrical current4 ,in addition to these primary functions which underlie the art of telephony1 the electromagnetic coil or helix serves a wide field of usefulness in cases where no mechanical motion is involved4 ,as impedance coils1 they serve to exert important influences on the flow of currents in circuits1 and as induction coils1 they serve to translate the energy of a current flowing in one circuit into the energy of a current flowing in another circuit1 the translation usually1 but not always1 being accompanied by a change in voltage4 ,when a current flows through the convolutions of an ordinary helix1 the helix will exhibit the properties of a magnet even though the substance forming the core of the helix is of non-magnetic material1 such as air1 or wood1 or brass4 ,if1 however1 a mass of iron1 such as a rod or a bundle of soft iron wires1 for instance1 is substituted as a core1 the magnetic properties will be enormously increased4 ,the reason for this is1 that a given magnetizing force will set up in iron a vastly greater number of lines of magnetic force than in air or in any other non-magnetic material4 ,magnetizing ,force4 ,the magnetizing force of a given helix is that force which tends to drive magnetic lines of force through the magnetic circuit interlinked with the helix4 ,it is called magnetomotive force and is analogous to electromotive force1 that is1 the force which tends to drive an electric current through a circuit4 ,the magnetizing force of a given helix depends on the product of the current strength and the number of turns of wire in the helix4 ,thus1 when the current strength is measured in amperes1 this magnetizing force is expressed as ampere-turns1 being the product of the number of amperes flowing by the number of turns4 ,the magnetizing force exerted by a given current1 therefore1 is independent of anything except the number of turns1 and the material within the core or the shape of the core has no effect upon it4 ,magnetic ,flux4 ,the total magnetization resulting from a magnetizing force is called the magnetic flux1 and is analogous to current4 ,the intensity of a magnetic flux is expressed by the number of magnetic lines of force in a square centimeter or square inch4 ,while the magnetomotive force or magnetizing force of a given helix is independent of the material of the core1 the flux which it sets up is largely dependent on the material and shape of the core--not only upon this but on the material that lies in the return path for the flux outside of the core4 ,we may say1 therefore1 that the amount of flux set up by a given current in a given coil or helix is dependent on the material in the magnetic path or magnetic circuit1 and on the shape and length of that circuit4 ,if the magnetic circuit be of air or brass or wood or any other non-magnetic material1 the amount of flux set up by a given magnetizing force will be relatively small1 while it will be very much greater if the magnetic circuit be composed in part or wholly of iron or steel1 which are highly magnetic substances4 ,permeability4 ,the quality of material1 which permits of a given magnetizing force setting up a greater or less number of lines of force within it1 is called its permeability4 ,more accurately1 the permeability is the ratio existing between the amount of magnetization and the magnetizing force which produces such magnetization4 ,the permeability of a substance is usually represented by the ,greek letter 7pronounced mu74 ,the intensity of the magnetizing force is commonly symbolized by ,h1 and since the permeability of air is always taken as unity1 we may express the intensity of magnetizing force by the number of lines of force per square centimeter which it sets up in air4 ,now1 if the space on which the given magnetizing force ,h were acting were filled with iron instead of air1 then1 owing to the greater permeability of iron1 there would be set up a very much greater number of lines of force per square centimeter1 and this number of lines of force per square centimeter in the iron is the measure of the magnetization produced and is commonly expressed by the letter ,b4 ,from this we have ,b_/,h ,thus1 when we say that the permeability of a given specimen of wrought iron under given conditions is #b1jjj1 we mean that #b1jjj times as many lines of force would be induced in a unit cross-section of this sample as would be induced by the same magnetizing force in a corresponding unit cross-section of air4 ,evidently for air ,b ,h1 hence becomes unity4 ,the permeability of air is always a constant4 ,this means that whether the magnetic density of the lines of force through the air be great or small the number of lines will always be proportional to the magnetizing force4 ,unfortunately for easy calculations in electromagnetic work1 however1 this is not true of the permeability of iron4 ,for small magnetic densities the permeability is very great1 but for large densities1 that is1 under conditions where the number of lines of force existing in the iron is great1 the permeability becomes smaller1 and an increase in the magnetizing force does not produce a corresponding increase in the total flux through the iron4 ,magnetization ,curves4 ,this quality of iron is best shown by the curves of ,fig4 #hi1 which illustrate the degree of magnetization set up in various kinds of iron by different magnetizing forces4 ,in these curves the ordinates represent the total magnetization ,b1 while the abscissas represent the magnetizing force ,h4 ,it is seen from an inspection of these curves that as the magnetizing force ,h increases1 the intensity of flux also increases1 but at a gradually lessening rate1 indicating a reduction in permeability at the higher densities4 ,these curves are also instructive as showing the great differences that exist between the permeability of the different kinds of iron2 and also as showing how1 when the magnetizing force becomes very great1 the iron approaches what is called saturation1 that is1 a point at which the further increase in magnetizing force will result in no further magnetization of the core4 ,from the data of the curves of ,fig4 #hi1 which are commonly called magnetization curves1 it is easy to determine other data from which so-called permeability curves may be plotted4 ,in permeability curves the total magnetization of the given pieces of iron are plotted as abscissas1 while the corresponding permeabilities are plotted as ordinates4 ,illustration3 ,fig4 #hi4 ,magnetization ,curve ,direction of ,lines of ,force4 ,the lines of force set up within the core of a helix always have a certain direction4 ,this direction always depends upon the direction of the flow of current around the core4 ,an easy way to remember the direction is to consider the helix as grasped in the right hand with the fingers partially encircling it and the thumb pointing along its axis4 ,then1 if the current through the convolutions of the helix be in the direction in which the fingers of the hand are pointed around the helix1 the magnetic lines of force will proceed through the core of the helix along the direction in which the thumb is pointed4 ,in the case of a simple bar electromagnet1 such as is shown in ,fig4 #ij1 the lines of force emerging from one end of the bar must pass back through the air to the other end of the bar1 as indicated by dotted lines and arrows4 ,the path followed by the magnetic lines of force is called the magnetic circuit1 and1 therefore1 the magnetic circuit of the magnet shown in ,fig4 #ij is composed partly of iron and partly of air4 ,from what has been said concerning the relative permeability of air and of iron1 it will be obvious that the presence of such a long air path in the magnetic circuit will greatly reduce the number of lines of force that a given magnetizing force can set up4 ,the presence of an air gap in a magnetic circuit has much the same effect on the total flow of lines of force as the presence of a piece of bad conductor in a circuit composed otherwise of good conductor1 in the case of the flow of electric current4 ,reluctance4 ,as the property which opposes the flow of electric current in an electrical circuit is called resistance1 so the property which opposes the flow of magnetic lines of force in a magnetic circuit is called reluctance4 ,in the case of the electric circuit1 the resistance is the reciprocal of the conductivity2 in the case of the magnetic circuit1 the reluctance is the reciprocal of the permeability4 ,as in the case of an electrical circuit1 the amount of flow of current is equal to the electromotive force divided by the resistance2 so in a magnetic circuit1 the magnetic flux is equal to the magnetizing force or magnetomotive force divided by the reluctance4 ,illustration3 ,fig4 #ij4 ,bar ,electromagnet ,types of ,low-,reluctance ,circuits4 ,as the pull of an electromagnet upon its armature depends on the total number of lines of force passing from the core to the armature--that is1 on the total flux--and as the total flux depends for a given magnetizing force on the reluctance of the magnetic circuit1 it is obvious that the design of the electromagnetic circuit is of great importance in influencing the action of the magnet4 ,obviously1 anything that will reduce the amount of air or other non-magnetic material that is in the magnetic circuit will tend to reduce the reluctance1 and1 therefore1 to increase the total magnetization resulting from a given magnetizing force4 ,horseshoe ,form4 ,one of the easiest and most common ways of reducing reluctance in a circuit is to bend the ordinary bar electromagnet into horseshoe form4 ,in order to make clear the direction of current flow1 attention is called to ,fig4 #ia4 ,this is intended to represent a simple bar of iron with a winding of one direction throughout its length4 ,the gap in the middle of the bar1 which divides the winding into two parts1 is intended merely to mark the fact that the winding need not cover the whole length of the bar and still will be able to magnetize the bar when the current passes through it4 ,in ,fig4 #ib a similar bar is shown with similar winding upon it1 but bent into ,u-form1 exactly as if it had been grasped in the hand and bent without further change4 ,the magnetic polarity of the two ends of the bar remain the same as before for the same direction of current1 and it is obvious that the portion of the magnetic circuit which extends through air has been very greatly shortened by the bending4 ,as a result1 the magnetic reluctance of the circuit has been greatly decreased and the strength of the magnet correspondingly increased4 ,illustration3 ,fig4 #ia4 ,bar ,electromagnet ,illustration3 ,fig4 #ib4 ,horseshoe ,electromagnet ,illustration3 ,fig4 #ic4 ,horseshoe ,electromagnet ,if the armature of the electromagnet shown in ,fig4 #ib is long enough to extend entirely across the air gap from the south to the north pole1 then the air gap in the magnetic circuit is still further shortened1 and is now represented only by the small gap between the ends of the armature and the ends of the core4 ,such a magnet1 with an armature closely approaching the poles1 is called a closed-circuit magnet1 since the only gap in the iron of the magnetic circuit is that across which the magnet pulls in attracting its armature4 ,in ,fig4 #ic is shown the electrical and magnetic counterpart of ,fig4 #ib4 ,the fact that the magnetic circuit is not a single iron bar but is made up of two cores and one backpiece rigidly secured together1 has no bearing upon the principle1 but only shows that a modification of construction is possible4 ,in the construction of ,fig4 #ic the armature #a is shown as being pulled directly against the two cores #b and #c1 these two cores being joined by a yoke #d1 which1 like the armature and the core1 is of magnetic material4 ,the path of the lines of force is indicated by dotted lines4 ,this is a very important form of electromagnet and is largely used in telephony4 ,iron-,clad ,form4 ,another way of forming a closed-circuit magnet that is widely used in telephony is to enclose the helix or winding in a shell of magnetic material which joins the core at one end4 ,this construction results in what is known as the tubular or iron-clad electromagnet1 which is shown in section and in end view in ,fig4 #id4 ,in this the core #a is a straight bar of iron and it lies centrally within a cylindrical shell #b1 also of iron4 ,the bar is usually held in place within the shell by a screw1 as shown4 ,the lines of force set up in the core by the current flowing through the coil1 pass to the center of the bottom of the iron shell and thence return through the metal of the shell1 through the air gap between the edges of the shell and the armature1 and then concentrate at the center of the armature and pass back to the end of the core4 ,this is a highly efficient form of closed-circuit magnet1 since the magnetic circuit is of low reluctance4 ,illustration3 ,fig4 #id4 ,iron-,clad ,electromagnet ,such forms of magnets are frequently used where it is necessary to mount a large number of them closely together and where it is desired that the current flowing in one magnet shall produce no inductive effect in the coils of the adjacent magnets4 ,the reason why mutual induction between adjacent magnets is obviated in the case of the iron-clad or tubular magnet is that practically all stray field is eliminated1 since the return path for the magnetic lines is so completely provided for by the presence of the iron shell4 ,special ,horseshoe ,form4 ,in ,fig4 #ie is shown a type of relay commonly employed in telephone circuits4 ,the purpose of illustrating it in this chapter is not to discuss relays1 but rather to show an adaptation of an electromagnet wherein low reluctance of the magnetic circuit is secured by providing a return leg for the magnetic lines developed in the core1 thus forming in effect a horseshoe magnet with a winding on one of its limbs only4 ,to the end of the core #a there is secured an ,l-shaped piece of soft iron #b4 ,this extends upwardly and then forwardly throughout the entire length of the magnet core4 ,an ,l-shaped armature #c rests on the front edge of the piece #b so that a slight rocking motion will be permitted on the knife-edge bearing thus afforded4 ,it is seen from the dotted lines that the magnetic circuit is almost a closed one4 ,the only gap is that between the lower end of the armature #c and the front end of the core4 ,when the coil is energized1 this gap is closed by the attraction of the armature4 ,as a result1 the rearwardly projecting end of the armature #c is raised and this raises the spring #d and causes it to break the normally existing contact with the spring #e and to establish another contact with the spring #f4 ,thus the energy developed within the coil of the magnet is made to move certain parts which in turn operate the switching devices to produce changes in electrical circuits4 ,these relays and other adaptations of the electromagnet will be discussed more fully later on4 ,illustration3 ,fig4 #ie4 ,electromagnet of ,relay ,there are almost numberless forms of electromagnets1 but we have illustrated here examples of the principal types employed in telephony1 and the modifications of these types will be readily understood in view of the general principles laid down4 ,direction of ,armature ,motion4 ,it may be said in general that the armature of an electromagnet always moves or tends to move1 when the coil is energized1 in such a way as to reduce the reluctance of the magnetic circuit through the coil4 ,thus1 in all of the forms of electromagnets discussed1 the armature1 when attracted1 moves in such a direction as to shorten the air gap and to introduce the iron of the armature as much as possible into the path of the magnetic lines1 thus reducing the reluctance4 ,in the case of a solenoid type of electromagnet1 or the coil and plunger type1 which is a better name than solenoid1 the coil1 when energized1 acts in effect to suck the iron core or plunger within itself so as to include more and more of the iron within the most densely occupied portion of the magnetic circuit4 ,illustration3 ,fig4 #if4 ,parallel ,differential ,electromagnet ,differential ,electromagnet4 ,frequently in telephony1 the electromagnets are provided with more than one winding4 ,one purpose of the double-wound electromagnet is to produce the so-called differential action between the two windings1 i4e41 making one of the windings develop magnetization in the opposite direction from that of the other1 so that the two will neutralize each other1 or at least exert different and opposite influences4 ,the principle of the differential electromagnet may be illustrated in connection with ,fig4 #if4 ,here two wires #a and #b are shown wrapped in the same direction about an iron core1 the ends of the wire being joined together at #c4 ,obviously1 if one of these windings only is employed and a current sent through it1 as by connecting the terminals of a battery with the points #d and #c1 for instance1 the core will be magnetized as in an ordinary magnet4 ,likewise1 the core will be energized if a current be sent from #e to #c4 ,assuming that the two windings are of equal resistance and number of turns1 the effects so produced1 when either the coil #a or the coil #b is energized1 will be equal4 ,if the battery be connected between the terminals #d and #e with the positive pole1 say1 at #e1 then the current will proceed through the winding #b and tend to generate magnetism in the core in the direction of the arrow4 ,after traversing the winding #b1 however1 it will then begin to traverse the other winding #a and will pass around the core in the opposite direction throughout the length of that winding4 ,this will tend to set up magnetism in the core in the opposite direction to that indicated by the arrow4 ,since the two currents are equal and also the number of turns in each winding1 it is obvious that the two magnetizing influences will be exactly equal and opposite and no magnetic effect will be produced4 ,such a winding1 as is shown in ,fig4 #if1 where the two wires are laid on side by side1 is called a parallel differential winding4 ,another way of winding magnets differentially is to put one winding on one end of the core and the other winding on the other end of the core and connect these so as to cause the currents through them to flow around the core in opposite directions4 ,such a construction is shown in ,fig4 #ig and is called a tandem differential winding4 ,the tandem arrangement1 while often good enough for practical purposes1 cannot result in the complete neutralization of magnetic effect4 ,this is true because of the leakage of some of the lines of force from intermediate points in the length of the core through the air1 resulting in some of the lines passing through more of the turns of one coil than of the other4 ,complete neutralization can only be attained by first twisting the two wires together with a uniform lay and then winding them simultaneously on the core4 ,illustration3 ,fig4 #ig4 ,tandem ,differential ,electromagnet ,mechanical ,details4 ,we will now consider the actual mechanical construction of the electromagnet4 ,this is a very important feature of telephone work1 because1 not only must the proper electrical and magnetic effects be produced1 but also the whole structure of the magnet must be such that it will not easily get out of order and not be affected by moisture1 heat1 careless handling1 or other adverse conditions4 ,the most usual form of magnet construction employed in telephony is shown in ,fig4 #ih4 ,on the core1 which is of soft ,norway iron1 usually cylindrical in form1 are forced two washers of either fiber or hard rubber4 ,fiber is ordinarily to be preferred because it is tougher and less liable to breakage4 ,around the core1 between the two heads1 are then wrapped several layers of paper or specially prepared cloth in order that the wire forming the winding may be thoroughly insulated from the core4 ,one end of the wire is then passed through a hole in one of the spool heads or washers1 near the core1 and the wire is then wound on in layers4 ,sometimes a thickness of paper is placed around each layer of wire in order to further guard against the breaking down of the insulation between layers4 ,when the last layer is wound on1 the end of the wire is passed out through a hole in the head1 thus leaving both ends projecting4 ,illustration3 ,fig4 #ih ,construction of ,electromagnet ,magnet ,wire4 ,the wire used in winding magnets is1 of course1 an important part of the electromagnet4 ,it is always necessary that the adjacent turns of the wire be insulated from each other so that the current shall be forced to pass around the core through all the length of wire in each turn rather than allowing it to take the shorter and easier path from one turn to the next1 as would be the case if the turns were not insulated4 ,for this purpose the wire is usually covered with a coating of some insulating material4 ,there are1 however1 methods of winding magnet coils with bare wire and taking care of the insulation between the turns in another way1 as will be pointed out4 ,insulated wire for the purpose of winding magnet coils is termed magnet wire4 ,copper is the material almost universally employed for the conductor4 ,its high conductivity1 great ductility1 and low cost are the factors which make it superior to all other metals4 ,however1 in special cases1 where exceedingly high conductivity is required with a limited winding space1 silver wire is sometimes employed1 and on the other hand1 where very high resistance is desired within a limited winding space1 either iron or ,german silver or some other high-resistance alloy is used4 ,wire ,gauges4 ,wire for electrical purposes is drawn to a number of different standard gauges4 ,each of the so-called wire gauges consists of a series of graded sizes of wire1 ranging from approximately one-half an inch in diameter down to about the fineness of a lady's hair4 ,in certain branches of telephone work1 such as line construction1 the existence of the several wire gauges or standards is very likely to lead to confusion4 ,fortunately1 however1 so far as magnet wire is concerned1 the so-called ,brown and ,sharpe1 or ,american1 wire gauge is almost universally employed in this country4 ,the abbreviations for this gauge are ,b4,s4 or ,a4,w4,g4 ,,table ,,iii ,copper ,wire ,table ,giving weights1 lengths1 and resistances of wire #fh ,f41 of ,matthiessen's ,standard ,conductivity4 ---------------------------------------- ---------------------------------------- ------------- ,,resistance ,,length ,,weight ,a4,w4,g4 ,,diameter ,,area ---------------------------------------- -------------------------- ,b4,s4 ,,mils ,,circular ,,ohms ,,per ,,ohms ,,per ,,feet ,,per ,,feet ,,per ,,pounds ,,per ,,pounds ,,per ,,mils ,,pound ,,foot ,,pound ,,ohm ,,foot ,,ohm ---------------------------------------- ---------------------------------------- ---------- #jjjj #dfj4 #baa1fjj4 #j4jjjjgfci #j4jjjjdhi #a4efa #bj1ddj4 #j4fdje #ac1jij4 #jjj #dji4f #afg1hjj4 #j4jjjabae #j4jjjjfag #a4ifi #af1baj4 #j4ejhj #h1bcb4 #jj #cfd4h #acc1ajj4 #j4jjjaica #j4jjjjggh #b4dhb #ab1hej4 #j4djbh #e1agg4 #j #cbd4i #aje1ejj4 #j4jjjcjga #j4jjjjiha #c4acj #aj1aij4 #j4caie #c1bef4 ---------------------------------------- ---------------------------------------- ---------- #a #bhi4c #hc1fij4 #j4jjjdhhc #j4jjjabcg #c4idg #h1jhc4 #j4becc #b1jdh4 #b #beg4f #ff1cgj4 #j4jjjggfe #j4jjjaefj #d4igg #f1daj4 #j4bjji #a1bhh4 #c #bbi4d #eb1fcj4 #j4jjabce #j4jjjaifg #f4bgf #e1jhd4 #j4aeic #haj4j #d #bjd4c #da1gdj4 #j4jjaifc #j4jjjbdhj #g4iad #d1jca4 #j4abfd #eji4d #e #aha4i #cc1ajj4 #j4jjcabb #j4jjjcabh #i4ihj #c1aig4 #j4ajjb #cbj4d #f #afb4j #bf1bej4 #j4jjdifc #j4jjjcidd #ab4eh #b1ece4 #j4jgidf #bja4e #g #add4c #bj1hbj4 #j4jjghib #j4jjjdigc #ae4hg #b1jaa4 #j4jfcjb #abf4g #h #abh4e #af1eaj4 #j4jabee #j4jjjfbga #bj4ja #a1eie4 #j4jdiih #gi4fi #i #aad4d #ac1jij4 #j4jaiie #j4jjjgijh #be4bc #a1bfe4 #j4jcifc #ej4ab #aj #aja4i #aj1chj4 #j4jcagc #j4jjjibgc #ca4hb #a1jjc4 #j4jcadc #ca4eb ---------------------------------------- ---------------------------------------- ---------- #aa #ij4gd #h1bcd4 #j4jejde #j4jjabeg #dj4ab #gie4c #j4jbdic #ai4hb #ab #hj4ha #f1ecj4 #j4jhjbb #j4jjaehf #ej4ei #fcj4g #j4jaigg #ab4dg #ac #ga4if #e1agh4 #j4abgf #j4jjaiii #fc4gi #ejj4a #j4jaefh #g4hdj #ad #fd4jh #d1ajg4 #j4bjbh #j4jjbeba #hj4dd #cif4f #j4jabdc #d4ica #ae #eg4jg #c1beg4 #j4cbbe #j4jjcagi #aja4d #cad4e #j4jjiheh #c4aja #af #ej4hb #b1ehc4 #j4eabh #j4jjdjji #abg4i #bdi4d #j4jjghah #a4iej #ag #de4bf #b1jdh4 #j4haec #j4jjejee #afa4c #aig4h #j4jjfbjj #a4bbf #ah #dj4cj #a1fbd4 #a4bif #j4jjfcgd #bjc4d #aef4i #j4jjdiag #j4ggac #ai #ce4hi #a1bhh4 #b4jfa #j4jjhjch #bef4e #abd4d #j4jjchii #j4dhea #bj #ca4if #a1jbb4 #c4bgh #j4jajad #cbc4d #ih4ff #j4jjcjib #j4cjea ---------------------------------------- ---------------------------------------- ---------- #ba #bh4df #haj4a #e4bab #j4jabgh #djg4h #gh4bd #j4jjbdeb #j4aiai #bb #be4ce #fdb4d #h4bhg #j4jafab #ead4b #fb4je #j4jjaide #j4abjg #bc #bb4eg #eji4e #ac4ah #j4jbjcb #fdh4d #di4ba #j4jjaedb #j4jgehi #bd #bj4aj #djd4j #bj4ie #j4jbefc #hag4f #ci4jb #j4jjabbc #j4jdggc #be #ag4ij #cbj4d #cc4cb #j4jcbca #a1jca4 #cj4ie #j4jjjifii #j4jcjjb #bf #ae4id #bed4a #eb4ig #j4jdjge #a1cjj4 #bd4ed #j4jjjgfib #j4aahg #bg #ad4b #bja4e #hd4bc #j4jeach #a1fci4 #ai4df #j4jjjfajj #j4jahhh #bh #ab4fd #aei4h #acc4i #j4jfdgi #b1jfg4 #ae4dc #j4jjjdhcg #j4jjgdff #bi #aa4bf #abf4g #bac4j #j4jhagj #b1fjg4 #ab4bd #j4jjjchcf #j4jjdfif #cj #aj4jc #ajj4e #cch4f #j4ajcj #c1bhg4 #i4gjg #j4jjjcjdb #j4jjbiec ---------------------------------------- ---------------------------------------- ---------- #ca #h4ibh #gi4gj #ech4d #j4abii #d1ade4 #g4fih #j4jjjbdac #j4jjaheg #cb #g4iej #fc4ba #hef4b #j4afch #e1bbg4 #f4aje #j4jjjaiac #j4jjaafh #cc #g4jhj #ej4ac #a1cfa4 #j4bjff #f1eia4 #d4hda #j4jjjaeag #j4jjjgcdf #cd #f4cje #ci4ge #b1afe4 #j4bfje #h1caa4 #c4hci #j4jjjabjc #j4jjjdfbj #ce #e4fae #ca4eb #c1dda4 #j4cbhd #aj1dhj4 #c4jde #j4jjjjiedc #j4jjjbije #cf #e4j #be4j #e1dgc4 #j4dadb #ac1baj4 #b4dad #j4jjjjgefh #j4jjjahbg #cg #d4dec #ai4hc #h1gjb4 #j4ebbb #af1ffj4 #a4iae #j4jjjjfjja #j4jjjaadi #ch #c4ife #ae4gb #ac1hgj4 #j4fehe #ba1jaj4 #a4eai #j4jjjjdgei #j4jjjjgbaj #ci #c4eca #ab4dg #bb1jjj4 #j4hcjd #bf1ejj4 #a4bjd #j4jjjjcggd #j4jjjjdede #dj #c4ade #i4hhh #cd1ihj4 #a4jdg #cc1daj4 #j4ieej #j4jjjjbiic #j4jjjjbheh ---------------------------------------- ---------------------------------------- ---------- ,illustration3 ,,south ,,office ,,of ,,home ,,telephone ,,company1 ,,san ,,francisco ,in the ,brown and ,sharpe gauge the sizes1 beginning with the largest1 are numbered #jjjj1 #jjj1 #jj1 #j1 #a1 #b1 and so on up to #dj4 ,sizes larger than about ,no4 #af ,b4,s4 gauge are seldom used as magnet wire in telephony1 but for the purpose of making the list complete1 ,table ,,iii is given1 including all of the sizes of the ,b4,s4 gauge4 ,in ,table ,,iii there is given for each gauge number the diameter of the wire in mils 7thousandths of an inch72 the cross-sectional area in circular mils 7a unit area equal to that of a circle having a diameter of one one-thousandth of an inch72 the resistance of the wire in various units of length and weight2 the length of the wire in terms of resistance and of weight2 and the weight of the wire in terms of its length and resistance4 ,it is to be understood that in ,table ,,iii the wire referred to is bare wire and is of pure copper4 ,it is not commercially practicable to use absolutely pure copper1 and the ordinary magnet wire has a conductivity equal to about #ih per cent of that of pure copper4 ,the figures given in this table are sufficiently accurate for all ordinary practical purposes4 ,silk and ,cotton ,insulation4 ,the insulating material usually employed for covering magnet wire is of silk or cotton4 ,of these1 silk is by far the better material for all ordinary purposes1 since it has a much higher insulating property than cotton1 and is very much thinner4 ,cotton1 however1 is largely employed1 particularly in the larger sizes of magnet wire4 ,both of these materials possess the disadvantage of being hygroscopic1 that is1 of readily absorbing moisture4 ,this disadvantage is overcome in many cases by saturating the coil after it is wound in some melted insulating compound1 such as wax or varnish or asphaltum1 which will solidify on cooling4 ,where the coils are to be so saturated the best practice is to place them in a vacuum chamber and exhaust the air1 after which the hot insulating compound is admitted and is thus drawn into the innermost recesses of the winding space4 ,silk-insulated wire1 as regularly produced1 has either one or two layers of silk4 ,this is referred to commercially as single silk wire or as double silk wire4 ,the single silk has a single layer of silk fibers wrapped about it1 while the double silk has a double layer1 the two layers being put on in reverse direction4 ,the same holds true of cotton insulated wire4 ,frequently1 also1 there is a combination of the two1 consisting of a single or a double wrapping of silk next to the wire with an outer wrapping of cotton4 ,where this is done the cotton serves principally as a mechanical protection for the silk1 the principal insulating properties residing in the silk4 ,enamel4 ,a later development in the insulation of magnet wire has resulted in the so-called enamel wire4 ,in this1 instead of coating the wire with some fibrous material such as silk or cotton1 the wire is heated and run through a bath of fluid insulating material or liquid enamel1 which adheres to the wire in a very thin coating4 ,the wire is then run through baking ovens1 so that the enamel is baked on4 ,this process is repeated several times so that a number of these thin layers of the enamel are laid on and baked in succession4 ,the characteristics sought in good enamel insulation for magnet wire may be thus briefly set forth3 ,it is desirable for the insulation to possess the highest insulating qualities2 to have a glossy1 flawless surface2 to be hard without being brittle2 to adhere tenaciously and stand all reasonable handling without cracking or flaking2 to have a coefficient of elasticity greater than the wire itself2 to withstand high temperatures2 to be moisture-proof and inert to corrosive agencies2 and not to dry out or become brittle over a long period of time4 ,space ,utilization4 ,the utilization of the winding space in an electromagnet is an important factor in design1 since obviously the copper or other conductor is the only part of the winding that is effective in setting up magnetizing force4 ,the space occupied by the insulation is1 in this sense1 waste space4 ,an ideally perfect winding may be conceived as one in which the space is all occupied by wire2 and this would necessarily involve the conception of wire of square cross-section and insulation of infinite thinness4 ,in such a winding there would be no waste of space and a maximum amount of metal employed as a conductor4 ,of course1 such a condition is not possible to attain and in practice some insulating material must be introduced between the layers of wire and between the adjacent convolutions of wire4 ,the ratio of the space occupied by the conductor to the total space occupied by the winding1 that is1 by the conductor and the insulation1 is called the coefficient of space utilization of the coil4 ,for the ideal coil just conceived the coefficient of space utilization would be #a4 ,ordinarily the coefficient of space utilization is greater for coarse wire than for fine wire1 since obviously the ratio of the diameter of the wire to the thickness of the insulation increases as the size of the wire grows larger4 ,the chief advantage of enamel insulation for magnet wire is its thinness1 and the high coefficient of space utilization which may be secured by its use4 ,in good enamel wire the insulation will average about one-quarter the thickness of the standard single silk insulation1 and the dielectric strength is equal or greater4 ,where economy of winding space is desirable the advantages of this may readily be seen4 ,for instance1 in a given coil wound with ,no4 #cf single silk wire about one-half of the winding space is taken up with the insulation1 whereas when the same coil is wound with ,no4 #cf enameled wire only about one-fifth of the winding space is taken up by the insulation4 ,thus the coefficient of space utilization is increased from 4#ej to 4#hj4 ,the practical result of this is that1 in the case of any given winding space where ,no4 #cf wire is used1 about #fj per cent more turns can be put on with enameled wire than with single silk insulation1 and of course this ratio greatly increases when the comparison is made with double silk insulation or with cotton insulation4 ,again1 where it is desired to reduce the winding space and keep the same number of turns1 an equal number of turns may be had with a corresponding reduction of winding space where enameled wire is used in place of silk or cotton4 ,in the matter of heat-resisting properties the enameled wire possesses a great advantage over silk and cotton4 ,cotton or silk insulation will char at about #bfj ,fahrenheit1 while good enameled wire will stand #djj to #ejj ,fahrenheit without deterioration of the insulation4 ,it is in the matter of liability to injury in rough or careless handling1 or in winding coils having irregular shapes1 that enamel wire is decidedly inferior to silk or cotton-covered wire4 ,it is likely to be damaged if it is allowed to strike against the sharp corners of the magnet spool during winding1 or run over the edge of a hard surface while it is being fed on to the spool4 ,coils having other than round cores1 or having sharp corners on their spool heads1 should not ordinarily be wound with enamel wire4 ,the dielectric strength of enamel insulation is much greater than that of either silk or cotton insulation of equal thickness4 ,this is a distinct advantage and frequently a combination of the two kinds of insulation results in a superior wire4 ,if wire insulated with enamel is given a single wrapping of silk or of cotton1 the insulating and dielectric properties of the enamel is secured1 while the presence of the silk and cotton affords not only an additional safeguard against bare spots in the enamel but also a certain degree of mechanical protection to the enamel4 ,winding ,methods4 ,in winding a coil1 the spool1 after being properly prepared1 is placed upon a spindle which may be made to revolve rapidly4 ,ordinarily the wire is guided on by hand2 sometimes1 however1 machinery is used1 the wire being run over a tool which moves to and fro along the length of the spool1 just fast enough to lay the wire on at the proper rate4 ,the movement of this tool is much the same as that of the tool in a screw cutting lathe4 ,unless high voltages are to be encountered1 it is ordinarily not necessary to separate the layers of wire with paper1 in the case of silk-or cotton-insulated magnet wire2 although where especially high insulation resistance is needed this is often done4 ,it is necessary to separate the successive layers of a magnet that is wound with enamel wire1 by sheets of paper or thin oiled cloth4 ,illustration3 ,fig4 #ii4 ,electromagnet with ,bare ,wire ,in ,fig4 #ii is shown a method1 that has been used with some success1 of winding magnets with bare wire4 ,in this the various adjacent turns are separated from each other by a fine thread of silk or cotton wound on beside the wire4 ,each layer of wire and thread as it is placed on the core is completely insulated from the subsequent layer by a layer of paper4 ,this is essentially a machine-wound coil1 and machines for winding it have been so perfected that several coils are wound simultaneously1 the paper being fed in automatically at the end of each layer4 ,another method of winding the bare wire omits the silk thread and depends on the permanent positioning of the wire as it is placed on the coil1 due to the slight sinking into the layer of paper on which it is wound4 ,in this case the feed of the wire at each turn of the spool is slightly greater than the diameter of the wire1 so that a small distance will be left between each pair of adjacent turns4 ,upon the completion of the winding of a coil1 regardless of what method is used1 it is customary to place a layer of bookbinders' cloth over the coil so as to afford a certain mechanical protection for the insulated wire4 ,winding ,terminals4 ,the matter of bringing out the terminal ends of the winding is one that has received a great deal of attention in the construction of electromagnets and coils for various purposes4 ,where the winding is of fine wire1 it is always well to reinforce its ends by a short piece of larger wire4 ,where this is done the larger wire is given several turns around the body of the coil1 so that the finer wire with which it connects may be relieved of all strain which may be exerted upon it from the protruding ends of the wire4 ,great care is necessary in the bringing out of the inner terminal--i4e41 the terminal which connects with the inner layer--that the terminal wire shall not come in contact with any of the subsequent layers that are wound on4 ,illustration ,fig4 #ajj4 ,electromagnet with ,terminals ,where economy of space is necessary1 a convenient method of terminating the winding of the coil consists in fastening rigid terminals to the spool head4 ,this1 in the case of a fiber spool head1 may be done by driving heavy metal terminals into the fiber4 ,the connections of the two wires leading from the winding are then made with these heavy rigid terminals by means of solder4 ,a coil having such terminals is shown in its finished condition in ,fig4 #ajj4 ,winding ,data4 ,the two things principally affecting the manufacture of electromagnets for telephone purposes are the number of turns in a winding and the resistance of the wound wire4 ,the latter governs the amount of current which may flow through the coil with a given difference of potential at its end1 while the former control the amount of magnetism produced in the core by the current flowing4 ,while a coil is being wound1 it is a simple matter to count the turns by any simple form of revolution counter4 ,when the coil has been completed it is a simple matter to measure its resistance4 ,but it is not so simple to determine in advance how many turns of a given size wire may be placed on a given spool1 and still less simple to know what the resistance of the wire on that spool will be when the desired turns shall have been wound4 ,,table ,,iv ,winding ,data for ,insulated ,wires--,silk and ,cotton ,covering ,a4,w4,g4 ,b ,s #bj #ba #bb #bc #bd #be ---------------------------------------- ----------------------------- ,,diameter ,mils #ca4ifa #bh4dfb #be4cdg #bb4ega #bj4ajj #ag4ijj ---------------------------------------- ----------------------------- ,,area ,circular ,mils #ajba4bj #haj4aj #fdb4gj #eji4de #djd4ja #cbj4dj ---------------------------------------- ----------------------------- ,,diameter ,,over ,,insulation ,,single ,,cotton #cg4hfa #cd4cfb #ca4bdg #bh4dga #bf4jjj #bc4hjj ,,double ,,cotton #db4afa #ch4ffb #ce4edg #cb4gga #cj4cjj #bh4ajj ,,single ,,silk #cd4bfa #cj4gfb #bg4fdg #bd1hga #bb4dja #bj4bjj ,,double ,,silk #cf4afa #cb4ffb #bi4edg #bf4gga #bd4cjj #bb4ajj ---------------------------------------- ----------------------------- ,,turns ,,per ,,linear ,,inch ,,single ,,cotton #be4g #bh4c #ca4j #cd4d #cf4i #ch4j ,,double #bb4e #bd4e #bf4g #bh4ig #ca4ce #cc4ib ,,cotton ,,single ,,silk #bg4gj #cj4ig #cd4ci #ch4ai #db4cg #dg4jb ,,double ,,silk #bf4bb #bi4jg #cb4aa #ce4ec #ci4ad #db4id ---------------------------------------- ----------------------------- ,,turns ,,per ,,square ,,inch ,,single ,,cotton #ffj4e #hjj4i #ifa4j #aahc4j #acba4f #addd4j ,,double ,,cotton #ejf4c #fjj4b #gab4i #hci4b #ihb4h #aaej4h ,,single ,,silk #gfg4c #iei4a #aahb4g #adeh4e #agie4b #bbaj4i ,,double ,,silk #fhg4e #hde4j #ajca4j #abfb4d #aecb4j #ahdc4h ---------------------------------------- ----------------------------- ,,ohms ,,per ,,cubic ,,inch ,,single ,,cotton 4#fdf 4#iha #a4ejb #b4cei #c4ebh #e4hca ,,double ,,cotton 4#ecc 4#gie #a4ahh #a4ggb #b4eie #c4hjb ,,single ,,silk 4#hja #a4bfa #a4ief #c4jdi #d4gci #g4dhi ---------------------------------------- ----------------------------- ,a4,w4,g4 ,b ,s #bf #bg #bh #bi #cj #ca ---------------------------------------- ----------------------------- ,,diameter ,mils #ae4idj #ad4aie #ab4fda #aa4beg #aj4jbe #h4ibh ---------------------------------------- ----------------------------- ,,area ,circular ,mils #bed4ja #bja4ej #aei4gi #abf4gb #ajj4ej #gi4ga ---------------------------------------- ----------------------------- ,,diameter ,,over ,,insulation ,,single ,,cotton #ba4hdj #bj4jie #ah4eda #ag4aeg #ae4ibe #ad4hbh ,,double ,,cotton #bf4adj #bd4cie #bb4hda #ba4deg #bj4bbe #ai4abh ,,single ,,silk #ah4bdj #af4die #ad4ida #ac4eeg #ab4cbe #aa4bbh ,,double ,,silk #bj4adj #ah4cie #af4hda #ae4deg #ad4bbe #ac4abh ---------------------------------------- ----------------------------- ,,turns ,,per ,,linear ,,inch ,,single ,,cotton #db4j #dh4j #ec4j #ef4e #ei4ff #fd4abe ,,double ,,cotton #cf4bi #ch4ie #da4fa #dd4bg #df4ic #di4gh ,,single ,,silk #eb4jf #eg4fg #fc4cf #gj4aa #gg4ad #hd4fd ,,double ,,silk #df4ha #ea4ei #ef4dc #fa4ef #ff4gi #gb4ci ---------------------------------------- ----------------------------- ,,turns ,,per ,,square ,,inch ,,single ,,cotton #agfd4j #bcjd4j #bhji4i #caib4c #ccei4b #daab4b ,,double ,,cotton #acag4j #aeag4b #agca4j #aiei4i #bbjb4e #bdgh4j ,,single ,,silk #bgaj4c #ccbf4j #djad4e #diae4e #eiej4b #gafd4j ,,double ,,silk #baia4b #bffa4f #cahd4e #cghi4h #ddfa4j #ebdj4j ---------------------------------------- ----------------------------- ,,ohms ,,per ,,cubic ,,inch ,,single ,,cotton #f4ida #aj4had #ag4fag #be4ejj #cd4hjj #dh4e ,,double ,,cotton #e4eeb #h4jgh #aa4ed #af4dg #bc4dc #cb4hc ,,single ,,silk #i4jca #ac4ib #bf4hf #da4bi #fb4ih #ie4gj ---------------------------------------- ----------------------------- ,a4,w4,g4 ,b ,s #cb #cc #cd #ce #cf #cg ---------------------------------------- ------------------------------ ,,diameter ,mils #g4iej #g4jhj #f4cjd #e4fad #e4jjj #d4dec ---------------------------------------- ------------------------------ ,,area ,circular ,mils #fc4bj #ej4ac #ci4gd #ca4eb #be4jj #ai4hc ---------------------------------------- ------------------------------ ,,diameter ,,over ,,insulation ,,single ,,cotton #ac4hej #ab4ihj #ab4bjd #aa4ead #aj4ijj #aj4cec ,,double ,,cotton #ah4aej #ag4bhj #af4ejd #ae4had #ae4bjj #ad4fec ,,single ,,silk #aj4bej #i4chj #h4ejd #g4iad #g4cjj #f4gec ,,double ,,silk #ab4aej #aa4bhj #aj4ejd #i4had #i4bjj #h4fec ---------------------------------------- ------------------------------ ,,turns ,,per ,,linear ,,inch ,,single ,,cotton #fh4fjj #gc4jej #gg4ijj #hb4fjj #hg4ajj #ia4hgj ,,double ,,cotton #eb4cd #ee4aj #eg4eg #fj4jd #fb4ea #fd4gj ,,single ,,silk #ib4gb #aja4fe #aab4aa #aai4g #acj4ae #adj4f ,,double ,,silk #gh4ai #hd4ag #ij4dd #if4ij #ajc4ee #aaj4bj ---------------------------------------- ------------------------------ ,,turns ,,per ,,square ,,inch ,,single #dfib4e #eccc4e #fjfh4e #fggc4c #gehf4e #hddj4j ,,cotton ,,double ,,cotton #bgci4e #cjcf4a #ccad4b #cfje4j #cijg4e #dahf4a ,,single ,,silk #heig4e #ajccb4j #abegj4j #adcbg4j #afidj4j #aiggj4j ,,double ,,silk #faad4j #gjhe4j #hagi4e #ichi4e #ajggb4j #abade4j ---------------------------------------- ----------------------------- ,,ohms ,,per ,,cubic ,,inch ,,single ,,cotton #gc4h #ajd4e #aea4d #bjb4j #bih4h #dah4j ,,double ,,cotton #df4ai #fd4cj #gj4eh #abe4i #aff4c #bbe4f ,,single ,,silk #add4gj #bag4h #cdb4a #dhi4j #gba4a #ajfb4j ---------------------------------------- ----------------------------- ,a4,w4,g4 ,b ,s #ch #ci #dj ---------------------------------------- ---- ,,diameter ,mils #c4ife #c4eca #c4add ---------------------------------------- ---- ,,area ,circular ,mils #ae4gb #ab4dg #i4hi ---------------------------------------- ---- ,,diameter ,,over ,,insulation ,,single ,,cotton #i4hfe #i4dca #i4jdd ,,double ,,cotton #ad4afe #ac4gca #ac4cdd ,,single ,,silk #f4bfe #e4hca #e4cdd ,,double ,,silk #h4afe #g4gca #g4cdd ---------------------------------------- ---- ,,turns ,,per ,,linear ,,inch ,,single ,,cotton #ie4jjj #ajj4gjj #ajf4jjj ,,double ,,cotton #ff4hj #fh4hj #ga4bj ,,single ,,silk #aea4je #afc4jd #agg4fe ,,double ,,silk #aaf4he #abb4ee #abi4bj ---------------------------------------- ---- ,,turns ,,per ,,square ,,inch ,,single ,,cotton #ijbe4j #ajadj4e #aabcf4j ,,double #ddfb4b #dgcc4f #ejfi4h ,,cotton ,,single ,,silk #bbhbj4j #bfgjj4j #caeei4j ,,double ,,silk #acfee4j #aejah4j #affib4j ---------------------------------------- ---- ,,ohms ,,per ,,cubic ,,inch ,,single ,,cotton #efg4j #haa4j #aaac4j ,,double #cje4e #dji4h #ede4e ,,cotton ,,single ,,silk #aeeg4j #bbff4j #cdjj4j ---------------------------------------- --- ,if the length and the depth of the winding space of the coil as well as the diameter of the core are known1 it is not difficult to determine how much bare copper wire of a given size may be wound on it1 but it is more difficult to know these facts concerning copper wire which has been covered with cotton or silk4 ,yet something may be done1 and tables have been prepared for standard wire sizes with definite thicknesses of silk and cotton insulation4 ,as a result of facts collected from a large number of actually wound coils1 the number of turns per linear inch and per square inch of ,b4,s4 gauge wires from ,no4 #bj to ,no4 #dj have been tabulated1 and these1 supplemented by a tabulation of the number of ohms per cubic inch of winding space for wires of three different kinds of insulation1 are given in ,table ,,iv4 ,bearing in mind that the calculations of ,table ,,iv are all based upon the diameter over insulation1 which it states at the outset for each of four different kinds of covering1 it is evident what is meant by turns per linear inch4 ,the columns referring to turns per square inch mean the number of turns1 the ends of which would be exposed in one square inch if the wound coil were cut in a plane passing through the axis of the core4 ,knowing the distance between the head1 and the depth to which the coil is to be wound1 it is easy to select a size of wire which will give the required number of turns in the provided space4 ,it is to be noted that the depth of winding space is one-half of the difference between the core diameter and the complete diameter of the wound coil4 ,the resistance of the entire volume of wound wire may be determined in advance by knowing the total cubic contents of the winding space and multiplying this by the ohms per cubic inch of the selected wire2 that is1 one must multiply in inches the distance between the heads of the spool by the difference between the squares of the diameters of the core and the winding space1 and this in turn by 4#ghed4 ,this result1 times the ohms per cubic inch1 as given in the table1 gives the resistance of the winding4 ,there is a considerable variation in the method of applying silk insulation to the finer wires1 and it is in the finer sizes that the errors1 if any1 pile up most rapidly4 ,yet the table throughout is based on data taken from many samples of actual coil winding by the present process of winding small coils4 ,it should be said further that the table does not take into account the placing of any layers of paper between the successive layers of the wires4 ,this table has been compared with many examples and has been used in calculating windings in advance1 and is found to be as close an approximation as is afforded by any of the formulas on the subject1 and with the further advantage that it is not so cumbersome to apply4 ,winding ,calculations4 ,in experimental work1 involving the winding of coils1 it is frequently necessary to try one winding to determine its effect in a given circuit arrangement1 and from the knowledge so gained to substitute another just fitted to the conditions4 ,it is in such a substitution that the table is of most value4 ,assume a case in which are required a spool and core of a given size with a winding of1 say ,no4 #be single silk-covered wire1 of a resistance of #ej ohms4 ,assume also that the circuit regulations required that this spool should be rewound so as to have a resistance of1 say #a1jjj ohms4 ,what size single silk-covered wire shall be used8 ,manifestly1 the winding space remains the same1 or nearly so4 ,the resistance is to be increased from #ej to #a1jjj ohms1 or twenty times its first value4 ,therefore1 the wire to be used must show in the table twenty times as many ohms per cubic inch as are shown in ,no4 #be1 the known first size4 ,this amount would be twenty times #g4dhi1 which is #adi4h1 but there is no size giving this exact resistance4 ,no4 #cb1 however1 is very nearly of that resistance and if wound to exactly the same depth would give about #igj ohms4 ,a few turns more would provide the additional thirty ohms4 ,similarly1 in a coil known to possess a certain number of turns1 the table will give the size to be selected for rewinding to a greater or smaller number of turns4 ,in this case1 as in the case of substituting a winding of different resistance1 it is unnecessary to measure and calculate upon the dimensions of the spool and core4 ,assume a spool wound with ,no4 #cj double silk-covered wire1 which requires to be wound with a size to double the number of turns4 ,the exact size to do this would have #hibb4 turns per square inch and would be between ,no4 #cd and ,no4 #ce4 ,a choice of these two wires may be made1 using an increased winding depth with the smaller wire and a shallower winding depth for the larger wire4 ,impedance ,coils4 ,in telephony electromagnets frequently serve1 as already stated1 to perform other functions than the producing of motion by attracting or releasing their armatures4 ,they are required to act as impedance coils to present a barrier to the passage of alternating or other rapidly fluctuating currents1 and at the same time to allow the comparatively free passage of steady currents4 ,where it is desired that an electromagnet coil shall possess high impedance1 it is usual to employ a laminated instead of a solid core4 ,this is done by building up a core of suitable size by laying together thin sheets of soft iron1 or by forming a bundle of soft iron wires4 ,the use of laminated cores is for the purpose of preventing eddy currents1 which1 if allowed to flow1 would not only be wasteful of energy but would also tend to defeat the desired high impedance4 ,sometimes in iron-clad impedance coils1 the iron shell is slotted longitudinally to break up the flow of eddy currents in the shell4 ,frequently electromagnetic coils have only the function of offering impedance1 where no requirements exist for converting any part of the electric energy into mechanical work4 ,where this is the case1 such coils are termed impedance1 or retardation1 or choke coils1 since they are employed to impede or to retard or to choke back the flow of rapidly varying current4 ,the distinction1 therefore1 between an impedance coil and the coil of an ordinary electromagnet is one of function1 since structurally they may be the same1 and the same principles of design and construction apply largely to each4 ,number of ,turns4 ,it should be remembered that an impedance coil obstructs the passage of fluctuating current1 not so much by ohmic resistance as by offering an opposing or counter-electromotive force4 ,other things being equal1 the counter-electromotive force of self-induction increases directly as the number of turns on a coil and directly as the number of lines of force threading the coil1 and this latter factor depends also on the reluctance of the magnetic circuit4 ,therefore1 to secure high impedance we need many turns or low reluctance1 or both4 ,often1 owing to requirements for direct-current carrying capacity and limitations of space1 a very large number of turns is not permissible1 in which case sufficiently high impedance to such rapid fluctuations as those of voice currents may be had by employing a magnetic circuit of very low reluctance1 usually a completely closed circuit4 ,kind of ,iron4 ,an important factor in the design of impedance coils is the grade of iron used in the magnetic circuit4 ,obviously1 it should be of the highest permeability and1 furthermore1 there should be ample cross-section of core to prevent even an approach to saturation4 ,the iron should1 if possible1 be worked at that density of magnetization at which it has the highest permeability in order to obtain the maximum impedance effects4 ,types4 ,open-,circuit3--,where very feeble currents are being dealt with1 and particularly where there is no flow of direct current1 an open magnetic circuit is much used4 ,an impedance coil having an open magnetic circuit is shown in section in ,fig4 #aja1 ,fig4 #ajb showing its external appearance and illustrating particularly the method of bringing out the terminals of the winding4 ,illustration3 ,fig4 #aja4 ,section of ,open-,circuit ,impedance ,coil ,illustration3 ,fig4 #ajb4 ,open-,circuit ,impedance ,coil ,illustration3 ,fig4 #ajc4 ,closed-,circuit ,impedance ,coil ,closed-,circuit3--,a type of retardation coil which is largely used in systems of simultaneous telegraphy and telephony1 known as composite systems1 is shown in ,fig4 #ajc4 ,in the construction of this coil the core is made of a bundle of fine iron wires first bent into ,u-shape1 and then after the coils are in place1 the free ends of the core are brought together to form a closed magnetic circuit4 ,the coils have a large number of turns of rather coarse wire4 ,the conditions surrounding the use of this coil are those which require very high impedance and rather large current-carrying capacity1 and fortunately the added requirement1 that it shall be placed in a very small space1 does not exist4 ,toroidal3--,another type of retardation coil1 called the toroidal type due to the fact that its core is a torus formed by winding a continuous length of fine iron wire1 is shown in diagram in ,fig4 #ajd4 ,the two windings of this coil may be connected in series to form in effect a single winding1 or it may be used as a split-winding coil1 the two windings being in series but having some other element1 such as a battery1 connected between them in the circuit4 ,evidently such a coil1 however connected1 is well adapted for high impedance1 on account of the low reluctance of its core4 ,illustration3 ,fig4 #ajd4 ,symbol of ,toroidal ,impedance ,coil ,this coil is usually mounted on a base-board1 the coil being enclosed in a protecting iron case1 as shown in ,fig4 #aje4 ,the terminal wires of both windings of each coil are brought out to terminal punchings on one end of the base-board to facilitate the making of the necessary circuit connections4 ,illustration3 ,fig4 #aje4 ,toroidal ,impedance ,coil ,the usual diagrammatic symbol for an impedance coil is shown in ,fig4 #ajf4 ,this is the same as for an ordinary bar magnet1 except that the parallel lines through the core may be taken as indicating that the core is laminated1 thus conveying the idea of high impedance4 ,the symbol of ,fig4 #ajd is a good one for the toroidal type of impedance coil4 ,illustration3 ,fig4 #ajf4 ,symbol of ,impedance ,coil ,induction ,coil4 ,an induction coil consists of two or more windings of wire interlinked by a common magnetic circuit4 ,in an induction coil having two windings1 any change in the strength of the current flowing in one of the windings1 called the primary1 will cause corresponding changes in the magnetic flux threading the magnetic circuit1 and1 therefore1 changes in flux through the other winding1 called the secondary4 ,this1 by the laws of electromagnetic induction1 will produce corresponding electromotive forces in the secondary winding and1 therefore1 corresponding currents in that winding if its circuit be closed4 ,current and ,voltage ,ratios4 ,in a well-designed induction coil the energy in the secondary1 i4e41 the induced current1 is for all practical purposes equal to that of the primary current1 yet the values of the voltage and the amperage of the induced current may vary widely from the values of the voltage and the amperage of the primary current4 ,with simple periodic currents1 such as the commercial alternating lighting currents1 the ratio between the voltage in the primary and that in the secondary will be equal to the ratio of the number of turns in the primary to the number of turns in the secondary4 ,since the energy in the two circuits will be practically the same1 it follows that the ratio between the current in the primary and that in the secondary will be equal to the ratio of the number of turns in the secondary to the number of turns in the primary4 ,in telephony1 where the currents are not simple periodic currents1 and where the variations in current strength take place at different rates1 such a law as that just stated does not hold for all cases2 but it may be stated in general that the induced currents will be of higher voltage and smaller current strength than those of the primary in all coils where the secondary winding has a greater number of turns than the primary1 and vice vers4 ,functions4 ,the function of the induction coil in telephony is1 therefore1 mainly one of transformation1 that is1 either of stepping up the voltage of a current1 or in other cases stepping it down4 ,the induction coil1 however1 does serve another purpose in cases where no change in voltage and current strength is desired1 that is1 it serves as a means for electrically separating two circuits so far as any conductive relation exists1 and yet of allowing the free transmission by induction from one of these circuits to the other4 ,this is a function that in telephony is scarcely of less importance than the purely transforming function4 ,design4 ,induction coils1 as employed in telephony1 may be divided into two general types3 first1 those having an open magnetic circuit2 and1 second1 those having a closed magnetic circuit4 ,in the design of either type it is important that the core should be thoroughly laminated1 and this is done usually by forming it of a bundle of soft ,swedish or ,norway iron wire about 4#jb of an inch in diameter4 ,the diameter and the length of the coil1 and the relation between the number of turns in the primary and in the secondary1 and the mechanical construction of the coil1 are all matters which are subject to very wide variation in practice4 ,while the proper relationship of these factors is of great importance1 yet they may not be readily determined except by actual experiment with various coils1 owing to the extreme complexity of the action which takes place in them and to the difficulty of obtaining fundamental data as to the existing facts4 ,it may be stated1 therefore1 that the design of induction coils is nearly always carried out by cut-and-try methods1 bringing to bear1 of course1 such scientific and practical knowledge as the experimenter may possess4 ,illustration3 ,fig4 #ajg4 ,induction ,coil ,illustration3 ,fig4 #ajh4 ,section of ,induction ,coil ,use and ,advantage4 ,the use and advantages of the induction coil in so-called local-battery telephone sets have already been explained in previous chapters4 ,such induction coils are nearly always of the open magnetic circuit type1 consisting of a long1 straight core comprised of a bundle of small annealed iron wires1 on which is wound a primary of comparatively coarse wire and having a small number of turns1 and over which is wound a secondary of comparatively fine wire and having a very much larger number of turns4 ,a view of such a coil mounted on a base is shown in ,fig4 #ajg1 and a sectional view of a similar coil is shown in ,fig4 #ajh4 ,the method of bringing out the winding terminals is clearly indicated in this figure1 the terminal wires #b and #d being those of the primary winding and #a and #c those of the secondary winding4 ,it is customary to bring out these wires and attach them by solder to suitable terminal clips4 ,in the case of the coil shown in ,fig4 #ajh these clips are mounted on the wooden heads of the coil1 while in the design shown in ,fig4 #ajg they are mounted on the base1 as is clearly indicated4 ,repeating ,coil4 ,the so-called repeating coil used in telephony is really nothing but an induction coil4 ,it is used in a variety of ways and usually has for its purpose the inductive association of two circuits that are conductively separated4 ,usually the repeating coil has a one to one ratio of turns1 that is1 there are the same number of turns in the primary as in the secondary4 ,however1 this is not always the case1 since sometimes they are made to have an unequal number of turns1 in which case they are called step-up or step-down repeating coils1 according to whether the primary has a smaller or a greater number of turns than the secondary4 ,repeating coils are almost universally of the closed magnetic circuit type4 ,ringing and ,talking ,considerations4 ,since repeating coils often serve to connect two telephones1 it follows that it is sometimes necessary to ring through them as well as talk through them4 ,by this is meant that it is necessary that the coil shall be so designed as to be capable of transforming the heavy ringing currents as well as the very much smaller telephone or voice currents4 ,ringing currents ordinarily have a frequency ranging from about #af to #ge cycles per second1 while voice currents have frequencies ranging from a few hundred up to perhaps ten thousand per second4 ,ordinarily1 therefore1 the best form of repeating coil for transforming voice currents is not the best for transforming the heavy ringing currents and vice vers4 ,if the comparatively heavy ringing currents alone were to be considered1 the repeating coil might well be of heavy construction with a large amount of iron in its magnetic circuit4 ,on the other hand1 for carrying voice currents alone it is usually made with a small amount of iron and with small windings1 in order to prevent waste of energy in the core1 and to give a high degree of responsiveness with the least amount of distortion of wave form1 so that the voice currents will retain as far as possible their original characteristics4 ,when1 therefore1 a coil is required to carry both ringing and talking currents1 a compromise must be effected4 ,types4 ,the form of repeating coil largely used for both ringing and talking through is shown in ,fig4 #aji4 ,this coil comprises a soft iron core made up of a bundle of wires about 4#jb inch in diameter1 the ends of which are left of sufficient length to be bent back around the windings after they are in place and thus form a completely closed magnetic path for the core4 ,the windings of this particular coil are four in number1 and contain about #b1djj turns each1 and have a resistance of about #fj ohms4 ,in this coil1 when connected for local battery work1 the windings are connected in pairs in series1 thus forming effectively two windings having about #abj ohms resistance each4 ,the whole coil is enclosed in a protecting case of iron4 ,the terminals are brought out to suitable clips on the wooden base1 as shown4 ,an external perspective view of this coil is shown in ,fig4 #aaj4 ,by bringing out each terminal of each winding1 eight in all1 as shown in this figure1 great latitude of connection is provided for1 since the windings may be connected in circuit in any desirable way1 either by connecting them together in pairs to form virtually a primary and a secondary1 or1 as is frequently the case1 to split the primary and the secondary1 connecting a battery between each pair of windings4 ,illustration3 ,fig4 #aji4 ,repeating ,coil ,illustration3 ,fig4 #aaj4 ,repeating ,coil ,fig4 #aaa illustrates in section a commercial type of coil designed for talking through only4 ,this coil is provided with four windings of #a1ceg turns each1 and when used for local battery work the coils are connected in pairs in series1 thus giving a resistance of about #aij ohms in each half of the repeating coil4 ,the core of this coil consists of a bundle of soft iron wires1 and the shell which forms the return path for the magnetic lines is of very soft sheet iron4 ,this shell is drawn into cup shape and its open end is closed1 after the coil is inserted1 by the insertion of a soft iron head1 as indicated4 ,as in the case of the coil shown in ,figs4 #aji and #aaj1 eight terminals are brought out on this coil1 thus providing the necessary flexibility of connection4 ,illustration3 ,fig4 #aaa4 ,repeating ,coil ,illustration3 ,fig4 #aab4 ,diagram of ,toroidal ,repeating ,coil ,illustration3 ,fig4 #aac4 ,toroidal ,repeating ,coil ,still another type of repeating coil is illustrated in diagram in ,fig4 #aab1 and in view in ,fig4 #aac4 ,this coil1 like the impedance coil shown in ,fig4 #ajd1 comprises a core made up of a bundle of soft iron wires wound into the form of a ring4 ,it is usually provided with two primary windings placed opposite each other upon the core1 and with two secondary windings1 one over each primary4 ,in practice these two primary windings are connected in one circuit and the two secondaries in another4 ,this is the standard repeating coil now used by the ,bell companies in their common-battery cord circuits4 ,illustration3 ,,the ,,operating ,,room ,,of ,,the ,,exchange ,,at ,,webb ,,city1 ,,missouri ,illustration3 ,fig4 #aad4 ,symbol of ,induction ,coil ,conventional ,symbols4 ,the ordinary symbol for the induction coil used in local battery work is shown in ,fig4 #aad4 ,this consists merely of a pair of parallel zig-zag lines4 ,the primary winding is usually indicated by a heavy line having a fewer number of zig-zags1 and the secondary by a finer line having a greater number of zig-zags4 ,in this way the fact that the primary is of large wire and of comparatively few turns is indicated4 ,this diagrammatic symbol may be modified to suit almost any conditions1 and where a tertiary as well as a secondary winding is provided it may be shown by merely adding another zig-zag line4 ,illustration3 ,fig4 #aae4 ,repeating-,coil ,symbols ,the repeating coil is indicated symbolically in the two diagrams of ,fig4 #aae4 ,where there is no necessity for indicating the internal connections of the coil1 the symbol shown in the left of this figure is usually employed4 ,where1 however1 the coil consists of four windings rather than two and the method of connecting them is to be indicated1 the symbol at the right hand is employed4 ,in ,fig4 #aaf another way of indicating a four-winding repeating coil or induction coil is shown4 ,sometimes such windings may be combined by connection to form merely a primary and a secondary winding1 and in other cases the four windings all act separately1 in which case one may be considered the primary and the others1 respectively1 the secondary1 tertiary1 and quaternary4 ,illustration3 ,fig4 #aaf4 ,symbol of ,four-,winding ,repeating ,coil ,where the toroidal type of repeating coil is employed1 the diagram of ,fig4 #aab1 already referred to1 is a good symbolic representation4 ,,chapter ,,xi ,,non-,,inductive ,,resistance ,,devices ,it is often desired to introduce simple ohmic resistance into telephone circuits1 in order to limit the current flow1 or to create specific differences of potential at given points in the circuit4 ,temperature ,coefficient4 ,the design or selection of resistance devices for various purposes frequently involves the consideration of the effect of temperature on the resistance of the conductor employed4 ,the resistance of conductors is subject to change by changes in temperature4 ,while nearly all metals show an increase1 carbon shows a decrease in its resistance when heated4 ,the temperature coefficient of a conductor is a factor by which the resistance of the conductor at a given temperature must be multiplied in order to determine the change in resistance of that conductor brought about by a rise in temperature of one degree4 ,,table ,v ,temperature ,coefficients ---------------------------------------- ---------------- ,,pure ,,metals ,,temperature ,,coefficients ---------------------------------------- --------------- ,,centigrade ,,fahrenheit ---------------------------------------- --------------- ,silver 7annealed7 #j4jjdjj #j4jjbbb ,copper 7annealed7 #j4jjdbh #j4jjbdb ,gold 7#ii4i7 #j4jjcgg #j4jjbaj ,aluminum 7#ii7 #j4jjdbc #j4jjbce ,zinc #j4jjdjf #j4jjbbf ,platinum 7annealed7 #j4jjbdg #j4jjacg ,iron #j4jjfbe #j4jjcdg ,nickel #j4jjfb #j4jjcde ,tin #j4jjddj #j4jjbde ,lead #j4jjdaa #j4jjbbh ,antimony #j4jjchi #j4jjbaf ,mercury #j4jjjgb #j4jjjdd ,bismuth #j4jjced #j4jjaig ---------------------------------------- --------------- ,positive and ,negative ,coefficients4 ,those conductors1 in which a rise in temperature produces an increase in resistance1 are said to have positive temperature coefficients1 while those in which a rise in temperature produces a lowering of resistance are said to have negative temperature coefficients4 ,the temperature coefficients of pure metals are always positive and for some of the more familiar metals1 have values1 according to ,foster1 as in ,table ,v4 ,iron1 it will be noticed1 has the highest temperature coefficient of all4 ,carbon1 on the other hand1 has a large negative coefficient1 as proved by the fact that the filament of an ordinary incandescent lamp has nearly twice the resistance when cold as when heated to full candle-power4 ,certain alloys have been produced which have very low temperature coefficients1 and these are of value in producing resistance units which have practically the same resistance for all ordinary temperatures4 ,some of these alloys also have very high resistance as compared with copper and are of value in enabling one to obtain a high resistance in small space4 ,one of the most valuable resistance wires is of an alloy known as ,german silver4 ,the so-called eighteen per cent alloy has approximately #ah4c times the resistance of copper and a temperature coefficient of 4#jjjaf per degree ,fahrenheit4 ,the thirty per cent alloy has approximately #bh times the resistance of copper and a temperature coefficient of 4#jjjbd per degree ,fahrenheit4 ,for facilitating the design of resistance coils of ,german silver wire1 ,tables ,,vi and ,,vii are given1 containing information as to length1 resistance1 and weight of the eighteen per cent and the thirty per cent alloys1 respectively1 for all sizes of wire smaller than ,no4 #bj ,b4 ,s4 gauge4 ,special resistance alloys may be obtained having temperature coefficients as low as 4#jjjjjc per degree ,fahrenheit4 ,other alloys of nickel and steel are adapted for use where the wire must carry heavy currents and be raised to comparatively high temperatures thereby2 for such use non-corrosive properties are specially to be desired4 ,such wire may be obtained having a resistance of about fifty times that of copper4 ,,table ,,vi #ah ,per ,cent ,german ,silver ,wire ---------------------------------------- --------------------------- ,no4 ,b4 ,s4 ,,diameter ,,weight ,,length ,,resistance ,,gauge ,,inches ,,pounds ,,per ,,foot ,,feet ,,per ,,pound ,,ohms ,,per ,,foot ---------------------------------------- --------------------------- #ba 4#jbhdf 4#jjbchi #dah4f 4#bccc #bb 4#jbece 4#jjahid #ebg4i 4#bida #bc 4#jbbeg 4#jjaejb #ffe4h 4#cgaj #bd 4#jbjaj 4#jjaaia #hci4e 4#dfgh #be 4#jagij 4#jjjiddi #ajeh4 4#ehii #bf 4#jaeid 4#jjjgdic #acce4 4#gdch #bg 4#jadai 4#jjjeidc #afhc4 4#ichf #bh 4#jabfd 4#jjjdgaa #babc4 #a4ahc #bi 4#jaabf 4#jjjcgce #bfgg4 #a4dia #cj 4#jajjc 4#jjjbifb #ccgf4 #a4hgi #ca 4#jjhibh 4#jjjbcej #dbee4 #b4cga #cb 4#jjgiej 4#jjjahfd #ecff4 #b4iij #cc 4#jjgjhj 4#jjjadgh #fgff4 #c4gga #cd 4#jjfcjd 4#jjjaagb #hecb4 #d4gef #ce 4#jjefad 4#jjjjibie #ajgeh4 #e4iig #cf 4#jjejjj 4#jjjjgcfi #acefi4 #g4efj #cg 4#jjddec 4#jjjjehde #agajh4 #i4ecb #ch 4#jjcife 4#jjjjdfcf #baefi4 #ab4jb #ci 4#jjceca 4#jjjjcfge #bgbji4 #ae4af #dj 4#jjcade 4#jjjjbiag #cdbhb4 #ai4aa ---------------------------------------- --------------------------- ,inductive ,neutrality4 ,where the resistance unit is required to be strictly non-inductive1 and is to be in the form of a coil1 special designs must be employed to give the desired inductive neutrality4 ,provisions ,against ,heating4 ,in cases where a considerable amount of heat is to be generated in the resistance1 due to the necessity of carrying large currents1 special precautions must be taken as to the heat-resisting properties of the structure1 and also as to the provision of sufficient radiating surface or its equivalent to provide for the dissipation of the heat generated4 ,types4 ,mica ,card ,unit4 ,one of the most common resistance coils used in practice is shown in ,fig4 #aag4 ,this comprises a coil of fine1 bare ,german silver wire wound on a card of mica1 the windings being so spaced that the loops are not in contact with each other4 ,the winding is protected by two cards of mica and the whole is bound in place by metal strips1 to which the ends of the winding are attached4 ,binding posts are provided on the extended portions of the terminals to assist in mounting the resistance on a supporting frame1 and the posts terminate in soldering terminals by which the resistance is connected into the circuit4 ,,table ,,vii #cj ,per ,cent ,german ,silver ,wire ---------------------------------------- --------------------------- ,no4 ,b4 ,s4 ,,diameter ,,weight ,,length ,,resistance ,,gauge ,,inches ,,pounds ,,per ,,foot ,,feet ,,per ,,pound ,,ohms ,,per ,,foot ---------------------------------------- --------------------------- #ba 4#jbhdf 4#jjbdje #dae4h 4#ceha #bb 4#jbece 4#jjaijg #ebd4d 4#deac #bc 4#jbbeg 4#jjaeab #ffa4c 4#efic #bd 4#jbjaj 4#jjaaii #hcc4i 4#gagh #be 4#jagij 4#jjjieac #ajea4 4#ijea #bf 4#jaeid 4#jjjgedd #acbf4 #a4ada #bg 4#jadai 4#jjjeihc #afga4 #a4ddj #bh 4#jabfd 4#jjjdgdc #bajh4 #a4hae #bi 4#jaabf 4#jjjcgfa #bfei4 #b4bhg #cj 4#jajjc 4#jjjbihb #ccec4 #b4hhc #ca 4#jjhibh 4#jjjbcff #dbbg4 #c4fch #cb 4#jjgiej 4#jjjahgf #eccj4 #d4ehh #cc 4#jjgjhj 4#jjjadhh #fgba4 #e4ghf #cd 4#jjfcjd 4#jjjaahj #hdge4 #g4big #ce 4#jjefad 4#jjjjiceh #ajfhf4 #i4bja #cf 4#jjejjj 4#jjjjgdai #acdgh4 #aa4fj #cg 4#jjddec 4#jjjjehhe #afiid4 #ad4fc #ch 4#jjcife 4#jjjjdffh #badbd4 #ah4de #ci 4#jjceca 4#jjjjcgjj #bgjbf4 #bc4bf #dj 4#jjcade 4#jjjjbicg #cdjec4 #bi4cb ---------------------------------------- --------------------------- ,differentially-,wound ,unit4 ,another type of resistance coil is that in which the winding is placed upon an insulating core of heat-resisting material and wound so as to overcome inductive effects4 ,in order to accomplish this1 the wire to be bound on the core is doubled back on itself at its middle portion to form two strands1 and these are wound simultaneously on the core1 thus forming two spirals of equal number of turns4 ,the current in traversing the entire coil must flow through one spiral in one direction with relation to the core1 and in the opposite direction in the other spiral1 thereby nullifying the inductive effects of one spiral by those of the other4 ,this is called a non-inductive winding and is in reality an example of differential winding4 ,lamp ,filament4 ,an excellent type of non-inductive resistance is the ordinary carbon-filament incandescent lamp4 ,this is used largely in the circuits of batteries1 generators1 and other sources of supply to prevent overload in case of short circuits on the line4 ,these are cheap1 durable1 have large current-carrying capacities1 and are not likely to set things afire when overheated4 ,an additional advantage incident to their use for this purpose is that an overload on a circuit in which they are placed is visibly indicated by the glowing of the lamp4 ,illustration3 ,fig4 #aag4 ,mica ,card ,resistance ,illustration3 ,fig4 #aah4 ,iron-,wire ,ballast ,obviously1 the carbon-filament incandescent lamp1 when used as a resistance1 has1 on account of the negative temperature coefficient of carbon1 the property of presenting the highest resistance to the circuit when carrying no current1 and of presenting a lower and lower resistance as the current and consequent heating increases4 ,for some conditions of practice this is not to be desired1 and the opposite characteristic of presenting low resistance to small currents and comparatively high resistance to large currents would best meet the conditions of practice4 ,iron-,wire ,ballast4 ,claude ,d4 ,enochs took advantage of the very high positive temperature coefficient of iron to produce a resistance device having these characteristics4 ,his arrangement possesses the compactness of the carbon-filament lamp and is shown in ,fig4 #aah4 ,the resistance element proper is an iron wire1 wound on a central stem of glass1 and this is included in an exhausted bulb so as to avoid oxidation4 ,such a resistance is comparatively low when cold1 but when traversed by currents sufficient to heat it considerably will offer a very large increase of resistance to oppose the further increase of current4 ,in a sense1 it is a self-adjusting resistance1 tending towards the equalization of the flow of current in the circuit in which it is placed4 ,,chapter ,,xii ,,condensers ,charge4 ,a conducting body insulated from all other bodies will receive and hold a certain amount of electricity 7a charge71 if subjected to an electrical potential4 ,thus1 referring to ,fig4 #aai1 if a metal plate1 insulated from other bodies1 be connected with1 say1 the positive pole of a battery1 the negative pole of which is grounded1 a current will flow into the plate until the plate is raised to the same potential as that of the battery pole to which it is connected4 ,the amount of electricity that will flow into the plate will depend1 other things being equal1 on the potential of the source from which it is charged2 in fact1 it is proportional to the potential of the source from which it is charged4 ,this amount of electricity is a measure of the capacity of the plate1 just as the amount of water that a bath-tub will hold is a measure of the capacity of the bath-tub4 ,capacity4 ,instead of measuring the amount of electricity by the quart or pound1 as in the case of material things1 the unit of electrical quantity is the coulomb4 ,the unit of capacity of an insulated conductor is the farad1 and a given insulated conductor is said to have unit capacity1 that is1 the capacity of one farad1 when it will receive a charge of one coulomb of electricity at a potential of one volt4 ,referring to ,fig4 #aai1 the potential of the negative terminal of the battery may be said to be zero1 since it is connected to the earth4 ,if the battery shown be supposed to have exactly one volt potential1 then the plate would be said to have the capacity of one farad if one coulomb of electricity flowed from the battery to the plate before the plate was raised to the same potential as that of the positive pole1 that is1 to a potential of one volt above the potential of the earth2 it being assumed that the plate was also at zero potential before the connection was made4 ,another conception of this quantity may be had by remembering that a coulomb is such a quantity of current as will result from one ampere flowing one second4 ,the capacity of a conductor depends1 among other things1 on its area4 ,if the plate of ,fig4 #aai should be made twice as large in area1 other things remaining the same1 it would have twice the capacity4 ,but there are other factors governing the capacity of a conductor4 ,consider the diagram of ,fig4 #abj1 which is supposed to represent two such plates as are shown in ,fig4 #aai1 placed opposite each other and connected respectively with the positive and the negative poles of the battery4 ,when the connection between the plates and the battery is made1 the two plates become charged to a difference of potential equal to the electromotive force of the battery4 ,in order to obtain these charges1 assume that the plates were each at zero potential before the connection was made2 then current flows from the battery into the plates until they each assume the potential of the corresponding battery terminal4 ,if the two plates be brought closer together1 it will be found that more current will now flow into each of them1 although the difference of potential between the two plates must obviously remain the same1 since each of them is still connected to the battery4 ,illustration3 ,fig4 #aai4 ,condenser ,plate ,theory4 ,due to the proximity of the plates1 the positive electricity on plate ,a is drawn by the negative charge on plate ,b towards plate ,b1 and likewise the negative electricity on plate ,b is drawn to the side towards plate ,a by the positive charge on that plate4 ,these two charges so drawn towards each other will1 so to speak1 bind each other1 and they are referred to as bound charges4 ,the charge on the right-hand side of plate ,a and on the left-hand side of plate ,b will1 however1 be free charges1 since there is nothing to attract them1 and these are1 therefore1 neutralized by a further flow of electricity from the battery to the plate4 ,illustration3 ,fig4 #abj4 ,theory of ,condenser ,obviously1 the closer together the plates are the stronger will be the attractive influence of the two charges on each other4 ,from this it follows that in the case of plate ,a1 when the two plates are being moved closer together1 more positive electricity will flow into plate ,a to neutralize the increasing free negative charges on the right-hand side of the plate4 ,as the plates are moved closer together still1 a new distribution of charges will take place1 resulting in more positive electricity flowing into plate ,a and more negative electricity flowing into plate ,b4 ,the closer proximity of the plates1 therefore1 increases the capacity of the plates for holding charges1 due to the increased inductive action across the dielectric separating the plates4 ,condenser ,defined4 ,a condenser is a device consisting of two adjacent plates of conducting material1 separated by an insulating material1 called a dielectric4 ,the purpose is to increase by the proximity of the plates1 each to the other1 the amount of electricity which each plate will receive and hold when subjected to a given potential4 ,dielectric4 ,we have already seen that the capacity of a condenser depends upon the area of its plates1 and also upon their distance apart4 ,there is still another factor on which the capacity of a condenser depends1 i4e41 on the character of the insulating medium separating its plates4 ,the inductive action which takes place between a charged conductor and other conductors nearby it1 as between plate ,a and plate ,b of ,fig4 #abj1 is called electrostatic induction1 and it plays an important part in telephony4 ,it is found that the ability of a given charged conductor to induce charges on other neighboring conductors varies largely with the insulating medium or dielectric that separates them4 ,this quality of a dielectric1 by which it enables inductive action to take place between two separated conductors1 is called inductive capacity4 ,usually this quality of dielectrics is measured in terms of the same quality in dry air1 this being taken as unity4 ,when so expressed1 it is termed specific inductive capacity4 ,to be more accurate the specific inductive capacity of a dielectric is the ratio between the capacity of a condenser having that substance as a dielectric1 to the capacity of the same condenser using dry air at zero degrees ,centigrade and at a pressure of #ad4g pounds per square inch as the dielectric4 ,to illustrate1 if two condensers having plates of equal size and equal distance apart are constructed1 one using air as the dielectric and the other using hard crown glass as the dielectric1 the one using glass will have a capacity of #f4if times that of the one using air4 ,from this we say that crown glass has a specific inductive capacity of #f4if4 ,various authorities differ rather widely as to the specific inductive capacity of many common substances4 ,the values given in ,table ,,viii have been chosen from the ,smithsonian ,physical ,tables4 ,,table ,,viii ,specific ,inductive ,capacities ---------------------------------------- ------- ,,dielectric ,,referred ,,to ,,air ,,as #a ---------------------------------------- ------- ,vacuum 4#iiida ,hydrogen 4#iiifg ,carbonic ,acid #a4jjjcf ,dry ,paper #a4be to #a4ge ,paraffin #a4ie to #b4cb ,ebonite #a4i to #c4dh ,sulphur #b4bd to #c4ij ,shellac #b4ie to #c4gc ,gutta-percha #c4c to #d4i ,plate ,glass #c4ca to #g4e ,porcelain #d4ch ,mica #d4f to #h4j ,glass--,light ,flint #f4fa ,glass--,hard ,crown #f4if ,selenium #aj4b ---------------------------------------- ------- ,this data is interesting as showing the wide divergence in specific inductive capacities of various materials1 and also showing the wide divergence in different observations of the same material4 ,undoubtedly1 this latter is due mainly to the fact that various materials differ largely in themselves1 as in the case of paraffin1 for instance1 which exhibits widely different specific inductive capacities according to the difference in rapidity with which it is cooled in changing from a liquid to a solid state4 ,we see then that the capacity of a condenser varies as the area of its plates1 as the specific inductive capacity of the dielectric employed1 and also inversely as the distance between the plates4 ,obviously1 therefore1 in making a condenser of large capacity1 it is important to have as large an area of the plate as possible2 to have them as close together as possible2 to have the dielectric a good insulating medium so that there will be practically no leakage between the plates2 and to have the dielectric of as high a specific inductive capacity as economy and suitability of material in other respects will permit4 ,dielectric ,materials4 ,mica4 ,of all dielectrics mica is the most suitable for condensers1 since it has very high insulation resistance and also high specific inductive capacity1 and furthermore may be obtained in very thin sheets4 ,high-grade condensers1 such as are used for measurements and standardization purposes1 usually have mica for the dielectric4 ,illustration3 ,fig4 #aba4 ,rolled ,condenser ,dry ,paper4 ,the demands of telephonic practice are1 however1 such as to require condensers of very cheap construction with large capacity in a small space4 ,for this purpose thin bond paper1 saturated with paraffin1 has been found to be the best dielectric4 ,the conductors in condensers are almost always of tinfoil1 this being an ideal material on account of its cheapness and its thinness4 ,before telephony made such urgent demands for a cheap compact condenser1 the customary way of making them was to lay up alternate sheets of dielectric material1 either of oiled paper or mica and tinfoil1 the sheets of tinfoil being cut somewhat smaller than the sheets of dielectric material in order that the proper insulation might be secured at the edges4 ,after a sufficient number of such plates were built up the alternate sheets of tinfoil were connected together to form one composite plate of the condenser1 while the other sheets were similarly connected together to form the other plate4 ,obviously1 in this way a very large area of plates could be secured with a minimum degree of separation4 ,illustration3 ,fig4 #abb4 ,rolled ,condenser ,there has been developed for use in telephony1 however1 and its use has since extended into other arts requiring condensers1 what is called the rolled condenser4 ,this is formed by rolling together in a flat roll four sheets of thin bond paper1 #a1 #b1 #c1 and #d1 and two somewhat narrower strips of tinfoil1 #e and #f1 ,fig4 #aba4 ,the strips of tinfoil and paper are fed on to the roll in continuous lengths and in such manner that two sheets of paper will lie between the two strips of tinfoil in all cases4 ,thin sheet metal terminals #g and #h are rolled into the condenser as it is being wound1 and as these project beyond the edges of the paper they form convenient terminals for the condenser after it is finished4 ,after it is rolled1 the roll is boiled in hot paraffin so as to thoroughly impregnate it and expel all moisture4 ,it is then squeezed in a press and allowed to cool while under pressure4 ,in this way the surplus paraffin is expelled and the plates are brought very close together4 ,it then appears as in ,fig4 #abb4 ,the condenser is now sealed in a metallic case1 usually rectangular in form1 and presents the appearance shown in ,fig4 #abc4 ,illustration3 ,fig4 #abc4 ,rolled ,condenser ,a later method of condenser making which has not yet been thoroughly proven in practice1 but which bids fair to produce good results1 varies from the method just described in that a paper is used which in itself is coated with a very thin conducting material4 ,this conducting material is of metallic nature and in reality forms a part of the paper4 ,to form a condenser of this the sheets are merely rolled together and then boiled in paraffin and compressed as before4 ,sizes4 ,the condensers ordinarily used in telephone practice range in capacity from about #a_/d microfarad to #b microfarads4 ,when larger capacities than #b microfarads are desired1 they may be obtained by connecting several of the smaller size condensers in multiple4 ,table ,,ix gives the capacity1 shape1 and dimensions of a variety of condensers selected from those regularly on the market4 ,,table ,,ix ,condenser ,data ---------------------------------------- -------------------- ,,dimensions ,,in ,,inches ,,capacity ,,shape ------------------------------- ,height ,width ,thickness ---------------------------------------- ------------------ #b m4 f4 ,rectangular #i-a_/f #d-c_/d #aa_/af #a m4 f4 #i-a_/f #d-c_/d #aa_/af #a m4 f4 #d-c_/d #b-c_/cb #ac_/af #a_/b m4 f4 #b-c_/d #a-a_/d #c_/d #a m4 f4 #d-ac_/af #b-a_/cb #be_/cb #a_/b m4 f4 #d-c_/d #b-c_/cb #ac_/af #c_/aj m4 f4 #d-c_/d #b-c_/cb #ac_/af #a m4 f4 #b-c_/d #c l ---------------------------------------- ------------------ ,conventional ,symbols4 ,the conventional symbols usually employed to represent condensers in telephone diagrams are shown in ,fig4 #abd4 ,these all convey the idea of the adjacent conducting plates separated by insulating material4 ,illustration3 ,fig4 #abd4 ,condenser ,symbols ,functions4 ,obviously1 when placed in a circuit a condenser offers a complete barrier to the flow of direct current1 since no conducting path exists between its terminals1 the dielectric offering a very high insulation resistance4 ,if1 however1 the condenser is connected across the terminals of a source of alternating current1 this current flows first in one direction and then in the other1 the electromotive force in the circuit increasing from zero to a maximum in one direction1 and then decreasing back to zero and to a maximum in the other direction1 and so on4 ,with a condenser connected so as to be subjected to such alternating electromotive forces1 as the electromotive force begins to rise the electromotive force at the condenser terminals will also rise and a current will1 therefore1 flow into the condenser4 ,when the electromotive force reaches its maximum1 the condenser will have received its full charge for that potential1 and the current flow into it will cease4 ,when the electromotive force begins to fall1 the condenser can no longer retain its charge and a current will1 therefore1 flow out of it4 ,apparently1 therefore1 there is a flow of current through the condenser the same as if it were a conductor4 ,means for ,assorting ,currents4 ,in conclusion1 it is obvious that the telephone engineer has within his reach in the various coils--whether non-inductive or inductive1 or whether having one or several windings--and in the condenser1 a variety of tools by which he may achieve a great many useful ends in his circuit work4 ,obviously1 the condenser affords a means for transmitting voice currents or fluctuating currents1 and for excluding steady currents4 ,likewise the impedance coil affords a means for readily transmitting steady currents but practically excluding voice currents or fluctuating currents4 ,by the use of these very simple devices it is possible to sift out the voice currents from a circuit containing both steady and fluctuating currents1 or it is possible in the same manner to sift out the steady currents and to leave the voice currents alone to traverse the circuit4 ,great use is made in the design of telephone circuits of the fact that the electromagnets1 which accomplish the useful mechanical results in causing the movement of parts1 possess the quality of impedance4 ,thus1 the magnets which operate various signaling relays at the central office are often used also as impedance coils in portions of the circuit through which it is desired to have only steady currents pass4 ,if1 on the other hand1 it is necessary to place a relay magnet1 having considerable impedance1 directly in a talking circuit1 the bad effects of this on the voice currents may be eliminated by shunting this coil with a condenser1 or with a comparatively high non-inductive resistance4 ,the voice currents will flow around the high impedance of the relay coil through the condenser or resistance1 while the steady currents1 which are the ones which must be depended upon to operate the relay1 are still forced in whole or in part to pass through the relay coil where they belong4 ,in a similar way the induction coil affords a means for keeping two circuits completely isolated so far as the direct flow of current between them is concerned1 and yet of readily transmitting1 by electromagnetic induction1 currents from one of these circuits to the other4 ,here is a means of isolation so far as direct current is concerned1 with complete communication for alternating current4 ,,chapter ,,xiii ,,current ,,supply ,,to ,,transmitters ,the methods by which current is supplied to the transmitter of a telephone for energizing it1 may be classified under two divisions3 first1 those where the battery or other source of current is located at the station with the transmitter which it supplies2 and second1 those where the battery or other source of current is located at a distant point from the transmitter1 the battery in such cases serving as a common source of current for the supply of transmitters at a number of stations4 ,the advantages of putting the transmitter and the battery which supplies it with current in a local circuit with the primary of an induction coil1 and placing the secondary of the induction coil in the line1 have already been pointed out but may be briefly summarized as follows3 ,when the transmitter is placed directly in the line circuit and the line is of considerable length1 the current which passes through the transmitter is necessarily rather small unless a battery of high potential is used2 and1 furthermore1 the total change in resistance which the transmitter is capable of producing is but a small proportion of the total resistance of the line1 and1 therefore1 the current changes produced by the transmitter are relatively small4 ,on the other hand1 when the transmitter is placed in a local circuit with the battery1 this circuit may be of small resistance and the current relatively large1 even though supplied by a low-voltage battery2 so that the transmitter is capable of producing relatively large changes in a relatively large current4 ,to draw a comparison between these two general classes of transmitter current supply1 a number of cases will be considered in connection with the following figures1 in each of which two stations connected by a telephone line are shown4 ,brief reference to the local battery method of supplying current will be made in order to make this chapter contain1 as far as possible1 all of the commonly used methods of current supply to transmitters4 ,illustration3 ,a ,,typical ,,medium-,,sized ,,multiple ,,switchboard ,,equipment ,local ,battery4 ,in ,fig4 #abe two stations are shown connected by a grounded line wire4 ,the transmitter of each station is included in a low-resistance primary circuit including a battery and the primary winding of an induction coil1 the relation between the primary circuits and the line circuits being established by the inductive action between the primary and the secondary windings of induction coils1 the secondary in each case being in the line circuits with the receivers4 ,illustration3 ,fig4 #abe4 ,local-,battery ,stations with ,grounded ,circuit ,fig4 #abf shows exactly the same arrangement but with a metallic circuit rather than a grounded circuit4 ,the student should become accustomed to the replacing of one of the line wires of a metallic circuit by the earth1 and to the method1 employed in ,figs4 #abe and #abf1 of indicating a grounded circuit as distinguished from a metallic circuit4 ,illustration3 ,fig4 #abf4 ,local-,battery ,stations with ,metallic ,circuit ,in ,fig4 #abg is shown a slight modification of the circuit shown in ,fig4 #abf1 which consists of connecting one end of the primary winding to one end of the secondary winding of the induction coil1 thus linking together the primary circuit and the line circuit1 a portion of each of these circuits being common to a short piece of the local wiring4 ,there is no difference whatever in the action of the circuits shown in ,figs4 #abf and #abg1 the latter being shown merely for the purpose of bringing out this fact4 ,it is very common1 particularly in local-battery circuits1 to connect one end of the primary and the secondary windings1 as by doing so it is often possible to save a contact point in the hook switch and also to simplify the wiring4 ,illustration3 ,fig4 #abg4 ,local-,battery ,stations with ,metallic ,circuit ,the advantages to be gained by employing a local battery at each subscriber's station associated with the transmitter in the primary circuit of an induction coil are attended by certain disadvantages from a commercial standpoint4 ,the primary battery is not an economical way to generate electric energy4 ,in all its commercial forms it involves the consumption of zinc and zinc is an expensive fuel4 ,the actual amount of current in watts required by a telephone is small1 however1 and this disadvantage due to the inexpensive method of generating current would not in itself be of great importance4 ,a more serious objection to the use of local batteries at subscribers' stations appears when the subject is considered from the standpoint of maintenance4 ,batteries1 whether of the so-called dry or wet type1 gradually deteriorate1 even when not used1 and in cases where the telephone is used many times a day the deterioration is comparatively rapid4 ,this makes necessary the occasional renewals of the batteries with the attendant expense for new batteries or new material1 and of labor and transportation in visiting the station4 ,the labor item becomes more serious when the stations are scattered in a sparsely settled community1 in which case the visiting of the stations1 even for the performance of a task that would require but a few minutes' time1 may consume some hours on the part of the employes in getting there and back4 ,common ,battery4 ,advantages4 ,it would be more economical if all of the current for the subscribers' transmitters could be supplied from a single comparatively efficient generating source instead of from a multitude of inefficient small sources scattered throughout the community served by the exchange4 ,the advantage of such centralization lies not only in more economic generating means1 but also in having the common source of current located at one place1 where it may be cared for with a minimum amount of expense4 ,such considerations have resulted in the so-called common-battery system1 wherein the current for all the subscribers' transmitters is furnished from a source located at the central office4 ,where such a method of supplying current is practiced1 the result has also been1 in nearly all cases1 the doing away with the subscriber's magneto generators1 relying on the central-office source of current to furnish the energy for enabling the subscriber to signal the operator4 ,such systems1 therefore1 concentrate all of the sources of energy at the central office and for that reason they are frequently referred to as central-energy systems4 ,,note4 ,in this chapter the central-energy or common-battery system will be considered only in so far as the supply of current for energizing the subscribers' transmitters is concerned1 the discussion of the action of signaling being reserved for subsequent chapters4 ,series ,battery4 ,if but a single pair of lines had to be considered1 the arrangement shown in ,fig4 #abh might be employed4 ,in this the battery is located at the central office and placed in series with the two grounded lines leading from the central office to the two subscribers' stations4 ,the voltage of this battery is made sufficient to furnish the required current over the resistance of the entire line circuit with its included instruments4 ,obviously1 changes in resistance in the transmitter at ,station ,a will affect the flow of current in the entire line and the fluctuations resulting from the vibration of the transmitter diaphragm will1 therefore1 reproduce these sounds in the receiver at ,station ,b1 as well as in that at ,station ,a4 ,illustration3 ,fig4 #abh4 ,battery in ,series with ,two ,lines ,an exactly similar arrangement applied to a metallic circuit is shown in ,fig4 #abi4 ,in thus placing the battery in series in the circuit between the two stations1 as shown in ,figs4 #abh and #abi1 it is obvious that the transmitter at each station is compelled to vary the resistance of the entire circuit comprising the two lines in series1 in order to affect the receiver at distant stations4 ,this is in effect making the transmitter circuit twice as long as is necessary1 as will be shown in the subsequent systems considered4 ,furthermore1 the placing of the battery in series in the circuit of the two combined lines does not lend itself readily to the supply of current from a common source to more than a single pair of lines4 ,illustration3 ,fig4 #abi4 ,battery in ,series with ,two ,lines ,series ,substation ,circuit4 ,the arrangement at the substations--consisting in placing the transmitter and the receiver in series in the line circuit1 as shown in ,figs4 #abh and #abi--;is the simplest possible one1 and has been used to a considerable extent1 but it has been subject to the serious objection1 where receivers having permanent magnets were used1 of making it necessary to so connect the receiver in the line circuit that the steady current from the battery would not set up a magnetization in the cores of the receiver in such a direction as to neutralize or oppose the magnetization of the permanent magnets4 ,as long as the current flowed through the receiver coils in such a direction as to supplement the magnetization of the permanent magnets1 no harm was usually done1 but when the current flowed through the receiver coils in such a way as to neutralize or oppose the magnetizing force of the permanent magnets1 the action of the receiver was greatly interfered with4 ,as a result1 it was necessary to always connect the receivers in the line circuit in a certain way1 and this operation was called poling4 ,in order to obviate the necessity for poling and also to bring about other desirable features1 it has been1 until recently1 almost universal practice to so arrange the receiver that it would be in the circuit of the voice currents passing over the line1 but would not be traversed by direct currents1 this condition being brought about by various arrangements of condensers1 impedance coils1 or induction coils1 as will be shown later4 ,during the year #aiji1 however1 the adoption by several concerns of the so-called direct-current receiver has made it necessary for the direct current to flow through the receiver coils in order to give the proper magnetization to the receiver cores1 and this has brought about a return to the very simple form of substation circuit1 which includes the receiver and the transmitter directly in the circuit of the line4 ,this illustrates well an occurrence that is frequently observed by those who have opportunity to watch closely the development of an art4 ,at one time the conditions will be such as to call for complicated arrangements1 and for years the aim of inventors will be to perfect these arrangements2 then1 after they are perfected1 adopted1 and standardized1 a new idea1 or a slight alteration in the practice in some other respect1 will demand a return to the first principles and wipe out the necessity for the things that have been so arduously striven for4 ,illustration3 ,fig4 #acj4 ,bridging ,battery with ,repeating ,coil ,bridging ,battery with ,repeating ,coil4 ,as pointed out1 the placing of the battery in series in the line circuit in the central office is not desirable1 and1 so far as we are aware1 has never been extensively used4 ,the universal practice1 therefore1 is to place it in a bridge path across the line circuit1 and a number of arrangements employing this basic idea are in wide use4 ,in ,fig4 #acj is shown the standard arrangement of the ,western ,electric ,company1 employed by practically all the ,bell operating companies4 ,in this the battery at the central office is connected in the middle of the two sides of a repeating coil so that the current from the battery is fed out to the two connected lines in multiple4 ,referring to the middle portion of this figure showing the central-office apparatus1 #a and #b may be considered as the two halves of one side of a repeating coil divided so that the battery may be cut into their circuit4 ,likewise1 #c and #d may be considered as the two halves of the other side of the repeating coil similarly divided for the same purpose4 ,the windings of this repeating coil are ordinarily alike2 that is1 #a and #b combined have the same resistance1 number of turns1 and impedance as #c and #d combined4 ,the two sides of this coil are alternately used as primary and secondary1 #a and #b forming the primary when ,station ,a is talking1 and #c and #d1 the secondary2 and vice vers when ,station ,b is talking4 ,as will be seen1 the current flowing from the positive pole of the battery will divide and flow through the windings #b and #d2 thence over the upper limb of each line1 through the transmitter at each station1 and back over the lower limbs of the line1 through the windings #a and #c1 where the two paths reunite and pass to the negative pole of the battery4 ,it is evident that when neither transmitter is being used the current flowing through both lines will be a steady current and that1 therefore1 neither line will have an inductive effect on the other4 ,when1 however1 the transmitter at ,station ,a is used the variations in the resistance caused by it will cause undulations in the current4 ,these undulations1 passing through the windings #a and #b of the repeating coil1 will cause1 by electromagnetic induction1 alternating currents to flow in the windings #c and #d1 and these alternating currents will be superimposed on the steady currents flowing in that line and will affect the receiver at ,station ,b1 as will be pointed out4 ,the reverse conditions exist when ,station ,b is talking4 ,bell ,substation ,arrangement4 ,the substation circuits at the stations in ,fig4 #acj are illustrative of one of the commonly employed methods of preventing the steady current from the battery from flowing through the receiver coil4 ,this particular arrangement is that employed by the common-battery instruments of the various ,bell companies4 ,considering the action at ,station ,b1 it is evident that the steady current will pass through the transmitter and through the secondary winding of the induction coil1 and that as long as this current is steady no current will flow through the telephone receiver4 ,the receiver1 transmitter1 and primary winding of the induction coil are1 however1 included in a local circuit with the condenser4 ,the presence of the condenser precludes the possibility of direct current flowing in this path4 ,considering ,station ,a as a receiving station1 it is evident that the voice currents coming to the station over the line will pass through the secondary winding and will induce alternating currents in the primary winding which will circulate through the local circuit containing the receiver and the condenser1 and thus actuate the receiver4 ,the considerations are not so simple when the station is being treated as a transmitting station4 ,under this condition the steady current passes through the transmitter in an obvious manner4 ,it is clear that if the local circuit containing the receiver did not exist1 the circuit would be operative as a transmitting circuit because the transmitter would produce fluctuations in the steady current flowing in the line and thus be able to affect the distant station4 ,the transmitter1 therefore1 has a direct action on the currents flowing in the line by the variation in resistance which it produces in the line circuit4 ,there is1 however1 a subsidiary action in this circuit4 ,obviously1 there is a drop of potential across the transmitter terminals due to the flow of steady current4 ,this means that the upper terminal of the condenser will be charged to the same potential as the upper terminal of the transmitter1 while the lower terminal of the condenser will be of the same potential as the lower terminal of the transmitter4 ,when1 now1 the transmitter varies its resistance1 a variation in the potential across its terminals will occur2 and as a result1 a variation in potential across the terminals of the condenser will occur1 and this means that alternating currents will flow through the primary winding of the induction coil4 ,the transmitter1 therefore1 by its action1 causes alternating currents to flow through the primary of this induction coil and it causes1 by direct action on the circuit of the line1 fluctuations in the steady current flowing in the line4 ,the alternating currents flowing in the primary of the coil induce currents in the secondary of the coil which supplement and augment the fluctuations produced by the direct action of the transmitter4 ,this circuit may be looked at1 therefore1 in the light of combining the direct action which the transmitter produces in the current in the line with the action which the transmitter produces in the local circuit containing the primary of the induction coil1 this action being repeated in the line circuit through the secondary of the induction coil4 ,the receiver in this circuit is placed in the local circuit1 and is thus not traversed by the steady currents flowing in the line4 ,there is thus no necessity for poling it4 ,this circuit is very efficient1 but is subject to the objection of producing a heavy side tone in the receiver of the transmitting station4 ,by side tone is meant the noises which are produced in the receiver at a station by virtue of the action of the transmitter at that station4 ,side tone is objectionable for several reasons3 first1 it is sometimes annoying to the subscriber2 second1 and of more importance1 the subscriber who is talking1 hearing a very loud noise in his own receiver1 unconsciously assumes that he is talking too loud and1 therefore1 lowers his voice1 sometimes to such an extent that it will not properly reach the distant station4 ,illustration3 ,fig4 #aca4 ,bridging ,battery with ,impedance ,coils ,bridging ,battery with ,impedance ,coils4 ,the method of feeding current to the line from the common battery1 shown in ,fig4 #acj1 is called the split repeating-coil method4 ,as distinguished from this is the impedance-coil method which is shown in ,fig4 #aca4 ,in this the battery is bridged across the circuit of the combined lines in series with two impedance coils1 #a and #b1 one on each side of the battery4 ,the steady currents from the battery find ready path through these impedance coils which are of comparatively low ohmic resistance1 and the current divides and passes in multiple over the circuits of the two lines4 ,voice currents1 however1 originating at either one of the stations1 will not pass through the shunt across the line at the central office on account of the high impedance offered by these coils1 and as a result they are compelled to pass on to the distant station and affect the receiver there1 as desired4 ,this impedance-coil method seems to present the advantage of greater simplicity over the repeating-coil method shown in ,fig4 #acj1 and so far as talking efficiency is concerned1 there is little to choose between the two4 ,the repeating-coil method1 however1 has the advantage over this impedance-coil method1 because by it the two lines are practically divided except by the inductive connection between the two windings1 and as a result an unbalanced condition of one of the connected lines is not as likely to produce an unbalanced condition in the other as where the two lines are connected straight through1 as with the impedance-coil method4 ,the substation arrangement of ,fig4 #aca is the same as that of ,fig4 #acj4 ,illustration3 ,fig4 #acb4 ,double-,battery ,kellogg ,system ,double ,battery with ,impedance ,coils4 ,a modification of the impedance-coil method is used in all of the central-office work of the ,kellogg ,switchboard and ,supply ,company4 ,this employs a combination of impedance coils and condensers1 and in effect isolates the lines conductively from each other as completely as the repeating-coil method4 ,it is characteristic of all the ,kellogg common-battery systems that they employ two batteries instead of one1 one of these being connected in all cases with the calling line of a pair of connected lines and the other in all cases with the called line4 ,as shown in ,fig4 #acb1 the left-hand battery is connected with the line leading to ,station ,a through the impedance coils #a and #b4 ,likewise1 the right-hand battery is connected to the line of ,station ,b through the impedance coils #c and #d4 ,these four impedance coils are wound on separate cores and do not have any inductive relation whatsoever with each other4 ,condensers #e and #f are employed to completely isolate the lines conductively4 ,current from the left-hand battery1 therefore1 passes only to ,station ,a1 and current from the right-hand battery to ,station ,b4 ,whenever the transmitter at ,station ,a is actuated the undulations of current which it produces in the line cause a varying difference of potential across the outside terminals of the two impedance coils #a and #b4 ,this means that the two left-hand terminals of condensers #e and #f are subjected to a varying difference of potential and these1 of course1 by electrostatic induction1 cause the right-hand terminals of these condensers to be subject to a correspondingly varying difference of potential4 ,from this it follows that alternating currents will be impressed upon the right-hand line and these will affect the receiver at ,station ,b4 ,a rough way of expressing the action of this circuit is to consider it in the same light as that of the impedance-coil circuit shown in ,fig4 #aca1 and to consider that the voice currents originating in one line are prevented from passing through the bridge paths at the central office on account of the impedance1 and are1 therefore1 forced to continue on the line1 being allowed to pass readily by the condensers in series between the two lines4 ,kellogg ,substation ,arrangement4 ,an interesting form of substation circuit which is employed by the ,kellogg ,company in all of its common-battery telephones is shown in ,fig4 #acb4 ,in passing1 it may be well to state that almost any of the substation circuits shown in this chapter are capable of working with any of the central-office circuits4 ,the different ones are shown for the purpose of giving a knowledge of the various substation circuits that are employed1 and1 as far as possible1 to associate them with the particular central-office arrangements with which they are commonly used4 ,in this ,kellogg substation arrangement the line circuit passes first through the transmitter and then divides1 one branch passing through an impedance coil #g and the other through the receiver and the condenser #h1 in series4 ,the steady current from the central-office battery finds ready path through the transmitter and the impedance coil1 but is prevented from passing through the receiver by the barrier set up by the condenser #h4 ,voice currents1 however1 coming over the line to the station1 find ready path through the receiver and the condenser but are barred from passing through the impedance coil by virtue of its high impedance4 ,in considering the action of the station as a transmitting station1 the variations set up by the transmitter pass through the condenser and the receiver at the same station1 while the steady current which supplies the transmitter passes through the impedance coil4 ,impedance coils used for this purpose are made of low ohmic resistance but of a comparatively great number of turns1 and1 therefore1 present a good path for steady currents and a difficult path for voice currents4 ,this divided circuit arrangement employed by the ,kellogg ,company is one of the very simple ways of eliminating direct currents from the receiver path1 at the same time allowing the free passage of voice currents4 ,illustration3 ,fig4 #acc4 ,dean ,system ,dean ,substation ,arrangement4 ,in marked contrast to the scheme for keeping steady current out of the receiver circuit employed by the ,kellogg ,company1 is that shown in ,fig4 #acc1 which has been largely used by the ,dean ,electric ,company1 of ,elyria1 ,ohio4 ,the central-office arrangement in this case is that using the split repeating coil1 which needs no further description4 ,the substation arrangement1 however1 is unique and is a beautiful example of what can be done in the way of preventing a flow of current through a path without in any way insulating that path or placing any barrier in the way of the current4 ,it is an example of the prevention of the direct flow of current through the receiver by so arranging the circuits that there will always be an equal potential on each side of it1 and1 therefore1 no tendency for current to flow through it4 ,in this substation arrangement four coils of wire--#a1 #b1 #c1 and #d--;are so arranged as to be connected in the circuit of the line1 two in series and two in multiple4 ,the current flowing from the battery at the central office1 after passing through the transmitter1 divides between the two paths containing1 respectively1 the coils #a and #c and the coils #b and #d4 ,the receiver is connected between the junction of the coils #b and #d and that of #a and #c4 ,the resistances of the coils are so chosen that the drop of potential through the coil #b will be equal to that through the coil #a1 and likewise that through the coil #d will be equal to that through the coil #c4 ,as a result1 the receiver will be connected between two points of equal potential1 and no direct current will flow through it4 ,how1 then1 do voice currents find their way through the receiver1 as they evidently must1 if the circuit is to fulfill any useful function8 ,the coils #b and #c are made to have high impedance1 while #a and #d are so wound as to be non-inductive and1 therefore1 offer no impedance save that of their ohmic resistance4 ,what is true1 therefore1 of direct currents does not hold for voice currents1 and as a result1 the voice currents1 instead of taking the divided path which the direct currents pursued1 are debarred from the coils #b and #c by their high impedance and thus pass through the non-inductive coil #a1 the receiver1 and the non-inductive coil #d4 ,this circuit employs a ,wheatstone-bridge arrangement1 adjusted to a state of balance with respect to direct currents1 such currents being excluded from the receiver1 not because the receiver circuit is in any sense opaque to such direct currents1 but because there is no difference of potential between the terminals of the receiver circuit1 and1 therefore1 no tendency for current to flow through the receiver4 ,in order that fluctuating currents may not1 for the same reason1 be caused to pass by1 rather than through1 the receiver circuit1 the diametrically-opposed arms of the ,wheatstone bridge are made to possess1 in large degree1 self-induction1 thereby giving these two arms a high impedance to fluctuating currents4 ,the conditions which exist for direct currents do not1 therefore1 exist for fluctuating currents1 and it is this distinction which allows alternating currents to pass through the receiver and at the same time excludes direct currents therefrom4 ,in practice1 the coils #a1 #b1 #c1 and #d of the ,dean substation circuit are wound on the same core1 but coils #a and #d--the non-inductive ones--are wound by doubling the wire back on itself so as to neutralize their self-induction4 ,stromberg-,carlson4 ,another modification of the central-office arrangement and also of the subscribers' station circuits1 is shown in ,fig4 #acd1 this being a simplified representation of the circuits commonly employed by the ,stromberg-,carlson ,telephone ,manufacturing ,company4 ,the battery feed at the central office differs only from that shown in ,fig4 #acb1 in that a single battery rather than two batteries is used1 the current being supplied to one of the lines through the impedance coils #a and #b1 and to the other line through the impedance coils #c and #d2 condensers #e and #f serve conductively to isolate the two lines4 ,at the subscriber's station the line circuit passes through the secondary of an induction coil and the transmitter4 ,the receiver is kept entirely in a local circuit so that there is no tendency for direct current to flow through it1 but it is receptive to voice currents through the electromagnetic induction between the primary and the secondary of the induction coil4 ,illustration3 ,fig4 #acd4 ,stromberg-,carlson ,system ,illustration3 ,fig4 #ace4 ,north ,electric ,company ,system ,north4 ,another arrangement of central-office battery feed is employed by the ,north ,electric ,company1 and is shown in ,fig4 #ace4 ,in this two batteries are used which supply current respectively to the two connected lines1 condensers being employed to conductively isolate the lines4 ,this differs from the ,kellogg arrangement shown in ,fig4 #acb in that the two coils #a and #b are wound on the same core1 while the coils #c and #d are wound together upon another core4 ,in this case1 in order that the inductive action of one of the coils may not neutralize that of the other coil on the same core1 the two coils are wound in such relative direction that their magnetizing influence will always be cumulative rather than differential4 ,the central-office arrangements discussed in ,figs4 #acj to #ace1 inclusive1 are those which are in principal use in commercial practice in common-battery exchanges4 ,current ,supply over ,limbs of ,line in ,parallel4 ,as indicating further interesting possibilities in the method of supplying current from a common source to a number of substations1 several other systems will be briefly referred to as being of interest1 although these have not gone into wide commercial use4 ,the system shown in ,fig4 #acf is one proposed by ,dean in the early days of common-battery working1 and this arrangement was put into actual service and gave satisfactory results1 but was afterwards supplanted by the ,bell equipment operating under the system shown in ,fig4 #acj1 which became standardized by that company4 ,in this the current from the common battery at the central office is not fed over the two line wires in series1 but in multiple1 using a ground return from the subscriber's station to the central office4 ,across the metallic circuit formed by two connected lines there is bridged1 at the central office1 an impedance coil #a1 and between the center point of this impedance coil and the ground is connected the common battery4 ,at the subscriber's station is placed an impedance coil #b1 also bridged across the two limbs of the line1 and between the center point of this impedance coil and the ground is connected the transmitter1 which is shunted by the primary winding of an induction coil4 ,connected between the two limbs of the line at the substation there is also the receiver and the secondary of an induction coil in series4 ,illustration3 ,fig4 #acf4 ,current ,supply over ,parallel ,limbs of ,line ,the action of this circuit at first seems a little complex1 but if taken step by step may readily be understood4 ,the transmitter supply circuit may be traced from the central-office battery through the two halves of the impedance coil #a in multiple2 thence over the two limbs of the line in multiple to ,station ,a1 for instance2 thence in multiple through the two halves of impedance coil #b1 to the center point of that coil2 thence through the two paths offered respectively by the primary of the induction coil and by the transmitter2 then to ground and back to the other pole of the central-office battery4 ,by this circuit the transmitter at the substation is supplied with current4 ,variations in the resistance of the transmitter when in action1 cause complementary variations in the supply current flowing through the primary of the induction coil4 ,these variations induce similar alternating currents in the secondary of this coil1 which is in series in the line circuit4 ,the currents1 so induced in this secondary1 flow in series through one side of the line to the distant station2 thence through the secondary and the receiver at that station to the other side of the line and back through that side of the line to the receiver4 ,these currents are not permitted to pass through the bridged paths across the metallic circuit that are offered by the impedance coils #a and #b1 because they are voice currents and are1 therefore1 debarred from these paths by virtue of the impedance4 ,illustration3 ,fig4 #acg4 ,current ,supply over ,parallel ,limbs of ,line ,an objection to this form of current supply and to other similar forms1 wherein the transmitter current is fed over the two sides of the line in multiple with a ground return1 is that the ground-return circuit formed by the two sides of the line in multiple is subject to inductive disturbances from other lines in the same way as an ordinary grounded line is subject to inductive disturbance4 ,the current-supply circuit is thus subject to external disturbances and such disturbances find their way into the metallic circuit and1 therefore1 through the instruments by means of the electromagnetic induction between the primary and the secondary coils at the substations4 ,another interesting method of current supply from a central-office battery is shown in ,fig4 #acg4 ,this1 like the circuit just considered1 feeds the energy to the subscriber's station over the two sides of the line in multiple with a ground return4 ,in this case1 however1 a local circuit is provided at the substation1 in which is placed a storage battery #a and the primary #b of an induction coil1 together with the transmitter4 ,the idea in this is that the current supply from the central office will pass through the storage battery and charge it4 ,upon the use of the transmitter1 this storage battery acts to supply current to the local circuit containing the transmitter and the primary coil #b in exactly the same manner as in a local battery system4 ,the fluctuating current so produced by the action of the transmitter in this local circuit acts on the secondary winding #c of the induction coil1 and produces therein alternating currents which pass to the central office and are in turn repeated to the distant station4 ,supply ,many ,lines from ,common ,source4 ,we come now to the consideration of the arrangement by which a single battery may be made to supply current at the central office to a large number of pairs of connected lines simultaneously4 ,up to this point in this discussion it has been shown only how each battery served a single pair of connected lines and no others4 ,repeating ,coil3--,in ,fig4 #ach is shown how a single battery supplies current simultaneously to four different pairs of lines1 the lines of each pair being connected for conversation4 ,it is seen that the pairs of lines shown in this figure are arranged in each case in accordance with the system shown in ,fig4 #acj4 ,let us inquire why it is that1 although all of these four pairs of lines are connected with a common source of energy and are1 therefore1 all conductively joined1 the stations will be able to communicate in pairs without interference between the pairs4 ,in other words1 why is it that voice currents originating at ,station ,a will pass only to the receiver at ,station ,b and not to the receivers at ,station ,c or ,station ,h1 for instance8 ,the reason is that separate supply conductors lead from the points such as #a and #b at the junctions of the repeating-coil windings on each pair of circuits to the battery terminals1 and the resistance and impedance of the battery itself and of the common leads to it are so small that although the feeble voice currents originating in the pair of lines connecting ,station ,a and ,station ,b pass through the battery1 they are not able to alter the potential of the battery in any appreciable degree4 ,as a result1 therefore1 the supply wires leading from the common-battery terminals to the points #g and #h1 for instance1 cannot be subjected to any variations in potential by virtue of currents flowing through the battery from the points #a and #b of the lines joining ,station ,a and ,station ,b4 ,illustration3 ,,main ,,office1 ,,keystone ,,telephone ,,company1 ,,philadelphia1 ,,pa4 ,illustration3 ,fig4 #ach4 ,common ,source for ,many ,lines ,illustration3 ,fig4 #aci4 ,common ,source for ,many ,lines ,retardation ,coil--,single ,battery3--,in ,fig4 #aci is shown in similar manner the current supply from a single battery to four different pairs of lines1 the battery being associated with the lines by the combined impedance coil and condenser method1 which was specifically dealt with in connection with ,fig4 #acc4 ,the reasons why there will be no interference between the conversations carried on in the various pairs of connected lines in this case are the same as those just considered in connection with the system shown in ,fig4 #ach4 ,the impedance coils in this case serve to keep the telephone currents confined to their respective pairs of lines in which they originate1 and this same consideration applies to the system of ,fig4 #ach1 for each of the separate repeating-coil windings of ,fig4 #ach is in itself an impedance coil with respect to such currents as might leak away from one pair of lines on to another4 ,retardation ,coil--,double ,battery3--,the arrangement of feeding a number of pairs of lines according to the ,kellogg two-battery system is indicated in ,fig4 #adj1 which needs no further explanation in view of the description of the preceding figures4 ,it is interesting to note in this case that the left-hand battery serves only the left-hand lines and the right-hand battery only the right-hand lines4 ,as this is worked out in practice1 the left-hand battery is always connected to those lines which originate a call and the right-hand battery always to those lines that are called for4 ,the energy supplied to a calling line is always1 therefore1 from a different source than that which supplies a called line4 ,illustration3 ,fig4 #adj4 ,two ,sources for ,many ,lines ,illustration3 ,fig4 #ada4 ,current ,supply from ,distant ,point ,current ,supply from ,distant ,point4 ,sometimes it is convenient to supply current to a group of lines centering at a certain point from a source of current located at a distant point4 ,this is often the case in the so-called private branch exchange1 where a given business house or other institution is provided with its own switchboard for interconnecting the lines leading to the various telephones of that concern or institution among themselves1 and also for connecting them with lines leading to the city exchange4 ,it is not always easy or convenient to maintain at such private switchboards a separate battery for supplying the current needed by the local exchange4 ,in such cases the arrangement shown in ,fig4 #ada is sometimes employed4 ,this shows two pairs of lines connected by the impedance-coil system with common terminals #a and #b1 between which ordinarily the common battery would be connected4 ,instead of putting a battery between these terminals1 however1 at the local exchange1 a condenser of large capacity is connected between them and from these terminals circuit wires #c and #d are led to a battery of suitable voltage at a distant central office4 ,the condenser in this case is used to afford a short-circuit path for the voice currents that leak from one side of one pair of lines to the other1 through the impedance coils bridged across the line4 ,in this way the effect of the necessarily high resistance in the common leads #c and #d1 leading to the storage battery1 is overcome and the tendency to cross-talk between the various pairs of connected lines is eliminated4 ,frequently1 instead of employing this arrangement1 a storage battery of small capacity will be connected between the terminals #a and #b1 instead of the condenser1 and these will be charged over the wires #c and #d from a source of current at a distant point4 ,a consideration of the various methods of supplying current from a common source to a number of lines will show that it is essential that the resistance of the battery itself be very low4 ,it is also necessary that the resistance and the impedance of the common leads from the battery to the point of distribution to the various pairs of lines be very low1 in order that the voice currents which flow through them1 by virtue of the conversations going on in the different pairs of lines1 shall not produce any appreciable alteration in the difference of potential between the battery terminals4 ,,chapter ,,xiv ,,the ,,telephone ,,set ,we have considered what may be called the elemental parts of a complete telephone2 that is1 the receiver1 transmitter1 hook switch1 battery1 generator1 call bell1 condenser1 and the various kinds of coils which go to make up the apparatus by which one is enabled to transmit and receive speech and signals4 ,we will now consider the grouping of these various elements into a complete working organization known as a telephone4 ,before considering the various types it is well to state that the term telephone is often rather loosely used4 ,we sometimes hear the receiver proper called a telephone or a hand telephone4 ,since this was the original speaking telephone1 there is some reason for so calling the receiver4 ,the modern custom more often applies the term telephone to the complete organization of talking and signaling apparatus1 together with the associated wiring and cabinet or standard on which it is mounted4 ,the name telephone set is perhaps to be preferred to the word telephone1 since it tends to avoid misunderstanding as to exactly what is meant4 ,frequently1 also1 the telephone or telephone set is referred to as a subscriber's station equipment1 indicating the equipment that is to be found at a subscriber's station4 ,this1 as applying to a telephone alone1 is not proper1 since the subscriber's station equipment includes more than a telephone4 ,it includes the local wiring within the premises of the subscriber and also the lightning arrester and other protective devices1 if such exist4 ,to avoid confusion1 therefore1 the collection of talking and signaling apparatus with its wiring and containing cabinet or standard will be referred to in this work as a telephone or telephone set4 ,the receiver will1 as a rule1 be designated as such1 rather than as a telephone4 ,the term subscriber's station equipment will refer to the complete equipment at a subscriber's station1 and will include the telephone set1 the interior wiring1 and the protective devices1 together with any other apparatus that may be associated with the telephone line and be located within the subscriber's premises4 ,classification of ,sets4 ,telephones may be classified under two general headings1 magneto telephones and common-battery telephones1 according to the character of the systems in which they are adapted to work4 ,magneto ,telephone4 ,the term magneto telephone1 as it was originally employed in telephony1 referred to the type of instrument now known as a receiver1 particularly when this was used also as a transmitter4 ,as the use of this instrument as a transmitter has practically ceased1 the term magneto telephone has lost its significance as applying to the receiver1 and1 since many telephones are equipped with magneto generators for calling purposes1 the term magneto telephone has1 by common consent1 come to be used to designate any telephone including1 as a part of its equipment1 a magneto generator4 ,magneto telephones usually1 also1 include local batteries for furnishing the transmitter with current1 and this has led to these telephones being frequently called local battery telephones4 ,however1 a local battery telephone is not necessarily a magneto telephone and vice vers1 since sometimes magneto telephones have no local batteries and sometimes local battery telephones have no magnetos4 ,nearly all of the telephones which are equipped with magneto generators are1 however1 also equipped with local batteries for talking purposes1 and1 therefore1 the terms magneto telephone and local battery telephone usually refer to the same thing4 ,common-,battery ,telephone4 ,common-battery telephones1 on the other hand1 are those which have no local battery and no magneto generator1 all the current for both talking and signaling being furnished from a common source of current at the central office4 ,wall and ,desk ,telephones4 ,again we may classify telephones or telephone sets in accordance with the manner in which their various parts are associated with each other for use1 regardless of what parts are contained in the set4 ,we may refer to all sets adapted to be mounted on a wall or partition as wall telephones1 and to all in which the receiver1 transmitter1 and hook are provided with a standard of their own to enable them to rest on any flat surface1 such as a desk or table1 as desk telephones4 ,these latter are also referred to as portable telephones and as portable desk telephones4 ,in general1 magneto or local battery telephones differ from common-battery telephones in their component parts1 the difference residing principally in the fact that the magneto telephone always has a magneto generator and usually a local battery1 while the common-battery telephone has no local source of current whatever4 ,on the other hand1 the differences between wall telephones and desk telephones are principally structural1 and obviously either of these types of telephones may be for common-battery or magneto work4 ,the same component parts go to make up a desk telephone as a wall telephone1 provided the two instruments are adapted for the same class of service1 but the difference between the two lies in the structural features by which these same parts are associated with each other and protected from exposure4 ,illustration3 ,fig4 #adb4 ,magneto ,wall ,set ,illustration3 ,fig4 #adc4 ,magneto ,wall ,set ,magneto-,telephone ,sets4 ,wall4 ,in ,fig4 #adb is shown a familiar type of wall set4 ,the containing box includes within it all of the working parts of the apparatus except that which is necessarily left outside in order to be within the reach of the user4 ,fig4 #adc shows the same set with the door open4 ,this gives a good idea of the ordinary arrangement of the apparatus within4 ,it is seen that the polarized bell or ringer has its working parts mounted on the inside of the door or cover of the box1 the tapper projecting through so as to play between the gongs on the outside4 ,likewise the transmitter arm1 which supports the transmitter and allows its adjustment up and down to accommodate itself to the height of the user1 is mounted on the front of the door1 and the conductors leading to it may be seen fastened to the rear of the door in ,fig4 #adc4 ,in some wall sets the wires leading to the bell and transmitter are connected to the wiring of the rest of the set through the hinges of the door1 thus allowing the door to be opened and closed repeatedly without breaking off the wires4 ,in order to always insure positive electrical contact between the stationary and movable parts of the hinge a small wire is wound around the hinge pin1 one end being soldered to the stationary part and the other end to the movable part of the hinge4 ,in other forms of wall set the wires to the bell and the transmitter lead directly from the stationary portion of the cabinet to the back of the door1 the wires being left long enough to have sufficient flexibility to allow the door to be opened and closed without injuring the wires4 ,at the upper portion of the box there is mounted the hook switch1 this being1 in this case1 of the short lever type4 ,the lever of the hook projects through the side of the box so as to make the hook available as a support for the receiver4 ,immediately at the right of the hook switch is mounted the induction coil1 and immediately below this the generator1 its crank handle projecting through the right-hand side of the box so as to be available for use there4 ,the generator is usually mounted on a transverse shelf across the middle of the cabinet1 this shelf serving to form a compartment below it in which the dry battery of two or three cells is placed4 ,the wall telephone-set cabinets have assumed a multitude of forms4 ,when wet cells rather than dry cells were ordinarily employed1 as was the case up to about the year #ahie1 the magneto generator1 polarized bell1 and hook switch were usually mounted in a rectangular box placed at the top of a long backboard4 ,immediately below this on the backboard was mounted the transmitter arm1 and sometimes the base of this included the induction coil4 ,below this was the battery box1 this being a large affair usually adapted to accommodate two and sometimes three ordinary ,le,clanch cells side by side4 ,the dry cell has almost completely replaced the wet cell in this country1 and as a result1 the general type of wall set as shown in ,figs4 #adb and #adc1 has gradually replaced the old wet-cell type1 which was more cumbrous and unsightly4 ,it is usual on wall sets to provide some sort of a shelf1 as indicated in ,fig4 #adb1 for the convenience of the user in making notes and memoranda4 ,desk4 ,in the magneto desk-telephone sets1 the so-called desk stand1 containing the transmitter1 the receiver1 and the hook switch1 with the standard upon which they are mounted1 is shown in ,fig4 #add4 ,this desk stand evidently does not comprise the complete equipment for a magneto desk-telephone set1 since the generator1 polarized bell1 and battery are lacking4 ,the generator and bell are usually mounted together in a box1 either on the under side of the desk of the user or on the wall within easy reach of his chair4 ,connections are made between the apparatus in the desk stand proper and the battery1 generator1 and bell by means of flexible conducting cords1 these carrying a plurality of conductors1 as required by the particular circuit of the telephone in question4 ,such a complete magneto desk-telephone set is shown in ,fig4 #ade1 this being one of the types manufactured by the ,stromberg-,carlson ,manufacturing ,company4 ,illustration3 ,fig4 #add4 ,desk ,stand ,a great variety of arrangements of the various parts of magneto desk-telephone apparatus is employed in practice4 ,sometimes1 as shown in ,fig4 #ade1 the magneto bell box is equipped with binding posts for terminating all of the conductors in the cord1 the line wires also running to some of these binding posts4 ,in the magneto-telephone set illustrated the box is made large enough to accommodate only the generator and call bell1 and the batteries are mounted elsewhere1 as in a drawer of the desk1 while in other cases there is no other equipment but that shown in the cut1 the batteries being mounted within the magneto bell box itself4 ,in still other cases1 the polarized bell is contained in one box1 the generator in another1 the batteries in the drawer of the desk1 the induction coil being mounted either in the base of the desk stand1 in the bell box1 or in the generator box4 ,in such cases all of the circuits of the various scattered parts are wired to a terminal strip1 located at some convenient point1 this strip containing terminals for all the wires leading from the various parts and for the line wires themselves4 ,by combining the various wires on the terminals of this terminal strip1 the complete circuits of the telephone are built up4 ,in still other cases the induction coil is mounted on the terminal strip and separate wires or sets of wires are run to the polarized bell and generator1 to the desk stand itself1 and to the batteries4 ,these various arrangements are subject largely to the desire or personal ideas of the manufacturer or user4 ,all of them work on the same principle so far as the operation of the talking and signaling circuits is concerned4 ,illustration3 ,fig4 #ade4 ,magneto ,desk ,set ,circuits of ,magneto-,telephone ,sets4 ,magneto telephones1 whether of the wall or desk type1 may be divided into two general classes1 series and bridging1 according to whether the magnet of the bell is included in series or bridge relation with the telephone line when the hook is down4 ,series4 ,in the so-called series telephone line1 where several telephones are placed in series in a single line circuit1 the employment of the series type of telephone results in all of the telephone bells being in series in the line circuit4 ,this means that the voice currents originating in the telephones that are in use at a given time must pass in series through the magnets of the bells of the stations that are not in use4 ,in order that these magnets1 through which the voice currents must pass1 may interfere to as small a degree as possible with the voice currents1 it is common to employ low-resistance magnets in series telephones1 these magnets being wound with comparatively few turns and on rather short cores so that the impedance will be as small as possible4 ,likewise1 since the generators are required to ring all of the bells in series1 they need not have a large current output1 but must have sufficient voltage to ring through all of the bells in series and through the resistance of the line4 ,for this reason the generators are usually of the three-bar type and sometimes have only two bars4 ,in ,fig4 #adf are shown1 in simplified form1 the circuits of an ordinary series telephone4 ,the receiver in this is shown as being removed from the hook and thus the talking apparatus is brought into play4 ,the line wires #a and #b connect respectively to the binding posts #c and #d which form the terminals of the instrument4 ,when the hook is up1 the circuit between the binding posts #c and #d includes the receiver and the secondary winding of the induction coil1 together with one of the upper contacts #e of the switch hook and the hook lever itself4 ,this completes the circuit for receiving speech4 ,the hook switch is provided with another upper contact #f1 between which and the contact #e is connected the local circuit containing the transmitter1 the battery1 and the primary of the induction coil in series4 ,the primary and the secondary windings are connected together at one end and connected with the switch contact #e1 as shown4 ,it is thus seen that when the hook is up the circuit through the receiver is automatically closed and also the local circuit containing the primary1 the battery1 and the transmitter4 ,thus1 all the conditions for transmitting and receiving speech are fulfilled4 ,fig4 #adf4 ,circuit of ,series ,magneto ,set ,when the hook is down1 however1 the receiving and transmitting circuits are broken1 but another circuit is completed by the engagement of the hook-switch lever with the lower hook contact #g4 ,between this contact and one side of the line is connected the polarized ringer and the generator4 ,with the hook down1 therefore1 the circuit may be traced from the line wire #a to binding post #c1 thence through the generator shunt to the call bell1 and thence through the lower switching contact #g to the binding post #d and line wire #b4 ,the generator shunt1 as already described in ,chapter ,,viii1 normally keeps the generator shunted out of circuit4 ,when1 however1 the generator is operated the shunt is broken1 which allows the armature of the generator to come into the circuit in series with the winding of the polarized bell4 ,the normal shunting of the generator armature from the circuit of the line is advantageous in several ways4 ,in the first place1 the impedance of the generator winding is normally cut out of the circuit so that in the case of a line with several stations the talking or voice currents do not have to flow through the generator armatures at the stations which are not in use4 ,again1 the normal shunting of the generator tends to save the generator armature from injury by lightning4 ,illustration3 ,fig4 #adg4 ,circuit of ,series ,magneto ,set4 ,the more complete circuits of a series magneto telephone are shown in ,fig4 #adg4 ,in this the line binding posts are shown as #a and #b4 ,at the bottom of the telephone cabinet are four other binding posts marked #c1 #d1 #e1 and #f4 ,of these #c and #d serve for the receiver terminals and #e and #f for the transmitter and battery terminals4 ,the circuits of this diagram will be found to be essentially the same as those of ,fig4 #adf1 except that they are shown in greater detail4 ,this particular type of circuit is one commonly employed where the generator1 ringer1 hook switch1 and induction coil are all mounted in a so-called magneto bell box at the top of the instrument1 and where the transmitter is mounted on an arm just below this box1 and the battery in a separate compartment below the transmitter4 ,the only wiring that has to be done between the bell box and the other parts of the instrument in assembling the complete telephone is to connect the receiver to the binding posts #c and #d and to connect the battery and transmitter circuit to the binding posts #e and #f4 ,bridging4 ,in other cases1 where several telephones are placed on a single-line circuit1 the bells are arranged in multiple across the line4 ,for this reason their magnets are wound with a very great number of turns and consequently to a high resistance4 ,in order to further increase the impedance1 the cores are made long and heavy4 ,since the generators on these lines must be capable of giving out a sufficient volume of current to divide up between all of the bells in multiple1 it follows that these generators must have a large current output1 and at the same time a sufficient voltage to ring the bells at the farthest end of the line4 ,such instruments are commonly called bridging instruments1 on account of the method of connecting their bells across the circuit of the line4 ,illustration3 ,fig4 #adh4 ,circuit of ,bridging ,magneto ,set ,the fundamental characteristic of the bridging telephone is that it contains three possible bridge paths across the line wires4 ,the first of these bridge paths is through the talking apparatus1 the second through the generator1 and the third through the ringer4 ,this is shown in simplified form in ,fig4 #adh4 ,the talking apparatus is associated with the two upper contacts of the hook switch in the usual manner and needs no further description4 ,the generator is the second separate bridge path1 normally open1 but adapted to be closed when the generator is operated1 this automatic closure being performed by the movement of the crank shaft4 ,the third bridge contains the polarized bell1 and this1 as a rule1 is permanently closed4 ,sometimes1 however1 the arrangement is such that the bell path is normally closed through the switch which is operated by the generator crank shaft1 and this path is automatically broken when the generator is operated1 at which time1 also1 the generator path is automatically closed4 ,this arrangement brings about the result that the generator never can ring its own bell1 because its switch always operates to cut out the bell at its own station just before the generator itself is cut into the circuit4 ,in ,fig4 #adi is shown the complete circuit of a bridging telephone4 ,the circuit given in this figure is for a local-battery wall set similar in type to that shown in ,figs4 #adb and #adc4 ,a simplified diagrammatic arrangement is shown in the lower left-hand corner of this figure1 and from a consideration of this it will be seen that the bell circuit across the line is normally completed through the two right-hand normally closed contacts of the switch on the generator4 ,when1 however1 the generator is operated these two contacts are made to disengage each other while the long spring of the generator switch engages the left-hand spring and thus brings the generator itself into the circuit4 ,illustration3 ,fig4 #adi4 ,circuit of ,bridging ,magneto ,set ,of the three binding posts1 #a1 #b1 and #c1 at the top of ,fig4 #adi1 #a and #b are for connecting with the line wires1 while #h is for a ground connection1 acting in conjunction with the lightning arrester mounted at the top of the telephone and indicated at #d in ,fig4 #adi4 ,this has no function in talking or ringing1 and will be referred to more fully in ,chapter ,,xix4 ,suffice it to say at this point that these arresters usually consist of two conducting bodies1 one connected permanently to each of the line binding posts1 and a third conducting body connected to the ground binding post4 ,these three conducting bodies are in close proximity but carefully insulated from each other2 the idea being that when the line wires are struck by lightning or subjected otherwise to a dangerous potential1 the charge on the line will jump across the space between the conducting bodies and pass harmlessly to ground4 ,,note4 ,the student should practice making simplified diagrams from actual wiring diagrams4 ,the difference between the two is that one is laid out for ease in understanding it1 while the other is laid out to show the actual course of the wires as installed4 ,if the large detailed circuit of ,fig4 #adi be compared with the small theoretical circuit in the same figure1 the various conducting paths will be found to be the same4 ,such a simplified circuit does more to enable one to grasp the fundamental scheme of a complex circuit than much description1 since it shows at a glance the general arrangement4 ,the more detailed circuits are1 however1 necessary to show the actual paths followed by the wiring4 ,the circuits of desk stands do not differ from those of wall sets in any material degree1 except as may be necessitated by the fact that the various parts of the telephone set are not all mounted in the same cabinet or on the same standard4 ,to provide for the necessary relative movement between the desk stand and the other portions of the set1 flexible conductors are run from the desk stand itself to the stationary portions of the equipment1 such as the battery and the parts contained in the generator and bell box4 ,illustration3 ,fig4 #aej4 ,circuit of ,bridging ,magneto ,desk ,set ,in ,fig4 #aej is shown the circuit of the ,stromberg-,carlson magneto desk-telephone set1 illustrated in ,fig4 #ade4 ,this diagram needs no explanation in view of what has already been said4 ,the conductors1 leading from the desk-stand group of apparatus to the bell-box group of apparatus1 are grouped together in a flexible cord1 as shown in ,fig4 #ade1 and are connected respectively to the various binding posts or contact points within the desk stand at one end and at the base of the bell box at the other end4 ,these flexible conductors are insulated individually and covered by a common braided covering4 ,they usually are individualized by having a colored thread woven into their insulating braid1 so that it is an easy matter to identify the two ends of the same conductor at either end of the flexible cord or cable4 ,illustration3 ,fig4 #aea4 ,common-,battery ,wall ,set ,illustration3 ,fig4 #aeb4 ,common-,battery ,wall ,set ,common-,battery ,telephone ,sets4 ,owing to the fact that common-battery telephones contain no sources of current1 they are usually somewhat simpler than the magneto type4 ,the component parts of a common-battery telephone1 whether of the wall or desk type1 are the transmitter1 receiver1 hook switch1 polarized bell1 condenser1 and sometimes an induction coil4 ,the purpose of the condenser is to prevent direct or steady currents from passing through the windings of the ringer while the ringer is connected across the circuit of the line during the time when the telephone is not in use4 ,the requirements of common-battery signaling demand that the ringer shall be connected with the line so as to be receptive of a call at any time while the telephone is not in use4 ,the requirements also demand that no conducting path shall normally exist between the two sides of the line4 ,these two apparently contradictory requirements are met by placing a condenser in series with the ringer so that the ringer will be in a path that will readily transmit the alternating ringing currents sent out from the central-office generator1 while at the same time the condenser will afford a complete bar to the passage of steady currents4 ,sometimes the condenser is also used as a portion of the talking apparatus1 as will be pointed out4 ,illustration3 ,,main ,,office1 ,,kansas ,,city ,,home ,,telephone ,,co41 ,,kansas ,,city1 ,,mo4 ,wall4 ,in ,figs4 #aea and #aeb are given two views of a characteristic form of common-battery wall-telephone set1 made by the ,stromberg-,carlson ,manufacturing ,company4 ,the common-battery wall set has usually taken this general form4 ,in it the transmitter is mounted on an adjustable arm at the top of the backboard1 while the box containing the bell and all working parts of the instrument is placed below the transmitter1 the top of the box affording a shelf for writing purposes4 ,in ,fig4 #aea are shown the hook switch and the receiver2 just below these may be seen the magnets of the polarized bell1 back of which is shown a rectangular box containing the condenser4 ,immediately in front of the ringer magnets is the induction coil4 ,illustration3 ,fig4 #aec4 ,stromberg-,carlson ,common-,battery ,wall ,set ,in ,fig4 #aec are shown the details of the circuit of this instrument4 ,this figure also includes a simplified circuit arrangement from which the principles involved may be more readily understood4 ,it is seen that the primary of the induction coil and the transmitter are included in series across the line4 ,the secondary of the induction coil1 in series with the receiver1 is connected also across the line in series with a condenser and the transmitter4 ,hotel4 ,sometimes1 in order to economize space1 the shelf of common-battery wall sets is omitted and the entire apparatus mounted in a small rectangular box1 the front of which carries the transmitter mounted on the short arm or on no arm at all4 ,such instruments are commonly termed hotel sets1 because of the fact that their use was first confined largely to the rooms in hotels4 ,later1 however1 these instruments have become very popular in general use1 particularly in residences4 ,sometimes the boxes or cabinets of these sets are made of wood1 but of recent years the tendency has been growing to make them of pressed steel4 ,the steel box is usually finished in black enamel1 baked on1 the color being sometimes varied to match the color of the surrounding woodwork4 ,in ,figs4 #aed and #aee are shown two views of a common-battery hotel set manufactured by the ,dean ,electric ,company4 ,such sets are extremely neat in appearance and have the advantage of taking up little room on the wall and the commercial advantage of being light and compact for shipping purposes4 ,a possible disadvantage of this type of instrument is the somewhat crowded condition which necessarily follows from the placing of all the parts in so confined a space4 ,this interferes somewhat with the accessibility of the various parts1 but great ingenuity has been manifested in making the parts readily get-at-able in case of necessity for repairs or alterations4 ,illustration3 ,fig4 #aed4 ,steel ,box ,hotel ,illustration3 ,fig4 #aee4 ,steel ,box ,hotel ,set ,desk4 ,the common-battery desk telephone presents a somewhat simpler problem than the magneto desk telephone for the reason that the generator and local battery1 the two most bulky parts of a magneto telephone1 do not have to be provided for4 ,some companies1 in manufacturing desk stands for common-battery purposes1 mount the condenser and the induction coil or impedance coil1 or whatever device is used in connection with the talking circuit1 in the base of the desk stand itself1 and mount the polarized ringer and the condenser used for ringing purposes in a separate bell box adapted to be mounted on the wall or some portion of the desk4 ,other companies mount only the transmitter1 receiver1 and hook switch on the desk stand proper and put the condenser or induction coil1 or other device associated with the talking circuit1 in the bell box4 ,there is little to choose between the two general practices4 ,the number of conducting strands in the flexible cord is somewhat dependent on the arrangement of the circuit employed4 ,illustration3 ,fig4 #aef4 ,common-,battery ,desk ,set ,illustration3 ,fig4 #aeg4 ,bell for ,common-,battery ,desk ,set4 ,the ,kellogg ,switchboard and ,supply ,company is one which places all the parts1 except the polarized ringer and the associated condenser1 in the desk stand itself4 ,in ,fig4 #aef is shown a bottom view of the desk stand with the bottom plate removed4 ,in the upper portion of the circle of the base is shown a small condenser which is placed in the talking circuit in series with the receiver4 ,in the right-hand portion of the circle of the base is shown a small impedance coil1 which is placed in series with the transmitter but in shunt relation with the condenser and the receiver4 ,illustration3 ,fig4 #aeh4 ,bell for ,common-,battery ,desk ,set ,in ,figs4 #aeg and #aeh are shown two views of the type of bell box employed by the ,kellogg ,company in connection with the common-battery desk sets1 this box being of pressed-steel construction and having a removable lid1 as shown in ,fig4 #aeh1 by which the working parts of the ringer are made readily accessible1 as are also the terminals for the cord leading from the desk stand and for the wires of the line circuit4 ,the condenser that is placed in series with the ringer is also mounted in this same box4 ,by employing two condensers1 one in the bell box large enough to transmit ringing currents and the other in the base of the desk stand large enough only to transmit voice currents1 a duplication of condensers is involved1 but it has the corresponding advantages of requiring only two strands to the flexible cord leading from the bell box to the desk stand proper4 ,illustration3 ,fig4 #aei4 ,microtelephone ,set ,a form of desk-telephone set that is used largely abroad1 but that has found very little use in this country1 is shown in ,fig4 #aei4 ,in this the transmitter and the receiver are permanently attached together1 the receiver being of the watch-case variety and so positioned relatively to the transmitter that when the receiver is held at the ear1 the mouthpiece of the transmitter will be just in front of the lips of the user4 ,in order to maintain the transmitter in a vertical position during use1 this necessitates the use of a curved mouthpiece as shown4 ,this transmitter and receiver so combined is commonly called1 in this country1 the microtelephone set1 although there seems to be no logical reason for this name4 ,the combined transmitter and receiver1 instead of being supported on an ordinary form of hook switch1 are supported on a forked bracket as shown1 this bracket serving to operate the switch springs which are held in one position when the bracket is subjected to the weight of the microtelephone1 and in the alternate position when relieved therefrom4 ,this particular microtelephone set is the product of the ,l4,m4 ,ericsson ,telephone ,manufacturing ,company1 of ,buffalo1 ,new ,york4 ,the circuits of such sets do not differ materially from those of the ordinary desk telephone set4 ,illustration3 ,fig4 #afj4 ,kellogg ,common-,battery ,desk ,set ,illustration3 ,fig4 #afa4 ,dean ,common-,battery ,set ,circuits of ,common-,battery ,telephone ,sets4 ,the complete circuits of the ,kellogg desk-stand arrangement are shown in ,fig4 #afj1 the desk-stand parts being shown at the left and the bell-box parts at the right4 ,as is seen1 but two conductors extend from the former to the latter4 ,a simplified theoretical sketch is also shown in the upper right-hand corner of this figure4 ,the details of the common-battery telephone circuits of the ,dean ,electric ,company are shown in ,fig4 #afa4 ,this involves the use of the balanced ,wheatstone bridge4 ,the only other thing about this circuit that needs description1 in view of what has previously been said about it1 is that the polarized bell is placed in series with a condenser so that the two sides of the circuit may be insulated from each other while the telephone is not in use1 and yet permit the passage of ringing current through the bell4 ,illustration3 ,fig4 #afb4 ,monarch ,common-,battery ,wall ,set ,the use of the so-called direct-current receiver has brought about a great simplification in the common-battery telephone circuits of several of the manufacturing companies4 ,by this use the transmitter and the receiver are placed in series across the line1 this path being normally opened by the hook-switch contacts4 ,the polarized bell and condenser are placed in another bridge path across the line1 this path not being affected by the hook-switch contacts4 ,all that there is to such a complete common-battery telephone set1 therefore1 is a receiver1 transmitter1 hook switch1 bell1 condenser1 and cabinet1 or other support4 ,the extreme simplicity of the circuits of such a set is illustrated in ,fig4 #afb1 which shows how the ,monarch ,telephone ,manufacturing ,company connect up the various parts of their telephone set1 using the direct-current receiver already described in connection with ,fig4 #ed4 ,illustration3 ,,ventilating ,,plant ,,for ,,large ,,telephone ,,office ,,building ,,chapter ,,xv ,,non-,,selective ,,party-,,line ,,systems ,a party line is a line that is for the joint use of several stations4 ,it is1 therefore1 a line that connects a central office with two or more subscribers' stations1 or where no central office is involved1 a line that connects three or more isolated stations with each other4 ,the distinguishing feature of a party line1 therefore1 is that it serves more than two stations1 counting the central office1 if there is one1 as a station4 ,strictly speaking1 the term party line should be used in contradistinction to the term private line4 ,companies operating telephone exchanges1 however1 frequently lease their wires to individuals for private use1 with no central-office switchboard connections1 and such lines are1 by common usage1 referred to as private lines4 ,such lines may be used to connect two or more isolated stations4 ,a private line1 in the parlance of telephone exchange working1 may1 therefore1 be a party line1 as inconsistent as this may seem4 ,a telephone line that is connected with an exchange is an exchange line1 and it is a party line if it has more than one station on it4 ,it is an individual line or a single party line if it has but a single station on it4 ,a line which has no central-office connection is called an isolated line1 and it is a party line if it has more than two stations on it4 ,the problem of mere speech transmission on party lines is comparatively easy1 being scarcely more complex than that involved in private or single party lines4 ,this is not true1 however1 of the problem of signaling the various stations4 ,this is because the line is for the common use of all its patrons or subscribers1 as they are termed1 and the necessity therefore exists that the person sending a signal1 whether operator or subscriber1 shall be able in some way to inform a person at the desired station that the call is intended for that station4 ,there are two general ways of accomplishing this purpose4 7#a7 ,the first and simplest of these ways is to make no provision for ringing any one bell on the line to the exclusion of the others1 and thus allow all bells to ring at once whenever any station on the line is wanted4 ,where this is done1 in order to prevent all stations from answering1 it is necessary1 in some way1 to convey to the desired station the information that the call is intended for that station1 and to all of the other stations the information that the call is not intended for them4 ,this is done on such lines by what is called code ringing1 the code consisting of various combinations of long and short rings4 7#b7 ,the other and more complex way is to arrange for selective ringing1 so that the person sending the call may ring the bell at the station desired1 allowing the bells at all the other stations to remain quiet4 ,illustration3 ,fig4 #afc4 ,grounded-,circuit ,series ,line ,these two general classes of party-line systems may1 therefore1 be termed non-selective and selective systems4 ,non-selective party lines are largely used both on lines having connection with a central office1 and through the central office the privilege of connection with other lines1 and on isolated lines having no central-office connection4 ,the greatest field of usefulness of non-selective lines is in rural districts and in connection with exchanges in serving rather sparsely settled districts where the cost of individual lines or even lines serving but a few subscribers1 is prohibitive4 ,non-selective telephone party lines most often employ magneto telephones4 ,the early forms of party lines employed the ordinary series magneto telephone1 the bells being of low resistance and comparatively low impedance1 while the generators were provided with automatic shunting devices1 so that their resistance would normally be removed from the circuit of the line4 ,series ,systems4 ,the general arrangement of a series party line employing a ground return is shown in ,fig4 #afc4 ,in this three ordinary series instruments are connected together in series1 the end stations being grounded1 in order to afford a return path for the ringing and voice currents4 ,illustration3 ,fig4 #afd4 ,metallic-,circuit ,series ,line ,in ,fig4 #afd there is shown a metallic-circuit series line on which five ordinary series telephones are placed in series4 ,in this no ground is employed1 the return being through a line wire1 thus making the circuit entirely metallic4 ,illustration3 ,fig4 #afe4 ,series ,party ,line ,the limitations of the ordinary series party line may be best understood by reference to ,fig4 #afe1 in which the circuits of three series telephones are shown connected with a single line4 ,the receiver of ,station ,a is represented as being on its hook1 while the receivers of ,stations ,b and ,c are removed from their hooks1 as when the subscribers at those two stations are carrying on a conversation4 ,the hook switches of ,stations ,b and ,c being in raised positions1 the generators and ringers of those stations are cut out of the circuit1 and only the telephone apparatus proper is included1 but the hook switch of ,station ,a being depressed by the weight of its receiver1 includes the ringer of that station in circuit1 and through this ringer1 therefore1 the voice currents of ,stations ,b and ,c must pass4 ,the generator of ,station ,a is not in the circuit of voice currents1 however1 because of the automatic shunt with which the generator is provided1 as described in ,chapter ,,viii4 ,a slight consideration of the series system as shown in this figure1 indicates that the voice currents of any two stations that are in use1 must pass 7as indicated by the heavy lines7 through the ringers of all the stations that are not in use2 and when a great number of stations are placed upon a single line1 as has been frequently the case1 the impedance offered by these ringers becomes a serious barrier to the passage of the voice currents4 ,this defect in the series party line is fundamental1 as it is obvious that the ringers must be left in the circuit of the stations which are not in use1 in order that those stations may always be in such condition as to be able to receive a call4 ,this defect may in some measure be reduced by making the ringers of low impedance4 ,this is the general practice with series telephones1 the ringers ordinarily having short cores and a comparatively small number of turns1 the resistance being as a rule about #hj ohms4 ,bridging ,systems4 ,very much better than the series plan of party-line connections1 is the arrangement by which the instruments are placed in bridges across the line1 such lines being commonly known as bridged or bridging lines4 ,this was first strongly advocated and put into wide practical use by ,j4,j4 ,carty1 now the ,chief ,engineer of the ,american ,telephone and ,telegraph ,company4 ,a simple illustration of a bridging telephone line is shown in ,fig4 #aff1 where the three telephones shown are each connected in a bridge path from the line wire to ground1 a type known as a grounded bridging line4 ,its use is very common in rural districts4 ,a better arrangement is shown in ,fig4 #afg1 which represents a metallic-circuit bridging line1 three telephone instruments being shown in parallel or bridge paths across the two line wires4 ,the actual circuit arrangements of a bridging party line are better shown in ,fig4 #afh4 ,there are three stations and it will be seen that at each station there are three possible bridges1 or bridge paths1 across the two limbs of the line4 ,the first of these bridges is controlled by the hook switch and is normally open4 ,when the hook is raised1 however1 this path is closed through the receiver and secondary of the induction coil1 the primary circuit being also closed so as to include the battery and transmitter4 ,this constitutes an ordinary local-battery talking set4 ,illustration3 ,fig4 #aff4 ,grounded ,bridging ,line ,illustration3 ,fig4 #afg4 ,metallic ,bridging ,line ,illustration3 ,fig4 #afh4 ,metallic ,bridging ,line ,a second bridge at each station is led through the ringer or call-bell1 and this1 in most bridging telephones1 is permanently closed1 the continuity of this path between the two limbs of the line not being affected either by the hook switch or by the automatic switch in connection with the generator4 ,a third bridge path at each station is led through the generator4 ,this1 as indicated1 is normally open1 but the automatic cut-in switch of the generator serves1 when the generator is operated1 to close its path across the line1 so that it may send its currents to the line and ring the bells of all the stations4 ,when any generator is operated1 its current divides and passes over the line wires and through all of the ringers in multiple4 ,it is seen1 therefore1 that the requirements for a bridging generator are that it shall be capable of generating a large current1 sufficient when divided up amongst all the bells to ring each of them2 and that it shall be capable of producing a sufficient voltage to send the required current not only to the near-by stations1 but to the stations at the distant end of the line4 ,it might seem at first that the bridging system avoided one difficulty only to encounter another4 ,it clearly avoids the difficulty of the series system in that the voice currents1 in order to reach distant stations1 do not have to pass through all of the bells of the idle stations in series4 ,there is1 however1 presented at each station a leakage path through the bell bridged across the line1 through which it would appear the voice currents might leak uselessly from one side of the line to the other and not pass on in sufficient volume to the distant station4 ,this difficulty is1 however1 more apparent than real4 ,it is found that1 by making the ringers of high impedance1 the leakage of voice currents through them from one side of the line to the other is practically negligible4 ,it is obvious that in a heavily loaded bridged line1 the bell at the home station1 that is at the station from which the call is being sent1 will take slightly more than its share of the current1 and it is also obvious that the ringing of the home bell performs no useful function4 ,the plan is frequently adopted1 therefore1 of having the operation of the generator serve to cut its own bell out of the circuit4 ,the arrangement by which this is done is clearly shown in ,fig4 #afi4 ,the circuit of the bell is normally complete across the line1 while the circuit of the generator is normally open4 ,when1 however1 the generator crank is turned these conditions are reversed1 the bell circuit being broken and the generator circuit closed1 so as to allow its current all to pass the line4 ,this feature of having the local bell remain silent upon the operation of its own generator is also of advantage because other parties at the same station are not disturbed by the ringing of the bell when a call is being made by that station4 ,a difficulty encountered on non-selective bridging party lines1 which at first seems amusing rather than serious1 but which nevertheless is often a vexatious trouble1 is that due to the propensity of some people to listen in on the line on hearing calls intended for other than their own stations4 ,people whose ethical standards would not permit them to listen at1 or peep through1 a keyhole1 often engage in this telephonic eavesdropping4 ,frequently1 not only one but many subscribers will respond to a call intended for others and will listen to the ensuing conversation4 ,this is disadvantageous in several respects3 ,it destroys the privacy of conversation between any two parties2 it subjects the local batteries to an unnecessary and useless drain2 and it greatly impairs the ringing efficiency of the line4 ,the reason for this interference with ringing is that the presence of the low-resistance receivers across the line allows the current sent out by any of the generators to pass in large measure through the receivers1 thus depriving the ringers1 which are of comparatively high resistance and impedance1 of the energy necessary to operate them4 ,as a result of this it is frequently impossible for one party to repeat the call for another because1 during the interval between the first and second call1 a number of parties remove their receivers from their hooks in order to listen4 ,ring-off or clearing-out signals are likewise interfered with4 ,illustration3 ,fig4 #afi4 ,circuits of ,bridging ,station ,a partial remedy for this interference with ringing1 due to eavesdropping1 is to introduce a low-capacity condenser into the receiver circuit at each station1 as shown in ,fig4 #afi4 ,this does not seriously interfere with the speech transmission since the condensers will readily transmit the high-frequency voice currents4 ,such condensers1 however1 have not sufficient capacity to enable them readily to transmit the low-frequency ringing currents and hence these are forced1 in large measure1 to pass through the bells for which they are intended rather than leaking through the low-resistance receiver paths4 ,the best condenser for this use is of about #a_/b-microfarad capacity1 which is ample for voice-transmitting purposes1 while it serves to effectively bar the major portion of the generator currents4 ,a higher capacity condenser would carry the generator currents much more readily and thus defeat the purpose for which it was intended4 ,in order that the requisite impedance may be given to the ringers employed for bridging party lines1 it is customary to make the cores rather long and of somewhat larger diameter than in series ringers and at the same time to wind the coils with rather fine wire so as to secure the requisite number of turns4 ,bridging bells are ordinarily wound to a resistance of #a1jjj or #a1fjj ohms1 these two figures having become standard practice4 ,it is not1 however1 the high resistance so much as the high impedance that is striven for in bridging bells2 it is the number of turns that is of principal importance4 ,as has already been stated1 the generators used for bridging lines are made capable of giving a greater current output than is necessary in series instruments1 and for this purpose they are usually provided with at least four1 and usually five1 bar magnets4 ,the armature is made correspondingly long and is wound1 as a rule1 with about ,no4 #cc wire4 ,sometimes where a bridged party line terminates in a central-office switchboard it is desired to so operate the line that the subscribers shall not be able to call up each other1 but shall1 instead1 be able to signal only the central-office operator1 who1 in turn1 will be enabled to call the party desired1 designating his station by a suitable code ring4 ,one common way to do this is to use biased bells instead of the ordinary polarized bells4 ,in order that the bells may not be rung by the subscribers' generators1 these generators are made of the direct-current type and these are so associated with the line that the currents which they send out will be in the wrong direction to actuate the bells4 ,on the other hand1 the central-office generator is of direct-current type and is associated with the line in the right direction to energize the bells4 ,thus any subscriber on the line may call the central office by merely turning his generator crank1 which action will not ring the bells of the subscribers on the line4 ,the operator will then be able to receive the call and in turn send out currents of the proper direction to ring all the bells and1 by code1 call the desired party to the telephone4 ,illustration3 ,,one ,,wing ,,of ,,operating ,,room1 ,,berlin1 ,,germany ,ultimate ,capacity #bd1jjj ,subscribers' ,lines and #b1ajj ,trunk ,lines4 ,siemens-,halske ,equipment4 ,note ,horizontal ,disposal of ,multiple ,signal ,code4 ,the code by which stations are designated on non-selective party lines usually consists in combinations of long and short rings similar to the dots and dashes in the ,morse code4 ,thus1 one short ring may indicate ,station ,no4 #a2 two short rings ,station ,no4 #b2 and so on up to1 say1 five short rings1 indicating ,station ,no4 #e4 ,it is not good practice to employ more than five successive short rings because of the confusion which often arises in people's minds as to the number of rings that they hear4 ,when1 therefore1 the number of stations to be rung by code exceeds five1 it is better to employ combinations of long and short rings1 and a good way is to adopt a partial decimal system1 omitting the numbers higher than five in each ten1 and employing long rings to indicate the tens digits and short rings to indicate the units digit1 ,table ,x4 ,,table ,x ,signal ,code ---------------------------------------- ------------------ ,,station ,,number,,ring ,,station ,,number,,ring #a #a short #ab #a long1 #b short #b #b short #ac #a long1 #c short #c #c short #ad #a long1 #d short #d #d short #ae #a long1 #e short #e #e short #ba #b long1 #a short #aa #a long1 #a short#bb #b long1 #b short ---------------------------------------- ------------------ ,other arrangements are often employed and by almost any of them a great variety of readily distinguishable signals may be secured4 ,the patrons of such lines learn to distinguish1 with comparatively few errors1 between the calls intended for them and those intended for others1 but frequently they do not observe the distinction1 as has already been pointed out4 ,limitations4 ,with good telephones the limit as to the number of stations that it is possible to operate upon a single line is usually due more to limitations in ringing than in talking4 ,as the number of stations is increased indefinitely a condition will be reached at which the generators will not be able to generate sufficient current to ring all of the bells1 and this condition is likely to occur before the talking efficiency is seriously impaired by the number of bridges across the line4 ,neither of these considerations1 however1 should determine the maximum number of stations to be placed on a line4 ,the proper limit as to the number of stations is not the number that can be rung by a single generator1 or the number with which it is possible to transmit speech properly1 but rather the number of stations that may be employed without causing undue interference between the various parties who may desire to use the line4 ,overloaded party lines cause much annoyance1 not only for the reason that the subscribers are often not able to use the line when they want it1 but also1 in non-selective lines1 because of the incessant ringing of the bells1 and the liability of confusion in the interpretation of the signaling code1 which of course becomes more complex as the number of stations increases4 ,the amount of business that is done over a telephone line is usually referred to as the traffic4 ,it will be understood1 however1 in considering party-line working that the number of calls per day or per hour1 or per shorter unit1 is not the true measure of the traffic and1 therefore1 not the true measure of the amount of possible interference between the various subscribers on the line4 ,an almost equally great factor is the average length of the conversation4 ,in city lines1 that is1 in lines in city exchanges1 the conversation is usually short and averages perhaps two minutes in duration4 ,in country lines1 however1 serving people in rural districts1 who have poor facilities for seeing each other1 particularly during the winter time1 the conversations will average very much longer4 ,in rural communities the people often do much of their visiting by telephone1 and conversations of half an hour in length are not unusual4 ,it is obvious that under such conditions a party line having a great many stations will be subject to very grave interference between the parties1 people desiring to use the line for business purposes often being compelled to wait an undue time before they may secure the use of the line4 ,it is obvious1 therefore1 that the amount of traffic on the line1 whether due to many short conversations or to a comparatively few long ones1 is the main factor that should determine the number of stations that1 economically1 may be placed on a line4 ,the facilities also for building lines enter as a factor in this respect1 since it is obvious that in comparatively poor communities the money may not be forthcoming to build as many lines as are needed to properly take care of the traffic4 ,a compromise is1 therefore1 often necessary1 and the only rule that may be safely laid down is to place as few parties on a given line as conditions will admit4 ,no definite limit may be set to apply to all conditions but it may be safely stated that under ordinary circumstances no more than ten stations should be placed on a non-selective line4 ,twenty stations are1 however1 common1 and sometimes forty and even fifty have been connected to a single line4 ,in such cases the confusion which results1 even if the talking and the ringing efficiency are tolerable1 makes the service over such overloaded lines unsatisfactory to all concerned4 ,,chapter ,,xvi ,,selective ,,party-,,line ,,systems ,the problem which confronts one in the production of a system of selective ringing on party lines is that of causing the bell of any chosen one of the several parties on a circuit to respond to a signal sent out from the central office without sounding any of the other bells4 ,this1 of course1 must be accomplished without interfering with the regular functions of the telephone line and apparatus4 ,by this is meant that the subscribers must be able to call the central office and to signal for disconnection when desired1 and also that the association of the selective-signaling devices with the line shall not interfere with the transmission of speech over the line4 ,a great many ways of accomplishing selective ringing on party lines have been proposed1 and a large number of them have been used4 ,all of these ways may be classified under four different classes according to the underlying principle involved4 ,classification4 7#a7 ,polarity systems are so called because they depend for their operation on the use of bells or other responsive devices so polarized that they will respond to one direction of current only4 ,these bells or other devices are so arranged in connection with the line that the one to be rung will be traversed by current in the proper direction to actuate it1 while all of the others will either not be traversed by any current at all1 or by current in the wrong direction to cause their operation4 7#b7 ,the harmonic systems have for their underlying principle the fact that a pendulum or elastic reed1 so supported as to be capable of vibrating freely1 will have one particular rate of vibration which it may easily be made to assume4 ,this pendulum or reed is placed under the influence of an electromagnet associated with the line1 and owing to the fact that it will vibrate easily at one particular rate of vibration and with extreme difficulty at any other rate1 it is clear that for current impulses of a frequency corresponding to its natural rate the reed will take up the vibration1 while for other frequencies it will fail to respond4 ,selection on party lines by means of this system is provided for by tuning all of the reeds on the line at different rates of vibration and is accomplished by sending out on the line ringing currents of proper frequency to ring the desired bell4 ,the current-generating devices for ringing these bells are capable of sending out different frequencies corresponding respectively to the rates of vibration of each of the vibrating reed tongues4 ,to select any one station1 therefore1 the current frequency corresponding to the rate of vibration of the reed tongue at that station is sent and this1 being out of tune with the reed tongues at all of the other stations1 operates the tongue of the desired station1 but fails to operate those at all of the other stations4 7#c7 ,in the step-by-step system the bells on the line are normally not in operative relation with the line and the bell of the desired party on the line is made responsive by sending over the line a certain number of impulses preliminary to ringing it4 ,these impulses move step-by-step mechanisms at each of the stations in unison1 the arrangement being such that the bells at the several stations are each made operative after the sending of a certain number of preliminary impulses1 this number being different for all the stations4 7#d7 ,the broken-line systems are new in telephony and for certain fields of work look promising4 ,in these the line circuit is normally broken up into sections1 the first section terminating at the first station out from the central office1 the second section at the second station1 and so on4 ,when the line is in its normal or inactive condition only the bell at the first station is so connected with the line circuit as to enable it to be rung1 the line being open beyond4 ,sending a single preliminary impulse will1 however1 operate a switching device so as to disconnect the bell at the first station and to connect the line through to the second station4 ,this may be carried out1 by sending the proper number of preliminary impulses1 so as to build up the line circuit to the desired station1 after which the sending of the ringing current will cause the bell to ring at that station only4 ,polarity ,method4 ,the polarity method of selective signaling on party lines is probably the most extensively used4 ,the standard selective system of the ,american ,telephone and ,telegraph ,company operates on this principle4 ,two-,party ,line4 ,it is obvious that selection may be had between two parties on a single metallic-circuit line without the use of biased bells or current of different polarities4 ,thus1 one limb of a metallic circuit may be used as one grounded line to ring the bell at one of the stations1 and the other limb of the metallic circuit may be used as another grounded line to ring the bell of the other station2 and the two limbs may be used together as a metallic circuit for talking purposes as usual4 ,this is shown in ,fig4 #agj1 where the ringing keys at the central office are diagrammatically shown in the left-hand portion of the figure as ,k#a and ,k#b4 ,the operation of these keys will be more fully pointed out in a subsequent chapter1 but a correct understanding will be had if it be remembered that the circuits are normally maintained by these keys in the position shown4 ,when1 however1 either one of the keys is operated1 the two long springs may be considered as pressed apart so as to disengage the normal contacts between the springs and to engage the two outer contacts1 with which they are shown in the cut to be disengaged4 ,the two outer contacts are connected respectively to an ordinary alternating-current ringing generator and to ground1 but the connection is reversed on the two keys4 ,illustration3 ,fig4 #agj4 ,simple ,two-,party ,line ,selection ,at ,station ,a the ordinary talking set is shown in simplified form1 consisting merely of a receiver1 transmitter1 and hook switch in a single bridge circuit across the line4 ,an ordinary polarized bell is shown connected in series with a condenser between the lower limb of the line and ground4 ,at ,station ,b the same talking circuit is shown1 but the polarized bell and condenser are bridged between the upper limb of the line and ground4 ,if the operator desires to call ,station ,a1 she will press key ,k#a which will ground the upper side of the line and connect the lower side of the line with the generator ,g#a1 and this1 obviously1 will cause the bell at ,station ,a to ring4 ,the bell at ,station ,b will not ring because it is not in the circuit4 ,if1 on the other hand1 the operator desires to ring the bell at ,station ,b1 she will depress key ,k#b1 which will allow the current from generator ,g#b to pass over the upper side of the line through the bell and condenser at ,station ,b and return by the path through the ground4 ,the object of grounding the opposite sides of the keys at the central office is to prevent cross-ringing1 that is1 ringing the wrong bell4 ,were the keys not grounded this might occur when a ringing current was being sent out while the receiver at one of the stations was off its hook2 the ringing current from1 say1 generator ,g#a then passing not only through the bell at ,station ,a as intended1 but also through the bell at ,station ,b by way of the bridge path through the receiver that happened to be connected across the line4 ,with the ringing keys grounded as shown1 it is obvious that this will not occur1 since the path for the ringing current through the wrong bell will always be shunted by a direct path to ground on the same side of the line4 ,in such a two-party-line selective system the two generators ,g#a and ,g#b may be the same generator and may be of the ordinary alternating-current type4 ,the bells likewise may be of the ordinary alternating-current type4 ,the two-party selective line just described virtually employs two separate circuits for ringing4 ,now each of these circuits alone may be employed to accomplish selective ringing between two stations by using two biased bells oppositely polarized1 and employing pulsating ringing currents of one direction or the other according to which bell it is desired to ring4 ,one side of a circuit so equipped is shown in ,fig4 #aga4 ,in this the two biased bells are at ,station ,a and ,station ,b1 these being bridged to ground in each case and adapted to respond only to positive and negative impulses respectively4 ,at the central office the two keys ,k#a and ,k#b are shown4 ,a single alternating-current generator ,g is shown1 having its brush #a grounded and brush #b connected to a commutator disk #c mounted on the generator shaft so as to revolve therewith4 ,one-half of the periphery of this disk is of insulating material so that the brushes #d and #e1 which bear against the disk1 will be alternately connected with the disk and1 therefore1 with the brush #b of the generator4 ,now the brush #b1 being one terminal of an alternating-current machine1 is alternately positive and negative1 and the arrangement of the commutator is such that the disk1 which is always at the potential of the brush #b1 will be connected to the brush #e only while it is positively charged and with the brush #d only while it is negatively charged4 ,as a result1 brush #e has a succession of positive impulses and brush #d a succession of negative ones4 ,obviously1 therefore1 when key ,k#a is depressed only the bell at ,station ,a will be rung1 and likewise the depression of key ,k#b will result only in the ringing of the bell at ,station ,b4 ,illustration3 ,fig4 #aga4 ,principle of ,selection by ,polarity ,four-,party ,line4 ,from the two foregoing two-party line systems it is evident that a four-party line system may be readily obtained1 that is1 by employing two oppositely polarized biased bells on each side of the metallic circuit4 ,the selection of any of the four bells may be obtained1 choosing between the pairs connected1 respectively1 with the two limbs of the line1 by choosing the limb on which the current is to be sent1 and choosing between the two bells of the pair on that side of the line by choosing which polarity of current to send4 ,such a four-party line system is shown in ,fig4 #agb4 ,in this the generators are not shown1 but the wires leading from the four keys are shown marked plus or minus1 according to the terminal of the generator to which they are supposed to be connected4 ,likewise the two bells connected with the lower side of the line are marked positive and negative1 as are the two bells connected with the upper side of the line4 ,from the foregoing description of ,figs4 #agj and #aga1 it is clear that if key ,k#a is pressed the bell at ,station ,a will be rung1 and that bell only1 since the bells at ,station ,c and ,station ,d are not in the circuit and the positive current sent over the lower side of the line is not of the proper polarity to ring the bell at ,station ,b4 ,the system shown in ,fig4 #agb is subject to one rather grave defect4 ,in subsequent chapters it will be pointed out that in common-battery systems the display of the line signal at the central office is affected by any one of the subscribers merely taking his receiver off its hook and thus establishing a connection between the two limbs of the metallic circuit4 ,such common-battery systems should have the two limbs of the line1 normally1 entirely insulated from each other4 ,it is seen that this is not the case in the system just described1 since there is a conducting path from one limb of the line through the two bells on that side to ground1 and thence through the other pair of bells to the other limb of the line4 ,this means that unless the resistance of the bell windings is made very high1 the path of the signaling circuit will be of sufficiently low resistance to actuate the line signal at the central office4 ,illustration3 ,fig4 #agb4 ,four-,party ,polarity ,selection ,it is not feasible to overcome this objection by the use of condensers in series with the bells1 as was done in the system shown in ,fig4 #agj1 since the bells are necessarily biased and such bells1 as may readily be seen1 will not work properly through condensers1 since the placing of a condenser in their circuit means that the current which passes through the bell is alternating rather than pulsating1 although the original source may have been of pulsating nature only4 ,illustration3 ,fig #agc4 ,standard ,polarity ,system ,the remedy for this difficulty1 therefore1 has been to place in series with each bell a very high non-inductive resistance of about #ae1jjj or #bj1jjj ohms1 and also to make the windings of the bells of comparatively high resistance1 usually about #b1ejj ohms4 ,even with this precaution there is a considerable leakage of the central-office battery current from one side of the line to the other through the two paths to ground in series4 ,this method of selective signaling has1 therefore1 been more frequently used with magneto systems4 ,an endeavor to apply this principle to common-battery systems without the objections noted above has led to the adoption of a modification1 wherein a relay at each station normally holds the ground connection open4 ,this is shown in ,fig4 #agc and is the standard four-party line ringing circuit employed by the ,american ,telephone and ,telegraph ,company and their licensees4 ,in this system the biased bells are normally disconnected from the line1 and1 therefore1 the leakage path through them from one side of the line to the other does not exist4 ,at each station there is a relay winding adapted to be operated by the ringing current bridged across the line in series with a condenser4 ,as a result1 when ringing current is sent out on the line all of the relays1 i4e41 one at each station1 are energized and attract their armatures4 ,this establishes the connection of all the bells to line and really brings about temporarily a condition equivalent to that of ,fig4 #agb4 ,as a result1 the sending of a positive current on the lower line with a ground return will cause the operation of the bell at ,station ,a4 ,it will not ring the bell at ,station ,b because of the wrong polarity4 ,it will not ring the bells of ,station ,c and ,station ,d because they are in the circuit between the other side of the line and ground4 ,as soon as the ringing current ceases all of the relays release their armatures and disconnect all the bells from the line4 ,by this very simple device the trouble1 due to marginal working of the line signal1 is done away with1 since normally there is no leakage from one side of the line to the other on account of the presence of the condensers in the bridge at each station4 ,illustration3 ,fig4 #agd4 ,ringing-,key ,arrangement ,in ,fig4 #agd1 the more complete connections of the central-office ringing keys are shown1 by means of which the proper positive or negative ringing currents are sent to line in the proper way to cause the ringing of any one of the four bells on a party line of either of the types shown in ,figs4 #agb and #agc4 ,in this the generator ,g and its commutator disk #c1 with the various brushes1 #a1 #b1 #d1 and #e1 are arranged in the same manner as is shown in ,fig4 #aga4 ,it is evident from what has been said that wire #f leading from generator brush #b and commutator disk #c will carry alternating potential2 that wire #g will carry positive pulsations of potential2 and that wire #h will carry negative pulsations of potential4 ,there are five keys in the set illustrated in ,fig4 #agd1 of which four1 viz1 ,k#a1 ,k#b1 ,k#c1 and ,k#d1 are connected in the same manner as diagrammatically indicated in ,figs4 #agb and #agc1 and will1 obviously1 serve to send the proper current over the proper limb of the line to ring one of the bells4 ,key ,k#e1 the fifth one in the set1 is added so as to enable the operator to ring an ordinary unbiased bell on a single party line when connection is made with such line4 ,as the two outside contacts of this key are connected respectively to the two brushes of the alternating-current dynamo ,g1 it is clear that it will impress an alternating current on the line when its contacts are closed4 ,circuits of ,two-,party ,line ,telephones4 ,in ,fig4 #age is shown in detail the wiring of the telephone set usually employed in connection with the party-line selective-ringing system illustrated in ,fig4 #agj4 ,in the wiring of this set and the two following1 it must be borne in mind that the portion of the circuit used during conversation might be wired in a number of ways without affecting the principle of selective ringing employed2 however1 the circuits shown are those most commonly employed with the respective selective ringing systems which they are intended to illustrate4 ,in connecting the circuits of this telephone instrument to the line1 the two line conductors are connected to binding posts #a and #b and a ground connection is made to binding post #c4 ,in practice1 in order to avoid the necessity of changing the permanent wiring of the telephone set in connecting it as an ,a or ,b ,station 7,fig4 #agj71 the line conductors are connected to the binding posts in reverse order at the two stations2 that is1 for ,station ,a the upper conductor1 ,fig4 #agj1 is connected to binding post #a and the lower conductor to binding post #b1 while at ,station ,b the upper conductor is connected to binding post #b and the lower conductor to binding post #a4 ,the permanent wiring of this telephone set is the same as that frequently used for a set connected to a line having only one station1 the proper ringing circuit being made by the method of connecting up the binding posts4 ,for example1 if this telephone set were to be used on a single station line1 the binding posts #a and #b would be connected to the two conductors of the line as before1 while binding post #c would be connected to post #a instead of being grounded4 ,illustration3 ,fig4 #age4 ,circuit of ,two-,party ,station ,circuits of ,four-,party-,line ,telephones4 ,the wiring of the telephone set used with the system illustrated in ,fig4 #agb is shown in detail in ,fig4 #agf4 ,the wiring of this set is arranged for local battery or magneto working1 as this method of selective ringing is more frequently employed with magneto systems1 on account of the objectionable features which arise when applied to common-battery systems4 ,in this figure the line conductors are connected to binding posts #a and #b1 and a ground connection is made to binding post #c4 ,in order that all sets may be wired alike and yet permit the instrument to be connected for any one of the various stations1 the bell is not permanently wired to any portion of the circuit but has flexible connections which will allow of the set being properly connected for any desired station4 ,the terminals of the bell are connected to binding posts #i and #aj1 to which are connected flexible conductors terminating in terminals #g and #h4 ,these terminals may be connected to the binding posts #d1 #e1 and #f in the proper manner to connect the set as an ,a1 ,b1 ,c1 or ,d station1 as required4 ,for example1 in connecting the set for ,station ,a1 ,fig4 #agb1 terminal #g is connected to binding post #f and #h to #e4 ,for connecting the set for ,station ,b terminal #g is connected to binding post #e and #h to #f4 ,for connecting the set for ,station ,c terminal #g is connected to binding post #f and #h to #d4 ,for connecting the set for ,station ,d terminal #g is connected to binding post #d and #h to #f4 ,illustration3 ,fig4 #agf4 ,circuit of ,four-,party ,station without ,relay ,illustration3 ,fig4 #agg4 ,circuit of ,four-,party ,station with ,relay ,the detailed wiring of the telephone set employed in connection with the system illustrated in ,fig4 #agc is shown in ,fig4 #agg4 ,the wiring of this set is arranged for a common-battery system1 inasmuch as this arrangement of signaling circuit is more especially adapted for common-battery working4 ,however1 this arrangement is frequently adapted to magneto systems as even with magneto systems a permanent ground connection at a subscriber's station is objectionable inasmuch as it increases the difficulty of determining the existence or location of an accidental ground on one of the line conductors4 ,the wiring of this set is also arranged so that one standard type of wiring may be employed and yet allow any telephone set to be connected as an ,a1 ,b1 ,c1 or ,d station4 ,harmonic ,method4 ,principles4 ,to best understand the principle of operation of the harmonic party-line signaling systems1 it is to be remembered that a flexible reed1 mounted rigidly at one end and having its other end free to vibrate1 will1 like a violin string1 have a certain natural period of vibration2 that is1 if it be started in vibration1 as by snapping it with the fingers1 it will take up a certain rate of vibration which will continue at a uniform rate until the vibration ceases altogether4 ,such a reed will be most easily thrown into vibration by a series of impulses having a frequency corresponding exactly to the natural rate of vibration of the reed itself2 it may be thrown into vibration by very slight impulses if they occur at exactly the proper times4 ,it is familiar to all that a person pushing another in a swing may cause a considerable amplitude of vibration with the exertion of but a small amount of force1 if he will so time his pushes as to conform exactly to the natural rate of vibration of the swing4 ,it is of course possible1 however1 to make the swing take up other rates of vibrations by the application of sufficient force4 ,as another example1 consider a clock pendulum beating seconds4 ,by gentle blows furnished by the escapement at exactly the proper times1 the heavy pendulum is kept in motion4 ,however1 if a person grasps the pendulum weight and shakes it1 it may be made to vibrate at almost any desired rate1 dependent on the strength and agility of the individual4 ,the conclusion is1 therefore1 that a reed or pendulum may be made to start and vibrate easily by the application of impulses at proper intervals1 and only with great difficulty by the application of impulses at other than the proper intervals2 and these facts form the basis on which harmonic-ringing systems rest4 ,the father of harmonic ringing in telephony was ,jacob ,b4 ,currier1 an undertaker of ,lowell1 ,mass4 ,his harmonic bells were placed in series in the telephone line1 and were considerably used in ,new ,england in commercial practice in the early eighties4 ,somewhat later ,james ,a4 ,lighthipe of ,san ,francisco independently invented a harmonic-ringing system1 which was put in successful commercial use at ,sacramento and a few other smaller ,california towns4 ,lighthipe polarized his bells and bridged them across the line in series with condensers1 as in modern practice1 and save for some crudities in design1 his apparatus closely resembled1 both in principle and construction1 some of that in successful use today4 ,lighthipe's system went out of use and was almost forgotten1 when about #aijc1 ,wm4 ,w4 ,dean again independently redeveloped the harmonic system1 and produced a bell astonishingly like that of ,lighthipe1 but of more refined design1 thus starting the development which has resulted in the present wide use of this system4 ,the signal-receiving device in harmonic-ringing systems takes the form of a ringer1 having its armature and striker mounted on a rather stiff spring rather than on trunnions4 ,by this means the moving parts of the bell constitute in effect a reed tongue1 which has a natural rate of vibration at which it may easily be made to vibrate with sufficient amplitude to strike the gongs4 ,the harmonic ringer differs from the ordinary polarized bell or ringer1 therefore1 in that its armature will vibrate most easily at one particular rate1 while the armature of the ordinary ringer is almost indifferent1 between rather wide limits1 as to the rate at which it vibrates4 ,as a rule harmonic party-line systems are limited to four stations on a line4 ,the frequencies employed are usually #af-b_/c1 #cc-a_/c1 #ej1 and #ff-b_/c cycles per second1 this corresponding to #a1jjj1 #b1jjj1 #c1jjj1 and #d1jjj cycles per minute4 ,the reason why this particular set of frequencies was chosen is that they represent approximately the range of desirable frequencies1 and that the first ringing-current machines in such systems were made by mounting the armatures of four different generators on a single shaft1 these having1 respectively1 two poles1 four poles1 six poles1 and eight poles each4 ,the two-pole generator gave one cycle per revolution1 the four-pole two1 the six-pole three1 and the eight-pole four1 so that by running the shaft of the machine at exactly #a1jjj revolutions per minute the frequencies before mentioned were attained4 ,this range of frequencies having proved about right for general practice and the early ringers all having been attuned so as to operate on this basis1 the practice of adhering to these numbers of vibrations has been kept up with one exception by all the manufacturers who make this type of ringer4 ,tuning4 ,the process of adjusting the armature of a ringer to a certain rate of vibration is called tuning1 and it is customary to refer to a ringer as being tuned to a certain rate of vibration1 just as it is customary to refer to a violin string as being tuned to a certain pitch or rate of vibration4 ,the physical difference between the ringers of the various frequencies consists mainly in the size of the weights at the end of the vibrating reed1 that is1 of the weights which form the tapper for the bell4 ,the low-frequency ringers have the largest weights and the high-frequency the smallest1 of course4 ,the ringers are roughly tuned to the desired frequencies by merely placing on the tapper rod the desired weight and then a more refined tuning is given them by slightly altering the positions of the weights on the tapper rod4 ,to make the reed have a slightly lower natural rate of vibration1 the weight is moved further from the stationary end of the reed1 while to give it a slightly higher natural rate of vibration the weight is moved toward the stationary4 ,in this way very nice adjustments may be made1 and the aim of the various factories manufacturing these bells is to make the adjustment permanent so that it will never have to be altered by the operating companies4 ,several years of experience with these bells has shown that when once properly assembled they maintain the same rate of vibration with great constancy4 ,there are two general methods of operating harmonic bells4 ,one of these may be called the in-tune system and the other the under-tune system4 ,the under-tune system was the first employed4 ,illustration3 ,,operating ,,room ,,at ,,tokyo1 ,,japan ,under-,tune ,system4 ,the early workers in the field of harmonic-selective signaling discovered that when the tapper of the reed struck against gongs the natural rate of vibration of the reed was changed1 or more properly1 the reed was made to have a different rate of vibration from its natural rate4 ,this was caused by the fact that the elasticity of the gongs proved another factor in the set of conditions causing the reeds to take up a certain rate of vibration1 and the effect of this added factor was always to accelerate the rate of vibration which the reed had when it was not striking the gongs4 ,the rebound of the hammer from the gongs tended1 in other words1 to accelerate the rate of vibration1 which1 as might be expected1 caused a serious difficulty in the practical operation of the bells4 ,to illustrate3 ,if a reed were to have a natural rate of vibration1 when not striking the gongs1 of #ej per second and a current of #ej cycles per second were impressed on the line1 the reed would take up this rate of vibration easily1 but when a sufficient amplitude of vibration was attained to cause the tapper to strike the gongs1 the reed would be thrown out of tune1 on account of the tendency of the gongs to make the reed vibrate at a higher rate4 ,this caused irregular ringing and was frequently sufficient to make the bells cease ringing altogether or to ring in an entirely unsatisfactory manner4 ,in order to provide for this difficulty the early bells of ,currier and ,lighthipe were made on what has since been called the under-tuned principle4 ,the first bells of the ,kellogg ,switchboard and ,supply ,company1 developed by ,dean1 were based on this idea as their cardinal principle4 ,the reeds were all given a natural rate of vibration1 when not striking the gongs1 somewhat below that of the current frequencies to be employed2 and yet not sufficiently below the corresponding current frequency to make the bell so far out of tune that the current frequency would not be able to start it4 ,this was done so that when the tapper began to strike the gongs the tapper would be accelerated and brought practically into tune with the current frequency1 and the ringing would continue regularly as long as the current flowed4 ,it will be seen that the under-tuned system was1 therefore1 one involving some difficulty in starting in order to provide for proper regularity while actually ringing4 ,ringers of this kind were always made with but a single gong1 it being found difficult to secure uniformity of ringing and uniformity of adjustment when two gongs were employed4 ,although no ringers of this type are being made at present1 yet a large number of them are in use and they will consequently be described4 ,their action is interesting in throwing better light on the more improved types1 if for no other reason4 ,figs4 #agh and #agi show1 respectively1 side and front views of the original ,kellogg bell4 ,the entire mechanism is self-contained1 all parts being mounted on the base plate #a4 ,the electromagnet is of the two-coil type1 and is supported on the brackets #b and #c4 ,the bracket #b is of iron so as to afford a magnetic yoke for the field of the electromagnet1 while the bracket #c is of brass so as not to short-circuit the magnetic lines across the air-gap4 ,the reed tongue--consisting of the steel spring #e1 the soft-iron armature pieces #f1 the auxiliary spring #g1 and the tapper ball #h1 all of which are riveted together1 as shown in ,fig4 #agh--;constitutes the only moving part of the bell4 ,the steel spring #e is rigidly mounted in the clamping piece #i at the upper part of the bracket #c1 and the reed tongue is permitted to vibrate only by the flexibility of this spring4 ,the auxiliary spring #g is much lighter than the spring #e and has for its purpose the provision of a certain small amount of flexibility between the tapper ball and the more rigid portion of the armature formed by the iron strips #f-f4 ,the front ends of the magnet pole pieces extend through the bracket #c and are there provided with square soft-iron pole pieces #aj set at right angles to the magnet cores so as to form a rather narrow air-gap in which the armature may vibrate4 ,illustration3 ,fig4 #agh4 ,under-,tuned ,ringer ,the cores of the magnet and also the reed tongue are polarized by means of the ,l-shaped bar magnet #d1 mounted on the iron yoke #b at one end in such manner that its other end will lie quite close to the end of the spring #e1 which1 being of steel1 will afford a path for the lines of force to the armature proper4 ,we see1 therefore1 that the two magnet cores are1 by this permanent magnet1 given one polarity1 while the reed tongue itself is given the other polarity1 this being exactly the condition that has already been described in connection with the regular polarized bell or ringer4 ,the electromagnetic action by which this reed tongue is made to vibrate is1 therefore1 exactly the same as that of an ordinary polarized ringer1 but the difference between the two is that1 in this harmonic ringer1 the reed tongue will respond only to one particular rate of vibrations1 while the regular polarized ringer will respond to almost any4 ,as shown in ,fig4 #agh1 the tapper ball strikes on the inside surface of the single gong4 ,the function of the auxiliary spring #g between the ball and the main portion of the armature is to allow some resilience between the ball and the balance of the armature so as to counteract in some measure the accelerating influence of the gong on the armature4 ,in these bells1 as already stated1 the natural rate of vibration of the reed tongue was made somewhat lower than the rate at which the bell was to be operated1 so that the reed tongue had to be started by a current slightly out of tune with it1 and then1 as the tapper struck the gong1 the acceleration due to the gong would bring the vibration of the reed tongue1 as modified by the gong1 into tune with the current that was operating it4 ,in ether words1 in this system the ringing currents that were applied to the line had frequencies corresponding to what may be called the operative rates of vibration of the reed tongues1 which operative rates of vibration were in each case the resultant of the natural pitch of the reed as modified by the action of the bell gong when struck4 ,illustration3 ,fig4 #agi4 ,under-,tuned ,ringer ,in-,tune ,system4 ,the more modern method of tuning is to make the natural rate of vibration of the reed tongue1 that is1 the rate at which it naturally vibrates when not striking the gongs1 such as to accurately correspond to the rate of vibration at which the bells are to be operated--that is1 the natural rate of vibration of the reed tongues is made the same as the operative rate4 ,thus the bells are attuned for easy starting1 a great advantage over the under-tuned system4 ,in the under-tuned system1 the reeds being out of tune in starting require heavier starting current1 and this is obviously conducive to cross-ringing1 that is1 to the response of bells to other than the intended frequency4 ,again1 easy starting is desirable because when the armature is at rest1 or in very slight vibration1 it is at a maximum distance from the poles of the electromagnet1 and1 therefore1 subject to the weakest influence of the poles4 ,a current1 therefore1 which is strong enough to start the vibration1 will be strong enough to keep the bell ringing properly4 ,illustration3 ,fig4 #ahj4 ,dean ,in-,tune ,ringer ,when with this in-tune mode of operation1 the armature is thrown into sufficiently wide vibration to cause the tapper to strike the gong1 the gong may tend to accelerate the vibration of the reed tongue1 but the current impulses through the electromagnet coils continue at precisely the same rates as before4 ,under this condition of vibration1 when the reed tongue has an amplitude of vibration wide enough to cause the tapper to strike the gongs1 the ends of the armature come closest to the pole pieces1 so that the pole pieces have their maximum magnetic effect on the armature1 with the result that even if the accelerating tendency of the gongs were considerable1 the comparatively large magnetic attractive impulses occurring at the same rate as the natural rate of vibration of the reed tongue1 serve wholly to prevent any actual acceleration of the reed tongue4 ,the magnetic attractions upon the ends of the armature1 continuing at the initial rate1 serve1 therefore1 as a check to offset any accelerating tendency which the striking of the gong may have upon the vibrating reed tongue4 ,it is obvious1 therefore1 that in the in-tune system the electromagnetic effect on the armature should1 when the armature is closest to the pole pieces1 be of such an overpowering nature as to prevent whatever accelerating tendency the gongs may have from throwing the armature out of its stride in step with the current4 ,for this reason it is usual in this type to so adjust the armature that its ends will actually strike against the pole pieces of the electromagnet when thrown into vibration4 ,sufficient flexibility is given to the tapper rod to allow it to continue slightly beyond the point at which it would be brought to rest by the striking of the armature ends against the pole pieces and thus exert a whipping action so as to allow the ball to continue in its movement far enough to strike against the gongs4 ,the rebound of the gong is then taken up by the elasticity of the tapper rod1 which returns to an unflexed position1 and at about this time the pole piece releases the armature so that it may swing over in the other direction to cause the tapper to strike the other gong4 ,illustration3 ,fig4 #aha4 ,tappers for ,dean ,ringers ,the construction of the in-tune harmonic ringer employed by the ,dean ,electric ,company1 of ,elyria1 ,ohio1 is illustrated in ,figs4 #ahj1 #aha1 and #ahb4 ,it will be seen from ,fig4 #ahj that the general arrangement of the magnet and armature is the same as that of the ordinary polarized ringer2 the essential difference is that the armature is spring-mounted instead of pivoted4 ,the armature and the tapper rod normally stand in the normal central position with reference to the pole pieces of the magnet and the gongs4 ,fig4 #aha shows the complete vibrating parts of four ringers1 adapted1 respectively1 to the four different frequencies of the system4 ,the assembled armature1 tapper rod1 and tapper are all riveted together and are non-adjustable4 ,all of the adjustment that is done upon them is done in the factory and is accomplished1 first1 by choosing the proper size of weight1 and second1 by forcing this weight into the proper position on the tapper rod to give exactly the rate of vibration that is desired4 ,illustration3 ,fig4 #ahb4 ,dean ,in-,tune ,ringer ,an interesting feature of this ,dean harmonic ringer is the gong adjustment4 ,as will be seen1 the gongs are mounted on posts which are carried on levers pivoted to the ringer frame4 ,these levers have at their outer end a curved rack provided with gear teeth adapted to engage a worm or screw thread mounted on the ringer frame4 ,obviously1 by turning this worm screw in one direction or the other1 the gongs are moved slightly toward or from the armature or tapper4 ,this affords a very delicate means of adjusting the gongs1 and at the same time one which has no tendency to work loose or to get out of adjustment4 ,illustration3 ,fig4 #ahc4 ,kellogg ,in-,tune ,ringer ,in ,fig4 #ahc is shown a drawing of the in-tune harmonic ringer manufactured by the ,kellogg ,switchboard and ,supply ,company4 ,this differs in no essential respect from that of the ,dean ,company1 except in the gong adjustment1 this latter being affected by a screw passing through a nut in the gong post1 as clearly indicated4 ,in both the ,kellogg and the ,dean in-tune ringers1 on account of the comparative stiffness of the armature springs and on account of the normal position of the armature with maximum air gaps and consequent minimum magnetic pull1 the armature will practically not be affected unless the energizing current is accurately attuned to its own natural rate4 ,when the proper current is thrown on to the line1 the ball will be thrown into violent vibration1 and the ends of the armature brought into actual contact with the pole pieces1 which are of bare iron and shielded in no way4 ,the armature in this position is very strongly attracted and comes to a sudden stop on the pole pieces4 ,the gongs are so adjusted that the tapper ball will have to spring about one thirty-second of an inch in order to hit them4 ,the armature is held against the pole piece while the tapper ball is engaged in striking the gong and in partially returning therefrom1 and so strong is the pull of the pole piece on the armature in this position that the accelerating influence of the gong has no effect in accelerating the rate of vibration of the reed4 ,illustration3 ,fig4 #ahd4 ,circuits of ,dean ,harmonic ,system ,circuits4 ,in ,fig4 #ahd are shown in simplified form the circuits of a four-station harmonic party line4 ,it is seen that at the central office there are four ringing keys1 adapted1 respectively1 to impress on the line ringing currents of four different frequencies4 ,at the four stations on the line1 lettered ,a1 ,b1 ,c1 and ,d1 there are four harmonic bells tuned accordingly4 ,at ,station ,a there is shown the talking apparatus employing the ,wheatstone bridge arrangement4 ,the talking apparatus at all of the other stations is exactly the same1 but is omitted for the sake of simplicity4 ,a condenser is placed in series with each of the bells in order that there may be no direct-current path from one side of the line to the other when all of the receivers are on their hooks at the several stations4 ,in ,fig4 #ahe is shown exactly the same arrangement1 with the exception that the talking apparatus illustrated in detail at ,station ,a is that of the ,kellogg ,switchboard and ,supply ,company4 ,otherwise the circuits of the ,dean and the ,kellogg ,company1 and in fact of all the other companies manufacturing harmonic ringing systems1 are the same4 ,advantages4 ,a great advantage of the harmonic party-line system is the simplicity of the apparatus at the subscriber's station4 ,the harmonic bell is scarcely more complex than the ordinary polarized ringer1 and the only difference between the harmonic-ringing telephone and the ordinary telephone is in the ringer itself4 ,the absence of all relays and other mechanism and also the absence of the necessity for ground connections at the telephone are all points in favor of the harmonic system4 ,illustration3 ,fig4 #ahe4 ,circuits of ,kellogg ,harmonic ,system ,limitations4 ,as already stated1 the harmonic systems of the various companies1 with one exception1 are limited to four frequencies4 ,the exception is in the case of the ,north ,electric ,company1 which sometimes employs four and sometimes five frequencies and thus gets a selection between five stations4 ,in the four-party ,north system1 the frequencies1 unlike those in the ,dean and ,kellogg systems1 wherein the higher frequencies are multiples of the lower1 are arranged so as to be proportional to the whole numbers #e1 #g1 #i1 and #aa1 which1 of course1 have no common denominator4 ,the frequencies thus employed in the ,north system are1 in cycles per second1 #cj4c1 #db4d1 #ed4e1 and #ff4g4 ,in the five-party system1 the frequency of #af4g is arbitrarily added4 ,while all of the commercial harmonic systems on the market are limited to four or five frequencies1 it does not follow that a greater number than four or five stations may not be selectively rung4 ,double these numbers may be placed on a party line and selectively actuated1 if the first set of four or five is bridged across the line and the second set of four or five is connected between one limb of the line and ground4 ,the first set of these is selectively rung1 as already described1 by sending the ringing currents over the metallic circuit1 while the second set may be likewise selectively rung by sending the ringing currents over one limb of the line with a ground return4 ,this method is frequently employed with success on country lines1 where it is desired to place a greater number of instruments on a line than four or five4 ,step-by-,step ,method4 ,a very large number of step-by-step systems have been proposed and reduced to practice1 but as yet they have not met with great success in commercial telephone work1 and are nowhere near as commonly used as are the polarity and harmonic systems4 ,principles4 ,an idea of the general features of the step-by-step systems may be had by conceiving at each station on the line a ratchet wheel1 having a pawl adapted to drive it one step at a time1 this pawl being associated with the armature of an electromagnet which receives current impulses from the line circuit4 ,there is thus one of these driving magnets at each station1 each bridged across the line so that when a single impulse of current is sent out from the central office all of the ratchet wheels will be moved one step4 ,another impulse will move all of the ratchet wheels another step1 and so on throughout any desired number of impulses4 ,the ratchet wheels1 therefore1 are all stepped in unison4 ,let us further conceive that all of these ratchet wheels are provided with a notch or a hole or a projection1 alike in all respects at all stations save in the position which this notch or hole or projection occupies on the wheel4 ,the thing to get clear in this part of the conception is that all of these notches1 holes1 or projections are alike on all of the wheels1 but they occupy a different position on the wheel for each one of the stations4 ,consider further that the bell circuit at each of the stations is normally open1 but that in each case it is adapted to be closed when the notch1 hole1 or projection is brought to a certain point by the revolution of the wheel4 ,let us conceive further that this distinguishing notch1 hole1 or projection is so arranged on the wheel of the first station as to close the bell circuit when one impulse has been sent1 that that on the second station will close the bell circuit after the second impulse has been sent1 and so on throughout the entire number of stations4 ,it will1 therefore1 be apparent that the bell circuits at the various stations will1 as the wheels are rotated in unison1 be closed one after the other4 ,in order to call a given station1 therefore1 it is only necessary to rotate all of the wheels in unison1 by sending out the proper stepping impulses until they all occupy such a position that the one at the desired station is in such position as to close the bell circuit at that station4 ,since all of the notches1 holes1 or projections are arranged to close the bell circuits at their respective stations at different times1 it follows that when the bell circuit at the desired station is closed those at all of the other stations will be open4 ,if1 therefore1 after the proper number of stepping impulses has been sent to the line to close the bell circuit of the desired station1 ringing current be applied to the line1 it is obvious that the bell of that one station will be rung to the exclusion of all others4 ,it is1 of course1 necessary that provision be made whereby the magnets which furnish the energy for stepping the wheels will not be energized by the ringing current4 ,this is accomplished in one of several ways1 the most common of which is to have the stepping magnets polarized or biased in one direction and the bells at the various stations oppositely biased1 so that the ringing current will not affect the stepping magnet and the stepping current will not affect the ringer magnets4 ,after a conversation is finished1 the line may be restored to its normal position in one of several ways4 ,usually so-called release magnets are employed1 for operating on the releasing device at each station4 ,these1 when energized1 will withdraw the holding pawls from the ratchets and allow them all to return to their normal positions4 ,sometimes these release magnets are operated by a long impulse of current1 being made too sluggish in their action to respond to the quick-stepping impulses2 sometimes the release magnets are tapped from one limb of the line to ground1 so as not to be affected by the stepping or ringing currents sent over the metallic circuit2 and sometimes other expedients are used for obtaining the release of the ratchets at the proper time1 a large amount of ingenuity having been spent to this end4 ,as practically all step-by-step party-line systems in commercial use have also certain other features intended to assure privacy of conversation to the users1 and1 therefore1 come under the general heading of lock-out party-line systems1 the discussion of commercial examples of these systems will be left for the next chapter1 which is devoted to such lock-out systems4 ,broken-,line ,method4 ,the broken-line system1 like the step-by-step system1 is also essentially a lock-out system and for that reason only its general features1 by which the selective ringing is accomplished1 will be dealt with here4 ,principles4 ,in this system there are no tuned bells1 no positively and negatively polarized bells bridged to ground on each side of the line1 and no step-by-step devices in the ordinary sense1 by which selective signaling has ordinarily been accomplished on party lines4 ,instead of this1 each instrument on the line is exclusively brought into operative relation with the line1 and then removed from such operative relation until the subscriber wanted is connected1 at which time all of the other instruments are locked out and the line is not encumbered by any bridge circuits at any of the instruments that are not engaged in the conversation4 ,furthermore1 in the selecting of a subscriber or the ringing of his bell there is no splitting up of current among the magnets at the various stations as in ordinary practice1 but the operating current goes straight to the station desired and to that station alone where its entire strength is available for performing its proper work4 ,in order to make the system clear it may be stated at the outset that one side of the metallic circuit line is continued as in ordinary practice1 passing through all of the stations as a continuous conductor4 ,the other side of the line1 however1 is divided into sections1 its continuity being broken at each of the subscriber's stations4 ,fig4 #ahf is intended to show in the simplest possible way how the circuit of the line may be extended from station to station in such manner that only the ringer of one station is in circuit at a time4 ,the two sides of the line are shown in this figure1 and it will be seen that limb ,l extends from the central office on the left to the last station on the right without a break4 ,the limb ,r1 however1 extends to the first station1 at which point it is cut off from the extension ,rx by the open contacts of a switch4 ,for the purpose of simplicity this switch is shown as an ordinary hand switch1 but as a matter of fact it is a part of a relay1 the operating coil of which is shown at #f1 just above it1 in series with the ringer4 ,illustration3 ,fig4 #ahf4 ,principle of ,broken-,line ,system ,obviously1 if a proper ringing current is sent over the metallic circuit from the central office1 only the bell at ,station ,a will operate1 since the bells at the other stations are not in the circuit4 ,if by any means the switch lever #b at ,station ,a were moved out of engagement with contact #a and into engagement with contact #c1 it is obvious that the bell of ,station ,a would no longer be in circuit1 but the limb ,r of the line would be continued to the extension ,rx and the bell of ,station ,b would be in circuit4 ,any current then sent over the circuit of the line from the central office would ring the bell of this station4 ,in ,fig4 #ahg the switches of both ,station ,a and ,station ,b have been thus operated1 and ,station ,c is thus placed in circuit4 ,inspection of this figure will show that the bells of ,station ,a1 ,station ,b1 and ,station ,d are all cut out of circuit1 and that1 therefore1 no current from the central office can affect them4 ,this general scheme of selection is a new-comer in the field1 and for certain classes of work it is of undoubted promise4 ,illustration3 ,fig4 #ahg4 ,principle of ,broken-,line ,system ,,chapter ,,xvii ,,lock-,,out ,,party-,,line ,,systems ,the party-line problem in rural districts is somewhat different from that within urban limits4 ,in the latter cases1 owing to the closer grouping of the subscribers1 it is not now generally considered desirable1 even from the standpoint of economy1 to place more than four subscribers on a single line4 ,for such a line selective ringing is simple1 both from the standpoint of apparatus and operation2 and moreover owing to the small number of stations on a line1 and the small amount of traffic to and from such subscribers as usually take party-line service1 the interference between parties on the same line is not a very serious matter4 ,for rural districts1 particularly those tributary to small towns1 these conditions do not exist4 ,owing to the remoteness of the stations from each other it is not feasible from the standpoint of line cost to limit the number of stations to four4 ,a much greater number of stations is employed and the confusion resulting is distressing not only to the subscribers themselves but also to the management of the company4 ,there exists then the need of a party-line system which will give the limited user in rural districts a service1 at least approaching that which he would get if served by an individual line4 ,the principal investment necessary to provide facilities for telephone service is that required to produce the telephone line4 ,in many cases the cost of instruments and apparatus is small in comparison with the cost of the line4 ,by far the greater number of subscribers in rural districts are those who use their instruments a comparatively small number of times a day1 and to maintain an expensive telephone line for the exclusive use of one such subscriber who will use it but a few minutes each day is on its face an economic waste4 ,as a result1 where individual line service is practiced exclusively one of two things must be true3 either the average subscriber pays more for his service than he should1 or else the operating company sells the service for less than it costs1 or at best for an insufficient profit4 ,both of these conditions are unnatural and cannot be permanent4 ,the party-line method of giving service1 by which a single line is made to serve a number of subscribers1 offers a solution to this difficulty1 but the ordinary non-selective or even selective party line has many undesirable features if the attempt is made to place on it such a large number of stations as is considered economically necessary in rural work4 ,these undesirable features work to the detriment of both the user of the telephone and the operating company4 ,many attempts have been made to overcome these disadvantages of the party line in sparsely settled communities1 by producing what are commonly called lock-out systems4 ,these1 as their name implies1 employ such an arrangement of parts that when the line is in use by any two parties1 all other parties are locked out from the circuit and cannot gain access to it until the parties who are using it are through4 ,system after system for accomplishing this purpose has been announced but for the most part these have involved such a degree of complexity and have introduced so many undesirable features as to seriously affect the smooth operation of the system and the reliability of the service4 ,we believe1 however1 in spite of numerous failures1 that the lock-out selective-signaling party line has a real field of usefulness and that operating companies as well as manufacturing companies are beginning to appreciate this need1 and as a result that the relief of the rural subscriber from the almost intolerable service he has often had to endure is at hand4 ,a few of the most promising lock-out party-line systems now before the public will1 therefore1 be described in some detail4 ,poole ,system4 ,the ,poole system is a lock-out system pure and simple1 its devices being in the nature of a lock-out attachment for selective-signaling lines1 either of the polarity or of the harmonic type wherein common-battery transmission is employed4 ,it will be here described as employed in connection with an ordinary harmonic-ringing system4 ,in ,fig4 #ahh there is shown a four-station party line equipped with ,poole lock-out devices1 it being assumed that the ringers at each station are harmonic and that the keys at the central office are the ordinary keys adapted to impress the proper frequency on the line for ringing any one of the stations4 ,in addition to the ordinary talking and ringing apparatus at each subscriber's station1 there is a relay of special form and also a push-button key4 ,illustration3 ,fig4 #ahh4 ,poole ,lock-,out ,system ,each of the relays has two windings1 one of high resistance and the other of low resistance4 ,remembering that the system to which this device is applied is always a common-battery system1 and that1 therefore1 the normal condition of the line will be one in which there is a difference of potential between the two limbs1 it will be evident that whenever any subscriber on a line that is not in use raises his receiver from its hook1 a circuit will be established from the upper contact of the hook through the lever of the hook to the high-resistance winding #a of the relay and thence to the other side of the line by way of wire #f4 ,this will result in current passing through the high-resistance winding of the relay and the relay will pull up its armature4 ,as soon as it does so it establishes two other circuits by the closure of the relay armature against the contacts #d and #e4 ,the closing of the contact #d establishes a circuit from the upper side of the line through the upper contact of the switch hook1 thence through the contacts of the push button #c1 thence through the low-resistance winding #b of the relay to the terminal #d1 thence through the relay armature and the transmitter to the lower side of the line4 ,this low-resistance path across the line serves to hold the relay armature attracted and also to furnish current to the transmitter for talking4 ,the establishment of this low-resistance path across the line does another important thing1 however2 it practically short-circuits the line with respect to all the high-resistance relay windings1 and thus prevents any of the other high-resistance relay windings from receiving enough current to actuate them1 should the subscriber at any other station remove his receiver from the hook in an attempt to listen in or to make a call while the line is in use4 ,as a subscriber can only establish the proper conditions for talking and listening by the attraction of this relay armature at his station1 it is obvious that unless he can cause the pulling up of his relay armature he can not place himself in communication with the line4 ,the second thing that is accomplished by the pulling up of the relay armature is the closure of the contacts #e1 and that completes the talking circuit through the condenser and receiver across the line in an obvious fashion4 ,the result of this arrangement is that it is the first party who raises his receiver from its hook who is enabled to successfully establish a connection with the line1 all subsequent efforts1 by other subscribers1 failing to do so because of the fact that the line is short-circuited by the path through the low-resistance winding and the transmitter of the station that is already connected with the line4 ,a little target is moved by the action of the relay so that a visual indication is given to the subscriber in making a call to show whether or not he is successful in getting the use of the line4 ,if the relay operates and he secures control of the line1 the target indicates the fact by its movement1 while if someone else is using the line and the relay does not operate1 the target1 by its failure to move1 indicates that fact4 ,when one party desires to converse with another on the same line1 he depresses the button #c at his station until after the called party has been rung and has responded4 ,this holds the circuit of his low-resistance winding open1 and thus prevents the lock-out from becoming effective until the called party is connected with the line4 ,the relay armature of the calling party does not fall back with the establishment of the low-resistance path at the called station1 because1 even though shunted1 it still receives sufficient current to hold its armature in its attracted position4 ,after the called party has responded1 the button at the calling station is released and both low-resistance holding coils act in multiple4 ,illustration3 ,,one ,,wing ,,of ,,operating ,,room1 ,,berlin1 ,,germany ,ultimate ,capacity #bd1jjj ,subscribers' ,lines and #b1ajj ,trunk ,lines4 ,siemens-,halske ,equipment4 ,note ,horizontal ,disposal of ,multiple ,jack ,field4 ,no induction coil is used in this system and the impedance of the holding coil is such that incoming voice currents flow through the condenser and the receiver1 which1 by reference to the figure1 will be seen to be in shunt with the holding coil4 ,the holding coil is in series with the local transmitter1 thus making a circuit similar to that of the ,kellogg common-battery talking circuit already discussed4 ,a possible defect in the use of this system is one that has been common to a great many other lock-out systems1 depending for their operation on the same general plan of action4 ,this appears when the instruments are used on a comparatively long line4 ,since the locking-out of all the instruments that are not in use by the one that is in use depends on the low-resistance shunt that is placed across the line by the instrument that is in use1 it is obvious that1 in the case of a long line1 the resistance of the line wire will enter into the problem in such a way as to tend to defeat the locking-out function in some cases4 ,thus1 where the first instrument to use the line is at the remote end of the line1 the shunting effect that this instrument can exert with respect to another instrument near the central office is that due to the resistance of the line plus the resistance of the holding coil at the end instrument4 ,the resistance of the line wire may be so high as to still allow a sufficient current to flow through the high-resistance coil at the nearer station to allow its operation1 even though the more remote instrument is already in use4 ,coming now to a consideration of the complete selective-signaling lock-out systems1 wherein the selection of the party and the locking out of the others are both inherent features1 a single example of the step-by-step1 and of the broken-line selective lock-out systems will be discussed4 ,step-by-,step ,system4 ,the so-called ,k4,b4 system1 manufactured by the ,dayton ,telephone ,lock-out ,manufacturing ,company of ,dayton1 ,ohio1 operates on the step-by-step principle4 ,the essential feature of the subscriber's telephone equipment in this system is the step-by-step actuating mechanism which performs also the functions of a relay4 ,this device consists of an electromagnet having two cores1 with a permanent polarizing magnet therebetween1 the arrangement in this respect being the same as in an ordinary polarized bell4 ,the armature of this magnet works a rocker arm1 which1 besides stepping the selector segment around1 also1 under certain conditions1 closes the bell circuit and the talking circuit1 as will be described4 ,illustration3 ,fig4 #ahi4 ,k4,b4 ,lock-,out ,system ,referring first to ,fig4 #ahi1 which shows in simplified form a four-station ,k4,b4 lock-out line1 the electromagnet is shown at #a and the rocker arm at #b4 ,the ratchet #c in this case is not a complete wheel but rather a segment thereof1 and it is provided with a series of notches of different depths4 ,it is obvious that the depth of the notches will determine the degree of movement which the upper end of the rocker arm may have toward the left1 this being dependent on the extent to which the pawl #f is permitted to enter into the segment4 ,the first or normal notch1 i4e41 the top notch1 is always of such a depth that it will allow the rocker-arm lever #b to engage the contact lever #d1 but will not permit the rocker arm to swing far enough to the left to cause that contact to engage the bell contact #e4 ,as will be shown later1 the condition for the talking circuit to be closed is that the rocker arm #b shall rest against the contact #d2 and from this we see that the normal notch of each of the segments #c is of such a depth as to allow the talking circuit at each station to be closed4 ,the next notch1 i4e41 the second one in each disk1 is always shallow1 as are all of the other notches except one4 ,a deep notch is placed on each disk anywhere from the third to the next to the last on the segment4 ,this deep notch is called the selective notch1 and it is the one that allows of contact being made with the ringer circuit of that station when the pawl #f drops into it4 ,the position of this notch differs on all of the segments on a line1 and obviously1 therefore1 the ringer circuit at any station may be closed to the exclusion of all the others by stepping all of the segments in unison until the deep notch on the segment of the desired station lies opposite to the pawl #f1 which will permit the rocker arm #b to swing so far to the left as to close not only the circuit between #b and #d1 but also between #b1 #d1 and #e4 ,in this position the talking and the ringing circuits are both closed4 ,the position of the deepest notch1 i4e41 the selective notch1 on the circumference of the segment at any station depends upon the number of that station2 thus1 the segment of ,station #d will have a deep notch in the sixth position2 the segment for ,station #i will have a deep notch in the eleventh position2 the segment for any station will have a deep notch in the position corresponding to the number of that station plus two4 ,from what has been said1 therefore1 it is evident that the first1 or normal1 notch on each segment is of such a depth as to allow the moving pawl #f to fall to such a depth in the segment as to permit the rocker arm #b to close the talking circuit only4 ,all of the other notches1 except one1 are comparatively shallow1 and while they permit the moving pawl #f under the influence of the rocker arm #b to move the segment #c1 yet they do not permit the rocker arm #b to move so far to the left as to close even the talking circuit4 ,the exception is the deep notch1 or selective notch1 which is of such depth as to permit the pawl #f to fall so far into the segment as to allow the rocker arm #b to close both the talking and the ringing circuits4 ,besides the moving pawl #f there is a detent pawl #g4 ,this always holds the segment #c in the position to which it has been last moved by the moving pawl #f4 ,the actuating magnet #a1 as has been stated1 is polarized and when energized by currents in one direction1 the rocker arm moves the pawl #f so as to step the segment one notch4 ,when this relay is energized by current in the opposite direction1 the operation is such that both the moving pawl #f and the detent pawl #g will be pulled away from the segment1 thus allowing the segment to return to its normal position by gravity4 ,this is accomplished by the following mechanism3 ,an armature stop is pivoted upon the face of the rocker arm so as to swing in a plane parallel to the pole faces of the relay1 and is adapted1 when the relay is actuated by selective impulses of one polarity1 to be pulled towards one of the pole faces where it acts1 through impact with a plate attached to the pole face of the relay1 as a limiting means for the motion of the rocker arm when the rocker arm is actuated by the magnet4 ,when1 however1 the relay is energized by current in the opposite direction1 as on a releasing impulse1 the armature stop swings upon its pivot towards the opposite pole face1 in which position the lug on the end of the armature stop registers with a hole in the plate on the relay1 thus allowing the full motion of the rocker arm when it is attracted by the magnet4 ,this motion of the rocker arm withdraws the detent pawl from engagement with the segment as well as the moving pawl1 and thereby permits the segment to return to its normal position4 ,as will be seen from ,fig4 #ahi1 each of the relay magnets #a is permanently bridged across the two limbs of the line4 ,each station is provided with a push button1 not shown1 by means of which the subscriber who makes a call may prevent the rocker arm of his instrument from being actuated while selective impulses are being sent over the line4 ,the purpose of this is to enable one party to make a call for another on the same line1 depressing his push button while the operator is selecting and ringing the called party4 ,the segment at his own station1 therefore1 remains in its normal position1 in which position1 as we have already seen1 his talking circuit is closed2 all of the other segments are1 however1 stepped up until the ringing and talking circuits of the desired station are in proper position1 at which time ringing current is sent over the line4 ,the segments in ,fig4 #ahi1 except at ,station ,c1 are shown as having been stepped up to the sixth position1 which corresponds to the ringing position of the fourth station1 or ,station ,d4 ,the condition shown in this figure corresponds to that in which the subscriber at ,station ,c originated the call and pressed his button1 thus retaining his own segment in its normal position so that the talking circuits would be established with ,station ,d4 ,when the line is in normal position any subscriber may call central by his magneto generator1 not shown in ,fig4 #ahi1 which will operate the drop at central1 but will not operate any of the subscribers' bells1 because all bell circuits are normally open4 ,when a subscriber desires connection with another line1 the operator sends an impulse back on the line which steps up and locks out all instruments except that of the calling subscriber4 ,illustration3 ,fig4 #aij4 ,k4,b4 ,lock-,out ,station ,a complete ,k4,b4 lock-out telephone is shown in ,fig4 #aij4 ,this is the type of instrument that is usually furnished when new equipment is ordered4 ,if1 however1 it is desired to use the ,k4,b4 system in connection with telephones of the ordinary bridging type that are already in service1 the lock-out and selective mechanism1 which is shown on the upper inner face of the door in ,fig4 #aij1 is furnished separately in a box that may be mounted close to the regular telephone and connected thereto by suitable wires1 as shown in ,fig4 #aia4 ,it is seen that this instrument employs a local battery for talking and also a magneto generator for calling the central office4 ,the central-office equipment consists of a dial connected with an impulse wheel1 together with suitable keys by which the various circuits may be manipulated4 ,this dial and its associated mechanism may be mounted in the regular switchboard cabinet1 or it may be furnished in a separate box and mounted alongside of the cabinet in either of the positions shown at #a or #b of ,fig4 #aib4 ,in order to send the proper number of impulses to the line to call a given party1 the operator places her finger in the hole in the dial that bears the number corresponding to the station wanted and rotates the dial until the finger is brought into engagement with the fixed stop shown at the bottom of the dial in ,fig4 #aib4 ,the dial is then allowed to return by the action of a spring to its normal position1 and in doing so it operates a switch within the box to make and break the battery circuit the proper number of times4 ,operation4 ,a complete description of the operation may now be had in connection with ,fig4 #aic1 which is similar to ,fig4 #ahi1 but contains the details of the calling arrangement at the central office and also of the talking circuits at the various subscribers' stations4 ,illustration3 ,fig4 #aia4 ,k4,b4 ,lock-,out ,station ,referring to the central-office apparatus the usual ringing key is shown1 the inside contacts of which lead to the listening key and to the operator's telephone set as in ordinary switchboard practice4 ,between the outside contact of this ringing key and the ringing generator there is interposed a pair of contact springs #h-h and another pair #i-i4 ,the contact springs #h are adapted to be moved backward and forward by the impulse wheel which is directly controlled by the dial under the manipulation of the operator4 ,when these springs #h are in their normal position1 the ringing circuit is continued through the release-key springs #i to the ringing generator4 ,these springs #h occupy their normal position only when the dial is in its normal position1 this being due to the notch #aj in the contact wheel4 ,at all other times1 i4e41 while the impulse wheel is out of its normal position1 the springs #h-h are either depressed so as to engage the lower battery contacts1 or else held in an intermediate position so as to engage neither the battery contacts nor the generator contacts4 ,illustration3 ,fig4 #aib4 ,calling ,apparatus ,k4,b4 ,system ,when it is desired to call a given station1 the operator pulls the subscriber's number on the dial and holds the ringing key closed1 allowing the dial to return to normal4 ,this connects the impulse battery to the subscriber's line as many times as is required to move the subscriber's sectors to the proper position1 and in such direction as to cause the stepping movement of the various relays4 ,as the impulse wheel comes to its normal position1 the springs #h1 associated with it1 again engage their upper contacts1 by virtue of the notch #aj in the impulse wheel1 and this establishes the connection between the ringing generator and the subscriber's line1 the ringing key being still held closed4 ,the pulling of the transmitter dial and holding the ringing key closed1 therefore1 not only sends the stepping impulses to line1 but also follows it by the ringing current4 ,the sending of five impulses to line moves all of the sectors to the sixth notch1 and this corresponds to the position necessary to make the fourth station operative4 ,such a condition is shown in ,fig4 #aic1 it being assumed that the subscriber at ,station ,c originated the call and pressed his own button so as to prevent his sector from being moved out of its normal position4 ,as a result of this1 the talking circuit at ,station ,c is left closed1 and the talking and the ringing circuit of ,station ,d1 the called station1 are closed1 while both the talking and the ringing circuits of all the other stations are left open4 ,station ,d may1 therefore1 be rung and may communicate with ,station ,c1 while all of the other stations on the line are locked out1 because of the fact that both their talking and ringing circuits are left open4 ,illustration3 ,fig4 #aic4 ,circuit ,k4,b4 ,system ,when conversation is ended1 the operator is notified by the usual clearing-out signal1 and she then depresses the release button1 which brings the springs #i out of engagement with the generator contact but into engagement with the battery contact in such relation as to send a battery current on the line in the reverse direction from that sent out by the impulse wheel4 ,this sends current through all of the relays in such direction as to withdraw both the moving and the holding pawls from the segments and thus allow all of the segments to return to their normal positions4 ,of course1 in thus establishing the release current1 it is necessary for the operator to depress the ringing key as well as the release key4 ,a one-half microfarad condenser is placed in the receiver circuit at each station so that the line will not be tied up should some subscriber inadvertently leave his receiver off its hook4 ,this permits the passage of voice currents1 but not of the direct currents used in stepping the relays or in releasing them4 ,the circuit of ,fig4 #aic is somewhat simplified from that in actual practice1 and it should be remembered that the hook switch1 which is not shown in this figure1 controls in the usual way the continuity of the receiver and the transmitter circuits as well as of the generator circuits1 the generator being attached to the line as in an ordinary telephone4 ,broken-,line ,system4 ,the broken-line method of accomplishing selective signaling and locking-out on telephone party lines is due to ,homer ,roberts and his associates4 ,illustration3 ,fig4 #aid4 ,roberts ,latching ,relay ,to understand just how the principles illustrated in ,figs4 #ahf and #ahg are put into effect1 it will be necessary to understand the latching relay shown diagrammatically in its two possible positions in ,fig4 #aid1 and in perspective in ,fig4 #aie4 ,referring to ,fig4 #aid1 the left-hand cut of which shows the line relay in its normal position1 it is seen that the framework of the device resembles that of an ordinary polarized ringer4 ,under the influence of current in one direction flowing through the left-hand coil1 the armature of this device depresses the hard rubber stud #d1 and the springs #a1 #b1 and #c are forced downwardly until the spring #b has passed under the latch carried on the spring #e4 ,when the operating current through the coil #f ceases1 the pressure of the armature on the spring #a is relieved1 allowing this spring to resume its normal position and spring #c to engage with spring #b4 ,the spring #b cannot rise1 since it is held by the latch #e1 and the condition shown in the right-hand cut of ,fig4 #aid exists4 ,it will be seen that the spring #b has in this operation carried out just the same function as the switch lever performed as described in connection with ,figs4 #ahf and #ahg4 ,an analysis of this action will show that the normal contact between the springs #a and #b1 which contact controls the circuit through the relay coil and the bell1 is not broken until the coil #f is de-energized1 which means that the magnet is effective until it has accomplished its work4 ,it is impossible1 therefore1 for this relay to cut itself out of circuit before it has caused the spring #b to engage under the latch #e4 ,if current of the proper direction were sent through the coil #g of the relay1 the opposite end of the armature would be pulled down and the hard rubber stud at the left-hand end of the armature would bear against the bent portion of the spring #e in such manner as to cause the latch of this spring to release the spring #b and thus allow the relay to assume its normal1 or unlatched1 position4 ,a good idea of the mechanical construction of this relay may be obtained from ,fig4 #aie4 ,the entire selecting function of the ,roberts system is performed by this simple piece of apparatus at each station4 ,illustration3 ,fig4 #aie4 ,roberts ,latching ,relay ,the diagram of ,fig4 #aif shows1 in simplified form1 a four-station line1 the circuits being given more in detail than in the diagrams of ,chapter ,,xvi4 ,it will be noticed that the ringer and the relay coil #f at the first station are bridged across the sides of the line leading to the central office4 ,in like manner the bell and the relay magnets are bridged across the two limbs of the line leading into each succeeding station1 but this bridge at each of the stations beyond ,station ,a is ineffective because the line extension ,rx is open at the next station nearest the central office4 ,illustration3 ,fig4 #aif4 ,simplified ,circuits of ,roberts ,system ,in order to ring ,station ,a it is only necessary to send out ringing current from the central office4 ,this current is in such direction as not to cause the operation of the relay1 although it passes through the coil #f4 ,if1 on the other hand1 it is desired to ring ,station ,b1 a preliminary impulse would be sent over the metallic circuit from the central office1 which impulse would be of such direction as to operate the relay at ,station ,a1 but not to operate the bell at that station4 ,the operation of the relay at ,station ,a causes the spring #b of this relay to engage the spring #c1 thus extending the line on to the second station4 ,after the spring #b at ,station ,a has been forced into contact with the spring #c1 it is caught by the latch of the spring #e and held mechanically4 ,when the impulse from the central office ceases1 the spring #a resumes its normal position1 thus breaking the bridge circuit through the bell at that station4 ,it is apparent now that the action of coil #f at ,station ,a has made the relay powerless to perform any further action1 and at the same time the line has been extended on to the second station4 ,a second similar impulse from the central office will cause the relay at ,station ,b to extend the line on to ,station ,c1 and at the same time break the circuit through the operating coil and the bell at ,station ,b4 ,in this way any station may be picked out by sending the proper number of impulses to operate the line relays of all the stations between the station desired and the central office1 and having picked out a station it is only necessary to send out ringing current1 which current is in such direction as to ring the bell but not to operate the relay magnet at that station4 ,in ,fig4 #aig1 a four-station line1 such as is shown in ,fig4 #aif1 is illustrated1 but the condition shown in this is that existing when two preliminary impulses have been sent over the line1 which caused the line relays at ,station ,a and ,station ,b to be operated4 ,the bell at ,station ,c is1 therefore1 the only one susceptible to ringing current from the central office4 ,illustration3 ,fig4 #aig4 ,simplified ,circuits of ,roberts ,system ,since only one bell and one relay are in circuit at any one time1 it is obvious that all of the current that passes over the line is effective in operating a single bell or relay only4 ,there is no splitting up of the current among a large number of bells as in the bridging system of operating step-by-step devices1 which method sometimes so greatly reduces the effective current for each bell that it is with great difficulty made to respond4 ,all the energy available is applied directly to the piece of apparatus at the time it is being operated4 ,this has a tendency toward greater surety of action1 and the adjustment of the various pieces of apparatus may be made with less delicacy than is required where many pieces of apparatus1 each having considerable work to do1 must necessarily be operated in multiple4 ,the method of unlatching the relays has been briefly referred to4 ,after a connection has been established with a station in the manner already described1 the operator may clear the line when it is proper to do so by sending impulses of such a nature as to cause the line relays of the stations beyond the one chosen to operate1 thus continuing the circuit to the end of the line4 ,the operation of the line relay at the last station brings into circuit the coil #h1 ,figs4 #aif and #aig1 of a grounding device4 ,this is similar to the line relay1 but it holds its operating spring in a normally latched position so as to maintain the two limbs of the line disconnected from the ground4 ,the next impulse following over the metallic circuit passes through the coil #h and causes the operation of this grounding device which1 by becoming unlatched1 grounds the limb ,l of the line through the coil #h4 ,this temporary ground at the end of the line makes it possible to send an unlocking or restoring current from the central office over the limb ,l1 which current passes through all of the unlocking coils #g1 shown in ,figs4 #aid1 #aif1 and #aig1 thus causing the simultaneous unlocking of all of the line relays and the restoration of the line to its normal condition1 as shown in ,fig4 #aif4 ,illustration3 ,fig4 #aih4 ,details of ,latching ,relay ,connections ,as has been stated1 the windings #g on the line relays are the unlatching windings4 ,in ,figs4 #aif and #aig1 for the purpose of simplicity1 these windings are not shown connected1 but as a matter of fact each of them is included in series in the continuous limb ,l of the line4 ,this would introduce a highly objectionable feature from the standpoint of talking over the line were it not for the balancing coils #ga1 each wound on the same core as the corresponding winding #g1 and each included in series in the limb ,r of the line1 and in such direction as to be differential thereto with respect to currents passing in series over the two limbs of the line4 ,the windings #g are the true unlocking windings1 while the windings #ga have no other function than to neutralize the inductive effects of these unlocking windings necessarily placed in series in the talking circuit4 ,all of these windings are of low ohmic resistance1 a construction which1 as has previously been noted1 brings about the desired effect without introducing any self-induction in the line1 and without producing any appreciable effect upon the transmission4 ,a study of ,fig4 #aih will make clear the connections of these unlocking and balancing windings at each station4 ,the statement of operation so far given discloses the general method of building up the line in sections in order to choose any party and of again breaking it up into sections when the conversation is finished4 ,it has been stated that the same operation which selects the party wanted also serves to give that party the use of the line and to lock the others off4 ,that this is true will be understood when it is stated that the ringer is of such construction that when operated to ring the subscriber wanted1 it also operates to unlatch a set of springs similar to those shown in ,fig4 #aid1 this unlatching causing the proper connection of the subscriber's talking circuit across the limbs of the line1 and also closing the local circuit through his transmitter4 ,the very first motion of the bell armature performs this unlatching operation after which the bell behaves exactly as an ordinary polarized biased ringer4 ,illustration3 ,fig4 #aii4 ,broken-,back ,ringer ,the construction of this ringer is interesting and is shown in its two possible positions in ,fig4 #aii4 ,the group of springs carried on its frame is entirely independent of the movement of the armature during the ringing operation4 ,with reversed currents1 however1 the armature is moved in the opposite direction from that necessary to ring the bells1 and this causes the latching of the springs into their normal position4 ,in order that this device may perform the double function of ringer and relay the tapper rod of the bell is hinged on the armature so as to partake of the movements of the armature in one direction only4 ,this has been called by the inventor and engineers of the ,roberts system a broken-back ringer1 a name suggestive of the movable relation between the armature and the tapper rod4 ,the construction of the ringer is of the same nature as that of the standard polarized ringer universally employed1 but a hinge action between the armature and the tapper rod1 of such nature as to make the tapper partake positively of the movements of the armature in one direction1 but to remain perfectly quiescent when the armature moves in the other direction1 is provided4 ,illustration3 ,fig4 #bjj4 ,details of ,ringer ,connection ,how this broken-back ringer controls the talking and the locking-out conditions may best be understood in connection with ,fig4 #bjj4 ,the ringer springs are normally latched at all stations4 ,under these conditions the receiver is short-circuited by the engagement of springs #aj and #aa1 the receiver circuit is open between springs #aj and #ab1 and the local-battery circuit is open between springs #i and #ab4 ,the subscribers whose ringers are latched are1 therefore1 locked out in more ways than one4 ,when the bell is rung1 the first stroke it makes unlatches the springs1 which assume the position shown in the right-hand cut of ,fig4 #aii1 and this1 it will be seen from ,fig4 #bjj1 establishes proper conditions for enabling the subscriber to transmit and to receive speech4 ,the hook switch breaks both transmitter and receiver circuits when down and in raising it establishes a momentary circuit between the ground and the limb ,l of the line1 both upper and lower hook contacts engaging the hook lever simultaneously during the rising of the hook4 ,the mechanism at the central office by which selection of the proper station is made in a rapid manner is shown in ,fig4 #bja4 ,it has already been stated that the selection of the proper subscriber is brought about by the sending of a predetermined number of impulses from the central office1 these impulses passing in one direction only and over the metallic circuit4 ,after the proper party has been reached1 the ringing current is put on in the reverse direction4 ,illustration3 ,fig4 #bja4 ,central-,office ,impulse ,transmitter ,the operator establishes the number of impulses to be sent by placing the pointer opposite the number on the dial corresponding to the station wanted4 ,the ratchet wheel is stepped around automatically by each impulse of current from an ordinary pole changer such as is employed in ringing biased bells4 ,when the required number of impulses has been sent1 a projection1 carried on a group of springs1 drops into a notch on the drum of the selector shaft1 which operation instantly stops the selecting current impulses and at the same time throws on the ringing current which consists of impulses in the reverse direction4 ,so rapidly does this device operate that it will readily follow the impulses of an ordinary pole changer1 even when this is adjusted to its maximum rate of vibration4 ,illustration3 ,,view ,,of ,a ,,large ,,foreign ,,multiple ,,switchboard ,operation4 ,space will not permit a full discussion of the details of the central-office selective apparatus1 but a general resum of the operation of the system may now be given1 with the aid of ,fig4 #bjb1 which shows a four-station line with the circuits of three of the stations somewhat simplified4 ,in this figure ,station ,a1 ,station ,b1 and ,station ,d are shown in their locked-out positions1 ,a and ,b having been passed by the selection and ringing of ,station ,c1 while ,station ,d is inoperative because it was not reached in the selection and the line is still broken at ,station ,c4 ,station ,c1 therefore1 has possession of the line4 ,when the subscriber at ,station ,c raised his receiver in order to call central1 a flash contact was made as the hook moved up1 which momentarily grounded the limb ,l of the line4 7,see ,fig4 #bjj47 ,this flash contact is produced by the arrangement of the hook which assures that the lower contact shall1 by virtue of its flexibility1 follow up the hook lever until the hook lever engages the upper contact1 after which the lower contact breaks4 ,this results in the momentary connection of both the upper and the lower contacts of the hook with the lever1 and1 therefore1 the momentary grounding of the limb ,l of the line4 ,this limb always being continuous serves1 when this flash contact is made1 to actuate the line signal at the central office4 ,illustration3 ,fig4 #bjb4 ,circuits of ,roberts ,line ,since1 however1 all parties on the line are normally locked out of talking circuits1 some means must be provided whereby the operator may place the signaling party in talking connection and leave all the other instruments on the line in their normally locked-out position4 ,in fact1 the operator must be able automatically to pick out the station that signaled in1 and operate the ringer to unlatch the springs controlling the talking circuit of that station4 ,accordingly the operator sends impulses on the line1 from a grounded battery1 which are in the direction to operate the line relays and to continue the line circuit to the station calling4 ,when1 after a sufficient number of impulses1 this current reaches that station it finds a path to ground from the limb ,l4 ,this path is made possible by the fact that the subscriber's receiver is off its hook at that station4 ,in order to understand just how this ground connection is made1 it must be remembered that each of the ringer magnets is energized with each selecting impulse1 but in such a direction as not to ring the bells1 it being understood that all of the ringer mechanisms are normally latched4 ,when the selecting impulse for ,station ,c arrives1 it passes through the ringer and the selecting relay coils at that station and starts to operate the remainder of the ringers sufficiently to cause the spring #ab to engage the spring #ac4 ,this establishes the ground connection from the limb ,l of the line1 the circuit being traced through limb ,l through the upper contact of the switch1 thence through springs #ab and #ac to ground1 and this1 before the line relay has time to latch1 operates the quick-acting relay at the central office1 which acts to cut off further impulses1 and thus automatically stops at the calling station4 ,ringing current in the opposite direction is then sent to line2 this unlatches the ringer springs and places the calling subscriber in talking circuit4 ,when the operator has communicated with the calling subscriber1 and found1 for example1 that another party on another similar line is desired1 she turns the dial pointer on the selector to the number corresponding to the called-for party's number on that line1 and presses the signal key4 ,pressing this key causes impulses to run down the line1 selecting the proper party and ringing his bell in the manner already described4 ,the connection between the two parties is then established1 and no one else can in any possible way1 except by permission of the operator1 obtain access to the line4 ,it is obvious that some means must be provided for restoring the selecting relays to normal after a conversation is finished4 ,by referring to ,fig4 #aid it will be seen that the upper end of the latch spring #e is bent over in such a manner that when the armature is attracted by current flowing through the coil #g1 the knob on the left-hand end of the armature on rising engages with the bent cam surface and forces back the latch1 permitting spring #b to return to its normal position4 ,to restore the line the operator sends out sufficient additional selective impulses to extend the circuit to the end of the line1 and thus brings the grounder into circuit4 ,the winding of the grounder is connected in such a manner that the next passing impulse throws off its latch1 permitting the long spring to contact with the ground spring4 ,the operator now sends a grounded impulse over the continuous limb ,l of the line which passes through the restoring coils #g at all the stations and through the right-hand coil of the grounding device to ground4 ,the selecting relays are1 therefore1 simultaneously restored to normal4 ,the grounder is also energized and restored to its normal position by the same current4 ,if a party in calling finds that his own line is busy and he cannot get central1 he may leave his receiver off its hook4 ,when the party who is using the line hangs up his receiver the fact that another party desires a connection is automatically indicated to the operator1 who then locks out the instrument of the party who has just finished conversation and passes his station by4 ,when the operator again throws the key1 the waiting subscriber is automatically selected in the same manner as was the first party4 ,if there are no subscribers waiting for service1 the stop relay at central will not operate until the grounder end of the line is unlatched1 the selecting relays being then restored automatically to normal4 ,the circuits are so organized that at all times whether the line is busy or not1 the movement up and down of the switch hook1 at any sub-station1 operates a signal before the operator4 ,such a movement1 when made slowly and repeatedly1 indicates to the operator that the subscriber has an emergency call and she may use her judgment as to taking the line away from the parties who are using it1 and finding out what the emergency call is for4 ,if the operator finds that the subscriber has misused this privilege of making the emergency call1 she may restore the connection to the parties previously engaged in conversation4 ,one of the salient points of this ,roberts system is that the operator always has control of the line4 ,a subscriber is not able even to use his own battery till permitted to do so4 ,a subscriber who leaves his receiver off its hook in order that he may be signaled by the operator when the line is free1 causes no deterioration of the local battery because the battery circuit is held open by the switch contacts carried on the ringer4 ,it cannot be denied1 however1 that this system is complicated1 and that it has other faults4 ,for instance1 as described herein1 both sides of the line must be looped into each subscriber's station1 thus requiring four drop1 or service1 wires instead of two4 ,it is possible to overcome this objection by placing the line relays on the pole in a suitably protected casing1 in which case it is sufficient to run but two drop wires from the nearer line to station4 ,there are undoubtedly other objections to this system1 and yet with all its faults it is of great interest1 and although radical in many respects1 it teaches lessons of undoubted value4 ,,chapter ,,xviii ,,electrical ,,hazards ,all telephone systems are exposed to certain electrical hazards4 ,when these hazards become actively operative as causes1 harmful results ensue4 ,the harmful results are of two kinds3 those causing damage to property and those causing damage to persons4 ,the damage to persons may be so serious as to result in death4 ,damage to property may destroy the usefulness of a piece of apparatus or of some portion of the wire plant4 ,or the property damage may initiate itself as a harm to apparatus or wiring and may result in greater and extending damage by starting a fire4 ,electrical currents which endanger life and property may be furnished by natural or artificial causes4 ,natural electricity which does such damage usually displays itself as lightning4 ,in rare cases1 currents tending to flow over grounded lines because of extraordinary differences of potential between sections of the earth's surface have damaged apparatus in such lines1 or only have been prevented from causing such damage by the operation of protective devices4 ,telegraph and telephone systems have been threatened by natural electrical hazards since the beginning of the arts and by artificial electrical hazards since the development of electric light and power systems4 ,at the present time1 contrary to the general supposition1 it is in the artificial1 and not in the natural electrical hazards that the greater variety and degree of danger lies4 ,of the ways in which artificial electricity may injure a telephone system1 the entrance of current from an external electrical power system is a greater menace than an abnormal flow of current from a source belonging to the telephone system itself4 ,yet modern practice provides opportunities for a telephone system to inflict damage upon itself in that way4 ,telephone engineering designs need to provide means for protecting all parts of a system against damage1 from external 7foreign7 as well as internal 7domestic7 hazards1 and to cause this protection to be inclusive enough to protect persons against injury and property from damage by any form of overheating or electrolytic action4 ,a part of a telephone system for which there is even a remote possibility of contact with an external source of electrical power1 whether natural or artificial1 is said to be exposed to electrical hazard4 ,the degree or character of possible contact or other interference often is referred to in relative terms of exposure4 ,the same terms are used concerning inductive relations between circuits4 ,the whole tendency of design1 particularly of wire plants1 is to arrange the circuits in such a way as to limit the exposure as greatly as possible1 the intent being to produce a condition in which all parts of the system will be unexposed to hazards4 ,methods of design are not yet sufficiently advanced for any plant to be formed of circuits wholly unexposed1 so that protective means are required to safeguard apparatus and circuits in case the hazard1 however remote1 becomes operative4 ,lightning discharges between the clouds and earth frequently charge open wires to potentials sufficiently high to damage apparatus2 and less frequently1 to destroy the wires of the lines themselves4 ,lightning discharges between clouds frequently induce charges in lines sufficient to damage apparatus connected with the lines4 ,heavy rushes of current in lines1 from lightning causes1 occasionally induce damaging currents in adjacent lines not sufficiently exposed to the original cause to have been injured without this induction4 ,the lightning hazard is least where the most lines are exposed4 ,in a small city with all of the lines formed of exposed wires and all of them used as grounded circuits1 a single lightning discharge may damage many switchboard signals and telephone ringers if there be but #ajj or #bjj lines1 while the damage might have been nothing had there been #hjj to #a1jjj lines in the same area4 ,means of protecting lines and apparatus against damage by lightning are little more elaborate than in the earliest days of telegraph working4 ,they are adequate for the almost entire protection of life and of apparatus4 ,power circuits are classified by the rules of various governing bodies as high-potential and low-potential circuits4 ,the classification of the ,national ,board of ,fire ,underwriters in the ,united ,states defines low-potential circuits as having pressures below #eej volts2 high-potential circuits as having pressures from #eej to #c1ejj volts1 and extra high-potential circuits as having pressures above #c1ejj volts4 ,pressures of #ajj1jjj volts are becoming more common4 ,where power is valuable and the distance over which it is to be transmitted is great1 such high voltages are justified by the economics of the power problem4 ,they are a great hazard to telephone systems1 however4 ,an unprotected telephone system meeting such a hazard by contact will endanger life and property with great certainty4 ,a very common form of distribution for lighting and power purposes is the three-wire system having a grounded neutral wire1 the maximum potential above the earth being about #aae volts4 ,telephone lines and apparatus are subject to damage by any power circuit whether of high or low potential4 ,the cause of property damage in all cases is the flow of current4 ,personal damage1 if it be death from shock1 ordinarily is the result of a high potential between two parts of the body4 ,the best knowledge indicates that death uniformly results from shock to the heart4 ,it is believed that death has occurred from shock due to pressure as low as #ajj volts4 ,the critical minimum voltage which can not cause death is not known4 ,a good rule is never willingly to subject another person to personal contact with any electrical pressure whatever4 ,electricity can produce actions of four principal kinds3 physiological1 thermal1 chemical1 and magnetic4 ,viewing electricity as establishing hazards1 the physiological action may injure or kill living things2 the thermal action may produce heat enough to melt metals1 to char things which can be burned1 or to cause them actually to burn1 perhaps with a fire which can spread2 the chemical action may destroy property values by changing the state of metals1 as by dissolving them from a solid state where they are needed into a state of solution where they are not needed2 the magnetic action introduces no direct hazard4 ,the greatest hazard to which property values are exposed is the electro-thermal action2 that is1 the same useful properties by which electric lighting and electric heating thrive may produce heat where it is not wanted and in an amount greater than can safely be borne4 ,the tendency of design is to make all apparatus capable of carrying without overheating any current to which voltage within the telephone system may subject it1 and to provide the system so designed with specific devices adapted to isolate it from currents originating without4 ,apparatus which is designed in this way1 adapted not only to carry its own normal working currents but to carry the current which would result if a given piece of apparatus were connected directly across the maximum pressure within the telephone system itself1 is said to be self-protecting4 ,apparatus amply able to carry its maximum working current but likely to be overheated1 to be injured1 or perhaps to destroy itself and set fire to other things if subjected to the maximum pressure within the system1 is not self-protecting apparatus4 ,to make all electrical devices self-protecting by surrounding them with special arrangements for warding off abnormal currents from external sources1 is not as simple as might appear4 ,a lamp1 for example1 which can bear the entire pressure of a central-office battery1 is not suitable for direct use in a line several miles long because it would not give a practical signal in series with that line and with the telephone set1 as it is required to do4 ,a lamp suitable for use in series with such a line and a telephone set would burn out by current from its own normal source if the line should become short-circuited in or near the central office4 ,the ballast referred to in the chapter on ,signals was designed for the very purpose of providing rapidly-rising resistance to offset the tendency toward rapidly-rising current which could burn out the lamp4 ,as another example1 a very small direct-current electric motor can be turned on at a snap switch and will gain speed quickly enough so that its armature winding will not be overheated4 ,a larger motor of that kind can not be started safely without introducing resistance into the armature circuit on starting1 and cutting it out gradually as the armature gains speed4 ,such a motor could be made self-protecting by having the armature winding of much larger wire than really is required for mere running1 choosing its size great enough to carry the large starting current without overheating itself and its insulation4 ,it is better1 and for long has been standard practice1 to use starting boxes1 frankly admitting that such motors are not self-protecting until started1 though they are self-protecting while running at normal speeds4 ,such a motor1 once started1 may be overloaded so as to be slowed down4 ,so much more current now can pass through the armature that its winding is again in danger4 ,overload circuit-breakers are provided for the very purpose of taking motors out of circuit in cases where1 once up to speed1 they are mechanically brought down again and into danger4 ,such a circuit-breaker is a device for protecting against an internal hazard2 that is1 internal to the power system of which the motor is a part4 ,another example3 ,in certain situations1 apparatus intended to operate under impulses of large current may be capable of carrying its normal impulses successfully but incapable of carrying currents from the same pressure continuously4 ,protective means may be provided for detaching such apparatus from the circuit whenever the period in which the current acts is not short enough to insure safety4 ,this is cited as a case wherein a current1 normal in amount but abnormal in duration1 becomes a hazard4 ,the last mentioned example of damage from internal hazards brings us to the law of the electrical generation of heat4 ,the greater the current or the greater the resistance of the conductor heated or the longer the time1 the greater will he the heat generated in that conductor4 ,but this generated heat varies directly as the resistance and as the time and as the square of the current1 that is1 the law is ,heat generated ,c#b,rt in which ,c the current2 ,rthe resistance of the conductor2 and t the time4 ,it is obvious that a protective device1 such as an overload circuit-breaker for a motor1 or a protector for telephone apparatus1 needs to operate more quickly for a large current than for a small one1 and this is just what all well-designed protective devices are intended to do4 ,the general problem which these heating hazards present with relation to telephone apparatus and circuits is3 ,to cause all parts of the telephone system to be made so as to carry successfully all currents which may flow in them because of any internal or external pressure1 or to supplement them by devices which will stop or divert currents which could overheat them4 ,electrolytic hazards depend not on the heating effects of currents but on their chemical effects4 ,the same natural law which enables primary and secondary batteries to be useful provides a hazard which menaces telephone-cable sheaths and other conductors4 ,when a current leaves a metal in contact with an electrolyte1 the metal tends to dissolve into the electrolyte4 ,in the processes of electroplating and electrotyping1 current enters the bath at the anode1 passes from the anode through the solution to the cathode1 removing metal from the former and depositing it upon the latter4 ,in a primary battery using zinc as the positive element and the negative terminal1 current is caused to pass1 within the cell1 from the zinc to the negative element and zinc is dissolved4 ,following the same law1 any pipe buried in the earth may serve to carry current from one region to another4 ,as single-trolley traction systems with positive trolley wires constantly are sending large currents through the earth toward their power stations1 such a pipe may be of positive potential with relation to moist earth at some point in its length4 ,current leaving it at such a point may cause its metal to dissolve enough to destroy the usefulness of the pipe for its intended purpose4 ,lead-sheathed telephone cables in the earth are particularly exposed to such damage by electrolysis4 ,the reasons are that such cables often are long1 have a good conductor as the sheath-metal1 and that metal dissolves readily in the presence of most aqueous solutions when electrolytic differences of potential exist4 ,the length of the cables enables them to connect between points of considerable difference of potential4 ,it is lack of this length which prevents electrolytic damage to masses of structural metal in the earth4 ,electrical power is supplied to single-trolley railroads principally in the form of direct current4 ,usually all the trolley wires of a city are so connected to the generating units as to be positive to the rails4 ,this causes current to flow from the cars toward the power stations1 the return path being made up jointly of the rails1 the earth itself1 actual return wires which may supplement the rails1 and also all other conducting things in the earth1 these being principally lead-covered cables and other pipes4 ,these conditions establish definite areas in which the currents tend to leave the cables and pipes1 i4e41 in which the latter are positive to other things4 ,these positive areas usually are much smaller than the negative areas1 that is1 the regions in which currents tend to enter the cables form a larger total than the regions in which the currents tend to leave the cables4 ,these facts simplify the ways in which the cables may be protected against damage by direct currents leaving them and also they reduce the amount1 complication1 and cost of applying the corrective and preventive measures4 ,all electric roads do not use direct current4 ,certain simplifications in the use of single-phase alternating currents in traction motors have increased the number of roads using a system of alternating-current power supply4 ,where alternating current is used1 the electrolytic conditions are different and a new problem is set1 for1 as the current flows in recurrently different directions1 an area which at one instant is positive to others1 is changed the next instant into a negative area4 ,the protective means1 therefore1 must be adapted to the changed requirements4 ,,chapter ,,xix ,,protective ,,means ,any of the heating hazards described in the foregoing chapter may cause currents which will damage apparatus4 ,all devices for the protection of apparatus from such damage1 operate either to stop the flow of the dangerous current1 or to send that flow over some other path4 ,protection ,against ,high ,potentials4 ,lightning is the most nearly universal hazard4 ,all open wires are exposed to it in some degree4 ,damaging currents from lightning are caused by extraordinarily high potentials4 ,furthermore1 a lightning discharge is oscillatory2 that is1 alternating1 and of very high frequency4 ,drops1 ringers1 receivers1 and other devices subject to lightning damage suffer by having their windings burned by the discharge4 ,the impedance these windings offer to the high frequency of lightning oscillations is great4 ,the impedance of a few turns of heavy wire may be negligible to alternating currents of ordinary frequencies because the resistance of the wire is low1 its inductance small1 and the frequency finite4 ,on the other hand1 the impedance of such a coil to a lightning discharge is much higher1 due to the very high frequency of the discharge4 ,were it not for the extremely high pressure of lightning discharges1 their high frequency of oscillation would enable ordinary coils to be self-protecting against them4 ,but a discharge of electricity can take place through the air or other insulating medium if its pressure be high enough4 ,a pressure of #gj1jjj volts can strike across a gap in air of one inch1 and lower pressures can strike across smaller distances4 ,when lightning encounters an impedance1 the discharge seldom takes place through the entire winding1 as an ordinary current would flow1 usually striking across whatever short paths may exist4 ,very often these paths are across the insulation between the outer turns of a coil4 ,it is not unusual for a lightning discharge to plow its way across the outer layer of a wound spool1 melting the copper of the turns as it goes4 ,often the discharge will take place from inner turns directly to the core of the magnet4 ,this is more likely when the core is grounded4 ,air-,gap ,arrester4 ,the tendency of a winding to oppose lightning discharges and the ease with which such discharge may strike across insulating gaps1 points the way to protection against them4 ,such devices consist of two conductors separated by an air space or other insulator and are variously known as lightning arresters1 spark gaps1 open-space cutouts1 or air-gap arresters4 ,the conductors between which the gap exists may be both of metal1 may be one of metal and one of carbon1 or both of carbon4 ,one combination consists of carbon and mercury1 a liquid metal4 ,the space between the conductors may be filled with either air or solid matter1 or it may be a vacuum4 ,speaking generally1 the conductors are separated by some insulator4 ,two conductors separated by an insulator form a condenser4 ,the insulator of an open-space arrester often is called the dielectric4 ,illustration ,fig4 #bjc4 ,saw ,tooth ,arrester ,discharge ,across ,gaps3--,electrical discharges across a given distance occur at lower potentials if the discharge be between points than if between smooth surfaces4 ,arresters1 therefore1 are provided with points4 ,fig4 #bjc shows a device known as a saw-tooth arrester because of its metal plates being provided with teeth4 ,such an arrester brings a ground connection close to plates connected with the line and is adapted to protect apparatus either connected across a metallic circuit or in series with a single wire circuit4 ,fig4 #bja shows another form of metal plate air-gap arrester having the further possibility of a discharge taking place from one line wire to the other4 ,inserting a plug in the hole between the two line plates connects the line wires directly together at the arrester4 ,this practice was designed for use with series lines1 the plug short-circuiting the telephone set when in place4 ,a defect of most ordinary types of metal air-gap lightning arresters is that heavy discharges tend to melt the teeth or edges of the plates and often to weld them together1 requiring special attention to re-establish the necessary gap4 ,advantages of ,carbon3--,solid carbon is found to be a much better material than metal for the reasons that a discharge will not melt it and that its surface is composed of multitudes of points from which discharges take place more readily than from metals4 ,illustration ,fig4 #bjd4 ,saw-,tooth ,arrester ,illustration ,fig4 #bje4 ,carbon ,block ,arrester ,carbon arresters now are widely used in the general form shown in ,fig4 #bje4 ,a carbon block connected with a wire of the line is separated from a carbon block connected to ground by some form of insulating separator4 ,mica is widely used as such a separator1 and holes of some form in a mica slip enable the discharge to strike freely from block to block1 while preventing the blocks from touching each other4 ,celluloid with many holes is used as a separator between carbon blocks4 ,silk and various special compositions also have their uses4 ,illustration ,fig4 #bjf4 ,arrester ,separators ,dust ,between ,carbons3--,discharges between the carbon blocks tend to throw off particles of carbon from them4 ,the separation between the blocks being small--from 4#jje to 4#jae inch--the carbon particles may lodge in the air-gap1 on the edges of the separator1 or otherwise1 so as to leave a conducting path between the two blocks4 ,slight moisture on the separator may help to collect this dust1 thus placing a ground on that wire of the line4 ,this ground may be of very high resistance1 but is probably one of many such--one at each arrester connected to the line4 ,in special forms of carbon arresters an attempt has been made to limit this danger of grounding by the deposit of carbon dust4 ,the object of the ,u-shaped separator of ,fig4 #bjf is to enable the arrester to be mounted so that this opening in the separator is downward1 in the hope that loosened carbon particles may fall out of the space between the blocks4 ,the deposit of carbon on the inside edges of the ,u-shaped separator often is so fine and clings so tightly as not to fall out4 ,the separator projects beyond the blocks so as to avoid the collection of carbon on the outer edges4 ,commercial ,types3--,fig4 #bjg is a commercial form of the arrangement shown in ,fig4 #bje and is one of the many forms made by the ,american ,electric ,fuse ,company4 ,line wires are attached to outside binding posts shown in the figure and the ground wire to the metal binding post at the front4 ,the carbon blocks with their separator slide between clips and a ground plate4 ,the air-gap is determined by the thickness of the separator between the carbon blocks4 ,illustration3 ,fig4 #bjg4 ,carbon ,block ,arrester ,illustration3 ,fig4 #bjh ,roberts ,self-,cleaning ,arrester ,the ,roberts carbon arrester is designed with particular reference to the disposal of carbon dust and is termed self-cleaning for that reason4 ,the arrangement of carbons and dielectric in this device is shown in ,fig4 #bjh2 mica is cemented to the line carbon and is large enough to provide a projecting margin all around4 ,the spark gap is not uniform over the entire surface of the block but is made wedge-shaped by grinding away the line carbon as shown4 ,it is claimed that a continuous arcing fills the wedge-shaped chamber with heated air or gas1 converting the whole of the space into a field of low resistance to ground1 and that this gas in expanding drives out every particle of carbon that may be thrown off4 ,it seems obvious that the wedge-shaped space offers greater freedom for carbon dust to fall out than in the case of the parallel arrangement of the block faces4 ,an outdoor arrester for metallic circuits1 designed by ,f4,b4 ,cook1 is shown in ,fig4 #bji4 ,the device is adapted to mount on a pole or elsewhere and to be covered by a protecting cap4 ,the carbons are large and are separated by a special compound intended to assist the self-cleaning feature4 ,the three carbons being grouped together as a unit1 the device has the ability to care for discharges from one terminal to either of the others direct1 without having to pass through two gaps4 ,in this particular1 the arrangement is the same as that of ,fig4 #bjd4 ,illustration3 ,fig4 #bji4 ,cook ,air-,gap ,arrester ,a form of ,western ,electric arrester particularly adapted for outside use on railway lines is shown with its cover in ,fig4 #baj4 ,illustration3 ,fig4 #baj4 ,western ,electric ,air-,gap ,arrester ,the ,kellogg ,company regularly equips its magneto telephones with air-gap arresters of the type shown in ,fig4 #baa4 ,the two line plates are semicircular and of metal4 ,the ground plate is of carbon1 circular in form1 covering both line plates with a mica separator4 ,this is mounted on the back board of the telephone and permanently wired to the line and ground binding posts4 ,illustration3 ,,old ,,switchboard ,,of ,,bell ,,exchange ,,serving ,,chinatown1 ,,san ,,francisco1 ,,california ,illustration3 ,fig4 #baa4 ,kellogg ,air-,gap ,arrester ,vacuum ,arresters3--,all of the carbon arresters so far mentioned depend on the discharge taking place through air4 ,a given pressure will discharge further in a fairly good vacuum than in air4 ,the ,national ,electric ,specialty ,company mounts three conductors in a vacuum of the incandescent lamp type1 ,fig4 #bab4 ,a greater separation and less likelihood of short-circuiting can be provided in this way4 ,either carbon or metal plates are adapted for use in such vacuum devices4 ,the plates may be further apart for a given discharge pressure if the surfaces are of carbon4 ,illustration3 ,fig4 #bab4 ,vacuum ,arrester ,introduction of ,impedance3--,it has been noted that the existence of impedance tends to choke back the passage of lightning discharge through a coil4 ,fig4 #bac suggests the relation between such an impedance and air-gap arrester4 ,if the coil shown therein be considered an arrangement of conductors having inductance1 it will be seen that a favorable place for an air-gap arrester is between that impedance and the line4 ,this fact is made known in practice by frequent damage to arial cables by electricity brought into them over long open wires1 the discharge taking place at the first turn or bend in the arial cable2 this discharge often damages both core and sheath4 ,it is well to have such bends as near the end of the cable as possible1 and turns or goosenecks at entrances to terminals have that advantage4 ,illustration3 ,fig4 #bac4 ,impedance and ,air-,gap ,this same principle is utilized in some forms of arresters1 such as the one shown in ,fig4 #bad1 which provides an impedance of its own directly in the arrester element4 ,in this device an insulating base carries a grounded carbon rod and two impedance coils4 ,the impedance coils are wound on insulating rods1 which hold them near1 but not touching1 the ground carbon4 ,the coils are arranged so that they may be turned when discharges roughen the surfaces of the wires4 ,illustration3 ,fig4 #bad4 ,holtzer-,cabot ,arrester ,metallic ,electrodes3--,copper or other metal blocks with roughened surfaces separated by an insulating slip may be substituted for the carbon blocks of most of the arresters previously described4 ,metal blocks lack the advantage of carbon in that the latter allows discharges at lower potentials for a given separation1 but they have the advantage that a conducting dust is not thrown off from them4 ,illustration3 ,fig4 #bae4 ,carbon ,air-,gap ,arrester ,provision ,against ,continuous ,arc3--,for the purpose of short-circuiting an arc1 a globule of low-melting alloy may be placed in one carbon block of an arrester4 ,this feature is not essential in an arrester intended solely to divert lightning discharges4 ,its purpose is to provide an immediate path to ground if an arc arising from artificial electricity has been maintained between the blocks long enough to melt the globule4 ,fig4 #bae is a plan and section of the ,western ,electric ,company's arrester used as the high potential element in conjunction with others for abnormal currents and sneak currents2 the latter are currents too small to operate air-gap arresters or substantial fuses4 ,protection ,against ,strong ,currents4 ,fuses4 ,a fuse is a metal conductor of lower carrying capacity than the circuit with which it is in series at the time it is required to operate4 ,fuses in use in electrical circuits generally are composed of some alloy of lead1 which melts at a reasonably low temperature4 ,alloys of lead have lower conductivity than copper4 ,a small copper wire1 however1 may fuse at the same volume of current as a larger lead alloy wire4 ,proper ,functions3--,a fuse is not a good lightning arrester4 ,as lightning damage is caused by current and as it is current which destroys a fuse1 a lightning discharge can open a circuit over which it passes by melting the fuse metal4 ,but lightning may destroy a fuse and at the same discharge destroy apparatus in series with the fuse4 ,there are two reasons for this3 ,one is that lightning discharges act very quickly and may have destroyed apparatus before heating the fuse enough to melt it2 the other reason is that when a fuse is operated with enough current even to vaporize it1 the vapor serves as a conducting path for an instant after being formed4 ,this conducting path may be of high resistance and still allow currents to flow through it1 because of the extremely high pressure of the lightning discharge4 ,a comprehensive protective system may include fuses1 but it is not to be expected that they always will arrest lightning or even assist other things in arresting lightning4 ,they should be considered as of no value for that purpose4 ,furthermore1 fuses are best adapted to be a part of a general protective system when they do all that they must do in stopping abnormal currents and yet withstand lightning discharges which may pass through them4 ,other things being equal1 that system of protection is best in which all lightning discharges are arrested by gap arresters and in which no fuses ever are operated by lightning discharges4 ,mica ,fuse3--,a convenient and widely used form of fuse is that shown in ,fig4 #baf4 ,a mica slip has metal terminals at its ends and a fuse wire joins these terminals4 ,the fuse is inserted in the circuit by clamping the terminals under screws or sliding them between clips as in ,figs4 #bag and #bah4 ,advantages of this method of fuse mounting for protecting circuits needing small currents are that the fuse wire can be seen1 the fuses are readily replaced when blown1 and their mountings may be made compact4 ,as elements of a comprehensive protective system1 however1 the ordinary types of mica-slip fuses are objectionable because too short1 and because they have no means of their own for extinguishing an arc which may follow the blowing of the fuses4 ,as protectors for use in distributing low potential currents from central-office power plants they are admirable4 ,by simple means1 they may be made to announce audibly or visibly that they have operated4 ,illustration3 ,fig4 #baf4 ,mica ,slip ,fuse ,illustration3 ,fig4 #bag4 ,postal ,type ,mica ,fuse ,illustration3 ,fig4 #bah4 ,western ,union ,type ,mica ,fuse ,enclosed ,fuses3--,if a fuse wire within an insulating tube be made to connect metal caps on that tube and the space around the tube be filled with a non-conducting powder1 the gases of the vaporized fuse metal will be absorbed more quickly than when formed without such imbedding in a powder4 ,the filling of such a tubular fuse also muffles the explosion which occurs when the fuse is vaporized4 ,illustration3 ,fig4 #bai4 ,pair of ,enclosed ,fuses ,fuses of the enclosed type1 with or without filling1 are widely used in power circuits generally and are recommended by fire insurance bodies4 ,fig4 #bai illustrates an arrester having a fuse of the enclosed type1 this example being that of the ,h4 ,w4 ,johns-,manville ,company4 ,illustration ,fig4 #bbj4 ,bank of ,enclosed ,fuses ,in telephony it is frequently necessary to mount a large number of fuses or other protective devices together in a restricted space4 ,in ,fig4 #bbj a group of ,western ,electric tubular fuses1 so mounted1 is shown4 ,these fuses have ordinarily a carrying capacity of #f or #g amperes4 ,it is not expected that this arrester will blow because #f or #g amperes of abnormal currents are flowing through it and the apparatus to be protected4 ,what is intended is that the fuse shall withstand lightning discharges and when a foreign current passes through it1 other apparatus will increase that current enough to blow the fuse4 ,it will be noticed that the fuses of ,fig4 #bbj are open at the upper end1 which is the end connected to the exposed wire of the line ,the fuses are closed at the lower end1 which is the end connected to the apparatus4 ,when the fuse blows1 its discharge is somewhat muffled by the lining of the tube1 but enough explosion remains so that the heated gases1 in driving outward1 tend to break the arc which is established through the vaporized metal4 ,a pair of ,cook tubular fuses in an individual mounting is shown in ,fig4 #bba4 ,fuses of this type are not open at one end like a gun1 but opportunity for the heated gases to escape exists at the caps4 ,the tubes are made of wood1 of lava1 or of porcelain4 ,fig4 #bbb is another tubular fuse1 the section showing the arrangement of asbestos lining which serves the two purposes of muffling the sound of the discharge and absorbing and cooling the resulting gases4 ,illustration3 ,fig4 #bba4 ,pair of ,wooden ,tube ,fuses ,air-,gap vs4 ,fuse ,arresters4 ,it is hoped that the student grasps clearly the distinction between the purposes of air-gap and fuse arresters4 ,the air-gap arrester acts in response to high voltages1 either of lightning or of high-tension power circuits4 ,the fuse acts in response to a certain current value flowing through it and this minimum current in well-designed protectors for telephone lines is not very small4 ,usually it is several times larger than the maximum current apparatus in the line can safely carry4 ,fuses can be made so delicate as to operate on the very smallest current which could injure apparatus and the earlier protective systems depended on such an arrangement4 ,the difficulty with such delicate fuses is that they are not robust enough to be reliable1 and1 worse still1 they change their carrying capacity with age and are not uniform in operation in different surroundings and at different temperatures4 ,they are also sensitive to lightning discharges1 which they have no power to stop or to divert4 ,protection ,against ,sneak ,currents4 ,for these reasons1 a system containing fuses and air-gap arresters only1 does not protect against abnormal currents which are continuous and small1 though large enough to injure apparatus because continuous4 ,these currents have come to be known as sneak currents1 a term more descriptive than elegant4 ,sneak currents though small1 may1 when allowed to flow for a long time through the winding of an electromagnet for instance1 develop enough heat to char or injure the insulation4 ,they are the more dangerous because insidious4 ,illustration3 ,fig4 #bbb4 ,tubular ,fuse with ,asbestos ,filling ,sneak-,current ,arresters4 ,as typical of sneak-current arresters1 ,fig4 #bbc shows the principle1 though not the exact form1 of an arrester once widely used in telephone and signal lines4 ,the normal path from the line to the apparatus is through a small coil of fine wire imbedded in sealing wax4 ,a spring forms a branch path from the line and has a tension which would cause it to bear against the ground contact if it were allowed to do so4 ,it is prevented from touching that contact normally by a string between itself and a rigid support4 ,the string is cut at its middle and the knotted ends as thus cut are imbedded in the sealing wax which contains the coil4 ,illustration3 ,fig4 #bbc4 ,principle of ,sneak-,current ,arrester ,a small current through the little coil will warm the wax enough to allow the string to part4 ,the spring then will ground the line4 ,even so simple an apparatus as this operates with considerable accuracy4 ,all currents below a certain critical amount may flow through the heating coil indefinitely1 the heat being radiated rapidly enough to keep the wax from softening and the string from parting4 ,all currents above this critical amount will operate the arrester2 the larger the current1 the shorter the time of operating4 ,it will be remembered that the law of these heating effects is that the heat generated ,c#b,rt1 so that if a certain current operates the arrester in1 say #dj seconds1 twice as great a current should operate the arrester in #aj seconds4 ,in other words1 the time of operation varies inversely as the square of the current and inversely as the resistance4 ,to make the arrester more sensitive for a given current--i4e41 to operate in a shorter time--one would increase the resistance of the coil in the wax either by using more turns or finer wire1 or by making the wire of a metal having higher specific resistance4 ,the present standard sneak-current arrester embodies the two elements of the devices of ,fig4 #bbc3 a resistance material to transform the dangerous sneak current into localized heat2 and a fusible material softened by this heat to release some switching mechanism4 ,the resistance material is either a resistance wire or a bit of carbon1 the latter being the better material1 although both are good4 ,the fusible material is some alloy melting at a low temperature4 ,lead1 tin1 bismuth1 and cadmium can be combined in such proportions as will enable the alloy to melt at temperatures from #adj to #ahj ,f4 ,such an alloy is a solder which1 at ordinary temperatures1 is firm enough to resist the force of powerful springs2 yet it will melt so as to be entirely fluid at a temperature much less than that of boiling water4 ,illustration3 ,fig4 #bbd4 ,heat ,coil ,heat ,coil4 ,fig4 #bbd shows a practical way of bringing the heating and to-be-heated elements together4 ,a copper spool is wound with resistance wire4 ,a metal pin is soldered in the bore of the spool by an easily melting alloy4 ,when current heats the spool enough1 the pin may slide or turn in the spool4 ,it may slide or turn in many ways and this happily enables many types of arresters to result4 ,for example1 the pin may pull out1 or push in1 or push through1 or rotate like a shaft in a bearing1 or the spool may turn on it like a hub on an axle4 ,messrs4 ,hayes1 ,rolfe1 ,cook1 ,mc,berty1 ,kaisling1 and many other inventors have utilized these combinations and motions in the production of sneak-current arresters4 ,all of them depend on one action3 the softening of a low-melting alloy by heat generated in a resistance4 ,when a heat coil is associated with the proper switching springs1 it becomes a sneak-current arrester4 ,the switching springs always are arranged to ground the line wire4 ,in some arresters1 the line wire is cut off from the wire leading toward the apparatus by the same movement which grounds it4 ,in others1 the line is not broken at all1 but merely grounded4 ,each method has its advantages4 ,complete ,line ,protection4 ,fig4 #bbe shows the entire scheme of protectors in an exposed line and their relation to apparatus in the central-office equipment and at the subscriber's telephone4 ,the central-office equipment contains heat coils1 springs1 and carbon arresters4 ,at some point between the central office and the subscriber's premises1 each wire contains a fuse4 ,at the subscriber's premises each wire contains other fuses and these are associated with carbon arresters4 ,the figure shows a central battery equipment1 in which the ringer of the telephone is in series with a condenser4 ,a sneak-current arrester is not required at the subscriber's station with such equipment4 ,assume the line to meet an electrical hazard at the point ,x4 ,if this be lightning1 it will discharge to ground at the central office or at the subscriber's instrument or at both through the carbon arresters connected to that side of the line4 ,if it be a high potential from a power circuit and of more than #cej volts1 it will strike an arc at the carbon arrester connected to that wire of the line in the central office or at the subscriber's telephone or at both1 if the separation of the carbons in those arresters is 4#jje inch or less4 ,if the carbon arresters are separated by celluloid1 it will burn away and allow the carbons to come together1 extinguishing the arc4 ,if they are separated by mica and one of the carbons is equipped with a globule of low-melting alloy1 the heat of the arc will melt this1 short-circuiting the gap and extinguishing the arc4 ,the passage of current to ground at the arrester1 however1 will be over a path containing nothing but wire and the arrester4 ,the resulting current1 therefore1 may be very large4 ,the voltage at the arrester having been #cej volts or more1 in order to establish the arc1 short-circuiting the gap will make the current #g amperes or more1 unless the applied voltage miraculously falls to #ej volts or less4 ,the current through the fuse being more than #g amperes1 it will blow promptly1 opening the line and isolating the apparatus4 ,it will be noted that this explanation applies to equipment at either end of the line1 as the fuse lies between the point of contact and the carbon arrester4 ,illustration3 ,fig4 #bbe4 ,complete ,line ,protection ,assume1 on the other hand1 that the contact is made at the point ,y4 ,the central-office carbon arrester will operate1 grounding the line and increasing the amount of current flowing4 ,there being no fuse to blow1 a worse thing will befall1 in the overheating of the line wire and the probable starting of a fire in the central office4 ,it is obvious1 therefore1 that a fuse must be located between the carbon arrester and any part of the line which is subject to contact with a potential which can give an abnormal current when the carbon arrester acts4 ,assume1 as a third case1 that the contact at the point ,x either is with a low foreign potential or is so poor a contact that the difference of potential across the gap of the carbon arrester is lower than its arcing point4 ,current will tend to flow by the carbon arrester without operating it1 but such a current must pass through the winding of the heat coil if it is to enter the apparatus4 ,the sneak current may be large enough to overheat the apparatus if allowed to flow long enough1 but before it has flowed long enough it will have warmed the heat-coil winding enough to soften its fusible alloy and to release springs which ground the line1 just as did the carbon arrester in the case last assumed4 ,again the current will become large and will blow the fuse which lies between the sneak-current arrester and the point of contact with the source of foreign current4 ,in this case1 also1 contact at the point ,y would have operated mechanism to ground the line at the central office1 and1 no fuse interposing1 the wiring would have been overheated4 ,exposed and ,unexposed ,wiring4 ,underground cables1 cables formed of rubber insulated wires1 and interior wiring which is properly done1 all may be considered to be wiring which is unexposed1 that is1 not exposed to foreign high potentials1 discharges1 sneak1 or abnormal currents4 ,all other wiring1 such as bare wires1 arial cables1 etc41 should be considered as exposed to such hazards and a fuse should exist in each wire between its exposed portion and the central office or subscriber's instrument4 ,the rule of action1 therefore1 becomes3 ,the proper position of the fuse is between exposed and unexposed wiring4 ,it may appear to the student that wires in an arial cable with a lead sheath--that sheath being either grounded or ungrounded--are not exposed to electrical hazards2 in the case of the grounded sheath1 this would presume that a contact between the cable and a high potential wire would result merely in the foreign currents going to ground through the cable sheath1 the arc burning off the high-potential wire and allowing the contact to clear itself by the falling of the wire4 ,if the assumption be that the sheath is not grounded1 then the student may say that no current at all would flow from the high-potential wire4 ,both assumptions are wrong4 ,in the case of the grounded sheath1 the current flows to it at the contact with the high-potential wire2 the lead sheath is melted1 arcs strike to the wires within1 and currents are led directly to the central office and to subscribers' premises4 ,in the case of the ungrounded sheath1 the latter charges at once through all its length to the voltage of the high-potential wire2 at some point1 a wire within the cable is close enough to the sheath for an arc to strike across1 and the trouble begins4 ,all the wires in the cable are endangered if the cross be with a wire of the primary circuit of a high-tension transmission line4 ,any series arc-light circuit is a high-potential menace4 ,even a #dej-volt trolley wire or feeder can burn a lead-covered cable entirely in two in a few seconds4 ,the authors have seen this done by the wayward trolley pole of a street car1 one side of the pole touching the trolley wire and the extreme end just touching the telephone cable4 ,the answer lies in the foregoing rule4 ,place the fuse between the wires which can and the wires which can not get into contact with high potentials4 ,in application1 the rule has some flexibility4 ,in the case of a cable which is arial as soon as it leaves the central office1 place the fuses in the central office2 in a cable wholly underground1 from central office to subscriber--as1 for example1 the feed for an office building--use no fuses at all2 in a cable which leaves the central office underground and becomes arial1 fuse the wires just where they change from underground to arial4 ,the several branches of an underground cable into arial ones should be fused as they branch4 ,wires properly installed in subscribers' premises are considered unexposed4 ,the position of the fuse thus is at or near the point of entrance of the wires into that building if the wires of the subscriber's line outside the premises are exposed1 as determined by the definitions given4 ,if the line is unexposed1 by those definitions1 no protector is required4 ,if one is indicated1 it should be used1 as compliance with the best-known practice is a clear duty4 ,less than what is known to be best is not honest practice in a matter which involves life1 limb1 and indefinite degrees of property values4 ,protectors in central-battery subscribers' equipments need no sneak-current arresters1 as the condenser reduces that hazard to a negligible amount4 ,magneto subscribers' equipments usually lack condensers in ringer circuits1 though they may have them in talking circuits on party lines4 ,the ringer circuit is the only path through the telephone set for about #ih per cent of the time4 ,sneak-current arresters1 therefore1 should be a part of subscribers' station protectors in magneto equipment1 except in such rural districts as may have no lighting or power wires4 ,when sneak-current arresters are so used the arrangement of the parts then is the same as in the central-office portion of ,fig4 #bbe4 ,types of ,central-,office ,protectors4 ,a form of combined heat coil and air-gap arrester1 widely used by ,bell companies for central-office protection1 is shown in ,fig4 #bbf4 ,the two inner springs form the terminals for the two limbs of the metallic-circuit line1 while the two outside springs are terminals for the continuation of the line leading to the switchboard4 ,the heat coils1 one on each side1 are supported between the inner and outer springs4 ,high-tension currents jump to ground through the air-gap arrester1 while sneak currents permit the pin of the heat coil to slide within the sleeve1 thus grounding the outside line and the line to the switchboard4 ,illustration3 ,fig4 #bbf4 ,sneak-,current and ,air-,gap ,arrester ,self-,soldering ,heat ,coils4 ,another form designed by ,kaisling and manufactured by the ,american ,electric ,fuse ,company is shown in ,fig4 #bbg4 ,in this the pin in the heat coil projects unequally from the ends of the coil1 and under the action of a sneak current the melting of the solder which holds it allows the outer spring to push the pin through the coil until it presses the line spring against the ground plate and at the same time opens the path to the switchboard4 ,when the heat-coil pin assumes this new position it cools off1 due to the cessation of the current1 and resolders itself1 and need only be turned end for end by the attendant to be reset4 ,many are the variations that have been made on this self-soldering idea1 and there has been much controversy as to its desirability4 ,it is certainly a feature of convenience4 ,illustration3 ,fig4 #bbg4 ,self-,soldering ,heat-,coil ,arrester ,instead of using a wire-wound resistance element in heat-coil construction some manufacturers employ a mass of high-resistance material1 interposed in the path of the current4 ,the ,kellogg ,company has long employed for its sneak-current arrester a short graphite rod1 which forms the resistance element4 ,the ends of this rod are electroplated with copper to which the brass terminal heads are soldered4 ,these heads afford means for making the connection with the proper retaining springs4 ,illustration3 ,fig4 #bbh4 ,cook ,arrester ,another central-office protector1 which uses a mass of special metal composition for its heat producing element is that designed by ,frank ,b4 ,cook and shown in ,fig4 #bbh4 ,in this the carbon blocks are cylindrical in form and specially treated to make them self-cleaning4 ,instead of employing a self-soldering feature in the sneak-current arrester of this device1 ,cook provides for electrically resoldering them after operation1 a clip being designed for holding the elements in proper position and passing a battery current through them to remelt the solder4 ,in small magneto exchanges it is not uncommon to employ combined fuse and air-gap arresters for central-office line protection1 the fuses being of the mica-mounted type already referred to4 ,a group of such arresters1 as manufactured by the ,dean ,electric ,company1 is shown in ,fig4 #bbi4 ,illustration3 ,fig4 #bbi4 ,mica ,fuse and ,air-,gap ,arresters ,types of ,subscribers' ,station ,protectors4 ,figs4 #bcj and #bca show types of subscribers' station protectors adapted to the requirements of central-battery and magneto systems4 ,these1 as has been said1 should be mounted at or near the point of entrance of the subscriber's line into the premises1 if the line is exposed outside of the premises4 ,it is possible to arrange the fuses so that they will be safe and suitable for their purposes if they are mounted out-of-doors near the point of entrance to the premises4 ,the sneak-current arrester1 if one exists1 and the carbon arrester also1 must be mounted inside of the premises or in a protecting case1 if outside1 on account of the necessity of shielding both of these devices from the weather4 ,speaking generally1 the wider practice is to put all the elements of the subscriber's station protector inside of the house4 ,it is nearer to the ideal arrangement of conditions if the protector be placed immediately at the point of entrance of the outside wires into the building4 ,illustration3 ,fig4 #bcj4 ,western ,electric ,station ,arrester ,illustration3 ,fig4 #bca4 ,cook ,arrester for ,magneto ,stations ,ribbon ,fuses4 ,a point of interest with relation to tubular fuses is that in some of the best types of such fuses1 the resistance material is not in the form of a round wire but in the form of a flat ribbon4 ,this arrangement disposes the necessary amount of fusible metal in a form to give the greatest amount of surface1 while a round wire offers the least surface for a given weight of metal--a circle encloses its area with less periphery than any other figure4 ,the reason for giving the fuse the largest possible surface area is to decrease the likelihood of the fuse being ruptured by lightning4 ,the fact that such fuses do withstand lightning discharges much more thoroughly than round fuses of the same rating is an interesting proof of the oscillating nature of lightning discharges1 for the density of the current of those discharges is greater on and near the surface of the conductor than within the metal and1 therefore1 flattening the fuse increases its carrying capacity for high-frequency currents1 without appreciably changing its carrying capacity for direct currents4 ,the reason its capacity for direct currents is increased at all by flattening it1 is that the surface for the radiation of heat is increased4 ,however1 when enclosed in a tube1 radiation of heat is limited1 so that for direct currents the carrying capacity of fuses varies closely with the area of cross-section4 ,city-,exchange ,requirements4 ,the foregoing has set down the requirements of good practice in an average city-exchange system4 ,nothing short of the general arrangement shown in ,fig4 #bbe meets the usual assortment of hazards of such an exchange4 ,it is good modern practice to distribute lines by means of cables1 supplemented in part by short insulated drop wires twisted in pairs4 ,absence of bare wires reduces electrical hazards enormously4 ,nevertheless1 hazards remain4 ,though no less than the spirit of this plan of protection should be followed1 additional hazards may exist1 which may require additional elements of protection4 ,at the end of a cable1 either arial or underground1 long open wires may extend into the open country as rural or long-distance circuits4 ,if these be longer than a mile or two1 in most regions they will be subjected to lightning discharges4 ,these may be subjected to high-potential contacts as well4 ,if a specific case of such exposure indicates that the cables may be in danger1 the long open lines then are equipped with additional air-gap arresters at the point of junction of those open lines with the cable4 ,practice varies as to the type4 ,maintenance charges are increased if carbon arresters separated 4#jje inch are used1 because of the cost of sending to the end of the long cable to clear the blocks from carbon dust after each slight discharge4 ,roughened metal blocks do not become grounded as readily as do carbon blocks4 ,the occasions of visit to the arresters1 therefore1 usually follow actual heavy discharges through them4 ,the recommendations and the practice of the ,american ,telephone and ,telegraph ,company differ on this point1 while the practice of other companies varies with the temperaments of the engineers4 ,the ,american ,company specifies copper-block arresters where long country lines enter cables1 if those lines are exposed to lightning discharges only4 ,the exposed line is called long if more than one-half mile in length4 ,if it is exposed to high-potential hazards1 carbon blocks are specified instead of copper4 ,other specifications of that company have called for the use of copper-block arresters on lines exposed to hazards above #b1ejj volts4 ,illustration3 ,,one ,,of ,,the ,,four ,,wings ,,of ,,the ,,old ,,kellogg ,,divided ,,multiple ,,board ,,of ,,the ,,cuyahoga ,,telephone ,,company1 ,,cleveland1 ,,ohio ,ultimate ,capacity1 #bd1jjj ,lines4 ,one of the ,two ,examples in the ,united ,states of a ,multiple ,switchboard ,having an ,ultimate ,capacity over #ah1jjj ,lines4 ,replaced ,recently by a ,kellogg ,straight ,multiple ,board ,having an ,ultimate ,capacity of #ah1jjj ,lines and a ,present ,capacity of #aj1jjj ,lines4 ,the freedom of metal-block arresters from dust troubles gives them a large economical advantage over carbon4 ,for similar separations1 the ratio of striking voltages between carbon blocks and metal blocks respectively is as #g to #af4 ,in certain regions of the ,pacific ,coast where the lightning hazard is negligible and the high tension hazard is great1 metal-block arresters at the outer ends of cables give acceptable protection4 ,high winds which drive snow or dust against bare wires of a long line1 create upon or place upon those wires a charge of static electricity which makes its way from the line in such ways as it can4 ,usually it discharges across arresters and when this discharge takes place1 the line is disturbed in its balance and loud noises are heard in the telephones upon it4 ,fig4 #bcb4 ,drainage ,coils ,a telephone line which for a long distance is near a high-tension transmission line may have electrostatic or electromagnetic potentials1 or both1 induced upon it4 ,if the line be balanced in its properties1 including balance by transposition of its wires1 the electrostatic induction may neutralize itself4 ,the electromagnetic induction still may disturb it4 ,drainage ,coils4 ,the device shown in ,fig4 #bcb1 which amounts merely to an inductive leak to earth1 is intended to cure both the snowstorm and electromagnetic induction difficulties4 ,it is required that its impedance be high enough to keep voice-current losses low1 while being low enough to drain the line effectively of the disturbing charges4 ,such devices are termed drainage coils4 ,electrolysis4 ,the means of protection against the danger due to chemical action1 set forth in the preceding chapter1 form such a distinct phase of the subject of guarding property against electrical hazards as to warrant treatment in a separate chapter devoted to the subject of electrolysis4 ,illustration3 ,,main ,,exchange1 ,,cleveland1 ,,ohio4 ,largest ,four-,party ,selective ,ringing ,switchboard in the ,world4 ,kellogg ,switchboard and ,supply ,co4 ,,chapter ,,xx ,,general ,,features ,,of ,,the ,,telephone ,,exchange ,up to this point only those classes of telephone service which could be given between two or more stations on a single line have been considered4 ,very soon after the practical conception of the telephone1 came the conception of the telephone exchange2 that is1 the conception of centering a number of lines at a common point and there terminating them in apparatus to facilitate their interconnection1 so that any subscriber on any line could talk with any subscriber on any other line4 ,the complete equipment of lines1 telephone instruments1 and switching facilities by which the telephone stations of the community are given telephone service is called a telephone exchange4 ,the building where a group of telephone lines center for interconnection is called a central office1 and its telephonic equipment the central-office equipment4 ,the terms telephone office and telephone exchange are frequently confused4 ,although a telephone office building may be properly referred to as a telephone exchange building1 it is hardly proper to refer to the telephone office as a telephone exchange1 as is frequently done4 ,in modern parlance the telephone exchange refers not only to the central office and its equipment but to the lines and instruments connected therewith as well2 furthermore1 a telephone exchange may embrace a number of telephone offices that are interconnected by means of so-called trunk lines for permitting the communication of subscribers whose lines terminate in one office with those subscribers whose lines terminate in any other office4 ,since a given telephone exchange may contain one or more central offices1 it is proper to distinguish between them by referring to an exchange which contains but a single central office as a single office exchange1 and to an exchange which contains a plurality of central offices as a multi-office exchange4 ,in telephone exchange working1 three classes of lines are dealt with--subscribers' lines1 trunk lines1 and toll lines4 ,subscribers' ,lines4 ,the term subscriber is commonly applied to the patron of the telephone service4 ,his station is1 therefore1 referred to as a subscriber's station1 and the telephone equipment at any subscriber's station is referred to as a subscriber's station equipment4 ,likewise1 a line leading from a central office to one or more subscribers' stations is called a subscriber's line4 ,a subscriber's line may1 as has been shown in a previous chapter1 be an individual line if it serves but one station1 or a party line if it serves to connect more than one station with the central office4 ,trunk ,lines4 ,a trunk line is a line which is not devoted to the service of any particular subscriber1 but which may form a connecting link between any one of a group of subscribers' lines which terminate in one place and any one of a group of subscribers' lines which terminate in another place4 ,if the two groups of subscribers' lines terminate in the same building or in the same switchboard1 so that the trunk line forming the connecting link between them is entirely within the central-office building1 it is called a local trunk line1 or a local trunk4 ,if1 on the other hand1 the trunk line is for connecting groups of subscribers' lines which terminate in different central offices1 it is called an inter-office trunk4 ,toll ,lines4 ,a toll line is a telephone line for the use of which a special fee or toll is charged2 that is1 a fee that is not included in the charges made to the subscriber for his regular local exchange service4 ,toll lines extend from one exchange district to another1 more or less remote1 and they are commonly termed local toll and long-distance toll lines according to the degree of remoteness4 ,a toll line1 whether local or long-distance1 may be looked upon in the nature of an inter-exchange trunk4 ,districts4 ,the district in a given community which is served by a single central office is called an office district4 ,likewise1 the district which is served by a complete exchange is called an exchange district4 ,an exchange district may1 therefore1 consist of a number of central-office districts1 just as an exchange may comprise a number of central offices4 ,to illustrate1 the entire area served by the exchange of the ,chicago ,telephone ,company in ,chicago1 embracing the entire city and some of its suburbs1 is the ,chicago exchange district4 ,the area served by one of the central offices1 such as the ,hyde ,park office1 the ,oakland office1 the ,harrison office1 or any of the others1 is an office district4 ,switchboards4 ,the apparatus at the central office by which the telephone lines are connected for conversation and afterwards disconnected1 and by which the various other functions necessary to the giving of complete telephone service are performed1 is called a switchboard4 ,this may be simple in the case of small exchanges1 or of vast complexity in the case of the larger exchanges4 ,sometimes the switchboards are of such nature as to require the presence of operators1 usually girls1 to connect and disconnect the line and perform the other necessary functions1 and such switchboards1 whether large or small1 are termed manual4 ,sometimes the switchboards are of such a nature as not to require the presence of operators1 the various functions of connection1 disconnection1 and signaling being performed by the aid of special forms of apparatus which are under the control of the subscriber who makes the call4 ,such switchboards are termed automatic4 ,of recent years there has appeared another class of switchboards1 employing in some measure the features of the automatic and in some measure those of the manual switchboard4 ,these boards are commonly referred to as semi-automatic switchboards1 presumably because they are supposed to be half automatic and half manual4 ,manual4 ,manual switchboards may be subdivided into two classes according to the method of distributing energy for talking purposes4 ,thus we may have magneto switchboards1 which are those capable of serving lines equipped with magneto telephones1 local batteries being used for talking purposes4 ,on the other hand1 we may have common-battery switchboards1 adapted to connect lines employing common-battery telephones in which all the current for both talking and signaling is furnished from the central office4 ,in still another way we may classify manual switchboards if the method of distributing the energy for talking and signaling purposes is ignored4 ,thus1 entirely irrespective of whether the switchboards are adapted to serve common-battery or local-battery lines1 we may have non-multiple switchboards and multiple switchboards4 ,the term multiple switchboard is applied to that class of switchboards in which the connection terminals or jacks for all the lines are repeated at intervals along the face of the switchboard1 so that each operator may have within her reach a terminal for each line and may thus be able to complete by herself any connection between two lines terminating in the switchboard4 ,the term non-multiple switchboard is applied to that class of boards where the provision for repeating the line terminals at intervals along the face of the board is not employed1 but where1 as a consequence1 each line has but a single terminal on the face of the board4 ,non-multiple switchboards have their main use in small exchanges where not more than a few hundred lines terminate4 ,where such is the case1 it is an easy matter to handle all the traffic by one1 two1 or three operators1 and as all of these operators may reach all over the face of the switchboard1 there is no need for giving any line any more than one connection terminal4 ,such boards may be called simple switchboards4 ,there is another type of non-multiple switchboard adaptable for use in larger exchanges than the simple switchboard4 ,a correct idea of the fundamental principle involved in these may be had by imagining a row of simple switchboards each containing terminals or jacks for its own group of lines4 ,in order to provide for the connection of a line in one of these simple switchboards with a line in another one1 out of reach of the operator at the first1 short connecting lines extending between the two switchboards are provided1 these being called transfer or trunk lines4 ,in order that connections may be made between any two of the simple boards1 a group of transfer lines is run from each board to every other one4 ,in such switchboards an operator at one of the boards or positions may complete the connection herself between any two lines terminating at her own board4 ,if1 however1 the line called for terminates at another one of the boards1 the operator makes use of the transfer or trunk line extending to that board1 and the operator at this latter board completes the connection1 so that the two subscribers' lines are connected through the trunk or transfer line4 ,a distinguishing feature1 therefore1 in the operation of so-called transfer switchboards1 is that an operator can not always complete a connection herself1 the connection frequently requiring the attention of two operators4 ,transfer systems are not now largely used1 the multiple switchboard having almost entirely supplanted them in manual exchanges of such size as to be beyond the limitation of the simple switchboard4 ,at multi-office manual exchanges1 however1 where there are a number of multiple switchboards employed at various central offices1 the same sort of a requirement exists as that which was met by the provision of trunk lines between the various simple switchboards in a transfer system4 ,obviously1 the lines in one central office must be connected to those of another in order to give universal service in the community in which the exchange operates4 ,for this purpose inter-office trunk lines are used1 the arrangement being such that when an operator at one office receives a call for a subscriber in another office1 she will proceed to connect the calling subscriber's line1 not directly with the line of the called subscriber because that particular line is not within her reach1 but rather with a trunk line leading to the office in which the called-for subscriber's line terminates2 having done this she will then inform an operator at that second office of the connection desired1 usually by means of a so-called order-wire circuit4 ,the connection between the trunk line so used and the line of the called-for subscriber will then be completed by the connecting link or trunk line extending between the two offices4 ,in such cases the multiple switchboard at each office is divided into two portions1 termed respectively the ,a board and the ,b board4 ,each of these boards1 with the exception that will be pointed out in a subsequent chapter1 is provided with a full complement of multiple jacks for all of the lines entering that office4 ,at the ,a board are located operators1 called ,a operators1 who answer all the calls from the subscribers whose lines terminate in that office4 ,in the case of calls for lines in that same office1 they complete the connection themselves without the assistance of the other operators4 ,on the other hand1 the calls for lines in another office are handled through trunk lines leading to that other office1 as before described1 and these trunk lines always terminate in the ,b board at that office4 ,the ,b operators are1 therefore1 those operators who receive the calls over trunk lines and complete the connection with the line of the subscriber desired4 ,to define these terms more specifically1 an ,a board is a multiple switchboard in which the subscriber's lines of a given office district terminate4 ,for this reason the ,a board is frequently referred to as a subscribers' board1 and the operators who work at these boards and who answer the calls of the subscribers are called ,a operators or subscribers' operators4 ,b boards are switchboards in which terminate the incoming ends of the trunk lines leading from other offices in the same exchange4 ,these boards are frequently called incoming trunk boards1 or merely trunk boards1 and the operators who work at them and who receive the directions from the ,a operators at the other boards are called ,b operators1 or incoming trunk operators4 ,the circuits which are confined wholly to the use of operators and over which the instructions from one operator to another are sent1 as in the case of the ,a operator giving an order for a connection to a ,b operator at another switchboard1 are designated call circuits or order wire circuits4 ,sometimes trunk lines are so arranged that connections may be originated at either of their ends4 ,in other cases they are so arranged that one group of trunk lines connecting two offices is for the traffic in one direction only1 while another group leading between the same two offices is for handling only the traffic in the other direction4 ,trunk lines are called one-way or two-way trunks1 according to whether they handle the traffic in one direction or in two4 ,a trunking system1 where the same trunks handle traffic both ways1 is called a single-track system2 and1 on the other hand1 a system in which there are two groups of trunks1 one handling traffic in one direction and the other in the other1 is called a double-track system4 ,this nomenclature is obviously borrowed from railroad practice4 ,there is still another class of manual switchboards called the toll board of which it will be necessary to treat4 ,telephone calls made by one person for another within the limits of the same exchange district are usually charged for either by a flat rate per month1 or by a certain charge for each call4 ,this is usually regardless of the duration of the conversation following the call4 ,on the other hand1 where a call is made by one party for another outside of the limits of the exchange district and1 therefore1 in some other exchange district1 a charge is usually made1 based on the time that the connecting long-distance line is employed4 ,such calls and their ensuing conversations are charged for at a very much higher rate than the purely local calls1 this rate depending on the distance between the stations involved4 ,the making up of connections between a long-distance and a local line is usually done by means of operators other than those employed in handling the local calls1 who work either by means of special equipment located on the local board1 or by means of a separate board4 ,such equipments for handling long-distance or toll traffic are commonly termed toll switchboards4 ,they differ from local boards 7a7 in that they are arranged for a very much smaller number of lines2 7b7 in that they have facilities by which the toll operator may make up the connections with a minimum amount of labor on the part of the assisting local operators2 and 7c7 in that they have facilities for recording the identification of the parties and timing the conversations taking place over the toll lines1 so that the proper charge may be made to the proper subscriber4 ,,chapter ,,xxi ,,the ,,simple ,,magneto ,,switchboard ,definitions4 ,as already stated those switchboards which are adapted to work in conjunction with magneto telephones are called magneto switchboards4 ,the signals on such switchboards are electromagnetic devices capable of responding to the currents of the magneto generators at the subscribers' stations4 ,since1 as a rule1 magneto telephones are equipped with local batteries1 it follows that the magneto switchboard does not need to be arranged for supplying the subscribers' stations with talking current4 ,this fact is accountable for magneto switchboards often being referred to as local-battery switchboards1 in contradistinction to common-battery switchboards which are equipped so as to supply the connected subscribers' stations with talking current4 ,the term simple as applied in the headings of this and the next chapter1 is employed to designate switchboards adapted for so small a number of lines that they may be served by a single or a very small group of operators2 each line is provided with but a single connection terminal and all of them1 without special provision1 are placed directly within the reach of the operator1 or operators if there are more than one4 ,this distinction will be more apparent under the discussion of transfer and multiple switchboards4 ,mode of ,operation4 ,the cycle of operation of any simple manual switchboard may be briefly outlined as follows3 ,the subscriber desiring a connection transmits a signal to the central office1 the operator seeing the signal makes connection with the calling line and places herself in telephonic communication with the calling subscriber to receive his orders2 the operator then completes the connection with the line of the called subscriber and sends ringing current out on that line so as to ring the bell of that subscriber2 the two subscribers then converse over the connected lines and when the conversation is finished either one or both of them may send a signal to the central office for disconnection1 this signal being called a clearing-out signal2 upon receipt of the clearing-out signal1 the operator disconnects the two lines and restores all of the central-office apparatus involved in the connection to its normal position4 ,component ,parts4 ,before considering further the operation of manual switchboards it will be well to refer briefly to the component pieces of apparatus which go to make up a switchboard4 ,line ,signal4 ,the line signal in magneto switchboards is practically always in the form of an electromagnetic annunciator or drop4 ,it consists in an electromagnet adapted to be included in the line circuit1 its armature controlling a latch1 which serves to hold the drop or shutter or target in its raised position when the magnet is not energized1 and to release the drop or shutter or target so as to permit the display of the signal when the magnet is energized4 ,the symbolic representation of such an electromagnetic drop is shown in ,fig4 #bcc4 ,illustration3 ,fig4 #bcc4 ,drop ,symbol ,jacks and ,plugs4 ,each line is also provided with a connection terminal in the form of a switch socket4 ,this assumes many forms1 but always consists in a cylindrical opening behind which are arranged one or more spring contacts4 ,the opening forms a receptacle for plugs which have one or more metallic terminals for the conductors in the flexible cord in which the plug terminates4 ,the arrangement is such that when a plug is inserted into a jack the contacts on the plug will register with certain of the contacts in the jack and thus continue the line conductors1 which terminate in the jack contacts1 to the cord conductors1 which terminate in the plug contacts4 ,usually also when a plug is inserted certain of the spring contacts in the jack are made to engage with or disengage other contacts in the jack so as to make or break auxiliary circuits4 ,illustration3 ,fig4 #bcd4 ,spring ,jack ,a simple form of spring jack is shown in section in ,fig4 #bcd4 ,in ,fig4 #bce is shown a sectional view of a plug adapted to co-operate with the jack of ,fig4 #bcd4 ,in ,fig4 #bcf the plug is shown inserted into the jack4 ,the cylindrical portion of the jack is commonly called the sleeve or thimble and it usually forms one of the main terminals of the jack2 the spring1 forming the other principal terminal1 is called the tip spring1 since it engages the tip of the plug4 ,the tip spring usually rests on another contact which may be termed the anvil4 ,when the plug is inserted into the jack as shown in ,fig4 #bcf1 the tip spring is raised from contact with this anvil and thus breaks the circuit leading through it4 ,it will be understood that spring jacks are not limited to three contacts such as shown in these figures nor are plugs limited to two contacts4 ,sometimes the plugs have three1 and even more1 contacts1 and frequently the jacks corresponding to such plugs have not only a contact spring adapted to register with each of the contacts of the plug1 but several other auxiliary contacts also1 which will be made or broken according to whether the plug is inserted or withdrawn from the jack4 ,symbolic representations of plugs and jacks are shown in ,fig4 #bcg4 ,these are employed in diagrammatic representations of circuits and are supposed to represent the essential elements of the plugs and jacks in such a way as to be suggestive of their operation4 ,it will be understood that such symbols may be greatly modified to express the various peculiarities of the plugs and jacks which they represent4 ,illustration3 ,fig4 #bce4 ,plug ,illustration3 ,fig4 #bcf4 ,plug and ,jack ,illustration3 ,fig4 #bcg4 ,jack and ,plug ,symbols ,keys4 ,other important elements of manual switchboards are ringing and listening keys4 ,these are the devices by means of which the operator may switch the central-office generator or her telephone set into or out of the circuit of the connected lines4 ,the details of a simple ringing and listening key are shown in ,fig4 #bch4 ,this consists of two groups of springs1 one of four and one of six1 the springs in each group being insulated from each other at their points of mounting4 ,two of these springs #a and #b in one group--the ringing group--are longer than the others1 and act as movable levers engaging the inner pair of springs #c and #d when in their normal positions1 and the outer pair #e and #f when forced into their alternate positions4 ,movement is imparted to these springs by the action of a cam which is mounted on a lever1 manipulated by the operator4 ,when this lever is moved in one direction the cam presses the two springs #a and #b apart1 thus causing them to disengage the springs #c and #d and to engage the springs #e and #f4 ,illustration3 ,fig4 #bch4 ,ringing and ,listening ,key ,the springs of the other group constitute the switching element of the listening key and are very similar in their action to those of the ringing key1 differing in the fact that they have no inner pair of springs such as #c and #d4 ,the two long springs #g and #h1 therefore1 normally do not rest against anything1 but when the key lever is pressed1 so as to force the cam between them1 they are made to engage the two outer springs #i and #aj4 ,illustration3 ,fig4 #bci4 ,ringing-and ,listening-,key ,symbols ,the design and construction of ringing and listening keys assume many different forms4 ,in general1 however1 they are adapted to do exactly the same sort of switching operations as that of which the device of ,fig4 #bch is capable4 ,easily understood symbols of ringing and listening keys are shown in ,fig4 #bci2 the cam member which operates on the two long springs is usually omitted for ease of illustration4 ,it will be understood in considering these symbols1 therefore1 that the two long curved springs usually rest against a pair of inner contacts in case of the ringing key or against nothing at all in case of the listening key1 and that when the key is operated the two springs are assumed to be spread apart so as to engage the outer pair of contacts with which they are respectively normally disconnected4 ,line and ,cord ,equipments4 ,the parts of the switchboard that are individual to the subscriber's line are termed the line equipment2 this1 in the case of a magneto switchboard1 consists of the line drop and the jack together with the associated wiring necessary to connect them properly in the line circuit4 ,the parts of the switchboard that are associated with a connecting link--consisting of a pair of plugs and associated cords with their ringing and listening keys and clearing-out drop--are referred to as a cord equipment4 ,the circuit of a complete pair of cords and plugs with their associated apparatus is called a cord circuit4 ,in order that there may be a number of simultaneous connections between different pairs of lines terminating in a switchboard1 a number of cord circuits are provided1 this number depending on the amount of traffic at the busiest time of the day4 ,operator's ,equipment4 ,a part of the equipment that is not individual to the lines or to the cord circuits1 but which may1 as occasion requires1 be associated with any of them is called the operator's equipment4 ,this consists of the operator's transmitter and receiver1 induction coil1 and battery connections together with the wiring and other associated parts necessary to co-ordinate them with the rest of the apparatus4 ,still another part of the equipment that is not individual to the lines nor to the cord circuits is the calling-current generator4 ,this may be common to the entire office or a separate one may be provided for each operator's position4 ,operation in ,detail4 ,with these general statements in mind we may take up in some detail the various operations of a telephone system wherein the lines center in a magneto switchboard4 ,this may best be done by considering the circuits involved1 without special regard to the details of the apparatus4 ,the series of figures showing the cycle of operations of the magneto switchboard about to be discussed are typical of this type of switchboard almost regardless of make4 ,the apparatus is in each case represented symbolically1 the representations indicating type rather than any particular kind of apparatus within the general class to which it belongs4 ,normal ,condition of ,line4 ,in ,fig4 #bdj is shown the circuit of an ordinary magneto line4 ,the subscriber's sub-station apparatus1 shown at the left1 consists of the ordinary bridging telephone but might with equal propriety be indicated as a series telephone4 ,the subscriber's station is shown connected with the central office by the two limbs of a metallic-circuit line4 ,one limb of the line terminates in the spring #a of the jack1 and the other limb in the sleeve or thimble #b of the jack4 ,the spring #a normally rests on the third contact or anvil #c in the jack1 its construction being such that when a plug is inserted this spring will be raised by the plug so as to break contact with the anvil #c4 ,it is understood1 of course1 that the plug associated with this jack has two contacts1 referred to respectively as the tip and the sleeve2 the tip makes contact with the tip spring #a and the sleeve with the sleeve or thimble #b4 ,illustration3 ,fig4 #bdj4 ,normal ,condition of ,line ,the drop or line signal is permanently connected between the jack sleeve and the anvil #c4 ,as a result1 the drop is normally bridged across the circuit of the line so as to be in a receptive condition to signaling current sent out by the subscriber4 ,it is evident1 however1 that when the plug is inserted into the jack this connection between the line and the drop will be broken4 ,in this normal condition of the line1 therefore1 the drop stands ready at the central office to receive the signal from the subscriber and the generator at the sub-station stands ready to be bridged across the circuit of the line as soon as the subscriber turns its handle4 ,similarly the ringer--the call-receiving device at the sub-station--is permanently bridged across the line so as to be responsive to any signal that may be sent out from the central office in order to call the subscriber4 ,the subscriber's talking apparatus is1 in this normal condition of the line1 cut out of the circuit by the switch hook4 ,subscriber ,calling4 ,fig4 #bda shows the condition of the line when the subscriber at the sub-station is making a call4 ,in turning his generator the two springs which control the connection of the generator with the line are brought into engagement with each other so that the generator currents may pass out over the line4 ,the condition at the central office is the same as that of ,fig4 #bdj except that the drop is shown with its shutter fallen so as to indicate a call4 ,illustration3 ,fig4 #bda4 ,subscriber ,calling ,illustration3 ,a ,,specially ,,formed ,,cable ,,for ,,key ,,shelf ,,of ,,monarch ,,switchboard ,operator ,answering4 ,the next step is for the operator to answer the call and this is shown in ,fig4 #bdb4 ,the subscriber has released the handle of his generator and the generator has1 therefore1 been automatically cut out of the circuit4 ,he also has removed his receiver from its hook1 thus bringing his talking apparatus into the line circuit4 ,the operator on the other hand has inserted one of the plugs ,pa into the jack4 ,this action has resulted in the breaking of the circuit through the drop by the raising of the spring #a from the anvil #c1 and also in the continuance of the line circuit through the conductors of the cord circuits4 ,thus1 the upper limb of the line is continued by means of the engagement of the tip spring #a with the tip #d of the plug to the conducting strand #f of the cord circuit2 likewise the lower limb of the line is continued by the engagement of the thimble #b of the jack with the sleeve contact #e of the plug ,pa to the strand #g of the cord circuit4 ,the operator has also closed her listening key ,l4,k4 ,in doing so she has brought the springs #h and #i into engagement with the anvils #aj and #aa and has thus bridged her head telephone receiver with the secondary of her induction coil across the two strands #f and #g of the cord4 ,associated with the secondary winding of her receiver is a primary circuit containing a transmitter1 battery1 and the primary of the induction coil4 ,it will be seen that the conditions are now such as to permit the subscriber at the calling station to converse with the operator and this conversation consists in the familiar ,number ,please on the part of the operator and the response of the subscriber giving the number of the line that is desired4 ,neither the plug ,pc1 nor the ringing key ,r4,k41 shown in ,fig4 #bdb1 is used in this operation4 ,the clearing-out drop ,c4,o4 is bridged permanently across the strands #f-g of the cord1 but is without function at this time2 the fact that it is wound to a high resistance and impedance prevents its having a harmful effect on the transmission4 ,illustration3 ,fig4 #bdb4 ,operator ,answering ,it may be stated at this point that the two plugs of an associated pair are commonly referred to as the answering and calling plugs4 ,the answering plug is the one which the operator always uses in answering a call as just described in connection with ,fig4 #bdb4 ,the calling plug is the one which she next uses in connecting with the line of the called subscriber4 ,it lies idle during the answering of a call and is only brought into play after the order of the calling subscriber has been given1 in which case it is used in establishing connection with the called subscriber4 ,illustration3 ,fig4 #bdc4 ,operator ,calling ,operator ,calling4 ,we may now consider how the operator calls the called subscriber4 ,the condition existing for this operation is shown in ,fig4 #bdc4 ,the operator after receiving the order from the calling subscriber inserts the calling plug ,pc into the jack of the line of the called station4 ,this act at once connects the limbs of the line with the strands #f and #g of the cord circuit1 and also cuts out the line drop of the called station1 as already explained4 ,the operator is shown in this figure as having opened her listening key ,l4,k4 and closed her ringing key ,r4,k4 ,as a result1 ringing current from the central-office generator will flow out over the two ringing key springs #ab and #ac to the tip and sleeve contacts of the calling plug ,pc1 then to the tip spring #a and the sleeve or thimble #b of the jack1 and then to the two sides of the metallic-circuit line to the sub-station and through the bell there4 ,this causes the ringing of the called subscriber's bell1 after which the operator releases the ringing key and thereby allows the two springs #ab and #ac of that key to again engage their normal contacts #ad and #ae1 thus making the two strands #f and #g of the cord circuit continuous from the contacts of the answering plug ,pa to the contacts of the calling plug ,pc4 ,this establishes the condition at the central office for conversation between the two subscribers4 ,illustration3 ,fig4 #bdd4 ,subscribers ,connected for ,conversation4 ,subscribers ,conversing4 ,the only other thing necessary to establish a complete set of talking conditions between the two subscribers is for the called subscriber to remove his receiver from its hook1 which he does as soon as he responds to the call4 ,the conditions for conversation between the two subscribers are shown in ,fig4 #bdd4 ,it is seen that the two limbs of the calling line are connected respectively to the two limbs of the called line by the two strands of the cord circuit1 both the operator's receiver and the central-office generator being cut out by the listening and ringing keys1 respectively4 ,likewise the two line drops are cut out of circuit and the only thing left associated with the circuit at the central office is the clearing-out drop ,c4 ,o41 which remains bridged across the cord circuit4 ,this1 like the two ringers at the respective connected stations1 which also remain bridged across the circuit when bridging instruments are used1 is of such high resistance and impedance that it offers practically no path to the rapidly fluctuating voice currents to leak from one side of the line circuit to the other4 ,fluctuating currents generated by the transmitter at the calling station1 for instance1 are converted by means of the induction coil into alternating currents flowing in the secondary of the induction coil at that station4 ,considering a momentary current as passing up through the secondary winding of the induction coil at the calling station1 it passes through the receiver of that station through the upper limb of the line to the spring #a of the line jack belonging to that line at the central office2 thence through the tip #d of the answering plug to the conductor #f of the cord2 thence through the pair of contacts #ad and #ab forming one side of the ringing key to the tip #d of the calling plug2 thence to the tip spring #a of the jack of the called subscriber's line2 thence over the upper limb of his line through his receiver and through the secondary of the induction to one of the upper switch-hook contacts2 thence through the hook lever to the lower side of the line1 back to the central office and through the sleeve contact #b of the jack and the sleeve contact #e of the plug2 thence through the other ringing key contacts #ac and #ae2 thence through the strand #g of the cord to the sleeve contact #e and the sleeve contact #b of the answering plug and jack1 respectively2 thence through the lower limb of the calling subscriber's line to the hook lever at his station2 thence through one of the upper contacts of this hook to the secondary of the induction coil1 from which point the current started4 ,illustration3 ,fig4 #bde4 ,clearing-,out ,signal ,obviously1 when the called subscriber is talking to the calling subscriber the same path is followed4 ,it will be seen that at any time the operator may press her listening key ,l4,k41 bridge her telephone set across the circuit of the two connected lines1 and listen to the conversation or converse with either of the subscribers in case of necessity4 ,clearing ,out4 ,at the close of the conversation1 either one or both of the subscribers may send a clearing-out signal by turning their generators after hanging up their receivers4 ,this condition is shown in ,fig4 #bde4 ,the apparatus at the central office remains in exactly the same position during conversation as that of ,fig4 #bdd1 except that the clearing-out drop shutter is shown as having fallen4 ,the two subscribers are shown as having hung up their receivers1 thus cutting out their talking apparatus1 and as operating their generators for the purpose of sending the clearing-out signals4 ,in response to this act the operator pulls down both the calling and the answering plug1 thus restoring them to their normal seats1 and bringing both lines to the normal condition as shown in ,fig4 #bdj4 ,the line drops are again brought into operative relation with their respective lines so as to be receptive to subsequent calls and the calling generators at the sub-stations are removed from the bridge circuits across the line by the opening of the automatic switch contacts associated with those generators4 ,essentials of ,operation4 ,the foregoing sequence of operations while described particularly with respect to magneto switchboards is1 with certain modifications1 typical of the operation of nearly all manual switchboards4 ,in the more advanced types of manual switchboards1 certain of the functions described are sometimes done automatically1 and certain other functions1 not necessary in connection with the simple switchboard1 are added4 ,the essential mode of operation1 however1 remains the same in practically all manual switchboards1 and for this reason the student should thoroughly familiarize himself with the operation and circuits of the simple switchboard as a foundation for the more complex and consequently more-difficult-to-understand switchboards that will be described later on4 ,commercial ,types of ,drops and ,jacks4 ,early ,drops4 ,coming now to the commercial types of switchboard apparatus1 the first subject that presents itself is that of magneto line signals or drops4 ,the very early forms of switchboard drops had1 in most cases1 two-coil magnets1 the cores of which were connected at their forward ends by an iron yoke and the armature of which was pivoted opposite the rear end of the two cores4 ,to the armature was attached a latch rod which projected forwardly to the front of the device and was there adapted to engage the upper edge of the hinged shutter1 so as to hold it in its raised or undisplayed position when the armature was unattracted4 ,such a drop1 of ,western ,electric manufacture1 is shown in ,fig4 #bdf4 ,illustration3 ,fig4 #bdf ,old-,style ,drop ,liability to ,cross-,talk3--,this type of drop is suitable for use only on small switchboards where space is not an important consideration1 and even then only when the drop is entirely cut out of the circuit during conversation4 ,the reason for this latter requirement will be obvious when it is considered that there is no magnetic shield around the winding of the magnet and no means for preventing the stray field set up by the talking currents in one of the magnets from affecting by induction the windings of adjacent magnets contained in other talking circuits4 ,unless the drops are entirely cut out of the talking circuit1 therefore1 they are very likely to produce cross-talk between adjacent circuits4 ,furthermore1 such form of drop is obviously not economical of space1 two coils placed side by side consuming practically twice as much room as in the case of later drops wherein single magnet coils have been made to answer the purpose4 ,tubular ,drops4 ,in the case of line drops1 which usually can readily be cut out of the circuit during conversation1 this cross-talk feature is not serious1 but sometimes the line drops1 and always the clearing-out drops must be left in connection with the talking circuit4 ,on account of economy in space and also on account of this cross-talk feature1 there has come into existence the so-called tubular or iron-clad drop1 one of which is shown in section in ,fig4 #bdg4 ,this was developed a good many years ago by ,mr4 ,e4,p4 ,warner of the ,western ,electric ,company1 and has since1 with modifications1 become standard with practically all the manufacturing companies4 ,in this there is but a single bobbin1 and this is enclosed in a shell of soft ,norway iron1 which is closed at its front end and joined to the end of the core as indicated1 so as to form a complete return magnetic path for the lines of force generated in the coil4 ,the rear end of the shell and core are both cut off in the same plane and the armature is made in such form as to practically close this end of the shell4 ,the armature carries a latch rod extending the entire length of the shell to the front portion of the structure1 where it engages the upper edge of the pivoted shutter2 this1 when released by the latch upon the attraction of the armature1 falls so as to display a target behind it4 ,illustration3 ,fig4 #bdg4 ,tubular ,drop ,illustration3 ,fig4 #bdh4 ,strip of ,tubular ,drops ,these drops may be mounted individually on the face of the switchboard1 but it is more usual to mount them in strips of five or ten4 ,a strip of five drops1 as manufactured by the ,kellogg ,switchboard and ,supply ,company1 is shown in ,fig4 #bdh4 ,the front strip on which these drops are mounted is usually of brass or steel1 copper plated1 and is sufficiently heavy to provide a rigid support for the entire group of drops that are mounted on it4 ,this construction greatly facilitates the assembling of the switchboard and also serves to economize space--obviously1 the thing to economize on the face of a switchboard is space as defined by vertical and horizontal dimensions4 ,these tubular drops1 having but one coil1 are readily mounted on #a-;inch centers1 both vertically and horizontally4 ,sometimes even smaller dimensions than this are secured4 ,the greatest advantage of this form of construction1 however1 is in the absolute freedom from cross-talk between two adjacent drops4 ,so completely is the magnetic field of force kept within the material of the shell1 that there is practically no stray field and two such drops may be included in two different talking circuits and the drops mounted immediately adjacent to each other without producing any cross-talk whatever4 ,night ,alarm4 ,switchboard drops in falling make but little noise1 and during the day time1 while the operator is supposed to be needed continually at the board1 the visual signal which they display is sufficient to attract her attention4 ,in small exchanges1 however1 it is frequently not practicable to keep an operator at the switchboard at night or during other comparatively idle periods1 and yet calls that do arrive during such periods must be attended to4 ,for this reason some other than a visual signal is necessary1 and this need is met by the so-called night-alarm attachment4 ,this is merely an arrangement by which the shutter in falling closes a pair of contacts and thus completes the circuit of an ordinary vibrating bell or buzzer which will sound until the shutter is restored to its normal position4 ,such contacts are shown in ,fig4 #bdi at #a and #b4 ,night-alarm contacts have assumed a variety of forms1 some of which will be referred to in the discussion of other types of drops and jacks4 ,illustration3 ,fig4 #bdi4 ,drop with ,night-,alarm ,contacts ,jack ,mounting4 ,jacks1 like drops1 though frequently individually mounted are more often mounted in strips4 ,an individually mounted jack is shown in ,fig4 #bej1 and a strip of ten jacks in ,fig4 #bea4 ,in such a strip of jacks1 the strips supporting the metallic parts of the various jacks are usually of hard rubber reinforced by brass so as to give sufficient strength4 ,various forms of supports for these strips are used by different manufacturers1 the means for fastening them in the switchboard frame usually consisting of brass lugs on the end of the jack strip adapted to be engaged by screws entering the stationary portion of the iron framework2 or sometimes pins are fixed in the framework1 and the jack is held in place by nuts engaging screw-threaded ends on such pins4 ,illustration3 ,fig4 #bej4 ,individual ,jack ,illustration3 ,fig4 #bea4 ,strip of ,jacks ,methods of ,associating ,jacks and ,drops4 ,there are two general methods of arranging the drops and jacks in a switchboard4 ,one of these is to place all of the jacks in a group together at the lower portion of the panel in front of the operator and all of the drops together in another group above the group of jacks4 ,the other way is to locate each jack in immediate proximity to the drop belonging to the same line so that the operator's attention will always be called immediately to the jack into which she must insert her plug in response to the display of a drop4 ,this latter practice has several advantages over the former4 ,where the drops are all mounted in one group and the jacks in another1 an operator seeing a drop fall must make mental note of it and pick out the corresponding jack in the group of jacks4 ,on the other hand1 where the jacks and drops are mounted immediately adjacent to each other1 the falling of a drop attracts the attention of the operator to the corresponding jack without further mental effort on her part4 ,the immediate association of the drops and jacks has another advantage--it makes possible such a mechanical relation between the drop and its associated jack that the act of inserting the plug into the jack in making the connection will automatically and mechanically restore the drop to its raised position4 ,such drops are termed self-restoring drops1 and1 since a drop and jack are often made structurally a unitary piece of apparatus1 they are frequently called combined drops and jacks4 ,manual vs4 ,automatic ,restoration44 ,there has been much difference of opinion on the question of manual versus automatic restoration of drops4 ,some have contended that there is no advantage in having the drops restored automatically1 claiming that the operator has plenty of time to restore the drops by hand while receiving the order from the calling subscriber or performing some of her other work4 ,those who think this way have claimed that the only place where an automatically restored drop is really desirable is where1 on account of the lack of space on the front of the switchboard1 the drops are placed on such a portion of the board as to be not readily reached by the operator4 ,this resulted in the electrically restored drop1 mention of which will be made later4 ,others have contended that even though the drop is mounted within easy reach of the operator1 it is advantageous that the operator should be relieved of the burden of restoring it1 claiming that even though there are times in the regular performance of the operator's duties when she may without interfering with other work restore the drops manually1 such requirement results in a double use of her attention and in a useless strain on her which might better be devoted to the actual making of connections4 ,until recently the various ,bell operating companies have adhered1 in their small exchange work1 to the manual restoring method1 while most of the so-called independent operating companies have adhered to the automatic self-restoring drops4 ,methods of ,automatic ,restoration4 ,two general methods present themselves for bringing about the automatic restoration of the drop4 ,first1 the mechanical method1 which is accomplished by having some moving part of the jack or of the plug as it enters the jack force the drop mechanically into its restored position4 ,this usually means the mounting of the drop and the corresponding jack in juxtaposition1 and this1 in turn1 has usually resulted in the unitary structure containing both the drop and the jack4 ,second1 the electrical method wherein the plug in entering the jack controls a restoring circuit1 which includes a battery or other source of energy and a restoring coil on the drop1 the result being that the insertion of the plug into the jack closes this auxiliary circuit and thus energizes the restoring magnet1 the armature of which pulls the shutter back into its restored position4 ,this practice has been followed by ,bell operating companies whenever conditions require the drop to be mounted out of easy reach of the operator2 not otherwise4 ,mechanical--,direct ,contact with ,plug4 ,one widely used method of mechanical restoration of drops1 once employed by the ,western ,telephone ,construction ,company with considerable success1 was to hang the shutter in such position that it would fall immediately in front of the jack so that the operator in order to reach the jack with the plug would have to push the plug directly against the shutter and thus restore it to its normal or raised position4 ,in this construction the coil of the drop magnet was mounted directly behind the jack1 the latch rod controlled by the armature reaching forward1 parallel with the jack1 to the shutter1 which1 as stated1 was hung in front of the jack4 ,this resulted in a most compact arrangement so far as the space utilization on the front of the board was concerned and such combined drops and jacks were mounted on about #a-;inch centers1 so that a bank of one hundred combined drops and jacks occupied a space only a little over #aj inches square4 ,a modification of this scheme1 as used by the ,american ,electric ,telephone ,company1 was to mount the drop immediately over the jack so that its shutter1 when down1 occupied a position almost in front of1 but above1 the jack opening4 ,the plug was provided with a collar1 which1 as it entered the jack1 engaged a cam on the base of the shutter and forced the latter mechanically into its raised position4 ,neither of these methods of restoring--i4e41 by direct contact between the shutter or part of it and the plug or part of it--is now as widely used as formerly4 ,it has been found that there is no real need in magneto switchboards for the very great compactness which the hanging of the shutter directly in front of the drop resulted in1 and the tendency in later years has been to make the combined drops and jacks more substantial in construction at the expense of some space on the face of the switchboard4 ,illustration3 ,fig4 #beb4 ,kellogg ,drop and ,jack ,kellogg ,type3--,a very widely used scheme of mechanical restoration is that employed in the ,miller drop and jack manufactured by the ,kellogg ,switchboard and ,supply ,company1 the principles of which may be understood in connection with ,fig4 #beb4 ,in this figure views of one of these combined drops and jacks in three different positions are shown4 ,the jack is composed of the framework ,b and the hollow screw ,a1 the latter forming the sleeve or thimble of the jack and being externally screw-threaded so as to engage and bind in place the front end of the framework ,b4 ,the jack is mounted on the lower part of the brass mounting strip ,c but insulated therefrom4 ,the tip spring of the jack is bent down as usual to engage the tip of the plug1 as better shown in the lower cut of ,fig4 #beb1 and then continues in an extension ,d1 which passes through a hole in the mounting plate ,c4 ,this tip spring in its normal position rests against another spring as shown1 which latter spring forms one terminal of the drop winding4 ,the drop or annunciator is of tubular form1 and the shutter is so arranged on the front of the mounting strip ,c as to fall directly above the extension ,d of the tip spring4 ,as a result1 when the plug is inserted into the jack1 the upward motion of the tip spring forces the drop into its restored position1 as indicated in the lower cut of the figure4 ,these drops and jacks are usually mounted in banks of five1 as shown in ,fig4 #bec4 ,illustration3 ,fig4 #bec4 ,strip of ,kellogg ,drops and ,jacks ,western ,electric ,type3--,the combined drop and jack of the ,western ,electric ,company recently put on the market to meet the demands of the independent trade1 differs from others principally in that it employs a spherical drop or target instead of the ordinary flat shutter4 ,this piece of apparatus is shown in its three possible positions in ,fig4 #bed4 ,the shutter or target normally displays a black surface through a hole in the mounting plate4 ,the sphere forming the target is out of balance1 and when the latch is withdrawn from it by the action of the electromagnet it falls into the position shown in the middle cut of ,fig4 #bed1 thus displaying a red instead of a black surface to the view of the operator4 ,when the operator plugs in1 the plug engages the lower part of an ,s-shaped lever which acts on the pivoted sphere to restore it to its normal position4 ,a perspective view of one of these combined line signals and jacks is shown in ,fig4 #bee4 ,a feature that is made much of in recently designed drops and jacks for magneto service is that which provides for the ready removal of the drop coil1 from the rest of the structure1 for repair4 ,the drop and jack of the ,western ,electric ,company1 just described1 embodies this feature1 a single screw being so arranged that its removal will permit the withdrawal of the coil without disturbing any of the other parts or connections4 ,the coil windings terminate in two projections on the front head of the spool1 and these register with spring clips on the inside of the shell so that the proper connections for the coil are automatically made by the mere insertion of the coil into the shell4 ,illustration3 ,fig4 #bed4 ,western ,electric ,drop and ,jack ,illustration3 ,fig4 #bee4 ,western ,electric ,drop and ,jack ,dean ,type3--,the combined drop and jack of the ,dean ,electric ,company is illustrated in ,figs4 #bef and #beg4 ,the two perspective views show the general features of the drop and jack and the method by which the magnet coil may be withdrawn from the shell4 ,as will be seen the magnet is wound on a hollow core which slides over the iron core1 the latter remaining permanently fixed in the shell1 even though the coil be withdrawn4 ,fig4 #beh shows the structural details of the jack employed in this combination and it will be seen that the restoring spring for the drop is not the tip spring itself1 but another spring located above and insulated from it and mechanically connected therewith4 ,illustration3 ,fig4 #bef4 ,dean ,drop and ,jack ,illustration3 ,fig4 #beg4 ,dean ,drop and ,jack ,illustration3 ,fig4 #beh4 ,details of ,dean ,jack ,monarch ,type3--,still another combined drop and jack is that of the ,monarch ,telephone ,manufacturing ,company of ,chicago1 shown in sectional view in ,fig4 #bei4 ,this differs from the usual type in that the armature is mounted on the front end of the electromagnet1 its latch arm retaining the shutter in its normal position when raised1 and releasing it when depressed by the attraction of the armature4 ,as is shown1 there is within the core of the magnet an adjustable spiral spring which presses forward against the armature and which spring is compressed by the attraction of the armature of the magnet4 ,the night-alarm contact is clearly shown immediately below the strip which supports the drop1 this consisting of a spring adapted to be engaged by a lug on the shutter and pressed upwardly against a stationary contact when the shutter falls4 ,the method of restoration of the shutter in this case is by means of an auxiliary spring bent up so as to engage the shutter and restore it when the spring is raised by the insertion of a plug into the jack4 ,illustration3 ,fig4 #bei4 ,monarch ,drop and ,jack ,code ,signaling4 ,on bridging party lines1 where the subscribers sometimes call other subscribers on the same line and sometimes call the switchboard so as to obtain a connection with another line1 it is not always easy for the operator at the switchboard to distinguish whether the call is for her or for some other party on the line4 ,on such lines1 of course1 code ringing is used and in most cases the operator's only way of distinguishing between calls for her and those for some sub-station parties on the line is by listening to the rattling noise which the drop armature makes4 ,in the case of the ,monarch drop the adjustable spring tension on the armature is intended to provide for such an adjustment as will permit the armature to give a satisfactory buzz in response to the alternating ringing currents1 whether the line be long or short4 ,illustration3 ,fig4 #bfj4 ,code ,signal ,attachment ,the ,monarch ,company provides in another way for code signaling at the switchboard4 ,in some cases there is a special attachment1 shown in ,fig4 #bfj1 by means of which the code signals are repeated on the night-alarm bell4 ,this is in the nature of a special attachment placed on the drop1 which consists of a light1 flat spring attached to the armature and forming one side of a local circuit4 ,the other side of the circuit terminates in a fixture which is mounted on the drop frame and is provided with a screw1 having a platinum point forming the other contact point2 this allows of considerable adjustment4 ,at the point where the screw comes in contact with the spring there is a platinum rivet4 ,when an operator is not always in attendance1 this code-signaling attachment has some advantages over the drop as a signal interpreter1 in that it permits the code signals to be heard from a distance4 ,of course1 the addition of spring contacts to the drop armature tends to complicate the structure and perhaps to cut down the sensitiveness of the drop1 which are offsetting disadvantages4 ,illustration3 ,fig4 #bfa4 ,combined ,drop and ,ringer ,for really long lines1 this code signaling by means of the drop is best provided for by employing a combined drop and ringer1 although in this case whatever advantages are secured by the mechanical restoration of the shutter upon plugging in are lost4 ,such a device as manufactured by the ,dean ,electric ,company is shown in ,fig4 #bfa4 ,in this the ordinary polarized ringer is used1 but in addition the tapper rod carries a latch which1 when vibrated by the ringing of the bell1 releases a shutter and causes it to fall1 thus giving a visual as well as an audible signal4 ,electrical4 ,coming now to the electrical restoration of drop shutters1 reference is made to ,fig4 #bfb1 which shows in side section the electrical restoring drop employed by the ,bell companies and manufactured by the ,western ,electric ,company4 ,in this the coil #a is a line coil1 and it operates on the armature #b to raise the latch lever #c in just the same manner as in the ordinary tubular drop4 ,the latch lever #c acts1 however1 to release another armature #d instead of a shutter4 ,this armature #d is pivoted at its lower end at the opposite end of the device from the armature #b and1 by falling outwardly when released1 it serves to raise the light shutter #e4 ,the restoring coil of this device is shown at #f1 and when energized it attracts the armature #d so as to pull it back under the catch of the latch lever #c and also so as to allow the shutter #e to fall into its normal position4 ,the method of closing the restoring circuit is by placing coil #f in circuit with a local battery and with a pair of contacts in the jack1 which latter contacts are normally open but are bridged across by the plug when it enters the jack1 thus energizing the restoring coil and restoring the shutter4 ,illustration3 ,fig4 #bfb4 ,electrically ,restored ,drop ,a perspective view of this ,western ,electric electrical restoring drop is shown in ,fig4 #bfc1 a more complete mention being made of this feature under the discussion of magneto multiple switchboards1 wherein it found its chief use4 ,it is mentioned here to round out the methods that have been employed for accomplishing the automatic restoration of shutters by the insertion of the plug4 ,illustration3 ,fig4 #bfc4 ,electrically ,restored ,drop ,switchboard ,plugs4 ,a switchboard plug such as is commonly used in simple magneto switchboards is shown in ,fig4 #bfd and also in ,fig4 #bce4 ,the tip contact is usually of brass and is connected to a slender steel rod which runs through the center of the plug and terminates near the rear end of the plug in a connector for the tip conductor of the cord4 ,this central core of steel is carefully insulated from the outer shell of the plug by means of hard rubber bushings1 the parts being forced tightly together4 ,the outer shell1 of course1 forms the other conductor of the plug1 called the sleeve contact4 ,a handle of tough fiber tubing is fitted over the rear end of the plug and this also serves to close the opening formed by cutting away a portion of the plug shell1 thus exposing the connector for the tip conductor4 ,illustration3 ,fig4 #bfd4 ,switchboard ,plug ,cord ,attachment4 ,the rear end of the plug shell is usually bored out just about the size of the outer covering of the switchboard cord1 and it is provided with a coarse internal screw thread1 as shown4 ,the cord is attached by screwing it tightly into this screw-threaded chamber1 the screw threads in the brass being sufficiently coarse and of sufficiently small internal diameter to afford a very secure mechanical connection between the outer braiding of the cord and the plug4 ,the connection between the tip conductor of the cord and the tip of the plug is made by a small machine screw connection as shown1 while the connection between the sleeve conductor of the plug and the sleeve conductor of the cord is made by bending back the latter over the outer braiding of the cord before it is screwed into the shank of the plug4 ,this results in the close electrical contact between the sleeve conductor of the cord and the inner metal surface of the shank of the plug4 ,switchboard ,cords4 ,a great deal of ingenuity has been exerted toward the end of producing a reliable and durable switchboard cord4 ,while great improvement has resulted1 the fact remains that the cords of manual switchboards are today probably the most troublesome element1 and they need constant attention and repairs4 ,while no two manufacturers build their cords exactly alike1 descriptions of a few commonly used and successful cords may be here given4 ,concentric ,conductors4 ,in one the core is made from a double strand of strong lock stitch twine1 over which is placed a linen braid4 ,then the tip conductor1 which is of stranded copper tinsel1 is braided on4 ,this is then covered with two layers of tussah silk1 laid in reverse wrappings1 then there is a heavy cotton braid1 and over the latter a linen braid4 ,the sleeve conductor1 which is also of copper tinsel1 is then braided over the structure so formed1 after which two reverse wrappings of tussah silk are served on1 and this is covered by a cotton braid and this in turn by a heavy linen or polished cotton braid4 ,the plug end of the cord is reinforced for a length of from #ab to #ah inches by another braiding of linen or polished cotton1 and the whole cord is treated with melted beeswax to make it moisture-proof and durable4 ,illustration3 ,fig4 #bfe4 ,switchboard ,cord ,steel ,spiral ,conductors4 ,in another cord that has found much favor the two conductors are formed mainly by two concentric spiral wrappings of steel wire1 the conductivity being reinforced by adjacent braidings of tinsel4 ,the structure of such a cord is well shown in ,fig4 #bfe4 ,beginning at the right1 the different elements shown are1 in the order named1 a strand of lock stitch twine1 a linen braiding1 into the strands of which are intermingled tinsel strands1 the inner spiral steel wrapping1 a braiding of tussah silk1 a linen braiding1 a loose tinsel braiding1 the outer conductor of round spiral steel1 a cotton braid1 and an outside linen or polished cotton braid4 ,the inner tinsel braiding and the inner spiral together form the tip conductor while the outer braiding and spiral together form the sleeve conductor4 ,the cord is reinforced at the plug end for a length of about #ad inches by another braiding of linen4 ,the tinsel used is1 in each case1 for the purpose of cutting down the resistance of the main steel conductor4 ,these wrappings of steel wire forming the tip and sleeve conductors respectively1 have the advantage of affording great flexibility1 and also of making it certain that whatever strain the cord is subjected to will fall on the insulated braiding rather than on the spiral steel which has in itself no power to resist tensile strains4 ,parallel ,tinsel ,conductors4 ,another standard two-conductor switchboard cord is manufactured as follows3 ,one conductor is of very heavy copper tinsel insulated with one wrapping of sea island cotton1 which prevents broken ends of the tinsel or knots from piercing through and short-circuiting with the other conductor4 ,over this is placed one braid of tussah silk and an outer braid of cotton4 ,this combines high insulation with considerable strength4 ,the other conductor is of copper tinsel1 not insulated1 and this is laid parallel to the thrice insulated conductor already described4 ,around these two conductors is placed an armor of spring brass wire in spiral form1 and over this a close1 stout braid of glazed cotton4 ,this like the others is reinforced by an extra braid at the plug end4 ,ringing and ,listening ,keys4 ,the general principles of the ringing key have already been referred to4 ,ringing keys are of two general types1 one having horizontal springs and the other vertical4 ,illustration3 ,fig4 #bff4 ,horizontal-,spring ,listening and ,ringing ,key ,horizontal ,spring ,type4 ,various ,bell operating companies have generally adhered to the horizontal spring type except in individual and four-party-line keys4 ,the construction of a ,western ,electric ,company horizontal spring key is shown in ,fig4 #bff4 ,in this particular key1 as illustrated1 there are two cam levers operating upon three sets of springs4 ,the cam lever at the left operates the ordinary ringing and listening set of springs according to whether it is pushed one way or the other4 ,in ringing on single-party lines the cam lever at the left is the one to be used2 while on two-party lines the lever at the left serves to ring the first party and the ringing key at the right the second party4 ,in order that the operator may have an indication as to which station on a two-party line she has called1 a small target #a carried on a lever #b is provided4 ,this target may display a black or a white field1 according to which of its positions it occupies4 ,the lever #b is connected by the links #c and #d with the two key levers and the target is thus moved into one position or the other1 according to which lever was last thrown into ringing position4 ,it will be noticed that the springs are mounted horizontally and on edge4 ,this on-edge feature has the advantage of permitting ready inspection of the contacts and of avoiding the liability of dust gathering between the contacts4 ,as will be seen1 at the lower end of each switch lever there is a roller of insulating material which serves as a wedge1 when forced between the two long springs of any set1 to force them apart and into engagement with their respective outer springs4 ,illustration3 ,fig4 #bfg4 ,vertical-,spring ,listening and ,ringing ,key ,vertical ,spring ,type4 ,the other type of ringing and listening key employing vertical springs is almost universally used by the various independent manufacturing companies4 ,a good example of this is shown in ,fig4 #bfg1 which shows partly in elevation and partly in section a double key of the ,monarch ,company4 ,the operation of this is obvious from its mode of construction4 ,the right-hand set of springs of the right-hand key in this cut are the springs of the listening key1 while the left-hand set of the right-hand key are those of the calling-plug ringing key4 ,the left-hand set of the left-hand key may be those of a ring-back key on the answering plug1 while the right-hand set of the left-hand key may be for any special purpose4 ,it is obvious that these groups of springs may be grouped in different combinations or omitted in part1 as required4 ,this same general form of key is also manufactured by the ,kellogg ,company and the ,dean ,company1 that of the ,kellogg ,company being illustrated in perspective1 ,fig4 #bfh4 ,the keys of this general type have the same advantages as those of the horizontal on-edge arrangement with respect to the gathering of dust1 and while perhaps the contacts are not so readily get-at-able for inspection1 yet they have the advantage of being somewhat more simple1 and of taking up less horizontal space on the key shelf4 ,illustration3 ,fig4 #bfh4 ,vertical ,listening and ,ringing ,key ,illustration3 ,fig4 #bfi4 ,four-,party ,listening and ,ringing ,key ,party-,line ,ringing ,keys4 ,for party-line ringing the key matter becomes somewhat more complicated4 ,usually the arrangement is such that in connection with each calling plug there are a number of keys1 each arranged with respect to the circuits of the plug so as to send out the proper combination and direction of current1 if the polarity system is used2 or the proper frequency of current if the harmonic system is used2 or the proper number of impulses if the step-by-step or broken-line system is used4 ,the number of different kinds of arrangements and combinations is legion1 and we will here illustrate only an example of a four-party line ringing key adapted for harmonic ringing4 ,a ,kellogg party-line listening and ringing key is shown in ,fig4 #bfi4 ,in this1 besides the regular listening key1 are shown four push-button keys1 each adapted1 when depressed1 to break the connection back of the key1 and at the same time connect the proper calling generator with the calling plug4 ,self-,indicating ,keys4 ,a complication that has given a good deal of trouble in the matter of party-line ringing is due to the fact that it is sometimes necessary to ring a second or a third time on a party-line connection1 because the party called may not respond the first time4 ,the operator is not always able to remember which one of the four keys associated with the plug connected with the desired party she has pressed on the first occasion and1 therefore1 when it becomes necessary to ring again1 she may ring the wrong party4 ,this is provided for in a very ingenious way in the key shown in ,fig4 #bfi1 by making the arrangement such that after a given key has been depressed to its full extent in ringing1 and then released1 it does not come quite back to its normal position but remains slightly depressed4 ,this always serves as an indication to the operator1 therefore1 as to which key she depressed last1 and in the case of a re-ring1 she merely presses the key that is already down a little way4 ,on the next call if she is required to press another one of the four keys1 the one which remained down a slight distance on the last call will be released and the one that is fully depressed will be the one that remains down as an indication4 ,such keys1 where the key that was last used leaves an indication to that effect1 are called indicating ringing keys4 ,in other forms the indication is given by causing the key lever to move a little target which remains exposed until some other key in the same set is moved4 ,the key shown in ,fig4 #bff is an example of this type4 ,,note4 ,the matter of automatic ringing and other special forms of ringing will be referred to and discussed at their proper places in this work1 but at this point they are not pertinent as they are not employed in simple switchboards4 ,operator's ,telephone ,equipment4 ,little need be said concerning the matter of the operator's talking apparatus1 i4e41 the operator's transmitter and receiver1 since as transmitters and receivers they are practically the same as those in ordinary use for other purposes4 ,the watch-case receiver is nearly always employed for operators' purposes on account of its lightness and compactness4 ,it is used in connection with a head band so as to be held continually at the operator's ear1 allowing both of her hands to be free4 ,the transmitter used by operators does not in itself differ from the transmitters employed by subscribers1 but the methods by which it is supported differ1 two general practices being followed4 ,one of these is to suspend the transmitter by flexible conducting cords so as to be adjustable in a vertical direction4 ,a good illustration of this is given in ,fig4 #bgj4 ,the other method1 and one that is coming into more and more favor1 is to mount the transmitter on a light bracket suspended by a flexible band from the neck of the operator1 a breast plate being furnished so that the transmitter will rest on her breast and be at all times within proper position to receive her speech4 ,to facilitate this1 a long curved mouthpiece is commonly employed1 as shown clearly in ,fig4 #dg4 ,illustration3 ,fig4 #bgj4 ,operator's ,transmitter ,suspension ,cut-in ,jack4 ,it is common to terminate that portion of the apparatus which is worn on the operator's person--that is1 the receiver only if the suspended type of transmitter is employed1 and the receiver and transmitter if the breast plate type of transmitter is employed--in a plug1 and a flexible cord connecting the plug terminates with the apparatus4 ,the portions of the operator's talking circuit that are located permanently in the switchboard cabinet are in such cases terminated in a jack1 called an operator's cut-in jack4 ,this is usually mounted on the front rail of the switchboard cabinet just below the key shelf4 ,such a cut-in jack is shown in ,fig4 #bga and it is merely a specialized form of spring jack adapted to receive the short1 stout plug in which the operator's transmitter1 or transmitter and receiver1 terminate4 ,by this arrangement the operator is enabled readily to connect or disconnect her talking apparatus1 which is worn on her person1 whenever she comes to the board for work or leaves it at the end of her work4 ,a complete operator's telephone set1 or that portion that is carried on the person of the operator1 together with the cut-in plug1 is shown in ,fig4 #bgb4 ,illustration3 ,fig4 #bga4 ,operator's ,cut-in ,jack ,illustration3 ,fig4 #bgb4 ,operator's ,talking ,set ,circuits of ,complete ,switchboard4 ,we may now discuss the circuits of a complete simple magneto switchboard4 ,the one shown in ,fig4 #bgc is typical4 ,before going into the details of this1 it is well to inform the student that this general form of circuit representation is one that is commonly employed in showing the complete circuits of any switchboard4 ,ordinarily two subscribers' lines are shown1 these connecting their respective subscribers' stations with two different line equipments at the central office4 ,the jacks and signals of these line equipments are turned around so as to face each other1 in order to clearly represent how the connection between them may be made by means of the cord circuit4 ,the elements of the cord circuit are also spread out1 so that the various parts occupy relative positions which they do not assume at all in practice4 ,in other words it must be remembered that1 in circuit diagrams1 the relative positions of the parts are sacrificed in order to make clear the circuit connections4 ,however1 this does not mean that it is often not possible to so locate the pieces of apparatus that they will in a certain way indicate relative positions1 as may be seen in the case of the drop and jack in ,fig4 #bgc1 the drop being shown immediately above the jack1 which is the position in which these parts are located in practice4 ,illustration3 ,fig4 #bgc4 ,circuit of ,simple ,magneto ,switchboard ,little need be said concerning this circuit in view of what has already been said in connection with ,figs4 #bdj to #bde4 ,it will be seen in the particular sub-station circuit here represented1 that the talking apparatus is arranged in the usual manner and that the ringer and generator are so arranged that when the generator is operated the ringer will be cut out of circuit1 while the generator will be placed across the circuit2 while1 when the generator is idle1 the ringer is bridged across the circuit and the generator is cut out4 ,the line terminates in each case in the tip and sleeve contacts of the jack1 and in the normal condition of the jack the line drop is bridged across the line4 ,the arrangement by which the drop is restored and at the same time cut out of circuit when the operator plugs in the jack1 is obvious from the diagrammatic illustration4 ,the cord circuit is the same as that already discussed1 with the exception that two ringing keys are provided1 one in connection with the calling plug1 as is universal practice1 and the other in connection with the answering plug as is sometimes practiced in order that the operator may1 when occasion requires1 ring back the calling subscriber without the necessity of changing the plug in the jack4 ,the outer contacts of these two ringing keys are connected to the terminals of the ringing generator and1 when either key is operated1 the connection between the plug1 on which the ringing is to be done1 and the rest of the cord circuit will be broken1 while the generator will be connected with the terminals of the plug4 ,the listening key and talking apparatus need no further explanation1 it being obvious that when the key is operated the subscriber's telephone set will be bridged across the cord circuit and1 therefore1 connected with either or both of the talking subscribers4 ,illustration3 ,fig4 #bgd4 ,night-,alarm ,circuit ,night-,alarm ,circuits4 ,the circuit of ,fig4 #bgc1 while referred to as a complete circuit1 is not quite that4 ,the night-alarm circuit is not shown4 ,in order to clearly indicate how a single battery and bell1 or buzzer1 may serve in connecting a number of line drops1 reference is made to ,fig4 #bgd which shows the connection between three different line drops and the night-alarm circuit4 ,the night-alarm apparatus consists in the battery #a and the buzzer1 or bell1 #b4 ,a switch #c adapted to be manually operated is connected in the circuit with the battery and the buzzer so as to open this circuit when the night alarm is not needed1 thus making it inoperative4 ,during the portions of the day when the operator is needed constantly at the board it is customary to leave this switch #c open1 but during the night period when she is not required constantly at the board this switch is closed so that an audible signal will be given whenever a drop falls4 ,the night-alarm contact #d on each of the drops will be closed whenever a shutter falls1 and as the two members of this contact1 in the case of each drop1 are connected respectively with the two sides of the night-alarm circuit1 any one shutter falling will complete the necessary conditions for causing the buzzer to sound1 assuming of course that the switch #c is closed4 ,night ,alarm with ,relay4 ,a good deal of trouble has been caused in the past by uncertainty in the closure of the night-alarm circuit at the drop contact4 ,some of the companies have employed the form of circuit shown in ,fig4 #bge to overcome this4 ,instead of the night-alarm buzzer being placed directly in the circuit that is closed by the drop1 a relay #e and a high-voltage battery #f are placed in this circuit4 ,the buzzer and the battery for operating it are placed in a local circuit controlled by this relay4 ,it will be seen by reference to ,fig4 #bge that when the shutter falls1 it will1 by closing the contact #d1 complete the circuit from the battery #f through the relay #e--;assuming switch #c to be closed--and thus cause the operation of the relay4 ,the relay1 in turn1 by pulling up its armature1 will close the circuit of the buzzer #b through the battery #g and cause the buzzer to sound4 ,illustration3 ,fig4 #bge4 ,night-,alarm ,circuit with ,relay ,the advantage of this method over the direct method of operating the buzzer is that any imperfection in the night-alarm contact at the drop is much less likely to prevent the flow of current of the high-voltage battery #f than of the low-voltage battery #a1 shown in connection with ,fig4 #bgd4 ,this is because the higher voltage is much more likely to break down any very thin bit of insulation1 such as might be caused by a minute particle of dust or oxide between contacts that are supposed to be closed by the falling of the shutter4 ,it has been common to employ for battery #f a dry-cell battery giving about #bj or #bd volts1 and for the operation of the buzzer itself1 a similar battery of about two cells giving approximately #c volts4 ,night-,alarm ,contacts4 ,the night-alarm contact #d of the drop shown diagrammatically in ,figs4 #bgd and #bge would1 if taken literally1 indicate that the shutter itself actually forms one terminal of the circuit and the contact against which it falls1 the other4 ,this has not been found to be a reliable way of closing the night-alarm contacts and this method is indicated in these figures and in other figures in this work merely as a convenient way of representing the matter diagrammatically4 ,as a matter of fact the night-alarm contacts are ordinarily closed by having the shutter fall against one spring1 which is thereby pressed into engagement with another spring or contact1 as shown in ,fig4 #bdi4 ,this method employs the shutter only as a means for mechanically causing the one spring to press against the other1 the shutter itself forming no part of the circuit4 ,the reason why it is not a good plan to have the shutter itself act as one terminal of the circuit is that this necessitates the circuit connections being led to the shutter through the trunnions on which the shutter is pivoted4 ,this is bad because1 obviously1 the shutter must be loosely supported on its trunnions in order to give it sufficiently free movement1 and1 as is well known1 loose connections are not conducive to good electrical contacts4 ,grounded-and ,metallic-,circuit ,lines4 ,when grounded circuits were the rule rather than the exception1 many of the switchboards were particularly adapted for their use and could not be used with metallic-circuit lines4 ,these grounded-circuit switchboards provided but a single contact in the jack and a single contact on the plug1 the cords having but a single strand reaching from one plug to the other4 ,the ringing keys and listening keys were likewise single-contact keys rather than double4 ,the clearing-out drop and the operator's talking circuit and the ringing generator were connected between the single strand of the cord and the ground as was required4 ,the grounded-circuit switchboard has practically passed out of existence1 and while a few of them may be in use1 they are not manufactured at present4 ,the reason for this is that while many grounded circuits are still in use1 there are very few places where there are not some metallic-circuit lines1 and while the grounded-circuit switchboard will not serve for metallic-circuit lines1 the metallic-circuit switchboard will serve equally well for either metallic-circuit or grounded lines1 and will interconnect them with equal facility4 ,this fact will be made clear by a consideration of ,figs4 #bgf1 #bgg1 and #bgh4 ,illustration3 ,fig4 #bgf4 ,connection ,between ,metallic ,lines ,illustration3 ,fig4 #bgg4 ,connection ,between ,grounded ,lines ,connection between ,two ,similar ,lines4 ,in ,fig4 #bgf a common magneto cord circuit is shown connecting two metallic-circuit lines2 in ,fig4 #bgg the same cord circuit is shown connecting two grounded lines4 ,in this case the line wire #a of the left-hand line is1 when the plugs are inserted1 continued to the tip of the answering plug1 thence through the tip strand of the cord circuit to the tip of the calling plug1 then to the tip spring of the right-hand jack and out to the single conductor of that line4 ,the entire sleeve portion of the cord circuit becomes grounded as soon as the plugs are inserted in the jacks of such a line4 ,hence1 we see that the sleeve contacts of the plug and the sleeve conductor of the cord are connected to ground through the permanent ground connection of the sleeve conductors of the jack as soon as the plug is inserted into the jack4 ,thus1 when the cord circuit of a metallic-circuit switchboard is used to connect two grounded circuits together1 the tip strand of the cord is the connecting link between the two conductors1 while the sleeve strand of the cord merely serves to ground one side of the clearing-out drop and one side each of the operator's telephone set and the ringing generator when their respective keys are operated4 ,connection between ,dissimilar ,lines4 ,fig4 #bgh shows how the same cord circuit and the same arrangement of line equipment may be used for connecting a grounded line to a metallic-circuit line4 ,the metallic circuit line is shown on the left and the grounded line on the right4 ,when the two plugs are inserted into the respective jacks of this figure1 the right-hand conductor of the metallic circuit shown on the left will be continued through the tip strand of the cord circuit to the line conductor of the grounded line shown on the right4 ,the left-hand conductor of the metallic-circuit line will be connected to ground because it will be continued through the sleeve strand of the cord circuit to the sleeve contact of the calling plug and thence to the sleeve contact of the jack of the grounded line1 which sleeve contact is shown to be grounded4 ,the talking circuit between the two connected lines in this case may be traced as follows3 ,from the subscriber's station at the left through the right-hand limb of the metallic-circuit line1 through the tip contact and tip conductor of the cord circuit1 to the single limb of the grounded-circuit line1 thence to the sub-station of that line and through the talking apparatus there to ground4 ,the return path from the right-hand station is by way of ground to the ground connection at the central office1 thence to the sleeve contact of the grounded line jack1 through the sleeve conductor of the cord circuit1 to the sleeve contact of the metallic-circuit line jack1 and thence by the left-hand limb of the metallic-circuit line to the subscriber's station4 ,illustration3 ,fig4 #bgh4 ,connection ,between ,dissimilar ,lines ,a better way of connecting a metallic-circuit line to a grounded line is by the use of a special cord circuit involving a repeating coil1 such a connection being shown in ,fig4 #bgi4 ,the cord circuit in this case differs in no respect from those already shown except that a repeating coil is associated with it in such a way as to conductively divide the answering side from the calling side4 ,obviously1 whatever currents come over the line connected with the answering plug will pass through the windings #a and #b of this coil and will induce corresponding currents in the windings #c and #d1 which latter currents will pass out over the circuit of the line connected with the calling plug4 ,when a grounded circuit is connected to a metallic circuit in this manner1 no ground is thrown onto the metallic circuit4 ,the balance of the metallic circuit is1 therefore1 maintained4 ,to ground one side of a metallic circuit frequently so unbalances it as to cause it to become noisy1 that is1 to have currents flowing in it1 by induction or from other causes1 other than the currents which are supposed to be there for the purpose of conveying speech4 ,illustration3 ,fig4 #bgi4 ,connection of ,dissimilar ,lines through ,repeating ,coil ,convertible ,cord ,circuits4 ,the consideration of ,fig4 #bgi brings us to the subject of so-called convertible cord circuits4 ,some switchboards1 serving a mixture of metallic and grounded lines1 are provided with cord circuits which may be converted at will by the operator from the ordinary type shown in ,fig4 #bgf to the type shown in ,fig4 #bgi4 ,the advantage of this will be obvious from the following consideration4 ,when a call originates on any line1 either grounded or metallic1 the operator does not know which kind of a line is to be called for4 ,she1 therefore1 plugs into this line with any one of her answering plugs and completes the connection in the usual way4 ,if the call is for the same kind of a circuit as that over which the call originated1 she places the converting key in such a position as will connect the conductors of the cord circuit straight through2 while if the connection is for a different kind of a line than that on which the call originated she throws the converting key into such a position as to include the repeating coil4 ,a study of ,fig4 #bhj will show that when the converting key1 which is commonly referred to as the repeating-coil key1 is in one position1 the cord conductors will be cut straight through1 the repeating coil being left open in both its windings2 and when it is thrown to its other position1 the connection between the answering and calling sides of the cord circuit will be severed and the repeating coil inserted so as to bring about the same effects and circuit arrangements as are shown in ,fig4 #bgi4 ,illustration3 ,fig4 #bhj4 ,convertible ,cord ,circuit ,cord-,circuit ,considerations4 ,simple ,bridging ,drop ,type4 ,the matter of cord circuits in magneto switchboards is deserving of much attention4 ,so far as talking requirements are concerned1 the ordinary form of cord circuit with a clearing-out drop bridged across the two strands is adequate for nearly all conditions except those where a grounded-and a metallic-circuit line are connected together1 in which case the inclusion of a repeating coil has some advantages4 ,illustration3 ,fig4 #bha4 ,bridging ,drop-,cord ,circuit ,from the standpoint of signaling1 however1 this type of cord circuit has some disadvantages under certain conditions4 ,in order to simplify the discussion of this and other cord-circuit matters1 reference will be made to some diagrams from which the ringing and listening keys and talking apparatus have been entirely omitted4 ,in ,fig4 #bha the regular bridging type of clearing-out drop-cord circuit is shown1 this being the type already discussed as standard4 ,for ordinary practice it is all right4 ,certain difficulties are experienced with it1 however1 where lines of various lengths and various types of sub-station apparatus are connected4 ,for instance1 if a long bridging line be connected with one end of this cord circuit and a short line having a low-resistance series ringer be connected with the other end1 then a station on the long line may have some difficulty in throwing the clearing-out drop1 because of the low-resistance shunt that is placed around it through the short line and the low-resistance ringer4 ,in other words1 the clearing-out drop is shunted by a comparatively low-resistance line and ringer and the feeble currents arriving from a distant station over the long line are not sufficient to operate the drop thus handicapped4 ,the advent of the various forms of party-line selective signaling and the use of such systems in connection with magneto switchboards has brought in another difficulty that sometimes manifests itself with this type of cord circuit4 ,if two ordinary magneto telephones are connected to the two ends of this cord circuit1 it is obvious that when one of the subscribers has hung up his receiver and the other subscriber rings off1 the bell of the other subscriber will very likely be rung even though the clearing-out drop operates properly2 it would be better in any event not to have this other subscriber's bell rung1 for he may understand it to be a recall to his telephone4 ,when1 however1 a party line is connected through such a cord circuit to an ordinary line having bridging instruments1 for instance1 the difficulty due to ringing off becomes even greater4 ,when the subscriber on the magneto line operates his generator to give the clearing-out signal1 he is very likely to ring some of the bells on the other line and this1 of course1 is an undesirable thing4 ,this may happen even in the case of harmonic bells on the party line1 since it is possible that the subscriber on the magneto line in turning his generator will1 at some phase of the operation1 strike just the proper frequency to ring some one of the bells on the harmonic party line4 ,it is obvious1 therefore1 that there is a real need for a cord circuit that will prevent through ringing4 ,one way of eliminating the through-ringing difficulty in the type of cord circuit shown in ,fig4 #bha would be to use such a very low-wound clearing-out drop that it would practically short-circuit the line with respect to ringing currents and prevent them from passing on to the other line4 ,this1 however1 is not a good thing to do1 since a winding sufficiently low to shunt the effective ringing current would also be too low for good telephone transmission4 ,illustration3 ,fig4 #bhb4 ,series ,drop-,cord ,circuit ,series ,drop ,type4 ,another type of cord circuit that was largely used by the ,stromberg-,carlson ,telephone ,manufacturing ,company at one time is shown in ,fig4 #bhb4 ,in this the clearing-out drop was not bridged but was placed in series in the tip side of the line and was shunted by a condenser4 ,the resistance of the clearing-out drop was #a1jjj ohms and the capacity of the condenser was #b microfarads4 ,it is obvious that this way of connecting the clearing-out drop was subject to the ringing-through difficulty1 since the circuit through which the clearing-out current necessarily passed included the telephone instrument of the line that was not sending the clearing-out signal4 ,this form was also objectionable because it was necessary for the subscriber to ring through the combined resistance of two lines1 and in case the other line happened to be open1 no clearing-out signal would be received4 ,while this circuit1 therefore1 was perhaps not quite so likely as the other to tie up the subscriber1 that is1 to leave him connected without the ability to send a clearing-out signal1 yet it was sure to ring through1 for the clearing-out drop could not be thrown without the current passing through the other subscriber's station4 ,illustration3 ,fig4 #bhc4 ,dean ,non-,ring-,through ,cord ,circuit ,non-,ring-,through ,type4 ,an early attempt at a non-ring-through cord is shown in ,fig4 #bhc1 this having once been standard with the ,dean ,electric ,company4 ,it made use of two condensers of #a microfarad each1 one in each side of the cord circuit4 ,the clearing-out drop was of #ejj ohms resistance and was connected from the answering side of the tip conductor to the calling side of the sleeve conductor4 ,in this way whatever clearing-out current reached the central office passed through at least one of the condensers and the clearing-out drop4 ,in order for the clearing-out current to pass on beyond the central office it was necessary for it to pass through the two condensers in series4 ,this arrangement had the advantage of giving a positive ring-off1 regardless of the condition of the connected line4 ,obviously1 even if the line was short-circuited1 the ringing currents from the other line would still be forced through the clearing-out drop on account of the high effective resistance of the #a-microfarad condenser connected in series with the short-circuited line4 ,also the clearing-out signal would be properly received if the connected line were open1 since the clearing-out drop would still be directly across the cord circuit4 ,this arrangement also largely prevented through ringing1 since the currents would pass through the #a-microfarad condenser and the #ejj-ohm drop more readily than through the two condensers connected in series4 ,illustration3 ,fig4 #bhd4 ,monarch ,non-,ring-,through ,cord ,circuit ,in ,fig4 #bhd is shown the non-ring-through arrangement of cord circuit adopted by the ,monarch ,company4 ,in this system the clearing-out drop has two windings1 either of which will operate the armature4 ,the two windings are bridged across the cord circuit1 with a #a_/b-microfarad condenser in series in the tip strand between the two winding connections4 ,while the low-capacity condenser will allow the high-frequency talking current to pass readily without affecting it to any appreciable extent1 it offers a high resistance to a low-frequency ringing current1 thus preventing it from passing out on a connected line and forcing it through one of the windings of the coil4 ,there is a tendency to transformer action in this arrangement1 one of the windings serving as a primary and the other as a secondary1 but this has not prevented the device from being highly successful4 ,a modification of this arrangement is shown in ,fig4 #bhe1 wherein a double-wound clearing-out drop is used1 and a #a_/b-microfarad condenser is placed in series in each side of the cord circuit between the winding connections of the clearing-out drop4 ,this circuit should give a positive ring-off under all conditions and should prevent through ringing except as it may be provided by the transformer action between the two windings on the same core4 ,illustration3 ,fig4 #bhe4 ,non-,ring-,through ,cord ,circuit ,another rather ingenious method of securing a positive ring-off and yet of preventing in a certain degree the undesirable ringing-through feature is shown in the cord circuit1 ,fig4 #bhf4 ,in this two non-inductive coils #a and #b are shown connected in series in the tip and sleeve strands of the coils1 respectively4 ,between the neutral point of these two non-inductive windings is connected the clearing-out drop circuit4 ,voice currents find ready path through these non-inductive windings because of the fact that1 being non-inductive1 they present only their straight ohmic resistance4 ,the impedance of the clearing-out drop prevents the windings being shunted across the two sides of the cord circuit4 ,with this circuit a positive ring-off is assured even though the line connected with the one sending the clearing-out signal is short-circuited or open4 ,if it is short-circuited1 the shunt around the clearing-out drop will still have the resistance of two of the non-inductive windings included in it1 and thus the drop will never be short-circuited by a very low-resistance path4 ,obviously1 an open circuit in the line will not prevent the clearing-out signal being received4 ,while this is an ingenious scheme1 it is not one to be highly recommended since the non-inductive windings1 in order to be effective so far as signaling is concerned1 must be of considerable resistance and this resistance is in series in the talking circuit4 ,even non-inductive resistance is to be avoided in the talking circuit when it is of considerable magnitude and where there are other ways of solving the problem4 ,illustration3 ,fig4 #bhf4 ,cord ,circuit with ,differential ,windings ,double ,clearing-out ,type4 ,some people prefer two clearing-out drops in each cord circuit1 so arranged that the one will be responsive to currents sent from the line with which the answering plug is connected and the other responsive only to currents sent from the line with which the calling plug is connected4 ,such a scheme1 shown in ,fig4 #bhg1 is sometimes employed by the ,dean1 the ,monarch1 and the ,kellogg companies4 ,two #ejj-ohm clearing-out drops of ordinary construction are bridged across the cord circuit and in each side of the cord circuit there is included between the drop connections a #a-microfarad condenser4 ,ringing currents originating on the line with which the answering plug is connected will pass through the clearing-out drop1 which is across that side of the cord circuit1 without having to pass through any condensers4 ,in order to reach the other clearing-out drop the ringing current must pass through the two #a-microfarad condensers in series1 this making in effect only #a_/b-microfarad4 ,as is well known1 a #a_/b-microfarad condenser not only transmits voice currents with ease but also offers a very high apparent resistance to ringing currents4 ,with the double clearing-out drop system the operator is enabled to tell which subscriber is ringing off4 ,if both shutters fall she knows that both subscribers have sent clearing-out signals and she1 therefore1 pulls down the connection without the usual precaution of listening to see whether one of the subscribers may be waiting for another connection4 ,this double clearing-out system is analogous to the complete double-lamp supervision that will be referred to more fully in connection with common-battery circuits4 ,there is not the need for double supervision in magneto work1 however1 that there is in common-battery work because of the fact that in magneto work the subscribers frequently fail to remember to ring off1 this act being entirely voluntary on their part1 while in common-battery work1 the clearing-out signal is given automatically by the subscriber when he hangs up his receiver1 thus accomplishing the desired end without the necessity of thoughtfulness on his part4 ,illustration3 ,fig4 #bhg4 ,double ,clearing-,out ,drops ,another form of double clearing-out cord circuit is shown in ,fig4 #bhh4 ,in this the calling and the answering plugs are separated by repeating coils1 a condenser of #a-microfarad capacity being inserted between each pair of windings on the two ends of the circuit4 ,the clearing-out drops are placed across the calling and answering cords in the usual manner4 ,the condenser in this case prevents the drop being short-circuited with respect to ringing currents and yet permits the voice currents to flow readily through it4 ,the high impedance of the drop forces the voice currents to take the path through the repeating coil rather than through the drop4 ,this circuit has the advantage of a repeating-coil cord circuit in permitting the connection of metallic and grounded lines without causing the unbalancing of the metallic circuits by the connection to them of the grounded circuits4 ,illustration3 ,fig4 #bhh4 ,double ,clearing-,out ,drops ,recently there has been a growing tendency on the part of some manufacturers to control their clearing-out signals by means of relays associated with cord circuits1 these signals sometimes being ordinary clearing-out drops and sometimes incandescent lamps4 ,illustration3 ,fig4 #bhi4 ,relay-,controlled ,clearing-,out ,drop ,in ,fig4 #bhi is shown the cord circuit sometimes used by the ,l4,m4 ,ericsson ,telephone ,manufacturing ,company4 ,a high-wound relay is normally placed across the cord and this1 besides having a high-resistance and impedance winding has a low-resistance locking winding so arranged that when the relay pulls up its armature it will close a local circuit including this locking winding and local battery4 ,when once pulled up the relay will1 therefore1 stay up due to the energizing of this locking coil4 ,another contact operated by the relay closes the circuit of a low-wound clearing-out drop placed across the line1 thus bridging it across the line4 ,the condition of high impedance is maintained across the cord circuit normally while the subscribers are talking2 but when either of them rings off1 the high-wound relay pulls up and locks1 thus completing the circuit of the clearing-out drop across the cords4 ,the subsequent impulses sent from the subscribers' generators operate this drop4 ,the relay is restored or unlocked and the clearing-out drop disconnected from the cord circuit by means of a key which opens the locking circuit of the relay4 ,this key is really a part of the listening key and serves to open this locking circuit whenever the listening key is operated4 ,the clearing-out drop is also automatically restored by the action of the listening key1 this connection being mechanical rather than electrical4 ,recall ,lamp3--,the ,monarch ,company sometimes furnishes what it terms a recall lamp in connection with the clearing-out drops on its magneto switchboards4 ,the circuit arrangement is shown in ,fig4 #bij1 wherein the drop is the regular double-wound clearing-out drop like that of ,fig4 #bhd4 ,the armature carries a contact spring adapted to close the local circuit of a lamp whenever it is attracted4 ,the object of this is to give the subscriber1 whose line still remains connected by a cord circuit1 opportunity to recall the central office if the operator has not restored the clearing-out drop4 ,illustration3 ,fig4 #bij4 ,cord ,circuit with ,recall ,lamp ,lamp-,signal ,type4 ,there has been a tendency on the part of some manufacturing companies to advocate1 instead of drop signals1 incandescent lamp signals for the cord circuits1 and sometimes for the line circuits on magneto boards4 ,in most cases this may be looked upon as a frill4 ,where line lamps instead of drops have been used on magneto switchboards1 it has been the practice to employ1 instead of a drop1 a locking relay associated with each lamp1 which was so arranged that when the relay was energized by the magneto current from the subscriber's station1 it would pull up and lock1 thus closing the lamp circuit4 ,the local circuit1 or locking circuit1 which included the lamp was carried through a pair of contacts in the corresponding jacks so arranged that when the plug was inserted in answer to the call1 this locking lamp circuit would be open1 thereby extinguishing the lamp and also unlocking the relay4 ,there seems to be absolutely no good reason why lamp signals should be substituted for mechanical drops in magneto switchboards4 ,there is no need for the economy in space which the lamp signal affords1 and the complications brought in by the locking relays1 and the requirements for maintaining a local battery suitable for energizing the lamps are not warranted for ordinary cases4 ,illustration3 ,fig4 #bia4 ,cord ,circuit with ,double ,lamp ,signals ,in ,fig4 #bia is shown a cord circuit1 adaptable to magneto switchboards1 provided with double lamp signals instead of clearing-out drops4 ,two high-wound locking relays are bridged across the line1 the cord strands being divided by #a-microfarad condensers4 ,when the high-wound coil of either relay is energized by the magneto current from the subscriber's station1 the relay pulls up and closes a locking circuit including a battery and a coil #b1 the contact #c of the locking relay1 and also the contact #d of a restoring key4 ,this circuit may be traced from the ground through battery1 coil #b1 contact #c controlled by the relay1 and contact #d controlled by the restoring key1 and back to ground4 ,in multiple with the locking coil #b is the lamp1 which is illuminated1 therefore1 whenever the locking circuit is closed4 ,pressure on the restoring key breaks the locking circuit of either of the lamps1 thereby putting out the lamp and at the same time restoring the locking relay to its normal position4 ,lamps vs4 ,drops in ,cord ,circuits4 ,so much has been said and written about the advantages of incandescent lamps as signals in switchboards and about the merits of the common-battery method of supplying current to the subscribers1 that there has been a tendency for people in charge of the operation of small exchanges to substitute the lamp for the drop in a magneto switchboard in order to give the general appearance of common-battery operations4 ,there has also been a tendency to employ the common-battery system of operation in many places where magneto service should have been used1 a mistake which has now been realized and corrected4 ,in places where the simple magneto switchboard is the thing to use1 the simpler it is the better1 and the employment of locking relays and lamp signals and the complications which they carry with them1 is not warranted4 ,switchboard ,assembly4 ,the assembly of all the parts of a simple magneto switchboard into a complete whole deserves final consideration4 ,the structure in which the various parts are mounted1 referred to as the cabinet1 is usually of wood4 ,functions of ,cabinet4 ,the purpose of the cabinet is not only to form a support for the various pieces of apparatus but also to protect them from dust and mechanical injury1 and to hold those parts that must be manipulated by the operator in such relation that they may be most convenient for use1 and thus best adapted for carrying out their various functions4 ,other points to be provided for in the design of the cabinet and the arrangement of the various parts within are3 that all the apparatus that is in any way liable to get out of order may be readily accessible for inspection and repairs2 and that provision shall be made whereby the wiring of these various pieces of apparatus may be done in a systematic and simple way so as to minimize the danger of crossed1 grounded1 or open circuits1 and so as to provide for ready repair in case any of these injuries do occur4 ,wall-,type ,switchboards4 ,the simplest form of switchboard is that for serving small communities in rural districts4 ,ordinarily the telephone industry in such a community begins by a group of farmers along a certain road building a line connecting the houses of several of them and installing their own instruments4 ,this line is liable to be extended to some store at the village or settlement1 thus affording communication between these farmers and the center of their community4 ,later on those residing on other roads do the same thing and connect their lines to the same store or central point4 ,then it is that some form of switchboard is established1 and perhaps the storekeeper's daughter or wife is paid a small fee for attendance4 ,illustration3 ,fig4 #bib4 ,wall ,switchboard with ,telephone ,a switchboard well-adapted for this class of service where the number of lines is small1 is shown in ,fig4 #bib4 ,in this the operator's talking apparatus and her calling apparatus are embodied in an ordinary magneto wall telephone4 ,the switchboard proper is mounted alongside of this1 and the two line binding posts of the telephone are connected by a pair of wires to terminals of the operator's plug1 which plug is shown hanging from the left-hand portion of the switchboard4 ,the various lines centering at this point terminate in the combined drops and jacks on the switchboard1 of which there are #bj shown in this illustration4 ,beside the operator's plug there are a number of pairs of plugs shown hanging from the switchboard cabinet4 ,these are connected straight through in pairs1 there being no clearing-out drops or keys associated with them in the arrangement4 ,each line shown is provided with an extra jack1 the purpose of which will be presently understood4 ,the method of operation is as follows3 ,when a subscriber on a certain line desires to get connection through the switchboard he turns his generator and throws the drop4 ,the operator in order to communicate with him inserts the plug in which her telephone terminates into the jack1 and removes her receiver from its hook4 ,having learned that it is for a certain subscriber on another line1 she withdraws her plug from the jack of the calling line and inserts it into the jack of the called line1 then1 hanging up her receiver1 she turns the generator crank in accordance with the proper code to call that subscriber4 ,when that subscriber responds she connects the two lines by inserting the two plugs of a pair into their respective jacks1 and the subscribers are thus placed in communication4 ,the extra jack associated with each line is merely an open jack having its terminals connected respectively with the two sides of the line4 ,whenever an operator desires to listen in on two connected lines she does so by inserting the operator's plug into one of these extra jacks of the connected lines1 and she may thus find out whether the subscribers are through talking or whether either one of them desires another connection4 ,the drops in such switchboards are commonly high wound and left permanently bridged across the line so as to serve as clearing-out drops4 ,the usual night-alarm attachment is provided1 the buzzer being shown at the upper right-hand portion of the cabinet4 ,illustration3 ,fig4 #bic4 ,combined ,telephone and ,switchboard ,another type of switchboard commonly employed for this kind of service is shown in ,fig4 #bic1 in which the telephone and the switchboard cabinet are combined4 ,the operation of this board is practically the same as that of ,fig4 #bib1 although it has manually-restored drops instead of self-restoring drops2 the difference between these two types1 however1 is not material for this class of service4 ,for such work the operator has ample time to attend to the restoring of the drop and the only possible advantage in the combined drop-and-jack for this class of work is that it prevents the operator from forgetting to restore the drops4 ,however1 she is not likely to do this with the night-alarm circuit in operation1 since the buzzer or bell would continue to ring as long as the drop was down4 ,illustration3 ,fig4 #bid4 ,upright ,magneto ,switchboard ,illustration3 ,fig4 #bie4 ,upright ,magneto ,switchboard--,rear ,view ,upright ,type ,switchboard4 ,by far the most common type of magneto switchboard is the so-called upright type1 wherein the drops and jacks are mounted on the face of upright panels rising from a horizontal shelf1 which shelf contains the plugs1 the keys1 and any other apparatus which the operator must manipulate4 ,front and rear views of such a switchboard1 as manufactured by the ,kellogg ,company1 are shown in ,figs4 #bid and #bie4 ,this particular board is provided with fifty combined drops and jacks and1 therefore1 equipped for fifty subscribers' lines4 ,the drops and jacks are mounted in strips of five1 and arranged in two panels4 ,the clearing-out drops1 of which there are ten1 are arranged at the bottom of the two panels in a single row and may be seen immediately above the switchboard plugs4 ,there are ten pairs of cords and plugs with their associated ringing and listening keys1 the plugs being mounted on the rear portion of the shelf1 while the ringing and listening keys are mounted on the hinged portion of the shelf in front of the plugs4 ,illustration3 ,fig4 #bif4 ,details of ,drop1 ,jack1 ,plug1 and ,key ,arrangement ,illustration3 ,fig4 #big4 ,cross-,section of ,upright ,switchboard ,a better idea of the arrangement of drops1 jacks1 plugs1 and keys may be had from an illustration of a ,dean magneto switchboard shown in ,fig4 #bif4 ,the clearing-out drops and the arrangement of the plugs and keys are clearly shown4 ,the portion of the switchboard on which the plugs are mounted is always immovable1 the plugs being provided with seats through which holes are bored of sufficient size to permit the switchboard cord to pass beneath the shelf4 ,when one of these plugs is raised1 the cord is pulled up through this hole thus allowing the plug to be placed in any of the jacks4 ,the key arrangement shown in this particular cut is instructive4 ,it will be noticed that the right-hand five pairs of plugs are provided with ordinary ringing and listening keys1 while the left-hand five are provided with party-line ringing keys and listening keys4 ,the listening key in each case is the one in the rear and is alike for all of the cord pairs4 ,the right-hand five ringing keys are so arranged that pressing the lever to the rear will ring on the answering cord1 while pressing it toward the front will cause ringing current to flow on the calling plug4 ,in the left-hand five pairs of cords shown in this cut1 the pressure of any one of the keys causes a ringing current of a certain frequency to flow on the calling cord1 this frequency depending upon which one of the keys is pressed4 ,illustration3 ,fig4 #bih4 ,cord ,weight ,an excellent idea of the grouping of the various pieces of apparatus in a complete simple magneto switchboard may be had from ,fig4 #big4 ,while the arrangement here shown is applicable particularly to the apparatus of the ,dean ,electric ,company1 the structure indicated is none-the-less generally instructive1 since it represents good practice in this respect4 ,in this drawing the stationary plug shelf with the plug seat is clearly shown and also the hinged key shelf4 ,the hinge of the key shelf is an important feature and is universally found in all switchboards of this general type4 ,the key shelf may be raised and thus expose all of the wiring leading to the keys1 as well as the various contacts of the keys themselves1 to inspection4 ,illustration3 ,fig4 #bii4 ,magneto ,switchboard1 ,target ,signals ,as will be seen1 the switchboard cords leading from the plugs extend down to a point near the bottom of the cabinet where they pass through pulley weights and then up to a stationary cord rack4 ,on this cord rack are provided terminals for the various conductors in the cord1 and it is at this point that the cord conductors join the other wires leading to the other portions of the apparatus as required4 ,a good form of cord weight is shown in ,fig4 #bih2 and obviously the function of these weights is to keep the cords taut at all times and to prevent their tangling4 ,illustration3 ,fig4 #cjj4 ,rear ,view of ,target ,signal1 ,magneto ,switchboard ,the drawing1 ,fig4 #big1 also gives a good idea of the method of mounting the hand generator that is ordinarily employed with such magneto switchboards4 ,the shaft of the generator is merely continued out to the front of the key shelf where the usual crank is provided1 by means of which the operator is able to generate the necessary ringing current4 ,beside the hand generator at each operator's position1 it is quite common in magneto boards1 of other than the smallest sizes1 to employ some form of ringing generator1 either a power-driven generator or a pole changer driven by battery current for furnishing ringing current without effort on the part of the operator4 ,illustration3 ,fig4 #cja4 ,dean ,two-,position ,switchboard ,switchboards as shown in ,figs4 #bid and #bie1 are called single-position switchboards because they afford room for a single operator4 ,ordinarily for this class of work a single operator may handle from one to two hundred lines1 although of course this depends on the amount of traffic on the line1 and this1 in turn1 depends on the character of the subscribers served1 and also on the average number of stations on a line4 ,another single-position switchboard is shown in ,figs4 #bii and #cjj1 being a front and rear view of the simple magneto switchboard of the ,western ,electric ,company1 which is provided with the target signals of that company rather than the usual form of drop4 ,where a switchboard must accommodate more lines than can be handled by a single operator1 the cabinet is made wider so as to afford room for more than one operator to be seated before it4 ,sometimes this is accomplished by building the cabinet wider1 or by putting two such switchboard sections as are shown in ,figs4 #bid or #bii side by side4 ,a two-position switchboard section is shown in front and rear views in ,figs4 #cja and #cjb4 ,illustration3 ,fig4 #cjb4 ,rear ,view of ,dean ,two-,position ,switchboard ,sectional ,switchboards4 ,the problem of providing for growth in a switchboard is very much the same as that which confronts one in buying a bookcase for his library4 ,the ,western ,electric ,company has met this problem1 for very small rural exchanges1 in much the same way that the sectional bookcase manufacturers have provided for the possible increase in bookcase capacity4 ,like the sectional bookcase1 this sectional switchboard may start with the smallest of equipment--a single sectional unit--and may be added to vertically as the requirements increase1 the original equipment being usable in its more extended surroundings4 ,illustration3 ,fig4 #cjc4 ,sectional ,switchboard--,wall ,type ,this line of switchboards is illustrated in ,figs4 #cjc to #cjf4 ,the beginning may be made with either a wall type or an upright type of switchboard1 the former being mounted on brackets secured to the wall1 and the latter on a table4 ,a good idea of the wall type is shown in ,fig4 #cjc4 ,three different kinds of sectional units are involved in this3 first1 the unit which includes the cords1 plugs1 clearing-out drops1 listening jacks1 operator's telephone set and generator2 second1 the unit containing the line equipment1 including a strip of ten magneto line signals and their corresponding jacks2 third1 the finishing top1 which includes no equipment except the support for the operator's talking apparatus4 ,illustration3 ,fig4 #cja4 ,sectional ,switchboard--,wall ,type ,the first of the units in ,fig4 #cjc forms the foundation on which the others are built4 ,two of the line-equipment units are shown2 these provide for a total of twenty lines4 ,the top rests on the upper line-equipment unit1 and when it becomes necessary to add one or more line-equipment units as the switchboard grows1 this top is merely taken off1 the other line-equipment units put in place on top of those already existing1 and the top replaced4 ,the wall type of sectional switchboard is so arranged that the entire structure may be swung out from the wall1 as indicated in ,fig4 #cjd1 exposing all of the apparatus and wiring for inspection4 ,each of the sectional units is provided with a separate door1 as indicated1 so that the rear door equipment is added to automatically as the sections are added4 ,in the embodiment of the sectional switchboard idea shown in these two figures just referred to1 no ringing and listening keys are provided1 but the operator's telephone and generator terminate in a special plug--the left-hand one shown in ,fig4 #cjc--;and when the operator desires to converse with the connected subscribers1 she does so by inserting the operator's plug into one of the jacks immediately below the clearing-out drop corresponding to the pair of plugs used in making the connection4 ,the arrangement in this case is exactly the same in principle as that described in ,fig4 #bib4 ,the operator's generator is so arranged in connection with this left-hand operator's plug that the turning of the generator crank automatically switches the operator's telephone set off and switches the generator on1 just the same as a switch hook may do in a subscriber's series telephone4 ,illustration3 ,fig4 #cje4 ,sectional ,switchboard--,table ,type ,illustration3 ,fig4 #cjf4 ,sectional ,switchboard--,table ,type ,the upright type of sectional switchboard is shown in ,figs4 #cje and #cjf1 which need no explanation in view of the foregoing1 except to say that1 in the particular instrument illustrated1 ringing and listening keys are provided instead of the jack-and-plug arrangement of the wall type4 ,in this case also1 the top section carries an arm for supporting a swinging transmitter instead of the hook support for the combined transmitter and receiver4 ,,review ,,questions ,blank ,page ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #aa--fb 99 99 99 99 99 #a4 ,when was the telephone invented and by whom8 #b4 ,state the velocity of sound in air4 ,is it higher in air than in a denser medium8 #c4 ,state and define the characteristics of sound4 #d4 ,make sketch of ,bell's original magneto telephone without permanent magnets4 #e4 ,describe and sketch ,hughes' microphone4 #f4 ,which is1 at present1 the best material for varying the resistance in transmitters8 #g4 ,give the fundamental differences between the magneto transmitter and the carbon transmitter4 #h4 ,what is the function of the induction coil in the telephone circuit8 #i4 ,describe and sketch the different kinds of visible signals4 #aj4 ,what should be the diameter of hard drawn copper wire in order to allow economical spacing of poles8 #aa4 ,state the four principal properties of a telephone line4 #ab4 ,if in testing a line the capacity is changed what are the results found on the receiver and transmitter end8 #ac4 ,why is paper used as an insulator of telephone cables8 #ad4 ,how does a conductor behave in connection with direct current and how with alternating current8 #ae4 ,what influence has inductance on the telephone8 #af4 ,define impedance and give the formula for it4 #ag4 ,what is the usual specification for insulation of resistance in telephone cables8 #ah4 ,if #gej feet of cable have an insulation resistance of #i1ace megohms1 how great is the insulation resistance for #g miles and #a1gdd feet of cable8 #ai4 ,what is the practical limiting conversation distance for ,no4 #aj ,b4 and ,s4 wire8 #bj4 ,describe ,professor ,pupin's method of inserting inductance into the telephone line4 #ba4 ,what does mho denote8 #bb4 ,why are ,pupin's coils not so successful on open wires8 #bc4 ,what is a repeater8 #bd4 ,define reactive interference4 #be4 ,state the frequencies of the pitches of the human voice4 #bf4 ,what is the office of a diaphragm in a telephone apparatus8 #bg4 ,what transmitter material has greatly increased the ranges of speech8 #bh4 ,describe the different methods of measurements of telephone circuits4 #bi4 ,what are the two kinds of electric calls8 #cj4 ,how many conductors has a telephone line8 #ca4 ,give formula for capacity reactance and the meaning of the symbols4 #cb4 ,which ,american cities are joined by underground lines at present8 #cc4 ,state the two practical ways of improving telephone transmission4 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #fc--ada 99 99 99 99 99 #a4 ,on what general principle are most of the telephone transmitters of today constructed8 #b4 ,make sketch of the new ,western ,electric transmitter and describe its working4 #c4 ,make sketch and describe the ,kellogg transmitter4 #d4 ,what troubles were encountered in the earlier forms of granular carbon transmitters and how were they overcome8 #e4 ,what limits the current-carrying capacity of the transmitter8 ,how may this capacity be increased8 #f4 ,state in what kind of transmitters a maximum degree of sensitiveness is desirable4 #g4 ,show the conventional symbols for transmitters4 #h4 ,describe a telephone receiver4 #i4 ,sketch a ,western ,electric receiver and point out its deficiencies4 #aj4 ,make a diagram of the ,kellogg receiver4 #aa4 ,describe the direct-current receiver of the ,automatic ,electric ,company4 #ab4 ,describe and sketch the ,dean receiver4 #ac4 ,show the conventional symbols of a receiver4 #ad4 ,describe exactly how1 in a cell composed of a tin and a silver plate with dilute sulphuric acid as electrolyte1 the current inside and outside of the cell will flow4 #ae4 ,describe the phenomenon of polarization4 #af4 ,what is local action of a cell8 ,how may it be prevented8 #ag4 ,into how many classes may cells be divided8 ,which class is most used in telephony8 #ah4 ,describe the ,le,clanch cell4 #ai4 ,sketch and describe an excellent form of dry cell4 #bj4 ,show the conventional symbols for batteries4 #ba4 ,sketch and describe the generator shunt switch and the generator cut-in switch4 #bb4 ,how may a pulsating current be derived from a magneto generator8 #bc4 ,show conventional symbols for magneto generators4 #bd4 ,sketch and describe the ,western ,electric polarized bell4 #be4 ,give conventional ringer symbols4 #bf4 ,what is the purpose of the hook switch8 #bg4 ,make sketch and give description of ,kellogg's long lever hook switch4 #bh4 ,describe and sketch the ,western ,electric short lever hook switch4 #bi4 ,point out the principal difference between the desk stand hook switches of the ,western ,electric ,company and of the ,kellogg ,switchboard and ,supply ,company4 #cj4 ,give conventional symbols of hook switches4 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #adc--bbe 99 99 99 99 99 #a4 ,describe an electromagnet and its function in telephony4 #b4 ,sketch an iron-clad electromagnet4 #c4 ,what is a differential electromagnet8 ,sketch and describe one type4 #d4 ,state the desirable characteristics of good enamel insulation for magnet wire4 #e4 ,if you have a coil of ,no4 #bc double cotton ,b4 and ,s4 wire of #aae ohms resistance and you have to rewind it for #a1jgj ohms resistance with double cotton wire1 what number of wire would you take8 ,show calculation4 ,,note4 ,no4 #bc d4 c wire has res4 #a4ggb ohms per cubic inch2 for the core1 #aae ohms4 ,there are required in the coil #a1jgj ohms1 that is1 #i4c times as much4 #a4ggb x #i4c #af4dg ohms1 which must be the resistance per cu4 in4 ,this resistance gives1 according to ,table ,,iv1 ,no4 #bi wire4 #f4 ,what is an impedance coil8 ,state how it differs from an electromagnet coil4 #g4 ,describe the different kinds of impedance coils4 #h4 ,give symbol of impedance coil4 #i4 ,what are the principal parts of an induction coil8 #aj4 ,what is the function of an induction coil in telephony8 #aa4 ,what is a repeating coil and how does it differ from an induction coil8 #ab4 ,give conventional symbols of induction coils and repeating coils4 #ac4 ,enumerate the different types of non-inductive resistance devices and give a short description of each4 #ad4 ,define condenser4 #ae4 ,what is the meaning of the word dielectrics8 #af4 ,state what you understand by the specific inductive capacity of a dielectric4 #ag4 ,upon what factors does the capacity of a condenser depend8 #ah4 ,what is the usual capacity of condensers in telephone practice8 #ai4 ,give conventional condenser symbols4 #bj4 ,by what two methods may the current be supplied to a telephone transmitter8 #ba4 ,make sketch of local-battery stations with metallic circuit4 #bb4 ,sketch common-battery circuit in series with two lines4 #bc4 ,state the objections against the preceding arrangement4 #bd4 ,make sketch of the standard arrangement of the ,western ,electric ,company in bridging the common battery with repeating coils4 #be4 ,sketch the arrangement of bridging the battery with impedance coils and state the purpose of the coils4 #bf4 ,make diagram of a common-source current supply for many lines with repeating coils and point out the travel of the voice currents4 #bg4 ,name the different parts which comprise a telephone set4 #bh4 ,what is a magneto telephone8 #bi4 ,make diagram of the circuit of a series magneto set with receiver on the hook and explain how the different currents are flowing4 #cj4 ,show diagram of the ,stromberg-,carlson magneto desk telephone circuit and describe its working4 #ca4 ,give sketch of the ,stromberg-,carlson common-battery wall set circuit4 #cb4 ,describe briefly the microtelephone set4 #cc4 ,make sketch of the ,monarch common-battery wall set4 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #bbg--bhf 99 99 99 99 99 #a4 ,what is a party line8 #b4 ,what is usually understood by private lines8 #c4 ,what problem is there to overcome in connection with party lines8 #d4 ,state the two general classes of party-line systems4 #e4 ,point out the defects of the series system4 #f4 ,make sketch of a metallic bridging line and show the circuit for the voice currents4 #g4 ,what is a signal code8 #h4 ,give classification of selective party-line systems with short definitions4 #i4 ,describe the principle of selection by polarity and make sketch illustrating this principle4 #aj4 ,make diagram of the circuit of a four-party station with relay4 #aa4 ,describe the process of tuning in the harmonic system4 #ab4 ,what is the difference between the under-tune and in-tune systems8 #ac4 ,sketch circuit of ,kellogg's harmonic system4 #ad4 ,illustrate the principle of a broken-line system by a sketch4 #ae4 ,in what particulars does the party-line system in rural districts differ from that within urban limits8 #af4 ,describe and sketch ,pool's lock-out system4 #ag4 ,make diagram of the ,k4,b4 lock-out system4 #ah4 ,what is the object of the ratchet in this system8 #ai4 ,make diagram of simplified circuits of ,roberts system4 #bj4 ,sketch and describe ,roberts latching key and connections4 #ba4 ,sketch circuits of bridging station for non-selective party line4 #bb4 ,how would you arrange the signal code for six stations on a non-selective party line8 #bc4 ,what is the limit of number of stations on a non-selective party line under ordinary circumstances8 #bd4 ,state the objections against the party polarity system as shown in ,fig4 #agb4 #be4 ,what are the advantages of the harmonic party-line system8 #bf4 ,to how many frequencies is the harmonic system usually limited8 #bg4 ,what can you say about the commercial success of the step-by-step method8 #bh4 ,state the principles of a lock-out party line4 #bi4 ,for what purpose is a condenser placed in the receiver circuit of each station in the ,k4,b4 lock-out system8 #cj4 ,how are the selecting relays in ,roberts line restored to their normal position after a conversation is finished8 #ca4 ,what are the objections against the ,roberts system8 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #bhg--cae 99 99 99 99 99 #a4 ,what are electrical hazards8 #b4 ,when is the lightning hazard least8 #c4 ,what actions can electricity produce8 ,which involves the greater hazard to the value of property8 #d4 ,when is a piece of apparatus called self-protecting8 #e4 ,why must a protector for telephone apparatus work more quickly for a large current than for a small one8 #f4 ,state the general problem which heating hazards present with relation to telephone apparatus4 #g4 ,what is the most nearly universal electrical hazard8 #h4 ,sketch and describe the saw-tooth lightning arrester4 #i4 ,make diagram of the carbon-block arrester and state its advantages4 #aj4 ,describe a vacuum arrester4 #aa4 ,explain the reason for placing an impedance in connection with the lightning arrester4 #ab4 ,what is the purpose of the globule of low-melting alloy in the ,western ,electric ,company's arrester8 #ac4 ,why are not fuses good lightning arresters8 #ad4 ,what is the proper function of a fuse8 #ae4 ,make sketch of a mica slip fuse4 #af4 ,define sneak currents4 #ag4 ,make a diagram of a sneak-current arrester and describe its principles and working4 #ah4 ,describe a heat coil4 #ai4 ,sketch a complete line protection4 #bj4 ,where is the proper position of the fuse8 #ba4 ,which wires are considered exposed and which unexposed8 #bb4 ,why is it not necessary to install sneak-current arresters in central-battery subscribers' stations8 #bc4 ,sketch and describe the action of a combined sneak-current and air-gap arrester1 as widely used by ,bell companies4 #bd4 ,describe the self-soldering heat-coil arrester4 #be4 ,what is the purpose of ribbon fuses8 #bf4 ,what is a drainage coil8 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #cag--chf 99 99 99 99 99 #a4 ,what is a central office8 #b4 ,what are 7a7 subscriber's lines8 7b7 ,trunk lines8 7c7 ,toll lines8 #c4 ,for what purpose is the switchboard8 #d4 ,give short descriptions of the different classes of switchboards4 #e4 ,how are manual switchboards subdivided8 ,describe briefly the different types4 #f4 ,define ,a and ,b boards4 #g4 ,what is a call circuit8 #h4 ,what kind of calls are handled on a toll switchboard8 #i4 ,give drop symbol and describe its principles4 #aj4 ,what is a jack8 #aa4 ,make a sketch of a plug inserted into a jack4 #ab4 ,give jack and plug symbols4 #ac4 ,what are ringing and listening keys8 #ad4 ,show symbols for ringing and listening keys4 #ae4 ,state the parts of which a cord equipment consists4 #af4 ,show step by step the various operations of a telephone system wherein the lines center in a magneto switchboard4 ,make all the necessary diagrams and give brief descriptions to show that you understand each operation4 #ag4 ,on what principle does a drop with night-alarm contact operate8 #ah4 ,what is the advantage of associating jacks and drops8 #ai4 ,describe the mechanical restoration as employed in the ,miller drop and jack4 #bj4 ,describe the electrical restoration of drop shutters as manufactured by the ,western ,electric ,company4 #ba4 ,what complications arise in ringing of party lines and how are they overcome8 #bb4 ,give diagram of the complete circuit of a simple magneto switchboard4 #bc4 ,sketch night-alarm circuit with relay4 #bd4 ,what is a convertible cord circuit8 #be4 ,state what disadvantages may be encountered under certain conditions with a bridging drop-cord circuit4 #bf4 ,are lamps in cord circuits to be advocated on magneto switchboards8 #bg4 ,what is the function of the cabinet8 #bh4 ,give cross-section of upright switchboard as used in the magneto system4 #bi4 ,what is the purpose of a sectional switchboard8 #cj4 ,give a short description of the essential parts of a sectional switchboard4 ,,index ,,index ,the page numbers of this volume will be found at the bottom of the pages2 the numbers at the top refer only to the section4 ,a ,acousticon transmitter ,acoustics characteristics of sound loudness pitch timbre human ear human voice propagation of sound ,air-gap vs4 fuse arresters ,amalgamated zincs ,arrester separators ,audible signals magneto bell telegraph sounder telephone receiver vibrating bell ,automatic ,electric ,company direct-current receiver transmitter ,automatic shunt ,b ,bar electromagnet ,battery bell ,battery symbols ,blake single electrode ,brazed bell ,broken-back ringer ,broken-line method of selective signaling ,c ,capacity reactance ,carbon adaptability limitations preparation of superiority ,carbon air-gap arrester ,carbon-block arrester ,carrying capacity of transmitter ,central-office protectors ,characteristics of sound loudness pitch timbre ,chloride of silver cell ,closed-circuit cells ,closed-circuit impedance coil ,common-battery telephone sets ,condensers capacity charge conventional symbols definition of dielectric dielectric materials functions means for assorting current sizes theory ,conductivity of conductors ,conductors1 conductivity of ,conventional symbols ,cook air-gap arrester arrester arrester for magneto stations ,crowfoot cell ,current supply to transmitters common battery advantages bell substation arrangement bridging battery with impedance coils bridging battery with repeating coil current supply from distant point current supply over limbs of line in parallel ,dean substation arrangement double battery with impedance coil ,kellogg substation arrangement ,north ,electric ,company system series battery series substation arrangement ,stromberg-,carlson system supply many lines from common source repeating coil retardation coil local battery ,d ,dean drop and jack receiver wall telephone hook ,desk stand hooks ,kellogg ,western ,electric ,dielectric ,dielectric materials dry paper mica ,differential electromagnet ,direct-current receiver ,drainage coils ,e ,electric lamp signal ,electrical hazards ,electrical reproduction of speech carbon conversion from sound waves to vibration of diaphragm conversion from vibration to voice currents conversion from voice currents to vibration cycle of conversion detrimental effects of capacity early conceptions electrostatic telephone induction coil limitations of magneto transmitter loose contact principle magneto telephone measurements of telephone currents variation of electrical pressure variation of resistance ,electrical signals audible magneto-bell telegraph sounder telephone receiver vibrating bell visible electric lamp signal electromagnetic signal ,electrodes arrangement of carbon preparation multiple single ,electrolysis ,electromagnetic method of measuring telephone currents ,electromagnetic signal ,electromagnets and inductive coils conventional symbols differential electromagnet direction of armature motion direction of lines of force electromagnets low-resistance circuits horseshoe form iron-clad form special horseshoe form impedance coils kind of iron number of turns types closed-circuit open-circuit toroidal induction coil current and voltage ratios design functions use and advantage magnet wire enamel silk and cotton insulation space utilization wire gauges magnetic flux magnetization curves magnetizing force mechanical details permeability reluctance repeating coil winding methods winding calculations winding data winding terminals ,electrostatic capacity unit of ,electrostatic telephone ,enamel ,f ,five-bar generator ,fuller cell ,g ,galvani ,generator armature ,generator cut-in switch ,generator shunt switch ,generator symbols ,granular carbon ,gravity cell ,h ,hand receivers ,harmonic method of selective signaling advantages circuits in-tune system limitations principles tuning under-tune system ,head receivers ,heat coil ,holtzer-,cabot arrester ,hook switch automatic operation contact material design desk stand hooks ,kellogg ,western ,electric purpose symbols wall telephone hooks ,dean ,kellogg ,western ,electric ,horseshoe electromagnet ,human ear ,human voice ,i ,impedance coils kind of iron number of turns symbols of types closed-circuit open-circuit toroidal ,inductance vs4 capacity ,induction coil current and voltage ratios design functions use and advantage ,inductive neutrality ,inductive reactance ,insulation of conductors ,introduction to telephony ,iron-clad electromagnet ,iron wire ballast ,k ,kellogg air-gap arrester desk stand hook drop and jack receiver ringer transmitter wall telephone hook ,l ,lalande cell ,lamp filament ,le ,clanch cell ,lenz law ,line signals ,lines of force1 direction of ,loading coils ,lock-out party-line systems broken-line method operation ,poole system step-by-step system ,loudness of sound ,low-reluctance circuits horseshoe form iron-clad form ,m ,magnetic flux ,magnetization curves ,magnetizing force ,magneto bell ,magneto operator ,magneto signaling apparatus armature automatic shunt battery bell generator symbols magneto bell magneto generator method of signaling polarized ringer pulsating current ringer symbols theory ,magneto switchboard automatic restoration mechanical ,dean type ,kellogg type ,monarch type ,western ,electric type circuits of complete switchboard code signaling commercial types of drops and jacks early drops jack mounting manual vs4 automatic restoration methods of associating night alarm tubular drops component parts jacks and plugs keys line and cord equipments line signal operators' equipment cord-circuit considerations double clearing-out type lamp-signal type non-ring through type series drop type simple bridging drop type definitions electrical restoration grounded and metallic-circuit lines mode of operation night-alarm circuits operation in detail clearing out essentials of operation normal condition of line operator answering operator calling subscriber calling subscribers conversing operator's telephone equipment cut-in jack ringing and listening keys horizontal spring type party-line ringing keys self-indicating keys vertical spring type switchboard assembly functions of cabinet sectional switchboards upright type of switchboard wall type switchboard switchboard cords concentric conductors parallel tinsel conductors steel spiral conductors switchboard plugs ,magneto telephone ,magneto telephone sets ,mica card resistance ,mica slip fuse ,microtelephone set ,monarch drop and jack ,monarch receiver ,monarch transmitter ,multiple electrode ,mutual induction ,n ,non-inductive resistance devices inductive neutrality provisions against heating temperature coefficient types differentially-wound unit iron wire ballast lamp filament mica card unit ,non-selective party-line systems bridging limitations series signal code ,o ,open-circuit cells ,open-circuit impedance coil ,operator's receiver ,p ,packing of transmitters ,permeability ,pitch ,doppler's principle vibration of diaphragms ,polarity method of selective signaling ,polarization of cells ,polarized ringer brazed bell ,kellogg ,western ,electric ,poole lock-out system ,primary cells conventional symbol series and multiple connections simple voltaic types of closed-circuit ,fuller gravity ,lalande prevention of creeping setting up open-circuit ,le ,clanch standard chloride of silver ,propagation of sound ,protective means against high potentials air-gap arrester advantages of carbon commercial types continuous arcs discharge across gaps dust between carbons introduction of impedance metallic electrodes vacuum arresters against sneak currents heat coil sneak-current arresters against strong currents fuses enclosed mica proper functions central-office protectors self-soldering heat coils sneak-current and air-gap arrester city exchange requirements complete line protection electrolysis subscribers' station protectors ribbon fuses ,pulsating-current commutator ,r ,receivers ,dean direct-current early ,kellogg modern ,monarch operator's single-pole symbols ,western ,electric ,reluctance ,repeating coil ,ribbon fuses ,ringer symbols ,ringing and listening key ,robert's latching relay ,robert's self-cleansing arrester ,rolled condenser ,s ,saw-tooth arrester ,selective party-line systems broken-line method classification broken-line systems harmonic systems polarity systems step-by-step systems harmonic method polarity method step-by-step method ,self-induction ,signal code ,signaling1 method of ,silk and cotton insulation ,single electrode ,single-pole receiver ,sneak-current arresters ,solid-back transmitter ,sound characteristics of loudness pitch timbre ,standard cell ,step-by-step lock-out system ,step-by-step method of selective signaling ,subscribers' station protectors ,switchboard cords ,switchboard plugs ,switchboard transmitter ,symbols battery condenser generator hook switch impedance coil induction coil receiver repeating coil ringer ringing and listening key transmitter ,t ,table condenser data copper wire ,german silver wire--#ah per cent ,german silver wire--#cj per cent metals1 behavior of1 in different electrolysis signal code specific inductive capacities temperature coefficients transmission distances1 limiting winding data for insulating wires ,tandem differential electromagnet ,telegraph sounder ,telephone currents1 measurements of electromagnetic method thermal method ,telephone exchange1 features of districts subscribers' lines switchboards toll lines trunk lines ,telephone lines conductivity of conductors electrostatic capacity inductance of circuit inductance vs4 capacity insulation of conductors transmission ,telephone sets classification of common-battery telephone magneto telephone wall and desk telephones common-battery desk hotel wall magneto circuits of bridging series desk wall ,temperature coefficients ,thermal method of measuring telephone currents ,timbre ,toroidal impedance coil ,toroidal repeating coil ,transmission1 ways of improving ,transmitters acousticon ,automatic ,electric ,company carrying capacity conventional diagram electrode arrangement of multiple single granular carbon ,kellogg materials ,monarch packing sensitiveness switchboard symbols variable resistance ,western ,electric solid-back ,u ,under-tuned ringer ,v ,vacuum arrester ,variable resistance ,vibrating bell ,visible signals electric lamp electromagnetic ,volta ,voltaic cell amalgamated zincs difference of potential local action polarization theory ,w ,wall telephone hooks ,dean ,kellogg ,western ,electric ,western ,electric air-gap arrester desk stand hook drop and jack receiver ringer solid-back transmitter station arrester wall telephone hook ,white transmitter ,wire gauges