,illustration3 ,,thomas ,a4 ,,edison ,pioneer ,electrical ,investigator and ,inventor of ,numerous ,telegraph1 ,telephone1 ,lighting1 and ,other ,electrical ,devices4 ,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 ,,copyright1 #aiaa1 #aiab1 ,,by ,,american ,,school ,,of ,,correspondence ,,copyright1 #aiaa1 #aiab ,,by ,,american ,,technical ,,society ,entered at ,stationers' ,hall1 ,london ,all ,rights ,reserved ,authors and ,collaborators 99 99 99 99 99 ,,kempster ,b4 ,,miller1 ,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 ,engineers 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 ,telephone1 ,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,sc41 ,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,sc41 ,,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 ,mc,m,i,l,l,a,n1 ,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 ,engineer2 ,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 ,college ,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 ,,grosse ,,point ,,exchange ,,rack ,detroit ,home ,telephone ,company1 ,detroit1 ,mich4 ,the ,dean ,electric ,co4 ,illustration3 ,,line ,,side ,,of ,,large ,,main ,,distributing ,,frame ,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 ,,ii ,,manual ,,switchboards ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen,a ,page,b #aa ,common-,battery ,switchboards--,line ,signals--,cord ,circuit--,lamps--,mechanical ,signals--,relays--,jacks--,switchboard ,assembly--,transfer ,switchboard--,transfer ,lines--,handling ,transfers--,multiple ,switchboard--,busy ,test--,influence of ,traffic--,magneto-,multiple ,switchboard--,multiple ,boards3 ,series1 ,branch-,terminal1 ,modern ,magneto1 ,common-,battery--,western ,electric ,no4 #a ,relay ,board--,western ,electric ,no4 #aj ,board--,types of ,multiple ,boards--,apparatus--,trunking--,western ,electric and ,kellogg ,trunk ,circuits ,,automatic ,,systems ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen ,page #ace ,automatic vs4 ,manual--,operation--,selecting ,switch--,line ,switch--,trunking ,systems--,two- and ,three-,wire ,systems--,subscriber's ,station ,apparatus--,first and ,second ,selector ,operation--,connector--,release after ,conversation--,multi-,office ,system--,automatic ,sub-,offices--,rotary ,connector--,party ,lines--,two-,wire ,automatic ,system--,lorimer ,system--,central-,office ,apparatus--,operation--,automanual ,system--,operation--,subscriber's ,apparatus--,operator's ,equipment--,switching ,equipment--,distribution of ,calls--,connection--,speed ,,power ,,plants ,,and ,,buildings ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen ,page #bbg ,currents ,employed--,types--,operator's ,transmitter ,supply--,ringing-,current ,supply--,auxiliary ,signaling ,current--,primary ,sources--,duplicate ,apparatus--,storage ,batteries--,power ,switchboards--,circuits--,central-,offi ce ,building--,arrangement of ,apparatus--,manual ,offices--,automatic ,offices ,,special ,,service ,,features ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen ,page #bga ,private-,branch ,exchanges--,switchboards--,supervision- -,with ,automatic ,offices--,battery ,supply--,ringing ,current--,inter-,communicating ,systems--,magneto ,system--,common-,battery ,systems--,types--,long-,distance ,switching--,operator's ,orders--,trunking--,way ,stations--,traffic--,measured ,service--,charging--,rates--,toll ,service--,local ,service ,,telegraph ,,and ,,railway ,,work ,by ,k4 ,b4 ,miller and ,s4 ,g4 ,mc,meen ,page #cba ,phantom1 ,simplex1 and ,composite ,circuits--,ringing--,railway ,composite--,telephone ,train ,dispatching--,railroad ,conditions--,transmitting ,orders--,apparatus--,telephone ,equipment--,types of ,circuits--,test ,boards--,blocking ,sets--,dispatching on ,electric ,railways ,,review ,,questions ,page #cei ,,index ,page #cgc ,footnote ,a3 ,for professional standing of authors1 see list of ,authors and ,collaborators at front of volume4 ,footnote ,b3 ,for page numbers1 see foot of pages4 ,illustration3 ,,portion ,,of ,,terminal ,,room ,,of ,,large ,,common-,,battery ,,office ,prospect ,office1 ,new ,york ,telephone ,co4 ,,chapter ,,xxii ,,the ,,simple ,,common-,,battery ,,switchboard ,advantages of ,common-,battery ,operation4 ,the advantages of the common-battery system of operation1 alluded to in ,chapter ,,xiii1 may be briefly summarized here4 ,the main gain in the common-battery system of supply is the simplification of the subscribers' instruments1 doing away with the local batteries and the magneto generators1 and the concentration of all these many sources of current into one single source at the central office4 ,a considerable saving is thus effected from the standpoint of maintenance1 since the simpler common-battery instrument is not so likely to get out of order and1 therefore1 does not have to be visited so often for repairs1 and the absence of local batteries1 of course1 makes the renewal of the battery parts by members of the maintenance department1 unnecessary4 ,another decided advantage in the common-battery system is the fact that the centralized battery stands ready always to send current over the line when the subscriber completes the circuit of the line at his station by removing his receiver from its hook4 ,the common-battery system1 therefore1 lends itself naturally to the purposes of automatic signaling1 since it is only necessary to place at the central office a device in the circuit of each line that will be responsive to the current which flows from the central battery when the subscriber removes his receiver from its hook4 ,it is thus that the subscriber is enabled automatically to signal the central office when he desires a connection2 and as will be shown1 it is by the same sort of means1 associated with the cord circuits used in connecting his line with some other line1 that the operator is automatically notified when a disconnection is desired1 the cessation of current through the subscriber's line when he hangs up his receiver being made to actuate certain responsive devices which are associated with the cord at that time connected with his line1 and which convey the proper disconnect signal to the operator4 ,concentration of sources of energy into a single large unit1 the simplification of the subscriber's station equipment1 and the ready adaptability to automatic signaling from the subscriber to the central office are1 therefore1 the reasons for the existence of the common-battery system4 ,common ,battery vs4 ,magneto4 ,it must not be supposed1 however1 that the common-battery system always has advantages over the magneto system1 and that it is superior to the magneto or local-battery system for all purposes4 ,it is the outward attractiveness of the common-battery system and the arguments in its favor1 so readily made by over-zealous salesmen1 that has led1 in many cases1 to the adoption of this system when the magneto system would better have served the purpose of utility and economy4 ,to say the least1 the telephone transmission to be had from common-battery systems is no better than that to be had from local-battery systems1 and as a rule1 assuming equality in other respects1 it is not as good4 ,it is perhaps true1 however1 that under average conditions common-battery transmission is somewhat better1 because whereas the local batteries at the subscribers' stations in the local-battery system are not likely to be in uniformly first-class condition1 the battery in a common-battery system will be kept up to its full voltage except under the grossest neglect4 ,the places in which the magneto1 or local-battery1 system is to be preferred to the common-battery system1 in the opinion of the writers1 are to be found in the small rural communities where the lines have a rather great average length2 where a good many subscribers are likely to be found on some of the lines2 where the sources of electrical power available for charging storage batteries are likely either not to exist1 or to be of a very uncertain nature2 and where it is not commercially feasible to employ a high-grade class of attendants1 or1 in fact1 any attendant at all other than the operator at the central office4 ,in large or medium-sized exchanges it is always possible to procure suitable current for charging the storage batteries required in common-battery systems1 and it is frequently economical1 on account of the considerable quantity of energy that is thus used1 to establish a generating plant in connection with the central office for developing the necessary electrical energy4 ,in very small rural places there are frequently no available sources of electrical energy1 and the expense of establishing a power plant for the purpose cannot be justified4 ,but even if there is an electric light or railway system in the small town1 so that the problem of available current supply does not exist1 the establishment of a common-battery system with its storage battery and the necessary charging machinery requires the daily attendance at the central office of some one to watch and care for this battery1 and this1 on account of the small gross revenue that may be derived from a small telephone system1 often involves a serious financial burden4 ,there is no royal road to a proper decision in the matter1 and no sharp line of demarcation may be drawn between the places where common-battery systems are superior to magneto and vice versa4 ,it may be said1 however1 that in the building of all new telephone plants having over about #ejj local subscribers1 the common-battery system is undoubtedly superior to the magneto4 ,if the plant is an old one1 however1 and is to be re-equipped1 the continuance of magneto apparatus might be justified for considerably larger exchanges than those having #ejj subscribers4 ,telephone operating companies who have changed over the equipment of old plants from magneto to common battery have sometimes been led into rather serious difficulty1 owing to the fact that their lines1 while serving tolerably well for magneto work1 were found inadequate to meet the more exacting demands of common-battery work4 ,again in an old plant the change from magneto to common-battery equipment involves not only the change of switchboards1 but also the change of subscribers' instruments that are otherwise good1 and this consideration alone often1 in our opinion1 justifies the replacing of an old magneto board with a new magneto board1 even if the exchange is of such size as to demand a small multiple board4 ,where the plant to be established is of such size as to leave doubt as to whether a magneto or a common-battery switchboard should be employed1 the questions of availability of the proper kind of power for charging the batteries1 the proper kind of help for maintaining the batteries and the more elaborate central-office equipment1 the demands and previous education of the public to be served1 all are factors which must be considered in reaching the decision4 ,it is not proper to say that anything like all exchanges having fewer than #ejj local lines1 should be equipped with magneto service4 ,where all the lines are short1 where suitable power is available1 and where a good grade of attendants is available--as1 for instance1 in the case of private telephone exchanges that serve some business establishment or other institution located in one building or a group of buildings--the common-battery system is to be recommended and is largely used1 even though it may have but a dozen or so subscribers' lines4 ,it is for such uses1 and for use in those regular public-service exchange systems where the conditions are such as to warrant the common-battery system1 and yet where the number of lines and the traffic are small enough to be handled by such a small group of operators that any one of them may reach over the entire face of the board1 that the simple non-multiple common-battery system finds its proper field of usefulness4 ,line ,signals4 ,the principles and means by which the subscriber is enabled to call the central-office operator in a common-battery system have been referred to briefly in ,chapter ,,iii4 ,we will review these at this point and also consider briefly the way in which the line signals are associated with the connective devices in the subscribers' lines4 ,direct-,line ,lamp4 ,the simplest possible way is to put the line signal directly in the circuit of the line in series with the central-office battery1 and so to arrange the jack of the corresponding line that the circuit through the line signal will be open when the operator inserts a plug into that jack4 ,this arrangement is shown in ,fig4 #cjg where the subscriber's station at the left is indicated in the simplest of its forms4 ,it is well to repeat here that in all common-battery manual systems1 the subscriber's station equipment1 regardless of the arrangement or type of its talking and signaling apparatus1 must have these features3 ,first1 that the line shall be normally open to direct currents at the subscriber's station2 second1 that the line shall be closed to direct currents when the subscriber removes his receiver from its hook in making or in answering a call2 third1 that the line normally1 although open to direct currents1 shall afford a proper path for alternating or varying currents through the signal receiving device at the sub-station4 ,the subscriber's station arrangement shown in ,fig4 #cjg1 and those immediately following1 is the simplest arrangement that possesses these three necessary features for common-battery service4 ,illustration3 ,fig4 #cjg4 ,direct-,line ,lamp ,considering the arrangement at the central office1 ,fig4 #cjg1 the two limbs of the line are permanently connected to the tip and sleeve contacts of the jack4 ,these two main contacts of the jack normally engage two anvils so connected that the tip of the jack is ordinarily connected through its anvil to ground1 while the sleeve of the jack is normally connected through its anvil to a circuit leading through the line signal--in this case a lamp--and the common battery1 and thence to ground4 ,the operation is obvious4 ,normally no current may flow from the common battery through the signal because the line is open at the subscriber's station4 ,the removal of the subscriber's receiver from its hook closes the circuit of the line and allows the current to flow through the lamp1 causing it to glow4 ,when the operator inserts the plug into the jack1 in response to the call1 the circuit through the lamp is cut off at the jack and the lamp goes out4 ,this arrangement1 termed the direct-line lamp arrangement1 is largely used in small common-battery telephone systems where the lines are very short1 such as those found in factories or other places where the confines of the exchange are those of a building or a group of neighboring buildings4 ,many of the so-called private-branch exchanges1 which will be considered more in detail in a later chapter1 employ this direct-line lamp arrangement4 ,illustration3 ,fig4 #cjh4 ,direct-,line ,lamp with ,ballast ,direct-,line ,lamp with ,ballast4 ,obviously1 however1 this direct-line lamp arrangement is not a good one where the lines vary widely in length and resistance4 ,an incandescent lamp1 as is well known1 must not be subjected to too great a variation in current4 ,if the current that is just right in amount to bring it to its intended degree of illumination is increased by a comparatively small amount1 the life of the lamp will be greatly shortened1 and too great an increase will result in the lamp's burning out immediately4 ,on the other hand1 a current that is too small will not result in the proper illumination of the lamp1 and a current of one-half the proper normal value will just suffice to bring the lamp to a dull red glow4 ,with lines that are not approximately uniform in length and resistance the shorter lines would afford too great a flow of current to the lamps and the longer lines too little1 and there is always the danger present1 unless means are taken to prevent it1 that if a line becomes short-circuited or grounded near the central office1 the lamp will be subjected to practically the full battery potential and1 therefore1 to such a current as will burn it out4 ,one of the very ingenious and1 we believe1 promising methods that has been proposed to overcome this difficulty is that of the iron-wire ballast1 alluded to in ,chapter ,,iii4 ,this1 it will be remembered1 consists of an iron-wire resistance enclosed in a vacuum chamber and so proportioned with respect to the flow of current that it will be subjected to a considerable heating effect by the amount of current that is proper to illuminate the lamp4 ,as has already been pointed out1 carbon has a negative temperature coefficient1 that is1 its resistance decreases when heated4 ,iron1 on the other hand1 has a positive temperature coefficient1 its resistance increasing when heated4 ,when such an iron-wire ballast is put in series with the incandescent lamp forming the line signal1 as shown in ,fig4 #cjh1 it is seen that the resistance of the carbon in the lamp filament and of the iron in the ballast will act in opposite ways when the current increases or decreases4 ,an increase of current will tend to heat up the iron wire of the ballast and1 therefore1 increase its resistance1 and the ballast is so proportioned that it will hold the current that may flow through the lamp within the proper maximum and minimum limits1 regardless of the resistance of the line in which the lamp is used4 ,this arrangement has not gone into wide use up to the present time4 ,line ,lamp with ,relay4 ,by far the most common method of associating the line lamp with the line is to employ a relay1 of which the actuating coil is in the line circuit1 this relay serving to control a local circuit containing the battery and the lamp4 ,this arrangement and the way in which these parts are associated with the jack are clearly indicated in ,fig4 #cji4 ,here the relay may receive any amount of current1 from the smallest which will cause it to pull up its armature1 to the largest which will not injure its winding by overheat4 ,relays may be made which will attract their armatures at a certain minimum current and which will not burn out when energized by currents about ten times as large1 and it is thus seen that a very large range of current through the relay winding is permissible1 and that1 therefore1 a very great latitude as to line resistance is secured4 ,on the other hand1 it is obvious that the lamp circuit1 being entirely local1 is of uniform resistance1 the lamp always being subjected1 in the arrangement shown1 to practically the full battery potential1 the lamp being selected to operate on that potential4 ,illustration3 ,fig4 #cji4 ,line ,lamp with ,relay ,pilot ,signals4 ,in the circuits of ,figs4 #cjg1 #cjh1 and #cji1 but a single line and its associated apparatus is shown1 and it may not be altogether clear to the uninitiated how it is that the battery shown in those figures may serve1 without interference of any function1 a larger number of lines than one4 ,it is to be remembered that this battery is the one which serves not only to operate the line signals1 but also to supply talking current to the subscribers and to supply current for the operation of the cord-circuit signals after the cord circuits are connected with the lines4 ,in ,fig4 #caj this matter is made clear with respect to the association of this common battery with the lines for operating the line signals1 and also another important feature of common-battery work is brought out1 viz1 the pilot lamp and its association with a group of line lamps4 ,three subscribers' lines only are shown1 but this serves clearly to illustrate the association of any larger number of lines with the common battery4 ,ignoring at first the pilot relay and the pilot lamp1 it will be seen that each of the tip-spring anvils of the jacks is connected to a common wire #a which is grounded4 ,each of the sleeve-contact anvils is connected through the coil of the line relay to another common wire #b1 which connects with the live side of the common battery4 ,obviously1 therefore1 this arrangement corresponds with that of ,fig4 #cji1 since the battery may furnish current to energize any one of the line relays upon the closure of the circuit of the corresponding line4 ,each of the relay armatures in ,fig4 #caj is connected to ground4 ,here we wish to bring out an important thing about telephone circuit diagrams which is sometimes confusing to the beginner1 but which really1 when understood1 tends to prevent confusion4 ,the showing of a separate ground for each of the line-relay armatures does not mean that literally each one of these armatures is connected by a separate wire to earth1 and it is to be understood that the three separate grounds shown in connection with these relay armatures is meant to indicate just such a set of affairs as is shown in connection with the tip-spring anvils of the jacks1 all of which are connected to a common wire which1 in turn1 is grounded4 ,obviously1 the result is the same1 but in the case of this particular diagram it is seen that a great deal of crossing of lines is prevented by showing a separate ground at each one of the relay armatures4 ,the same practice is followed in connection with the common battery4 ,sometimes it is very inconvenient in a complicated diagram to run all of the wires that are supposed to connect with one terminal of the battery across the diagram to represent this connection4 ,it is permissible1 therefore1 and in fact desirable1 that separate battery symbols be shown wherever by so doing the diagram will be simplified1 the understanding being1 in the absence of other information or of other indications1 that the same battery is referred to1 just as the same ground is referred to in connection with the relay armatures in the figure under discussion4 ,each line lamp in ,fig4 #caj is shown connected on one hand to its corresponding line relay contact and on the other hand to a common wire which leads through the winding of the pilot relay to the live side of the battery4 ,it is obvious here that whenever any one of the line relays attracts its armature the local circuit containing the corresponding lamp and the common battery will be closed and the lamp illuminated4 ,whenever any line relay operates1 the current1 which is supplied to its lamp1 must come through the pilot-relay winding1 and if a number of line relays are energized1 then the current flow of the corresponding lamps must flow through this relay winding4 ,therefore1 this relay winding must be of low resistance1 so that the drop through its winding may not be sufficient to interfere with the proper burning of the lamps1 even though a large number of lamps be fed simultaneously through it4 ,the pilot relay must be so sensitive that the current1 even through one lamp1 will cause it to attract its armature4 ,when it does attract its armature it causes illumination of the pilot lamp in the same way that the line relays cause the illumination of the line lamps4 ,the pilot lamp1 which is commonly associated with a group of line lamps that are placed on any one operator's position of the switchboard1 is located in a conspicuous place in the switchboard cabinet and is provided with a larger lens so as to make a more striking signal4 ,as a result1 whenever any line lamp on a given position lights1 the pilot lamp does also and serves to attract the attention1 even of those located in distant portions of the room1 to the fact that a call exists on that position of the board1 the line lamp itself1 which is simultaneously lighted1 pointing out the particular line on which the call exists4 ,pilot lamps1 in effect1 perform similar service to the night alarm in magneto boards1 but1 of course1 they are silent and do not attract attention unless within the range of vision of the operator4 ,they are used not only in connection with line lamps1 but also in connection with the cord-circuit lamps or signals1 as will be pointed out4 ,illustration3 ,fig4 #caa4 ,battery ,supply ,through ,impedance ,coils ,illustration3 ,fig4 #cab4 ,battery ,supply through ,repeating ,coils ,illustration3 ,fig4 #cac4 ,battery ,supply with ,impedance ,coils and ,condensers ,cord ,circuit4 ,battery ,supply4 ,were it not for the necessity of providing for cord-circuit signals in common-battery switchboards1 the common-battery cord circuit would be scarcely more complex than that for magneto working4 ,stripped of all details1 such as signals1 ringing and listening keys1 and operator's equipment1 cord circuits of three different types are shown in ,figs4 #caa1 #cab1 and #cac4 ,these merely illustrate the way in which the battery is associated with the cord circuits and through them with the line circuits for supplying current for talking purposes to the subscribers4 ,it is thought that this matter will be clear in view of the discussion of the methods by which current is supplied to the subscribers' transmitters in common-battery systems as discussed in ,chapter ,,xiii4 ,while the arrangements in this respect of ,figs4 #caa1 #cab1 and #cac illustrate only three of the methods1 these three are the ones that have been most widely and successfully used4 ,supervisory ,signals4 ,the signals that are associated with the cord circuits are termed supervisory signals because of the fact that by their means the operator is enabled to supervise the condition of the lines during times when they are connected for conversation4 ,the operation of these supervisory signals may be best understood by considering the complete circuits of a simple switchboard and must be studied in conjunction with the circuits of the lines as well as those of the cords4 ,illustration3 ,fig4 #cad4 ,simple ,common-,battery ,switchboard ,complete ,circuit4 ,such complete circuits are shown in ,fig4 #cad4 ,the particular arrangement indicated is that employed by the ,kellogg ,company1 and except for minor details may be considered as typical of other makes also4 ,two subscribers' lines are shown extending from ,station ,a and ,station ,b1 respectively1 to the central office4 ,the line wires are shown terminating in jacks in the same manner as indicated in ,figs4 #cjg1 #cjh1 and #cji1 and their circuits are normally continued from these jacks to the ground on one side and to the line relay and battery on the other4 ,the jack in this case has three contacts adapted to register with three corresponding contacts in each of the plugs4 ,the thimble of the jack in this case forms no part of the talking circuit and is distinct from the two jack springs which form the line terminals4 ,it and the auxiliary contact #a in each of the plugs with which it registers1 are solely for the purpose of co-operating in the control of the supervisory signals4 ,the tip and sleeve strands of the cord are continuous from one plug to the other except for the condensers4 ,the two batteries indicated in connection with the cord circuit are separate batteries1 a characteristic of the ,kellogg system4 ,one of these batteries serves to supply current to the tip and sleeve strand of the cord circuit through the two windings #c and #d1 respectively1 of the supervisory relay connected with the answering side of the cord circuit1 while the other battery similarly supplies current through the windings #e and #f of the supervisory relay associated with the calling side of the cord circuit4 ,the windings of these relays1 therefore1 act as impedance coils and the arrangement by which battery current is supplied to the cord circuits and1 therefore1 to the lines of the connected subscribers1 is seen to be the combined impedance coil and condenser arrangement discussed in ,chapter ,,xiii4 ,as soon as a plug is inserted into the jack of a line1 the line relay will be removed from the control of the line1 and since the two strands of the cord circuit now form continuations of the two line conductors1 the supervisory relay will be substituted for the line relay and will be under control of the line4 ,since all of the current which passes to the line after a plug is inserted must pass through the cord-circuit connection and through the relay windings1 and since current can only flow through the line when the subscriber's receiver is off its hook1 it follows that the supervisory relays will only be energized after the corresponding plug has been inserted into a jack of the line and after the subscriber has removed his receiver4 ,unlike the line relays1 the supervisory relays open their contacts to break the local circuits of the supervisory lamps #g and #h when the relay coils are energized1 and to close them when de-energized2 but the armatures of the supervisory relays do alone control the circuits of the supervisory lamps4 ,these circuits are normally held open in another place1 that is1 between the plug contacts #a and the jack thimbles4 ,it is only1 therefore1 when a plug is inserted into a jack and when the supervisory relay is de-energized1 that the supervisory lamp may be lighted4 ,when a plug is inserted into a jack and when the corresponding supervisory relay is de-energized1 the circuit may be traced from ground at the cord-circuit batteries through the left-hand battery1 for instance1 through lamp #g1 thence through the contacts of the supervisory relay to the contact #a of the plug1 thence through the thimble of the jack to ground4 ,when a plug is inserted into the jack1 therefore1 the necessary arrangements are completed for the supervisory lamp to be under the control of the subscriber4 ,under this condition1 whenever the subscriber's receiver is on its hook1 the circuit of the line will be broken1 the supervisory relay will be de-energized1 and the supervisory lamp will be lighted4 ,when1 on the other hand1 the subscriber's receiver is off its hook1 the circuit of the line will be complete1 the supervisory relay will be energized1 and the supervisory lamp will be extinguished4 ,salient ,features of ,supervisory ,operation4 ,it will facilitate the student's understanding of the requirements and mode of operation of common-battery supervisory signals in manual systems1 whether simple or multiple1 if he will firmly fix the following facts in his mind4 ,in order that the supervisory signal may become operative at all1 some act must be performed by the operator--this being usually the act of plugging into a jack--and then1 until the connection is taken down1 the supervisory signal is under the control of the subscriber1 and it is displayed only when the subscriber's receiver is placed on its hook4 ,cycle of ,operations4 ,we may now trace through the complete cycle of operations of the simple common-battery switchboard1 the circuits of which are shown in ,fig4 #cad4 ,assume all apparatus in its normal condition1 and then assume that the subscriber at ,station ,a removes his receiver from its hook4 ,this pulls up the line relay and lights the line lamp1 the pilot relay also pulling up and lighting the common pilot lamp which is not shown4 ,in response to this call1 the operator inserts the answering plug and throws her listening key ,l4,k4 ,the operator's talking set is thus bridged across the cord circuit and she is enabled to converse with the calling subscriber4 ,the answering supervisory lamp #g did not light when the operator inserted the answering plug into the jack1 because1 although the contacts in the lamp circuit were closed by the plug contact #a engaging the thimble of the jack1 the lamp circuit was held open by the attraction of the supervisory relay armature1 the subscriber's receiver being off its hook4 ,learning that the called-for subscriber is the one at ,station ,b1 the operator inserts the calling plug into the jack at that station and presses the ringing key ,r4,k41 in order to ring the bell4 ,the act of plugging in1 it will be remembered1 cuts off the line-signaling apparatus from connection with that line4 ,as the subscriber at ,station ,b was not at his telephone when called and his receiver was1 therefore1 on its hook1 the insertion of the calling plug did not energize the supervisory relay coils #e and #f1 and1 therefore1 that relay did not attract its armature4 ,the supervisory lamp #h was thus lighted1 the circuit being from ground through the right-hand cord-circuit battery1 lamp #h1 back contacts of the supervisory relay1 third strand of the cord to contact #a of the calling plug1 and thence to ground through the thimble of the jack4 ,the lighting of this lamp is continued until the party at ,station ,b responds by removing his receiver from its hook1 which completes the line circuit1 energizes relay windings #e and #f1 causes that relay to attract its armature1 and thus break the circuit of the lamp #h4 ,both supervisory lamps remain out as long as the two subscribers are conversing1 but when either one of them hangs up his receiver the corresponding supervisory relay becomes de-energized and the corresponding lamp lights4 ,when both of the lamps become illuminated1 the operator knows that both subscribers are through talking and she takes down the connection4 ,countless variations have been worked in the arrangement of the line and cord circuits1 but the general mode of operation of this particular circuit chosen for illustration is standard and should be thoroughly mastered4 ,the operation of other arrangements will be readily understood from an inspection of the circuits1 once the fundamental mode of operation that is common to all of them is well in mind4 ,lamps4 ,the incandescent lamps used in connection with line and supervisory signals are specially manufactured1 but differ in no sense from the larger lamps employed for general lighting purposes1 save in the details of size1 form1 and method of mounting4 ,usually these lamps are rated at about one-third candle-power1 although they have a somewhat larger candle-power as a rule4 ,they are manufactured to operate on various voltages1 the most usual operating pressures being #ab1 #bd1 and #dh volts4 ,the #bd-volt lamp consumes about one-tenth of an ampere when fully illuminated1 the lamp thus consuming about #b4d watts4 ,the #ab- and #dh-volt lamps consume about the same amount of energy and corresponding amounts of current4 ,illustration3 ,fig4 #cae4 ,switchboard ,lamp ,lamp ,mounting4 ,the usual form of screw-threaded mounting employed in lamps for commercial lighting was at first applied to the miniature lamps used for switchboard work1 but this was found unsatisfactory and these lamps are now practically always provided with two contact strips1 one on each side of the glass bulb1 these strips forming respectively the terminals for the two ends of the filament within4 ,such a construction of a common form of lamp is shown in ,fig4 #cae1 where these terminals are indicated by the numerals #a and #b1 #c being a dry wooden block arranged between the terminals at one end for securing greater rigidity between them4 ,illustration3 ,fig4 #caf4 ,line ,lamp ,mounting ,the method of mounting these lamps is subject to a good deal of variation in detail1 but the arrangement is always such that the lamp is slid in between two metallic contacts forming terminals of the circuit in which the lamp is to operate4 ,such an arrangement of springs and the co-operating mounting forming a sort of socket for the reception of switchboard lamps is referred to as a lamp jack4 ,these are sometimes individually mounted and sometimes mounted in strips in much the same way that jacks are mounted in strips4 ,a strip of lamp jacks as manufactured by the ,kellogg ,company is shown in ,fig4 #caf4 ,the opalescent lens is adapted to be fitted in front of the lamp after it has been inserted into the jack4 ,fig4 #cag gives an excellent view of an individually-mounted lamp jack with its lamp and lens1 this also being of ,kellogg manufacture4 ,this figure shows a section of the plug shelf which is bored to receive a lamp4 ,in order to protect the lamps and lenses from breakage1 due to the striking of the plugs against them1 a metal shield is placed over the lens1 as shown in this figure1 this being so cut away as to allow sufficient openings for the light to shine through4 ,sometimes instead of employing lenses in front of the lamps1 a flat piece of translucent material is used to cover the openings of the lamp1 this being protected by suitable perforated strips of metal4 ,a strip of lamp jacks employing this feature is shown in ,fig4 #cah1 this being of ,dean manufacture4 ,an advantage of this for certain types of work is that the flat translucent plate in front of the lamp may readily carry designating marks1 such as the number of the line or something to indicate the character of the line1 which marks may be readily changed as required4 ,illustration3 ,fig4 #cag4 ,supervisory ,lamp ,mounting ,illustration3 ,fig4 #cah4 ,line ,lamp ,mounting ,illustration3 ,fig4 #cai4 ,individual ,lamp ,jacks ,in the types made by some manufacturers the only difference between the pilot lamp and the line lamp is in the size of the lens in front of it1 the jack and the lamp itself being the same for each1 while others use a larger lamp for the pilot4 ,in ,fig4 #cai are shown two individual lamp jacks1 the one at the top being for supervisory lamps and the one at the bottom being provided with a large lens for serving as a pilot lamp4 ,illustration3 ,,terminal ,,room ,,apparatus ,,in ,,process ,,of ,,installation ,installed by ,dean ,electric ,company at ,detroit1 ,mich4 ,mechanical ,signals4 ,as has been stated the so-called mechanical signals are sometimes used in small common-battery switchboards instead of lamps4 ,where this is done the coil of the signal1 if it is a line signal1 is substituted in the line circuit in place of the relay coil4 ,if the signals are used in connection with cord circuits for supervisory signals1 their coils are put in the circuit in place of the supervisory relay coils4 7,these signals are referred to in ,chapter ,,iii in connection with ,fig4 #bc47 ,they are so arranged that the attraction of the armature lifts a target on the end of a lever1 and this causes a display of color or form4 ,the release of the armature allows this target to drop back1 thus obliterating the display4 ,such signals1 often called visual signals and electromagnet signals1 should be distinguished from the drops considered in connection with magneto switchboards in which the attraction of the armature causes the display of the signal by the falling of a drop1 the signal remaining displayed until restored by some other means1 the restoration depending in no wise on when the armature is released4 ,western ,electric4 ,the mechanical signal of the ,western ,electric ,company1 shown in ,fig4 #cbj1 has a target similar to that shown in ,fig4 #bed but without a latch4 ,it is turned to show a different color by the attraction of the armature and allowed to resume its normal position when the armature is released4 ,illustration3 ,fig4 #cbj4 ,mechanical ,signal ,kellogg4 ,fig4 #cba gives a good idea of a strip of mechanical signals as manufactured by the ,kellogg ,company4 ,this is known as the gridiron signal on account of the cross-bar striping of its target4 ,the white bars on the target normally lie just behind the cross-bars on the shield in front1 but a slight raising of the target--about one-eighth of an inch--exposes these white bars to view1 opposite the rectangular openings in the front shield4 ,illustration3 ,fig4 #cba4 ,strip of ,gridiron ,signals ,monarch4 ,in ,fig4 #cbb is shown the visual signal manufactured by the ,monarch ,telephone ,company4 ,illustration3 ,fig4 #cbb4 ,mechanical ,signal ,relays4 ,the line relays for common-battery switchboards likewise assume a great variety of forms4 ,the well-known type of relay employed in telegraphy would answer the purpose well but for the amount of room that it occupies1 as it is sometimes necessary to group a large number of relays in a very small space4 ,nearly all present-day relays are of the single-coil type1 and in nearly all cases the movement of the armature causes the movement of one or more switching springs1 which are thus made to engage or disengage their associated spring or springs4 ,one of the most widely used forms of relays has an ,l-shaped armature hung across the front of a forwardly projecting arm of iron1 on the knife-edge corner of which it rocks as moved by the attraction of the magnet4 ,the general form of this relay was illustrated in ,fig4 #ie4 ,sometimes this relay is made up in single units and frequently a large number of such single units are mounted on a single mounting plate4 ,this matter will be dealt with more in detail in the discussion of common-battery multiple switchboards4 ,in other cases these relays are built en bloc1 a rectangular strip of soft iron long enough to afford space for ten relays side by side being bored out with ten cylindrical holes to receive the electromagnets4 ,the iron of the block affords a return path for the lines of force4 ,the ,l-shaped armatures are hung over the front edge of this block1 so that their free ends lie opposite the magnet cores within the block4 ,this arrangement as employed by the ,kellogg ,company is shown in two views in ,figs4 #cbc and #cbd4 ,illustration3 ,fig4 #cbc4 ,strip of ,relays ,illustration3 ,fig4 #cbd4 ,strip of ,relays ,a bank of line relays especially adapted for small common-battery switchboards as made by the ,dean ,company1 is shown in ,fig4 #cbe4 ,illustration3 ,fig4 #cbe4 ,bank of ,relays ,jacks4 ,the jacks in common-battery switchboards are almost always mounted in groups of ten or twenty1 the arrangement being similar to that discussed in connection with lamp strips4 ,ordinarily in common-battery work the jack is provided with two inner contacts so as to cut off both sides of the signaling circuit when the operator plugs in4 ,a strip of such jacks is shown in ,fig4 #cbf4 ,illustration3 ,fig4 #cbf4 ,strip of ,cut-,off ,jacks ,ringing and listening keys for simple common-battery switchboards differ in no essential respect from those employed in magneto boards4 ,illustration3 ,fig4 #cbg4 ,details of ,lamp1 ,plug1 and ,key ,mounting ,switchboard ,assembly4 ,the general assembly of the parts of a simple common-battery switchboard deserves some attention4 ,the form of the switchboard need not differ essentially from that employed in magneto work1 but ordinarily the cabinet is somewhat smaller on account of the smaller amount of room required by its lamps and jacks4 ,an excellent idea of the line jacks and lamps1 plugs1 keys1 and supervisory signals may be obtained from ,fig4 #cbg1 which is a detail view taken from a ,kellogg board4 ,in the vertical panel of the board above the plug shelf are arranged the line jacks and the lamps in rows of twenty each1 each lamp being immediately beneath its corresponding jack4 ,such jacks are ordinarily mounted on #a_/b-;inch centers both vertically and horizontally1 so that a group of one hundred lamps and line jacks will occupy a space only slightly over #aj by #e inches4 ,such economy of space is not required in the simple magneto board1 because the space might easily be made larger without in any way taxing the reach of the operator4 ,the reason for this comparatively close mounting is a result1 not of the requirements of the simple non-multiple common-battery board itself1 but of the fact that the jack strips and lamp strips1 which are required in very large numbers in multiple boards1 have to be mounted extremely close together1 and as the same lamp strips and jack strips are often available for simple switchboards1 an economy in manufacture is effected by adherence to the same general dimensions4 ,illustration3 ,fig4 #cbh4 ,simple ,common-,battery ,switchboard with ,removable ,relay ,panel ,a rear view of a common form of switchboard cabinet1 known as the upright type and manufactured by the ,dean ,company1 is shown in ,fig4 #cbh4 ,in this all the relays are mounted on a hinged rack1 which1 when opened out as indicated1 exposes the wiring to view for inspection or repairs4 ,access to both sides of the relays is thus given to the repairman who may do all his work from the rear of the board without disturbing the operator4 ,fig4 #cbi shows a three-position cabinet of ,kellogg manufacture1 this being about the limit in size of boards that could properly be called simple4 ,obviously1 where a switchboard cabinet must be made of greater length than this1 i4 e41 than is required to accommodate three operators1 it becomes too long for the operators to reach all over it without undue effort or without moving from their seats4 ,the so-called transfer board and the multiple board 7to be considered in subsequent chapters71 constitute methods of relief from such a condition in larger exchanges4 ,illustration3 ,fig4 #cbi4 ,three-,position ,lamp ,board ,,chapter ,,xxiii ,,transfer ,,switchboard ,when the traffic originating in a switchboard becomes so great as to require so many operators that the board must be made so long that any one of the operators cannot reach over its entire face1 the simple switchboard does not suffice4 ,either some form of transfer switchboard or of multiple switchboard must be used4 ,in this chapter the transfer switchboard will be briefly discussed4 ,the transfer switchboard is so named because its arrangement is such that some of the connections through it are handled by means of two operators1 the operator who answers the call transferring it to another operator who completes the connection desired4 ,limitations of ,simple ,switchboard4 ,conceive a number of simple magneto switchboards1 or a number of common-battery switchboards1 arranged side by side1 their number being so great as to form1 by their combination1 a board too long for the ordinary cords and plugs to reach between its extremities4 ,on each of these simple switchboards1 which we will say are each of the one-position type1 there terminates a group of subscribers' lines so great in number1 considering the traffic on them1 that the efforts of one operator will just about be taxed to properly attend to their calls during the busiest hours of the day4 ,if1 now1 these subscribers would be sufficiently accommodating to call for no other subscribers than those whose lines terminate on the same switchboard section or on one of the immediately adjacent switchboard sections1 all would be well1 but subscribers will not be so restricted4 ,they demand universal service2 that is1 they demand the privilege of having their own lines connected with the line of any other person in the exchange4 ,obviously1 in the arrangement just conceived1 any operator may answer any call originating at her own board and complete the connection with the desired subscriber if that subscriber's jack terminates on her own section or on one of the adjacent ones4 ,beyond that she is powerless unless other means are provided4 ,transfer ,lines4 ,in the transfer board these other means consist in the provision of groups of local trunk lines or transfer lines extending from each switchboard position to each other non-adjacent switchboard position4 ,when an operator receives a call for some line on a non-adjacent position1 having answered this call with her answering plug1 she inserts the calling plug into the jack of one of these transfer lines that leads to the proper other section4 ,the operator at that section is notified either verbally or by signal1 and she completes the connection between the other end of the transfer line and the line of the called subscriber2 the connection between the two subscribers thus being effected through the cords of the two operators in question linked together by the transfer line4 ,such a transfer line as just described1 requiring the connection at each of its ends by one of the plugs of the operator's cord pair1 is termed a jack-ended trunk or a jack-ended transfer line because each of its ends terminates in a jack4 ,illustration3 ,fig4 #ccj4 ,jack-,ended ,transfer ,circuit ,there is another method of accomplishing the same general result by the employment of the so-called plug-ended trunk or plug-ended transfer line4 ,in this the trunk or transfer line terminates at one end1 the answering end1 in a jack as before1 and the connection is made with it by the answering operator by means of the calling plug of the pair with which she answered the originating call4 ,the other end of this trunk1 instead of terminating in a jack1 ends in a plug and the second operator involved in the connection1 after being notified1 picks up this plug and inserts it in the jack of the called subscriber1 thus completing the connection without employing one of her regular cord pairs4 ,jack-,ended ,trunk4 ,in ,fig4 #ccj are shown the circuits of a commonly employed jack-ended trunk for transfer boards4 ,the talking circuit1 as usual1 is shown in heavy lines and terminates in the tip and sleeve of the transfer jacks at each end4 ,the auxiliary contacts in these jacks and the circuits connecting them are absolutely independent of the talking circuit and are for the purpose of signaling only1 the arrangement of the jacks being such that when a plug is inserted1 the spring #a will break from spring #b and make with spring #c4 ,obviously1 the insertion of a plug in either of the jacks will establish such connections as to light both lamps1 since the engagement of spring #a with spring #c in either of the jacks will connect both of the lamps in multiple across the battery1 this connection including always the contacts #a and #b of the other jack4 ,from this it follows that the insertion of a plug in the other end of the trunk will1 by breaking contact between springs #a and #b1 put out both the lamps4 ,one plug inserted will1 therefore1 light both lamps2 two plugs inserted or two plugs withdrawn will extinguish both lamps4 ,illustration3 ,fig4 #cca4 ,jack-,ended ,transfer ,circuit ,if an operator located at one end of this trunk answers a call and finds that the called-for subscriber's line terminates within reach of the operator near the other end of this trunk1 she will insert a calling plug1 corresponding to the answering plug used in answering a call1 into the jack of this trunk and thus light the lamp at both its ends4 ,the operator at the other end upon seeing this transfer lamp illuminated inserts one of her answering plugs into the jack1 and by means of her listening key ascertains the number of the subscriber desired1 and immediately inserts her calling plug into the jack of the subscriber wanted and rings him in the usual manner4 ,the act of this second operator in inserting her answering plug into the jack extinguishes the lamp at her own end and also at the end where the call originated1 thus notifying the answering operator that the call has been attended to4 ,as long as the lamps remain lighted1 the operators know that there is an unattended connection on that transfer line4 ,such a transfer line is called a two-way line or a single-track line1 because traffic over it may be in either direction4 ,in ,fig4 #cca is shown a trunk that operates in a similar way except that the two lamps1 instead of being arranged in multiple1 are arranged in series4 ,illustration3 ,fig4 #ccb4 ,jack- and ,plug-,ended ,transfer ,circuit ,plug-,ended ,trunk4 ,in ,fig4 #ccb is shown a plug-ended trunk1 this particular arrangement of circuits being employed by the ,monarch ,company in its transfer boards4 ,this is essentially a one-way trunk1 and traffic over it can pass only in the direction of the arrow4 ,traffic in the opposite direction between any two operators is handled by another trunk or group of trunks similar to this but pointed in the other direction4 ,for this reason such a system is referred to as a double-track system4 ,the operation of signals is the same in this case as in ,fig4 #ccj1 except that the switching device at the left-hand end of the trunk instead of being associated with the jack is associated with the plug seat1 which is a switch closely associated with the seat of a plug so as to be operated whenever the plug is withdrawn from or replaced in its seat4 ,the operation of this arrangement is as follows3 ,whenever an operator at the right-hand end of this trunk receives a call for a subscriber whose line terminates within the reach of the operator at the left-hand end of the trunk1 she inserts the calling plug of the pair used in answering the calling subscriber into the jack of the trunk1 and thus lights both of the trunk lamps4 ,the operator at the other end of the trunk1 seeing the trunk lamp lighted1 raises the plug from its seat and1 having learned the wishes of the calling subscriber1 inserts this plug into the jack of the called subscriber without using one of her regular pairs4 ,when she raised the trunk plug from its seat1 she permitted the long spring #a of the plug seat switch to rise1 thus extinguishing both lamps and giving the signal to the originating operator that the trunk connection has received attention4 ,on taking down the connection1 the withdrawal of the plug from the right hand of the trunk lights both lamps1 and the restoring of the trunk plug to its normal seat again extinguishes both lamps4 ,plug-,seat ,switch4 ,the plug-seat switch is a device that has received a good deal of attention not only for use with transfer systems1 but also for use in a great variety of ways with other kinds of manual switching systems4 ,the placing of a plug in its seat or withdrawing it therefrom offers a ready means of accomplishing some switching or signaling operation automatically4 ,the plug-seat switch has1 however1 in spite of its possibilities1 never come into wide use1 and so far as we are aware the ,monarch ,telephone ,manufacturing ,company is the only company of prominence which incorporates it in its regular output4 ,the ,monarch plug-switch mechanism is shown in ,fig4 #ccc1 and its operation is obvious4 ,it may be stated at this point that one of the reasons why the plug-seat switch has not been more widely adopted for use1 is the difficulty that has been experienced due to lint from the switchboard cords collecting on or about the contact points4 ,in the construction given in the detailed cut1 upper part1 ,fig4 #ccc1 is shown the means adopted by the ,monarch ,company for obviating this difficulty4 ,the contact points are carried in the upper portion of an inverted cup mounted on the under side of the switchboard shelf1 and are thus protected1 in large measure1 from the damaging influence of dust and lint4 ,illustration3 ,fig4 #ccc4 ,plug-,seat ,switch ,illustration3 ,fig4 #ccd4 ,order-,wire ,arrangement ,methods of ,handling ,transfers4 ,one way of giving the number of the called subscriber to the second operator in a transfer system is to have that operator listen in on the circuit after it is continued to her position and receive the number either from the first operator or from the subscriber4 ,receiving it from the first operator has the disadvantage of compelling the first operator to wait on the circuit until the second operator responds2 receiving it from the subscriber has the disadvantage of sometimes being annoying to him4 ,this1 however1 is to be preferred to the loss of time on the part of the originating operator that is entailed by the first method4 ,a better way than either of these is to provide between the various operators working in a transfer system1 a so-called order-wire system4 ,an order wire1 as ordinarily arranged1 is a circuit terminating at one end permanently in the head receiver of an operator1 and terminating at the other end in a push button which1 when depressed1 will connect the telephone set of the operator at that end with the order wire4 ,the operator at the push-button end of the order wire may1 therefore1 at will1 communicate with the other operator in spite of anything that the other operator may do4 ,an order-wire system suitable for transfer switchboards consists in an order wire leading from each operator's receiver to a push button at each of the other operator's positions1 so that every operator has it within her power to depress a key or button and establish communication with a corresponding operator4 ,when1 therefore1 an operator in a transfer system answers a call that must be completed through a transfer circuit1 she establishes connection with that transfer circuit and then informs the operator at the other end of that circuit by order wire of the number of the trunk and the number of the subscriber with which that trunk is to be connected4 ,fig4 #ccd shows a system of order-wire buttons by means of which each operator may connect her telephone set with that of every other operator in the room1 the number in this case being confined to three4 ,assuming that each pair of wires leading from the lower portion of this figure terminates respectively in the operator's talking apparatus of the three respective operators1 then it is obvious that operator ,no4 #a1 by depressing button ,no4 #b1 will connect her telephone set with that of operator ,no4 #b2 likewise that any operator may communicate with any other operator by depressing the key bearing the corresponding number4 ,limitations of ,transfer ,system4 ,it may be stated that the transfer system at present has a limited place in the art of telephony4 ,the multiple switchboard has outstripped it in the race for popular approval and has demonstrated its superiority in practically all large manual exchange work4 ,this is not because of lack of effort on the part of telephone engineers to make the transfer system a success in a broad way4 ,a great variety of different schemes1 all embodying the fundamental idea of having one operator answer the call and another operator complete it through a trunk line1 have been tried4 ,in ,san ,francisco1 the ,sabin-,hampton system was in fairly successful service and served many thousands of lines for a number of years4 ,it was1 however1 afterwards replaced by modern multiple switchboards4 ,examples of ,obsolete ,systems4 ,the ,sabin-,hampton system was unique in many respects and involved three operators in each connection4 ,it was one of the very first systems which employed automatic signaling throughout and did away with the subscribers' generators4 ,it did not1 however1 dispense with the subscribers' local batteries4 ,another large transfer system1 used for years in an exchange serving at a time as many as #e1jjj1 was employed at ,grand ,rapids1 ,michigan4 ,this was later replaced by an automatic switchboard4 ,illustration3 ,fig4 #cce4 ,three-,position ,transfer ,switchboard ,field of ,usefulness4 ,the real field of utility for the transfer system today is to provide for the growth of simple switchboards that have extended beyond their originally intended limits4 ,by the adding of additional sections to the simple switchboard and the establishment of a comparatively cheap transfer system1 the simple boards may be made to do continued service without wasting the investment in them by discarding them and establishing a completely new system4 ,however1 switchboards are sometimes manufactured in which the transfer system is included as a part of the original equipment4 ,in ,fig4 #cce is shown a three-position transfer switchboard1 manufactured by the ,monarch ,telephone ,company4 ,at first glance the switchboard appears to be exactly like those described in ,chapter ,,xxi1 but on close observation1 the transfer jacks and signals may be seen in the first and third positions1 just below the line jacks and signals4 ,there is no transfer equipment in the second position of this switchboard because the operator at that position is able to reach the jacks of all the lines and1 therefore1 is able to complete all calls originating on her position without the use of any transfer equipment4 ,referring to ,fig4 #cja1 which illustrates a two-position simple switchboard1 it may readily be seen that if the demands for telephone service in the locality in which this switchboard is installed should increase so as to require the addition of more switchboard positions1 this switchboard could readily be converted to a transfer switchboard by placing the necessary transfer jacks and signals in the vacant space between the line jacks and clearing-out drops4 ,illustration3 ,,cable ,,turning ,,sections1 ,,between ,a ,,and ,b ,,boards ,cortlandt ,office1 ,new ,york ,telephone ,co4 ,,chapter ,,xxiv ,,principles ,,of ,,the ,,multiple ,,switchboard ,field of ,utility4 ,the multiple switchboard1 unlike the transfer board1 provides means for each operator to complete1 without assistance1 a connection with any subscriber's line terminating in the switchboard no matter how great the number of lines may be4 ,it is used only where the simple switchboard will not suffice2 that is1 where the number of lines and the consequent traffic is so great as to require so many operators and1 therefore1 so great a length of board as to make it impossible for any one operator to reach all over the face of the board without moving from her position4 ,the ,multiple ,feature4 ,the fundamental feature of the multiple switchboard is the placing of a jack for every line served by the switchboard within the reach of every operator4 ,this idea underlying the multiple switchboard may be best grasped by merely considering the mechanical arrangement and grouping of parts without regard to their details of operation4 ,the idea is sometimes elusive1 but it is really very simple4 ,if the student at the outset will not be frightened by the very large number of parts that are sometimes involved in multiple switchboards1 and by the great complexity which is apparent in the wiring and in the action of these parts2 and will remember that this apparent complexity results from the great number of repetitions of the same comparatively simple group of apparatus and circuits1 much will be done toward a mastery of the subject4 ,the multiple switchboard is divided into sections1 each section being about the width and height that will permit an ordinary operator to reach conveniently all over its face4 ,the usual width of a section brought about by this limitation is from five and one-half to six feet4 ,such a section affords room for three operators to sit side by side before it4 ,now each line1 instead of having a single jack as in the simple switchboard1 is provided with a number of jacks and one of these is placed on each of the sections1 so that each one of the operators may have within her reach a jack for each line4 ,it is from the fact that each line has a multiplicity of jacks1 that the term multiple switchboard arises4 ,number of ,sections4 ,since there is a jack for each line on each section of the switchboard1 it follows that on each section there are as many jacks as there are lines2 that is1 if the board were serving #e1jjj lines there would be #e1jjj jacks4 ,let us see now what it is that determines the number of sections in a multiple switchboard4 ,in the final analysis1 it is the amount of traffic that arises in the busiest period of the day4 ,assume that in a particular office serving #e1jjj lines1 the subscribers call at such a very low rate that even at the busiest time of the day only enough calls are made to keep1 say1 three operators busy4 ,in this case there would be no need for the multiple switchboard1 for a single section would suffice4 ,the three operators seated before that section would be able to answer and complete the connections for all of the calls that arose4 ,but subscribers do not call at this exceedingly low rate4 ,a great many more calls would arise on #e1jjj lines during the busiest hour than could be handled by three operators and1 therefore1 a great many more operators would be required4 ,space has to be provided for these operators to work in1 and as each section accommodates three operators the total number of sections must be at least equal to the total number of required operators divided by three4 ,let us assume1 for instance1 that each operator can handle #bjj calls during the busy hour4 ,assume further that during the busy hour the average number of calls made by each subscriber is two4 ,one hundred subscribers would1 therefore1 originate #bjj calls within this busy hour and this would be just sufficient to keep one operator busy4 ,since one operator can handle only the calls of one hundred subscribers during the busy hour1 it follows that as many operators must be employed as there are hundreds of subscribers whose lines are served in a switchboard1 and this means that in an exchange of #e1jjj subscribers1 #ej operators' positions would be required1 or #af-b_/c sections4 ,each of these sections would be equipped with the full #e1jjj jacks1 so that each operator could have a connection terminal for each line4 ,the ,multiple4 ,these groups of #e1jjj jacks1 repeated on each of the sections are termed multiple jacks1 and the entire equipment of these multiple jacks and their wiring is referred to as the multiple4 ,it will be shown presently that the multiple jacks are only used for enabling the operator to connect with the called subscriber4 ,in other words these jacks are for the purpose of enabling each operator to have within her reach any line that may be called for regardless of what line originates the call4 ,we will now consider what arrangements are provided for enabling the operator to receive the signal indicating a call and what provisions are made for her to answer the call in response to such a signal4 ,line ,signals4 ,obviously it is not necessary to have the line signals repeated on each section of the board as are the multiple jacks4 ,if a line has one definite place on the switchboard where its signal may be received and its call may be answered1 that suffices4 ,each line1 therefore1 in addition to having its multiple jacks distributed one on each section of the switchboard1 has a line signal and an individual jack immediately associated with it1 located on one only of the sections4 ,this signal usually is in the form of a lamp and is termed the line signal1 and this jack is termed the answering jack since it is by means of it that the operator always answers a call in response to the line signal4 ,distribution of ,line ,signals4 ,it is evident that it would not do to have all of these line signals and answering jacks located at one section of the board for then they would not be available to all of the operators4 ,they are1 therefore1 distributed along the board in such a way that one group of them will be available to one operator1 another group to another operator1 and so on2 the number of answering jacks and signals in any one group being so proportioned with respect to the number of calls that come in over them during the busy hour that it will afford just about enough calls to keep the operator at that position busy4 ,we may summarize these conditions with respect to the jack and line-signal equipment of the multiple switchboard by saying that each line has a multiple jack on each section of the board and in addition to this has on one section of the board an answering jack and a line signal4 ,these answering jacks and line signals are distributed in groups along the face of the board so that each operator will receive her proper quota of the originating calls which she will answer and1 by virtue of the multiple jack1 be able to complete the connections with the desired subscribers without moving from her position4 ,cord ,circuits4 ,each operator is also provided with a number of pairs of cords and plugs with proper supervisory or clearing-out signals and ringing and listening keys1 the arrangement in this respect being similar to that already described in connection with the simple switchboard4 ,guarding against ,double ,connections4 ,from what has been said it is seen that a call originating on a given line may be answered at one place only1 but an outgoing connection with that line may be made at any position4 ,this fact that a line may be connected with when called for at any one of the sections of the switchboard makes necessary the provision that two or more connections will not be made with the same line at the same time4 ,for instance1 if a call came in over a line whose signal was located on the first position of the switchboard for a connection with line ,no4 #a1jjj1 the operator at the first position would connect this calling line with ,no4 #a1jjj through the multiple jack on the first section of the switchboard4 ,assume now that some line1 whose signal was located on the #cith position of the switchboard1 should call also for line ,no4 #a1jjj while that line was still connected with the first calling subscriber4 ,obviously confusion would result if the operator at the #cith position1 not knowing that line ,no4 #a1jjj was already busy1 should connect this second line with it1 thereby leaving both of the calling subscribers connected with line ,no4 #a1jjj1 and as a result all of these three subscribers connected together4 ,the provisions for suitable means for preventing the making of a connection with a line that is already switched at some other section of the switchboard1 has offered one of the most fertile fields for invention in the whole telephone art4 ,the ways that have been proposed for accomplishing this are legion4 ,fortunately common practice has settled on one general plan of action and that is to so arrange the circuits that whenever a line is switched at one section1 such an electrical condition will be established on the forward contacts of all of its multiple jacks that any operator at any other section in attempting to make a connection with that line will be notified of the fact that it is already switched by an audible signal1 which she will receive in her head receiver4 ,on the other hand the arrangement is such that when a line is not busy1 i4 e41 it is not switched at any of the positions of the switchboard1 the operator on attempting to make a connection with such a line will receive no such guarding signal and will1 therefore1 proceed with the connection4 ,we may liken a line in a multiple switchboard to a lane having a number of gates giving access to it4 ,one of these gates--the answering jack--is for the exclusive use of the proprietor of that lane4 ,all of the other gates to the lane--the multiple jacks--are for affording means for the public to enter4 ,but whenever any person enters one of these gates1 a signal is automatically put up at all of the other gates forbidding any other person to enter the lane as long as the first person is still within4 ,illustration3 ,fig4 #ccf4 ,principle of ,multiple ,switchboard ,diagram ,showing ,multiple ,board ,principle4 ,for those to whom the foregoing description of the multiple board is not altogether clear1 the diagram of ,fig4 #ccf may offer some assistance4 ,five subscribers' lines are shown running through four sections of a switchboard4 ,each of these lines is provided with a multiple jack on each section of the board4 ,each line is also provided with an answering jack and a line signal on one of the sections of the board4 ,thus the answering jacks and the line signals of lines #a and #b are shown in ,section ,i1 that of line #d is shown in ,section ,,ii1 that of line #c in ,section ,,iii1 and that of line #e in ,section ,,iv4 ,at ,section ,i1 line #a is shown in the condition of having made a call and having had this call answered by the operator inserting one of her plugs into its answering jack4 ,in response to the instructions given by the subscriber1 the operator has inserted the other plug of this same pair in the multiple jack of line #b1 thus connecting these two lines for conversation4 ,at ,section ,,iii1 line #c is shown as having made a call1 and the operator as having answered by inserting one of her plugs into the answering jack4 ,it happens that the subscriber on line #c requests a connection with line #a1 and the condition at ,section ,,iii is that where the operator is about to apply the tip of the calling plug to the jack of line #a to ascertain whether or not that line is busy4 ,as before stated1 when the contact is made between the tip of the calling plug and the forward contact of the multiple jack1 the operator will receive a click in the ear 7by means that will be more fully discussed in later chapters71 this click indicating to her that line #a is not available for connection because it is already switched at some other section of the switchboard4 ,busy ,test4 ,the busy signal1 by which an operator in attempting to make a connection is informed that the line is already busy1 has assumed a great variety of forms and has brought forth many inventions4 ,it has been proposed by some that the insertion of a plug into any one of the jacks of a line would automatically close a little door in front of each of the other jacks of the line1 therefore making it impossible for any other operator to insert a plug as long as the line is in use4 ,it has been proposed by others to ring bells or to operate buzzers whenever the attempt was made by an operator to plug into a line that was already in use4 ,still others have proposed to so arrange the circuits that the operator would get an electric shock whenever she attempted to plug into a busy line4 ,the scheme that has met with universal adoption1 however1 is that the operator shall1 when the tip of her calling plug touches the forward contact of the jack of a line that is already switched1 receive a click in her telephone which will forbid her to insert the plug4 ,the absence of this click1 or silence in her telephone1 informs her that she may safely make the connection4 ,principle4 ,the means by which the operator receives or fails to receive this click1 according to whether the line is busy or idle1 vary widely1 but so far as the writers are aware they all have one fundamental feature in common4 ,the tip of the calling plug and the test contact of all of the multiple jacks of an idle line must be absolutely at the same potential before the test1 so that no current will flow through the test circuit when the test is actually made4 ,the test thimbles of all the jacks of a busy line must be at a different potential from the tip of the test plug so that a current will flow and a click result when the test is made4 ,potential of ,test ,thimbles4 ,it has been found an easy matter to so arrange the contacts in the jacks of a multiple switchboard that whenever the line is idle the test thimbles of that line will be a certain potential1 the same as that of all the unused calling plug tips4 ,it has also been easy to so arrange these contacts that the insertion of a plug into any one of the jacks will1 by virtue of the contacts established1 change the potential of all the test thimbles of that line so that they will be at a different potential from that of the tips of the calling plugs4 ,it has not been so easy1 however1 to provide that these conditions shall exist under all conditions of practice4 ,a great many busy tests that looked well on paper have been found faulty in practice4 ,as is always the case in such instances1 this has been true because the people who considered the scheme on paper did not foresee all of the conditions that would arise in practice4 ,many busy-test systems will operate properly while everything connected with the switchboard and the lines served by it remains in proper order4 ,but no such condition as this can be depended on in practice4 ,switchboards1 no matter how perfectly made and no matter with how great care they may be installed and maintained1 will get out of order4 ,telephone lines will become grounded or short-circuited or crossed or opened4 ,such conditions1 in a faulty busy-test system1 may result in a line that is really idle presenting a busy test1 and thus barring the subscriber on that line from receiving calls from other lines just as completely as if his line were broken4 ,on the other hand1 faulty conditions either in the switchboard or in the line may make a line that is really busy1 test idle1 and thus result in the confusion of having two or more subscribers connected to the same line at the same time4 ,busy-,test ,faults4 ,to show how elusive some of the faults of a busy test may be1 when considered on paper1 it has come within the observation of the writers that a new busy-test system was thought well enough of by a group of experienced engineers to warrant its installation in a group of very large multiple switchboards1 the cost of which amounted to hundreds of thousands of dollars1 and yet when so installed it developed that a single short-circuited cord in a position would make the test inoperative on all the cords of that position--obviously an intolerable condition4 ,luckily the remedy was simple and easily applied4 ,in a well-designed busy-test system there should be complete silence when the test is made of an idle line1 and always a well-defined click when the test is made of a busy line4 ,the test on busy lines should result in a uniform click regardless of length of lines or the condition of the apparatus4 ,it does not suffice to have a little click for an idle line and a big click for a busy line1 as practice has shown that this results in frequent errors on the part of the operators4 ,good operating requires that the tip of the calling plug be tapped against the test thimble several times in order to make sure of the state of the called line4 ,in some multiple switchboards the arrangement has been such that the jacks of a line would test busy as soon as the subscriber on that line removed his receiver from its hook to make a call1 as well as while any plug was in any jack of that line4 ,the advocates of this added feature1 in connection with the busy test1 have claimed that the receiver1 when removed from its hook in making a call1 should make the line test busy and that a line should not be connected with when the subscriber's receiver was off its hook any more than it should be when it was already connected with at some other section of the switchboard4 ,while it is true that a line may be properly termed busy when the subscriber has removed his receiver in order to make a call1 it is not true that there is any real necessity for guarding against a connection with it while he is waiting for the operator to answer4 ,leaving the line unguarded for this brief period may result in the subscriber1 who intended to make the call1 having to defer his call until he has conversed with the party who is trying to reach him4 ,this cannot be said to be a detriment to the service1 however1 since the second party gets the connection he desires much sooner than he otherwise would1 and the first party may still make his first intended call as soon as he has disposed of the party who reached him while he was waiting for his own operator to answer4 ,it may be said1 therefore1 in connection with this matter of making the line test busy as soon as a subscriber has removed his receiver from the hook1 that it is not considered an essential1 and in case of those switchboard systems which naturally work out that way it is not considered a disadvantage4 ,field of ,each ,operator4 ,it was stated earlier in this chapter that as each section accommodated three operators1 the total number of sections in a switchboard will be at least one-third the total number of required operators4 ,this thought needs further development1 for to stop at that statement is to arrive somewhat short of the truth4 ,in order to do this it is necessary to consider the field in the multiple1 reached by each operator4 ,the section is of such size1 or should be1 that an operator seated in the center position of it may1 without undue effort1 reach all over the multiple4 ,but the operator at the right-hand position cannot reach the extreme left portion of the multiple of that section1 nor can the operator at the left reach the extreme right4 ,how then may each operator reach a jack for every line8 ,remembering that the multiple jacks are arranged exactly the same in each section1 each jack always occupying the same relative position1 it is easy to see that while the operator at a right-hand position of a section cannot reach the left-hand third of the multiple in her own section1 she may reach the left-hand third of the multiple in the section at her right1 and this1 together with the center and right-hand thirds of her own section1 represents the entire number of lines4 ,so it is with the left-hand operator at any section1 she reaches two-thirds of all the lines in the multiple of her own section and one-third in that of the section at her left4 ,end ,positions4 ,this makes it necessary to inquire about the operators at the end positions of the entire board4 ,to provide for these the multiple is extended one-third of a section beyond them1 so as to supply at the ends of the switchboard jacks for those lines which the end operators cannot reach on their own sections4 ,sometimes instead of adding these end sections to the multiple for the end operators1 the same result is accomplished by using only the full and regular sections of the multiple1 and leaving the end positions without operators' equipment1 as well as without answering jacks1 line signals1 and cords and plugs1 so that in reality the end operator is at the middle position of the end section4 ,this1 in our opinion1 is the better practice1 since it leaves the sections standard1 and makes it easier to extend the switchboard in length1 as it grows1 by the mere addition of new sections without disturbing any of the old multiple4 ,influence of ,traffic4 ,we wish again to emphasize the fact that it is the traffic during the busiest time of day and not the number of lines that determine the size of a multiple switchboard so far as its length is concerned4 ,the number of lines determines the size of the multiple in any one section1 but it is the amount of traffic1 the number of calls that are made in the busiest period1 that determines the number of operators required1 and thus the number of positions4 ,had this now very obvious fact been more fully realized in the past1 some companies would be operating at less expense1 and some manufacturers would have sold less expensive switchboards4 ,the whole question as to the number of positions boils down to how many answering jacks and line signals may be placed at each operator's position without overburdening the operator with incoming traffic at the busy time of day4 ,obviously1 some lines will call more frequently than others1 and hence the proper number of answering jacks at the different positions will vary4 ,obviously1 also1 due to changes in the personnel of the subscribers1 the rates of calling of different groups of lines will change from time to time1 and this may necessitate a regrouping of the line signals and answering jacks on the positions2 and changes in the personnel of the operators or in their skill also demand such regrouping4 ,intermediate ,frame4 ,the intermediate distributing frame is provided for this purpose1 and will be more fully discussed in subsequent chapters4 ,suffice it to say here that the intermediate distributing frame permits the answering jacks and line signals to be shifted about among the operators' positions1 so that each position will have just enough originating traffic to keep each of the operators economically busy during the busiest time of the day4 ,,chapter ,,xxv ,,the ,,magneto ,,multiple ,,switchboard ,field of ,utility4 ,the principles of the multiple switchboard set forth in the last chapter were all developed long before the common-battery system came into existence1 and consequently all of the first multiple switchboards were of the magneto type4 ,although once very widely used1 the magneto multiple switchboard has almost passed out of existence1 since it has become almost universal practice to equip exchanges large enough to employ multiple boards with common-battery systems4 ,nevertheless there is a field for magneto multiple switchboards1 and in this field it has recently been coming into increasing favor4 ,in those towns equipped with magneto systems employing simple switchboards or transfer switchboards1 and which require new switchboards by virtue of having outgrown or worn out their old ones1 the magneto multiple switchboard is frequently found to best fit the requirements of economy and good practice4 ,the reason for this is that by its use the magneto telephones already in service may be continued1 no change being required outside of the central office4 ,furthermore1 with the magneto multiple switchboard no provision need be made for a power plant1 which1 in towns of small size1 is often an important consideration4 ,again1 many companies operate over a considerable area1 involving a collection of towns and hamlets4 ,it may be that all of these towns except one are clearly of a size to demand magneto equipment and that magneto equipment is the standard throughout the entire territory of the company4 ,if1 however1 one of the towns1 by virtue of growth1 demands a multiple switchboard1 this condition affords an additional argument for the employment of the magneto multiple switchboard1 since the same standards of equipment and construction may be maintained throughout the entire territory of the operating company1 a manifest advantage4 ,on the other hand1 it may be said that the magneto multiple switchboard has no proper place in modern exchanges of considerable size--say1 having upward of one thousand subscribers--at least under conditions found in the ,united ,states4 ,notwithstanding the obsolescence of the magneto multiple switchboard for large exchanges1 a brief discussion of some of the early magneto multiple switchboards1 and particularly of one of the large ones1 is worth while1 in that a consideration of the defects of those early efforts will give one a better understanding and appreciation of the modern multiple switchboard1 and particularly of the modern multiple common-battery switchboard1 the most highly organized of all the manual switching systems4 ,brief reference will1 therefore1 be made to the so-called series multiple switchboard1 and then to the branch terminal multiple switchboard1 which latter was the highest type of switchboard development at the time of the advent of common-battery working4 ,illustration3 ,fig4 #ccg4 ,series ,magneto ,multiple ,switchboard ,series-,multiple ,board4 ,in ,fig4 #ccg are shown the circuits of a series magneto multiple switchboard as developed by the engineers of the ,western ,electric ,company during the eighties4 ,as is usual1 two subscribers' lines and a single cord circuit are shown4 ,one side of each line passes directly from the subscriber's station to one side of the drop1 and also branches off to the sleeve contact of each of the jacks4 ,the other side of the line passes first to the tip spring of the first jack1 thence to the anvil of that jack and to the tip spring of the next jack1 and so on in series through all of the jacks belonging in that line to the other terminal of the drop coil4 ,normally1 therefore1 the drop is connected across the line ready to be responsive to the signal sent from the subscriber's generator4 ,the cord circuit is of the two-conductor type1 the plugs being provided with tip and sleeve contacts1 the tips being connected by one of the flexible conductors through the proper ringing and listening key springs1 and the sleeve being likewise connected through the other flexible conductor and the other springs of the ringing and listening keys4 ,it is obvious that when any plug is inserted into a jack1 the circuit of the line will be continued to the cord circuit and at the same time the line drop will be cut out of the circuit1 because of the lifting of the tip spring of the jack from its anvil4 ,permanently connected between the sleeve side of the cord circuit and ground is a retardation coil #a and a battery4 ,another retardation coil #b is connected between the ground and a point on the operator's telephone circuit between the operator's head receiver and the secondary of her induction coil4 ,these two retardation coils have to do with the busy test1 the action of which is as follows3 normally1 or when a line is not switched at the central office1 the test thimbles will all be at substantially ground potential1 i4 e41 they are supposed to be4 ,the point on the operator's receiver circuit which is grounded through the retardation coil #b will also be of ground potential because of that connection to ground4 ,in order to test1 the operator always has to throw her listening key ,l4,k4 into the listening position4 ,she also has to touch the tip of the calling plug ,pc to a sleeve or jack of the line that is being tested4 ,if1 therefore1 a test is made of an idle or non-busy line1 the touching of the tip of the calling plug with the test thimble of that line will result in no flow of current through the operator's receiver1 because there will be no difference of potential anywhere in the test circuit1 which test circuit may be traced from the test thimble of the line under test to the tip of the calling plug1 thence through the tip strand of the cord to the listening key1 thence to the outer anvil of the listening key on that side1 through the operator's receiver to ground through the impedance coil #b4 ,if1 however1 the line had already been switched at some other section by the insertion of either a calling or answering plug1 all of the test thimbles of that line would have been raised to a potential above that of the ground1 by virtue of the battery connected with the sleeve side of the cord circuit through the retardation coil #a4 ,if the operator had made a test of such a line1 the tip of her testing plug would have found the thimble raised to the potential of the battery and1 therefore1 a flow of current would occur which would give her the busy click4 ,the complete test circuit thus formed in testing a busy line would be from the ungrounded pole of the battery through the impedance coil #a associated with the cord that was already in connection with the line1 thence to the sleeve strand of that cord to the sleeve of the jack at which the line was already switched1 thence through that portion of the line circuit to which all of the sleeve contacts were connected1 and therefore to the sleeve or test thimble of the jack at which the test is made1 thence through the tip of the calling plug employed in making the test through the tip side of that cord circuit to the outer listening key contact of the operator making the test1 and thence to ground through the operator's receiver and the impedance coil #b4 ,the resultant click would be an indication to the operator that the line was already in use and that1 therefore1 she must not make the connection4 ,the condenser #c is associated with the operator's talking set and with the extra spring in the listening key ,l4,k4 in such a manner that when the listening key is thrown1 the tip strand of the cord circuit is divided and the condenser included between them4 ,this is for the purpose of preventing any potentials1 which might exist on the line with which the answering plug ,pa was connected1 from affecting the busy-test conditions4 ,operation4 ,the operation of the system aside from the busy-test feature is just like that described in connection with the simple magneto switchboard4 ,assuming that the subscriber at ,station ,a makes the call1 he turns his hand generator1 which throws the drop on his line at the central office4 ,the operator1 seeing the signal1 inserts the answering plug of one of her idle pairs of cords into the answering jack and throws her listening key ,l4,k4 ,this enables the operator to talk with the calling subscriber1 and having found that he desires a connection with the line extending to ,station ,b1 she touches the tip of her calling plug to the multiple jack of that line that is within her reach1 it being remembered that each one of the multiple jacks shown is on a different section4 ,she leaves the listening key in the listening position when she does this4 ,if the line is busy1 the click will notify her that she must not make the connection1 in which case she informs the calling subscriber that the line is busy and requests him to call again4 ,if1 however1 she received no click1 she would insert the calling plug into the jack1 thus completing the connection between the two lines4 ,she would then press the ringing key associated with the calling plug and that momentarily disconnects the calling plug from the answering plug and at the same time establishes connection between the ringing generator and the called line4 ,the release of the ringing key again connects the calling and answering plugs and1 therefore1 connects the two subscribers' lines ready for conversation4 ,all that is then necessary is that the called subscriber shall respond and remove his receiver from its hook1 the calling subscriber already having done this4 ,when the conversation is finished1 both of the subscribers 7if they remember it7 will operate their ringing generators1 which will throw the clearing-out drop as a signal to the operator for disconnection4 ,if it should become necessary for the operator to ring back on the line of the calling subscriber1 she may do so by pressing the ringing key associated with the calling plug4 ,frequently this multiple switchboard arrangement was used with grounded lines1 in which case the single line wire extending from the subscriber's station to the switchboard was connected with the tip spring of the first jack1 the circuit being continued in series through the jack to the drop and thence to ground through a high non-inductive resistance4 ,defects4 ,this series multiple magneto system was used with a great many variations1 and it had a good many defects4 ,one of these defects was due to the necessary extending of one limb of the line through a large number of series contacts in the jacks4 ,this is not to be desired in any case1 but it was particularly objectionable in the early days before jacks had been developed to their present high state of perfection4 ,a particle of dust or other insulating matter1 lodging between the tip spring and its anvil in any one of the jacks1 would leave the line open1 thus disabling the line to incoming signals1 and also for conversation in case the break happened to occur between the subscriber and the jack that was used in connecting with the line4 ,another defect due to the same cause was that the line through the switchboard was always unbalanced by the insertion of a plug1 one limb of the line always extending clear through the switchboard to the drop and the other1 when the plug was inserted1 extending only part way through the switchboard and being cut off at the jack where the connection was made4 ,the objection will be apparent when it is remembered that the wires in the line circuit connecting the multiple jacks are necessarily very closely bunched together and1 therefore1 there is very likely to be cross-talk between two adjacent lines unless the two limbs of each line are exactly balanced throughout their entire length4 ,again the busy-test conditions of this circuit were not ideal4 ,the fact that the test rings of the line were connected permanently with the outside line circuit subjected these test rings to whatever potentials might exist on the outside lines1 due to any causes whatever1 such as a cross with some other wire2 thus the test rings of an idle line might by some exterior cause be raised to such a potential that the line would test busy4 ,it may be laid down as a fundamental principle in good multiple switchboard practice that the busy-test condition should be made independent of any conditions on the line circuit outside of the central office1 and such is not the case in this circuit just described4 ,illustration3 ,,cable ,,run ,,from ,,intermediate ,,frame ,,to ,,multiple ,cortlandt ,office1 ,new ,york ,telephone ,co4 ,branch-,terminal ,multiple ,board4 ,the next important step in the development of the magneto multiple switchboard was that which produced the so-called branch-terminal board4 ,this came into wide use in the various ,bell operating companies before the advent of the common-battery systems4 ,its circuits and the principles of operation may be understood in connection with ,fig4 #cch4 ,in the branch-terminal system there are no series contacts in the jacks and no unbalancing of the line due to a cutting off of a portion of the line circuit when a connection was made with it4 ,furthermore1 the test circuits were entirely local to the central office and were not likely to be affected by outside conditions on the line4 ,this switchboard also added the feature of the automatic restoration of the drops1 thus relieving the operator of the burden of doing that manually1 and also permitting the drops to be mounted on a portion of the switchboard that was not available for the mounting of jacks1 and thus permitting a greater capacity in jack equipment4 ,illustration3 ,fig4 #cch4 ,branch-,terminal ,magneto ,multiple ,switchboard ,each jack has five contacts1 and the answering and multiple jacks are alike1 both in respect to their construction and their connection with the line4 ,the drops are the electrically self-restoring type shown in ,fig4 #bfc4 ,the line circuits extended permanently from the subscriber's station to the line winding of the drop and the two limbs of the line branched off to the tip and sleeve contacts #a and #b respectively of each jack4 ,another pair of wires extended through the multiple parallel to the line wires and these branched off respectively to the contact springs #c and #d of each of the jacks4 ,this pair of wires formed portions of the drop-restoring circuit1 including the restoring coil #f and the battery #g1 as indicated4 ,the test thimble #e of each of the jacks is connected permanently with the spring #c of the corresponding jack and1 therefore1 with the wire which connects through the restoring coil #f of the corresponding drop to ground through the battery #g4 ,the plugs were each provided with three contacts4 ,two of these were the usual tip and sleeve contacts connected with the two strands of the cord circuit4 ,the third contact #h was not connected with any portion of the cord circuit1 being merely an insulated contact on the plug adapted1 when the plug was fully inserted1 to connect together the springs #c and #d4 ,the cord circuit itself is readily understood from the drawing1 having two features1 however1 which merit attention4 ,one is the establishing of a grounded battery connection to the center portion of the winding of the receiver for the purposes of the busy test1 and the other is the provision of a restoring coil and restoring circuit for the clearing-out drop1 this circuit being closed by an additional contact on the listening key so as to restore the clearing-out drop whenever the listening key was operated4 ,operation4 ,an understanding of the operation of this system is easy4 ,the turning of the subscriber's generator1 when the line was in its normal condition1 caused the display of the line signal4 ,the insertion of the answering plug1 in response to this call1 did three things3 7#a7 ,it extended the line circuit to the tip and sleeve strand of the cord circuit4 7#b7 ,it energized the restoring coil #f of the drop by establishing the circuit from the contact spring #c through the plug contact #h to the other contact spring #d1 thus completing the circuit between the two normally open auxiliary wires4 7#c7 ,the connecting of the springs #c and #d established a connection from ground to the test thimbles of all the jacks on a line1 the spring #d being always grounded and the spring #c being always connected to the test thimble #e4 ,it is to be noted that on idle lines the test rings are always at the same potential as the ungrounded pole of the battery #g1 being connected thereto through the winding #f of the restoring coil4 ,on all busy lines1 however1 the test rings are dead grounded through the contact #h of the plug that is connected with the line4 ,the tip of the testing plug at the time of making a test will also be at the same potential as that of the ungrounded pole of the battery #g1 since this pole of the battery #g is always connected to the center portion of the operator's receiver winding1 and when the listening key is thrown the tip of the calling plug is connected therewith and is at the same potential4 ,when1 therefore1 the operator touches the tip of the calling plug to the test thimble of an idle line1 she will get no click1 since the tip of the plug and the test thimble will be at the same potential4 ,if1 however1 the line has already been switched at another section of the board1 there will be a difference of potential1 because the test thimble will be grounded1 and the circuit1 through which the current which causes the click flows1 may be traced from the ungrounded pole of the battery #g to the center portion of the operator's receiver1 thence through one-half of the winding to the tip of the calling plug1 thence to the test thimble of the jack under test1 thence to the spring #c of the jack on another section at which the connection exists1 through the contact #h on the plug of that jack to the spring #d1 and thence to ground and back to the other terminal of the battery #g4 ,magnet ,windings4 ,coils of the line and clearing-out drops by which these drops are thrown1 are wound to such high resistance and impedance as to make it proper to leave them permanently bridged across the talking circuit4 ,the necessity for cutting them out is1 therefore1 done away with1 with a consequent avoidance1 in the case of the line drops1 of the provision of series contacts in the jacks4 ,arrangement of ,apparatus4 ,in boards of this type the line and clearing-out drops were mounted in the extreme upper portion of the switchboard face so as to be within the range of vision of the operator1 but yet out of her reach4 ,therefore1 the whole face of the board that was within the limit of the operator's reach was available for the answering and multiple jacks4 ,a front view of a little over one of the sections of the switchboard1 involving three complete operator's positions1 is shown in ,fig4 #cci1 which is a portion of the switchboard installed by the ,western ,electric ,company in one of the large exchanges in ,paris1 ,france4 7,this has recently been replaced by a common-battery multiple board47 ,in this the line drops may be seen at the extreme top of the face of the switchboard1 and immediately beneath these the clearing-out drops4 ,beneath these are the multiple jacks arranged in banks of one hundred1 each hundred consisting of five strips of twenty4 ,at the extreme lower portion of the jack space are shown the answering jacks and beneath these on the horizontal shelf1 the plugs and keys4 ,these jacks were mounted on #a_/b-;inch centers1 both vertically and horizontally and each section had in multiple #ij banks of #ajj each1 making #i1jjj in all4 ,subsequent practice has shown that this involves too large a reach for the operators and that1 therefore1 #i1jjj is too large a number of jacks to place on one section if the jacks are not spaced closer than on #a_/b-;inch centers4 ,with the jack involving as many parts as that required by this branch terminal system1 it was hardly feasible to make them smaller than this without sacrificing their durability1 and one of the important features of the common-battery multiple system which has supplanted this branch-terminal magneto system is that the jacks are of such a simple nature as to lend themselves to mounting on #c_/h-;inch centers1 and in some cases on #c_/aj-;inch centers4 ,illustration3 ,fig4 #cci4 ,face of ,magneto ,multiple ,switchboard ,modern ,magneto ,multiple ,board4 ,coming now to a consideration of modern magneto multiple switchboards1 and bearing in mind that such boards are to be found in modern practice only in comparatively small installations and then only under rather peculiar conditions1 as already set forth1 we will consider the switchboard of the ,monarch ,telephone ,manufacturing ,company as typical of good practice in this respect4 ,illustration3 ,fig4 #cdj4 ,monarch ,magneto ,multiple ,switchboard ,circuits ,line ,circuit4 ,the line and cord circuits of the ,monarch system are shown in ,fig4 #cdj4 ,it will be seen that each jack has in all five contacts1 numbered from #a to #e respectively1 of which #a and #d are the springs which register with the tip and ring contacts of the plug and through which the talking circuit is continued1 while #b and #c are series contacts for cutting off the line drop when a plug is inserted1 and #e is the test contact or thimble adapted to register with the sleeve contact on the plug when the plug is fully inserted4 ,the line circuit through the drop may be traced normally from one side of the line through the drop coil1 thence through all of the pairs of springs #b and #c in the jacks of that line1 and thence to spring #a of the last jack1 this spring always being strapped to the spring #b in the last jack1 and thence to the other side of the line4 ,all the ring springs #a are permanently tapped on to one side of the line1 and all of the tip springs #d are permanently tapped to the other side of the line4 ,this system may1 therefore1 properly be called a branch-terminal system4 ,it is seen that as soon as a plug is inserted into any of the jacks1 the circuit through the drop will be broken by the opening of the springs #b and #c in that jack4 ,the drop shown immediately above the answering jack is so associated mechanically with that jack as to be mechanically self-restored when the answering plug is inserted into the answering jack in response to a call4 ,the arrangement in this respect is the same as that shown in ,fig4 #bei1 illustrating the ,monarch combined drop and jack4 ,cord ,circuit4 ,the cord circuit needs little explanation4 ,the tip and ring strands are the ones which carry the talking current and across these is bridged the double-wound clearing-out drop1 a condenser being included in series in the tip strand between the two drop windings in the manner already explained in connection with ,fig4 #bhd4 ,the third or sleeve strand of the cord is continuous from plug to plug1 and between it and the ground there is permanently connected a retardation coil4 ,test4 ,the test is dependent on the presence or absence of a path to ground from the test thimbles through some retardation coil associated with a cord circuit4 ,obviously1 in the case of an idle line there will be no path to ground from the test thimbles1 since normally they are merely connected to each other and are insulated from everything else4 ,when1 however1 a plug is inserted into a multiple or answering jack1 the test thimbles of that line are connected to ground through the retardation coil associated with the third strand of the plug used in making the connection4 ,when the operator applies the tip of the calling plug to a test contact of a multiple jack there will be no path to ground afforded if the line is idle1 while if it is busy the potential of the tip of the test plug will cause a current to flow to ground through the impedance coil associated with the plug used in making the connection4 ,this will be made clearer by tracing the test circuit4 ,with the listening key thrown this may be traced from the live side of the battery through the retardation coil #f1 which is common to an operator's position1 thence through the tip side of the listening key to the tip conductor of the calling cord1 and thence to the tip of the calling plug and the thimble of the jack under test4 ,if the line is idle there will be no path to ground from this point and no click will result1 but if the line is busy1 current will flow from the tip of the test plug to the thimble of the jack tested1 thence by the test wire in the multiple to the thimble of the jack at which a connection already exists1 and thence to ground through the third strand of the cord used in making that connection and the impedance coil associated therewith4 ,the current which flows in this test circuit changes momentarily the potential of the tip side of the operator's telephone circuit1 thus unbalancing her talking circuit and causing a click4 ,illustration3 ,fig4 #cda4 ,magneto ,multiple ,switchboard ,if this test system were used in a very large board where the multiple would extend through a great many sections1 there would be some liability of a false test due to the static capacity of the test contacts and the test wire running through the multiple4 ,for small boards1 however1 where the multiple is short1 this system has proven reliable4 ,a multiple magneto switchboard employing the form of circuits just described is shown in ,fig4 #cda4 ,this switchboard consists of three sections of two positions each4 ,the combined answering jacks and drops may be seen at the lower part of the face of the switchboard and occupying somewhat over one-half of the jack and drop space4 ,the multiple jacks are above the answering jacks and drops and it may be noted that the same arrangement and number of these jacks is repeated in each section4 ,this switchboard may be extended by adding more sections and increasing the multiple in those already installed to serve #a1fjj lines4 ,assembly4 ,in connection with the assembly of these magneto multiple switchboards1 as installed by the ,monarch ,company1 ,fig4 #cdb shows the details of the cord rack at the back of the board4 ,it shows how the ends of the switchboard cords opposite to the ends that are fastened to the plugs are connected permanently to terminals on the cord rack1 at which point the flexible conductors are brought out to terminal clips or binding posts1 to which the wires leading from the other portions of the cord circuit are led4 ,in order to relieve the conductors in the cords from strain1 the outer braiding of the cord at the rack end is usually extended to form what is called a strain cord1 and this attached to an eyelet under the cord rack1 so that the weight of the cord and the cord weights will be borne by the braiding rather than by the conductors4 ,this leaves the insulated conductors extending from the ends of the cords free to hang loose without strain and be connected to the terminals as shown4 ,this method of connecting cords1 with variations in form and detail1 is practically universal in all types of switchboards4 ,illustration3 ,fig4 #cdb4 ,cord-,rack ,connectors ,a detail of the assembly of the drops and jacks in such a switchboard is shown in ,fig4 #cdc4 ,the single pair of clearing-out drops is mounted in the lower part of the vertical face of the switchboard just above the space occupied by the plug shelf4 ,vertical stile strips extend above the clearing-out drop space for supporting the drops and jacks4 ,a single row of #aj answering jacks and the corresponding line drops are shown in place4 ,above these there would be placed1 in the completely assembled board1 the other answering jacks and line signals that were to occupy this panel1 and above these the strips of multiple jacks4 ,the rearwardly projecting pins from the stile strips are for the support of the multiple jack strips1 these pins supporting the strips horizontally by suitable multiple clips at the ends of the jack strips2 the jack strips being fastened from the rear by means of nuts engaging the screw threads on these pins4 ,this method of supporting drops and jacks is one that is equally adaptable for use in other forms of boards1 such as the simple magneto switchboard4 ,illustration3 ,fig4 #cdc4 ,drop and ,jack ,mounting ,illustration3 ,fig4 #cdd4 ,keyboard ,wiring ,in ,fig4 #cdd is shown a detail photograph of the key shelf wiring in one of these ,monarch magneto switchboards4 ,in this the under side of the keys is shown1 the key shelf being raised on its hinge for that purpose4 ,the cable1 containing all of the insulated wires leading to these keys1 enters the space under the key shelf at the extreme left and from the rear4 ,it then passes to the right of this space where a knee is formed1 after which the cable is securely strapped to the under side of the key shelf4 ,by this construction sufficient flexibility is provided for in the cable to permit the raising and lowering of the key shelf1 the long reach of the cable between the knee and the point of entry at the left serving as a torsion member1 so that the raising of the shelf will give the cable a slight twist rather than bend it at a sharp angle4 ,,chapter ,,xxvi ,,the ,,common-,,battery ,,multiple ,,switchboard ,western ,electric ,no4 #a ,relay ,board4 ,the common-battery multiple switchboard differs from the simple or non-multiple common-battery switchboard mainly in the provision of multiple jacks and in the added features which are involved in the provision for a busy test4 ,the principles of signaling and of supplying current to the subscribers for talking are the same as in the non-multiple common-battery board4 ,for purposes of illustrating the practical workings of the common-battery multiple switchboard1 we will take the standard form of the ,western ,electric ,company1 choosing this only because it is the standard with nearly all the ,bell operating companies throughout the ,united ,states4 ,illustration3 ,fig4 #cde4 ,line ,circuit ,western ,electric ,no4 #a4 ,board ,line ,circuit4 ,we will first consider the line circuit in simplified form1 as shown in ,fig4 #cde4 ,at the left in this figure the common-battery circuit is shown at the subscriber's station1 and at the right the central-office apparatus is indicated so far as equipment of a single line is concerned4 ,in this simplified diagram no attempt has been made to show the relative positions of the various parts1 these having been grouped in this figure in such a way as to give as clear and simple an idea as possible of the circuit arrangements4 ,it is seen at a glance that this is a branch terminal board1 the three contacts of each jack being connected by separate taps or legs to three wires running throughout the length of the board1 these three wires being individual to the jacks of one line4 ,on this account this line circuit is commonly referred to as a three-wire circuit4 ,by the same considerations it will be seen that the switchboard line circuit of the branch-terminal multiple magneto system1 shown in ,fig4 #cch1 would be called a four-wire circuit4 ,it will be shown later that other multiple switchboards in wide use have a still further reduction in the number of wires running through the jacks1 or through the multiple as it is called1 such being referred to as two-wire switchboards4 ,the two limbs of the line which extend from the subscriber's circuit1 beside being connected by taps to the tip and sleeve contacts of the jack respectively1 connect with the two back contacts of a cut-off relay1 and when this relay is in its normal or unenergized condition1 these two limbs of the line are continued through the windings of the line relay and thence one to the ungrounded or negative side of the common-battery and the other to the grounded side4 ,the subscriber's station circuit being normally open1 no current flows through the line1 but when the subscriber removes his receiver for the purpose of making a call the line circuit is completed and current flows through the coil of the line relay1 thus energizing that relay and causing it to complete the circuit of the line lamp4 ,the cut-off relay plays no part in the operation of the subscriber's calling1 but merely leaves the circuit of the line connected through to the calling relay and battery4 ,the coil of the cut-off relay is connected to ground on one side and on the other side to the third wire running through the switchboard multiple and which is tapped off to each of the test rings on the jacks4 ,as will be shown later1 when the operator plugs into the jack of a line1 such a connection is established that the test ring of that jack will be connected to the live or negative pole of the common battery1 which will cause current to flow through the coil of the cut-off relay1 which will then operate to cut off both of the limbs of the line from their normal connection with ground and the battery and the line relay4 ,hence the name cut-off relay4 ,the use of the cut-off relay to sever the calling apparatus from the line at all times when the line is switched serves to make possible a very much simpler jack than would otherwise be required1 as will be obvious to anyone who tries to design a common-battery multiple system without a cut-off relay4 ,the additional complication introduced by the cut-off relay is more than offset by the saving in complexity of the jacks4 ,it is desirable1 on account of the great number of jacks necessarily employed in a multiple switchboard1 that the jacks be of the simplest possible construction1 thus reducing to a minimum their first cost and making them much less likely to get out of order4 ,cord ,circuit4 ,the cord circuit of the ,western ,electric standard multiple common-battery switchboard is shown in ,fig4 #cdf4 ,this cord circuit involves the use of three strands in the flexible cords of both the calling and the answering plugs4 ,two of these are the ordinary tip and ring conductors over which speech is transmitted to the connected subscriber's wire4 ,the third1 the sleeve strand1 carries the supervisory lamps and has associated with it other apparatus for the control of these lamps and of the test circuit4 ,illustration3 ,fig4 #cdf4 ,cord ,circuit ,western ,electric ,no4 #a ,board ,the system of battery feed is the well-known split repeating-coil arrangement already discussed4 ,the tip strand runs straight through to the repeating coil1 while the ring strand contains1 in each case1 the winding of the supervisory relay corresponding to either the calling or the answering plug4 ,in order that the presence in the talking circuit of a magnet winding possessing considerable impedance may not interfere with the talking efficiency1 each of these supervisory relay windings is shunted by a non-inductive resistance4 ,in practice the supervisory relay windings have each a resistance of about #bj ohms and the shunt around them each a resistance of about #ca ohms4 ,in the third strand of each cord is placed a #ab-volt supervisory lamp1 and in series with it a resistance of about #hj ohms4 ,each supervisory relay is adapted1 when energized1 to close a #dj-ohm shunt about its supervisory lamp4 ,the arrangement and proportion of these resistances is such that when a plug is inserted into the jack of a line the lamp will receive current from a circuit traced from the negative pole of the battery in the center of the cord circuit through the lamp and the #hj-ohm series resistance1 through the third strand of the cord to the test thimble of the jack1 and thence to the positive or grounded pole of the battery through the third conductor in the multiple and the winding of the cut-off relay4 ,this current always flows as long as the plug is inserted1 and it is just sufficient to illuminate the lamp when the supervisory relay armature is not attracted4 ,when1 however1 the supervisory relay armature is attracted1 the shunting of the lamp by the #dj-ohm resistance cuts down the current to such a degree as to prevent the illumination of the lamp1 although some current still flows through it4 ,the usual ringing and listening key is associated with the calling plug1 and in some cases a ring-back key is associated with the answering plug1 but this is not standard practice4 ,operation4 ,the operation of this cord circuit in conjunction with the line circuit of ,fig4 #cde may best be understood by reference to ,fig4 #cdg4 ,this figure employs a little different arrangement of the line circuit in order more clearly to indicate how the two lines may be connected by a cord2 a study of the two line circuits1 however1 will show that they are identical in actual connections4 ,it is to be remembered that all of the battery symbols shown in this figure represent in reality the same battery1 separate symbols being shown for greater simplicity in circuit connections4 ,we will assume the subscriber at ,station ,a calls for the subscriber at ,station ,b4 ,the operation of the line relay and the consequent lighting of the line lamp1 and also the operation of the pilot relay will be obvious from what has been stated4 ,the response of the operator by inserting the answering plug into the answering jack1 and the throwing of her listening key so as to bridge her talking circuit across the cord in order to place herself in communication with the subscriber1 is also obvious4 ,the insertion of the answering plug into the answering jack completed the circuit through the third strand of the cord and the winding of the cut-off relay of the calling line1 and this accomplishes three desirable results4 ,the circuit so completed may be traced from the negative or ungrounded side of the battery to the center portion of the cord circuit1 thence through the supervisory lamp #a1 resistance #b1 to the third conductor on the plug1 test thimble on the jack1 thence through the winding of the cut-off relay to ground1 which forms the other terminal of the battery4 ,the results accomplished by the closing of this circuit are3 first1 the energizing of the cut-off relay to cut off the signaling portion of the line2 second1 the flowing of current through the lamp that is almost sufficient to illuminate it1 but not quite so because of the closure of the shunt about it1 for the reason that will be described2 third1 the raising of the potential of all the contact thimbles on the jacks from zero to a potential different from that of the ground and equal in amount to the fall of potential through the winding of the cut-off relay4 ,a condition is thus established at the test rings such that some other operator at some other section in testing the line will find it busy and will not connect with it4 ,illustration3 ,fig4 #cdg4 ,western ,electric ,no4 #a ,board ,the reason why the lamp #a1 connected with the answering plug1 was not lighted was that the supervisory relay #c1 associated with the answering plug1 became energized when the operator plugged in1 due to the flow of current from the battery through the calling subscriber's talking apparatus1 this flow of current being permitted by the removal of the calling subscriber's receiver from its hook4 ,the energizing of this relay magnet by causing the attraction of its armature1 closed the shunt about the lamp #a1 which shunt contains the #dj-ohm resistance #d1 and thus prevents the lamp from receiving enough current to illuminate it4 ,obviously1 as soon as the calling subscriber replaces his receiver on its hook1 the supervisory relay #c will be de-energized1 the shunt around the lamp will be broken1 and the lamp will be illuminated to indicate to the operator the fact that the subscriber with whose line her calling plug is connected has replaced his receiver on its hook4 ,testing--,called ,line ,idle4 ,having now shown how the operator connects with the calling subscriber's line and how that line automatically becomes guarded as soon as it is connected with1 so that no other operator will connect with it1 we will discuss how the operator tests the called line and subsequently connects with that line1 if it is found proper to do so4 ,if1 on making the test with one of the multiple jacks of the line leading to ,station ,b1 that line is idle and free to be connected with1 its test rings will all be at zero potential because of the fact that they are connected with ground through the cut-off relay winding with no source of current connected with them4 ,the tip of the calling plug will also be at zero potential in making this test1 because it is connected to ground through the tip side of the calling-plug circuit and one winding of the cord-circuit repeating coil4 ,as a result no flow of current will occur1 the operator will receive no click1 and she will know that she is free to connect with the line4 ,as soon as she does so1 by inserting the plug1 the third strand of the cord will be connected with the test thimble of the calling line and the resulting flow of current will bring about three results1 two of which are the same1 and one of which is slightly different from those described as resulting from the insertion of the answering plug into the jack of the calling line4 ,first1 the cut-off relay will be operated and cut off the line signaling apparatus from the called line2 second1 a flow of current will result through the calling supervisory lamp #e1 which in this case will be sufficient to illuminate that lamp for the reason that the called subscriber has not yet responded1 the calling supervisory relay #f has1 therefore1 not yet been energized1 and the lamp has not1 therefore1 been shunted by its associated resistance #g2 third1 the test thimbles of the called line will be raised to a potential above that of the earth1 and thus the line will be guarded against connection at another section of the switchboard4 ,as soon as the called subscriber responds to the ringing current sent out by the operator1 current will flow over the cord circuit and over his line through his transmitter4 ,this will cause the calling supervisory relay to be energized and the calling lamp to be extinguished4 ,both lamps #a and #e remain extinguished as long as the connected subscribers are in conversation1 but as soon as either one of them hangs up his receiver the corresponding lamp will be lighted1 due to the de-energization of the supervisory relay and the breaking of the shunt around the lamp4 ,the lighting of both lamps associated with a cord circuit is a signal to the operator for disconnection4 ,illustration3 ,,terminal ,,room ,,in ,,medium-,,sized ,,manual ,,office ,relay ,rack at ,right4 ,this ,employs the ,kellogg ,parallel ,arrangement of ,frames4 ,testing--,called ,line ,busy4 ,if we now assume that the called line was already busy1 by virtue of being connected with at another section1 the test rings of that line would accordingly all be raised to a potential above that of the earth4 ,as a result1 when the operator applied the tip of her calling plug to a test thimble on that line1 current would flow from this test thimble through the tip of the calling plug and tip strand of the cord and through one winding of the cord-circuit repeating coil to ground4 ,this would cause a slight raising of potential of the entire tip side of the cord circuit and a consequent momentary flow of current through the secondary of the operator's circuit bridged across the cord circuit at that time4 ,operator's ,circuit ,details4 ,the details of the operator's talking circuit shown in ,fig4 #cdg deserve some attention4 ,the battery supply to the operator's transmitter is through an impedance coil #i4 ,the condenser #ab is bridged around the transmitter and the two primary windings #aj and #aa1 which windings are in parallel so as to afford a local circuit for the passage of fluctuating currents set up by the transmitter4 ,the two primary windings #aj and #aa are on separate induction coils1 the secondary windings #ac and #ad being1 therefore1 on separate cores4 ,the winding #ae1 in circuit with the secondary winding #ad and the receiver1 is a non-inductive winding and is supposed to have a resistance about equal to the effective resistance to fluctuating currents of a subscriber's line of average length4 ,owing to the respective directions of the primary and secondary windings #aj and #aa1 #ac and #ad1 the result is that the outgoing currents set up by the operator's transmitter are largely neutralized in the operator's receiver4 ,incoming currents from either of the connected subscribers1 however1 pass1 in the main1 through the secondary coil #ac and the operator's receiver1 rather than through the shunt path formed by the secondary #ad1 and the non-inductive resistance #ae4 ,this is known as an anti-side tone arrangement1 and its object is to prevent the operator from receiving her own voice transmission so loudly as to make her ear insensitive to the feebler voice currents coming in from the subscribers4 ,order-,wire ,circuits4 ,the two keys #af and #ag1 shown in connection with the operator's talking circuit in ,fig4 #cdg1 play no part in the regular operation of connecting two local lines1 as described above4 ,they are order-wire keys1 and the circuits with which they connect lead to the telephone sets of other operators at distant central offices1 and by pressing either one of these keys the operator is enabled to place herself in communication over these so-called order-wire circuits with such other operators4 ,the function and mode of operation of these order-wire circuits will be described in the next chapter1 wherein inter-office connections will be discussed4 ,wiring of ,line ,circuit4 ,the line circuits shown in ,figs4 #cde and #cdg are1 as stated1 simplified to facilitate understanding1 although the connections shown are those which actually exist4 ,the more complete wiring of a single line circuit is shown in ,fig4 #cdh4 ,the line wires are shown entering at the left4 ,they pass immediately1 upon entering the central office1 through the main distributing frame1 the functions and construction of which will be considered in detail in a subsequent chapter4 ,the dotted portions of the circuit shown in connection with this main distributing frame indicate the path from the terminals on one side of the frame to those on the other through so-called jumper wires4 ,the two limbs of the line then pass to terminals #a and #b on one side of the so-called intermediate distributing frame4 ,here the circuit of each limb of the line divides1 passing1 on the one hand1 to the tip and sleeve springs of all the multiple jacks belonging to that line2 and1 on the other hand1 through the jumper wires indicated by dotted lines on the intermediate distributing frame1 and thence to the tip and ring contacts of the answering jack4 ,a consideration of this connection will show that the actual electrical connections so far as already described are exactly those of ,figs4 #cde and #cdg1 although those figures omitted the main and intermediate distributing frames4 ,only two limbs of the line are involved in the main frame4 ,in the intermediate frame the test wire running through the multiple is also involved4 ,this test wire1 it will be seen1 leads from the test thimbles of all the multiple jacks to the terminal #c on the intermediate frame1 thence through the jumper wire to the terminal #f of this frame1 and to the test thimble of the answering jack4 ,here again the electrical connections are exactly those represented in ,figs4 #cde and #cdg1 although those figures do not show the intermediate frame4 ,the two terminals #d and #e of the intermediate frame1 besides being connected to the tip and sleeve springs of the answering jack1 are connected to the contacts of the cut-off relay1 and thence through the coils of the line relay to ground on one side and to battery on the other4 ,thus the line relay and battery are normally included in the circuit of the line4 ,the contact #f on the intermediate distributing frame1 besides being connected to the test thimble of all the jacks1 is connected through the coil of the cut-off relay to ground1 thus establishing a path by which current is supplied to the cut-off relay when connection is made to the line at any jack4 ,there is another contact #g on the intermediate distributing frame which merely forms a terminal for joining one side of the line lamp to the back contact of the line relay4 ,functions of ,distributing ,frames4 ,since the line circuit thus far described in connection with ,fig4 #cdh is exactly the same as that of ,fig4 #cde in its electrical connections1 it becomes obvious that the main and intermediate distributing frames play no part in the operation of the circuit any more than a binding post of a telephone plays a part in its operation4 ,these frames carry terminals for facilitating the connection of the various wires in the line circuit and1 as will be shown later1 for facilitating certain changes in the line connection4 ,illustration3 ,fig4 #cdh4 ,line ,circuit ,no4 #a ,board ,remembering that the dotted lines in ,fig4 #cdh indicate jumper wires of the main and intermediate distributing frames1 and that these are in the nature of temporary or readily changeable connections1 and that the full lines1 whether heavy or light1 are permanent connections not readily changeable1 it will be seen that the wires leading through the multiple jacks of a certain line are permanently associated with each other1 and with certain terminals on the main distributing frame and certain other terminals on the intermediate distributing frame4 ,it will also be seen that the line lamp and the answering jack1 together with the cut-off relay and line relay1 are permanently associated with each other and with another group of terminals #d1 #e1 #f1 and #g on the intermediate distributing frame4 ,it will also be apparent that by changing the jumper wires on the main frame1 any outside line may be connected with any different set of line switchboard equipment1 and also that by making changes in the jumper wires on the intermediate frame1 any given answering jack and line lamp with its associated line cut-off relay may be associated with any set of multiple jacks4 ,pilot ,signals4 ,in a portion of the circuit leading from the battery that is common to a group of line lamps is the winding of the pilot relay1 which is common to this group of line lamps4 ,this controls1 as already described1 the circuit of the pilot lamp common to the same group of line lamps4 ,in addition1 a night-bell circuit is sometimes provided1 this usually being in the form of an ordinary polarized ringer1 the circuit of which is controlled by a night-bell relay common to the entire office4 ,normally1 this relay is shunted out of the circuit of the common portion of the lead to the pilot relay contacts by the key #h1 but when the key #h is opened all current that is fed to the pilot lamps passes through the night-bell relay1 and thus1 whenever any pilot lamp is lighted1 the night-bell relay will attract its armature and thus close the circuit of the calling generator through the night bell4 ,a study of this figure will make clear to the student how the portions of the circuit that are individual to the line are associated with such things as the battery1 that are common to the entire office1 and such as the pilot relay and lamp1 that are common to a group of lines terminating in one position4 ,modified ,relay ,windings4 ,in some cases1 the line relay instead of being double wound1 as shown1 is made with a single winding1 this winding being normally included between the ring side of the cut-off relay and the battery1 the tip side of the cut-off relay being run direct to ground4 ,the present practice of the ,western ,electric ,company is towards the double-wound relay1 however1 and that is considered standard in all of their large ,no4 #a multiple boards1 except where the customer1 owing to special reasons1 demands a single wound relay on the ring side of the line4 ,the prime reason for the two-winding line relay is the lessened click in the calling-subscriber's receiver which occurs when the operator answers4 ,all line relays prior to #aijb were single-wound1 but after that they were made double and used some turns of resistance wire to limit the normal calling current4 ,relay ,mounting4 ,in the standard ,no4 #a relay board of the ,western ,electric ,company and1 in fact1 in nearly all common-battery multiple boards that are manufactured by other companies1 the line and cut-off relays are mounted on separate racks outside the switchboard room and adjacent to the main and intermediate distributing frames1 the wiring being extended from the relays to the jacks and lamps on the switchboard proper by means of suitable cables4 ,the ,western ,electric ,company has recently instituted a departure from this practice in the case of some of their smaller ,no4 #a switchboard installations4 ,where it is thought that the ultimate capacity required by the board will not be above #c1jjj lines1 the relay rack is dispensed with and all of the line and cut-off relays1 as well as the supervisory relays1 are mounted in the rear of the switchboard frame4 ,for this purpose the line and cut-off relays are specially made with the view to securing the utmost compactness4 ,in still other cases1 in switchboards of relatively small ultimate capacity1 they use this small line and cut-off relay mounted on a separate relay rack1 in which case the board is the standard ,no4 #a board except for the type of relays4 ,in all of these modifications of the ,no4 #a board adapted for the use of the smaller and cheaper relays1 the line relay has but a single winding1 the small size of the relay winding not lending itself readily to double winding with the added necessary coil terminals4 ,capacity ,range4 ,the ,no4 #a ,western ,electric board is made in standard sizes up to an ultimate capacity of #i1fjj lines4 ,for all capacities above #d1ijj lines1 a #c_/h-;inch jack1 vertical and horizontal face dimensions1 is employed4 ,for this capacity the smaller types of cut-off and line relays are not employed4 ,up to ultimate capacities of #d1ijj lines1 #a_/b-;inch jacks are employed1 and either the small or the large relays mounted on a separate rack are available4 ,up to #c1jjj lines ultimate capacity1 the #a_/b-;inch jack is employed1 and either the small or the large cut-off and line relays are available1 but in case the small type is used the purchaser has the option of mounting them on a separate relay rack1 as in ordinary practice1 or mounting them in the switchboard cabinet and dispensing with the relay rack4 ,western ,electric ,no4 #aj ,board4 ,the ,no4 #a common-battery multiple switchboard1 regardless of its size and type of arrangement of line and cut-off relays1 involves two relays for each line1 the line relay energized by the taking of the receiver off its hook1 and the cut-off relay energized by the act of the operator on plugging in and serving to remove the line relay from the circuit whenever and as long as a plug is inserted into any jack of the line4 ,this seems to involve a considerable expense in relays1 but this1 as has been stated1 is warranted by the greater simplicity in jacks which the use of the cut-off relay makes possible4 ,in addition to this expense of investment in the line and cut-off relays1 the amount of current required to hold up the cut-off relays during conversations foots up to a considerable item of expense1 particularly as the system of supervisory signals is one in which the supervisory lamp takes current not only while burning1 but its circuit takes even more current when the lamp is extinguished during the time of a connection4 ,for all of these reasons1 and some other minor ones1 it was deemed expedient by the engineers of the ,western ,electric ,company to design a common-battery multiple switchboard for small and medium-sized exchanges in which certain sacrifices might be made to the end of accomplishing certain savings4 ,the result has been a type of switchboard1 designated the ,no4 #aj1 which may be found in a number of ,bell exchanges1 it being considered particularly adaptable to installations of from #ejj to #c1jjj lines4 ,although this board has been subject to a good deal of adverse criticism1 and although it seems probable that even for the cheaper boards the ,no4 #a type with some of the modifications just described will eventually supersede this ,no4 #aj board1 yet the present extent of use of the ,no4 #aj board and the instructive features which its type displays warrant its discussion here4 ,circuits4 ,the circuits of this switchboard are shown in ,fig4 #cdi1 this indicating two-line circuits and a connecting cord circuit1 together with the auxiliary apparatus employed in connection with the operator's telephone circuit1 the pilot and night alarm circuits4 ,the most noticeable feature is that cut-off jacks are employed1 the circuit of the line normally extending through the sets of jack springs in the multiple1 and answering jacks to the line relay and battery on one side of the line1 and to ground on the other side4 ,obviously1 the additional complexity of the jack saves the use of a cut-off relay and the relay equipment of each line consists1 therefore1 of but a single line relay1 which controls the lamp in an obvious manner4 ,illustration3 ,fig4 #cdi4 ,western ,electric ,no4 #aj ,board ,the cord circuit is of the three-conductor type1 the two talking strands extending to the usual split repeating-coil arrangement1 and battery current for talking purposes being fed through these windings as in the standard ,no4 #a board4 ,the supervisory relay is included in the ring strand of the cord circuit and is shunted by a non-inductive resistance1 so that its impedance will not interfere with the talking currents4 ,the armature of the supervisory relay closes the lamp contact on its back stroke1 so that the lamp is always held extinguished when the relay is energized4 ,the supervisory lamp is included in a connection between the back contact of the supervisory relay and ground1 this connection including the central-office battery4 ,as a result1 the illumination of the supervisory lamp is impossible until a plug has been inserted into a jack1 in which case1 assuming the supervisory relay to be de-energized1 the lamp circuit is completed through the wire connecting all of the test thimbles and the resistance permanently bridged to ground from that wire4 ,test4 ,for purposes of the test it is evident that the test rings of an idle line are always at ground potential1 due to their connection to ground through the resistance coil4 ,it is also evident that the tip of an unused calling plug will always be at ground potential and1 therefore1 that the testing of an idle line will result in no click in the operator's receiver4 ,when a line is switched1 however1 the potential of all the test rings will be raised due to their being connected with the live pole of the battery through the third strand of the cord4 ,when the operator in testing touches the test contact of the jack of a busy line1 a current will1 therefore1 flow from this test contact to the tip strand of the cord and thence to ground through one of the repeating coil windings4 ,the potential of the tip side of the cord will1 therefore1 be momentarily altered1 and this will result in a click in the operator's receiver bridged across the cord circuit at the time4 ,the details of the operator's cord circuit and of the pilot lamp and night alarm circuits will be clear from the diagram4 ,operation4 ,a brief summary of the operation of this system is as follows3 ,the subscriber removes his receiver from its hook1 thus drawing up the armature of the line relay and lighting his line lamp4 ,the operator answers4 ,the line lamp is extinguished by the falling back of the line-relay armature1 due to the breaking of the relay circuit at the jack contacts4 ,the subscriber then receives current for his transmitter through the cord-circuit battery connections4 ,the supervisory relay connected with the answering cord is not lighted1 because1 although the lamp-circuit connection is completed at the jack1 the supervisory relay is operated to hold the lamp circuit open4 ,conversation ensues between the operator and the subscriber1 after which the operator tests the line called for with the tip of the calling plug of the pair used in answering4 ,if the called line is not busy1 no click will ensue1 because both the tested ring and the calling plug are at the same potential4 ,finding no click1 the operator will insert the plug and ring by means of the ringing key4 ,when the operator plugs in1 the supervisory lamp1 associated with the calling plug1 becomes lighted because the circuit is completed at the jack and the supervisory relay remains de-energized1 since the line circuit is open at the subscriber's station4 ,when the called subscriber responds1 the calling supervisory lamp goes out because of the energization of the supervisory relay4 ,both lamps remain out during the conversation1 but when either subscriber hangs up1 the corresponding supervisory lamp will be lighted because of the falling back of the supervisory relay armature4 ,if the called line is busy1 a click will be heard1 for the reason described1 and the operator will so inform the calling subscriber4 ,it goes without saying1 that in any multiple-switchboard system a plug may be found in the actual multiple jack that is reached for1 in which case1 although no test will be made1 the busy condition will be reported back to the calling subscriber4 ,economy4 ,it has been the belief of the ,western ,electric engineers that a real economy is accomplished in this type of board by the saving in relay equipment4 ,it is1 of course1 apparent at a glance that with a switchboard long enough and of sections enough1 the cost of extra jack springs and their platinum contacts must become great enough to offset the saving accomplished by omitting the cut-off relay4 ,this makes it apparent that if there is any economy in this type of multiple switchboard1 it must be found in the very small boards where there are but few jacks per line and where the extra cost of the cut-off jack is not enough to offset the extra cost of an added relay4 ,it is the growing belief1 however1 among engineers1 that the multiple switchboard must be very small indeed in order that the added complexity of the cut-off jacks and wiring may be able to save anything over the two-relay type of line2 and it is believed that where economy is necessary in small boards1 it may be best effected by employing cheaper and more compact forms of relays and mounting them1 if necessary1 directly in the switchboard cabinet4 ,,note4 ,these two standard types of common-battery multiple switchboards of the ,western ,electric ,company represent the development through long years of careful work on the part of the ,western ,electric and ,bell engineers1 credit being particularly due to ,scribner1 ,mc,berty1 and ,mc,quarrie of the ,western ,electric ,company1 and ,hayes of the ,american ,telephone and ,telegraph ,company4 ,kellogg ,two-,wire ,multiple ,board4 ,the simplicity in the jacks permitted by the use of the cut-off relay in the ,western ,electric common-battery multiple switchboard for larger exchanges was carried a step further by ,dunbar and ,miller in the development of the so-called two-wire common-battery multiple switchboard1 which for many years has been the standard of the ,kellogg ,switchboard and ,supply ,company4 ,the particular condition which led to the development of the two-wire system was the demand at that time on the ,kellogg ,company for certain very large multiple switchboards1 involving as many as #ah1jjj lines in the multiple4 ,obviously1 this necessitated a small jack1 and obviously a jack having only two contacts1 a tip spring and a sleeve1 could be made more easily and with greater durability of this very small size than a jack requiring three or more contacts4 ,other reasons that were considered were1 of course1 cheapness in cost of construction and extreme simplicity1 which1 other things being equal1 lends itself to low cost of maintenance4 ,line ,circuit4 ,like the standard ,western ,electric board for large offices1 the ,kellogg two-wire board employs two relays for each line1 the line relay under the control of the subscriber and in turn controlling the lamp1 and a cut-off relay under the control of the operator and in turn controlling the connection of the line relay with the line4 ,the line circuit as originally developed and as widely used by the ,kellogg ,company is shown in ,fig4 #cej4 ,the extreme simplicity of the jacks is apparent1 as is also the fact that but two wires lead through the multiple4 ,another distinguishing feature is1 that all of the multiple and answering jacks are normally cut off from the line at the cut-off relay1 but when the cut-off relay operates it serves1 in addition to cutting off the line relay1 to attach the two limbs of the line to the two wires leading through the multiple and answering jacks4 ,the control of the line relay by the subscriber's switch hook is clear from the figure4 ,the control of the cut-off relay is secured by attaching one terminal of the cut-off relay winding permanently to that wire leading through the multiple which connects with the sleeve contacts of the jack1 the other terminal of the cut-off relay being grounded4 ,the way in which this relay is operated will be understood when it is stated that the sleeve contacts of both the answering and calling plugs always carry full battery potential and1 therefore1 whenever any plug is inserted into any jack1 current flows from the sleeve of the jack through the sleeve contact of the jack to ground1 through the winding of the cut-off relay1 which relay becomes energized and performs the functions just stated4 ,it is seen that the wire running through the multiple to which the sleeve jack contacts are attached1 is thus made to serve the double purpose of answering as one side of the talking circuit1 and also of performing the functions carried out by the separate or third wire in the three-wire system4 ,it will be shown also that1 in addition1 this wire is made to lend itself to the purposes of the busy test without any of these functions interfering with each other in any way4 ,illustration3 ,fig4 #cej4 ,two-,wire ,line ,circuit ,cord ,circuit4 ,the cord circuit in somewhat simplified form is shown in ,fig4 #cea4 ,here again there are but two conductors to the plugs and two strands to the cords4 ,this greater simplicity is in some measure offset by the fact that four relays are required1 two for each plug4 ,this so-called four-relay cord circuit may be most readily understood by considering half of it at a time1 since the two relays associated with the answering plug act in exactly the same way as those connected with the calling plug4 ,illustration3 ,fig4 #cea4 ,two-,wire ,cord ,circuit ,associated with each plug of a pair are two relays #a and #b1 in the case of the answering cord1 and #c and #d in the case of the calling cord4 ,the coils of the relays #a and #b are connected in series and bridged across the answering cord1 a battery being included between the coils in this circuit4 ,the coils of the relays #c and #d are similarly connected across the calling cord4 ,a peculiar feature of the ,kellogg system is that two batteries are used in connection with the cord circuit1 one of them being common to all answering cords and the other to all calling cords4 ,the operation of the system would1 however1 be exactly the same if a single battery were substituted for the two4 ,supervisory ,signals4 ,considering the relays associated with the answering cord1 it is obvious that these two relays #a and #b together control the circuit of the supervisory lamp #e1 the circuit of this lamp being closed only when the relay #a is de-energized and the relay #b is energized4 ,we will find in discussing the operation of these that the relay #b is wholly under the control of the operator1 and that the relay #a1 after its plug has been connected with a line1 is wholly under the control of the subscriber on that line4 ,it is through the windings of these two relays that current is fed to the line of the subscriber connected with the corresponding cord4 ,when a plug--the answering plug1 for instance--is inserted into a jack1 current at once flows from the positive pole of the left-hand battery through the winding of the relay #b to the sleeve of the plug1 thence to the sleeve of the jack and through the cut-off relay to ground4 ,this at once energizes the supervisory relay #b and the cut-off relay associated with the line4 ,the cut-off relay acts1 as stated1 to continue the tip and sleeve wires associated with the jacks to the line leading to the subscriber1 and also to cut off the line relay4 ,the supervisory relay #b acts at the same time to attract its armature and thus complete its part in closing the circuit of the supervisory lamp4 ,whether or not the lamp will be lighted at this time depends on whether the relay #a is energized or not1 and this1 it will be seen1 depends on whether the subscriber's receiver is off or on its hook4 ,if off its hook1 current will flow through the metallic circuit of the line for energizing the subscriber's transmitter1 and as whatever current goes to the subscriber's line must flow through the relay #a1 that relay will be energized and prevent the lighting of the supervisory lamp #e4 ,if1 on the other hand1 the subscriber's receiver is on its hook1 no current will flow through the line1 the supervisory relay will not be energized1 and the lamp #e will be lighted4 ,in a nutshell1 the sleeve supervisory relay normally prevents the lighting of the corresponding supervisory lamp1 but as soon as the operator inserts a plug into the jack of the line1 the relay #b establishes such a condition as to make possible the lighting of the supervisory lamp1 and the lighting of this lamp is then controlled entirely by the relay #a1 which is1 in turn1 controlled by the position of the subscriber's switch hook4 ,battery ,feed4 ,a #b-microfarad condenser is included in each strand of the cord1 and battery is fed through the relay windings to the calling and called subscribers on opposite sides of these condensers1 in accordance with the combined impedance coil and condenser method described in ,chapter ,,xiii4 ,here the relay windings do double duty1 serving as magnets for operating the relays and as retardation coils in the system of battery supply4 ,complete ,cord and ,line ,circuits4 ,the complete cord and line circuits of the ,kellogg two-wire system are shown in ,fig4 #ceb4 ,in the more recent installations of the ,kellogg ,company the cord and line circuits have been slightly changed from those shown in ,figs4 #cej and #cea1 and these changes have been incorporated in ,fig4 #ceb4 ,the principles of operation described in connection with the simplified figures remain1 however1 exactly the same4 ,one of the changes is1 that the tip side of the lines is permanently connected to the tips of the jacks instead of being normally cut off by the cut-off relay1 as was done in the system as originally developed4 ,another change is1 that the line relay is associated with the tip side of the line1 rather than with the sleeve side1 as was formerly done4 ,the cord circuit shown in ,fig4 #ceb shows exactly the same arrangement of supervisory relays and exactly the same method of battery feed as in the simplified cord circuit of ,fig4 #cea1 but in addition to this the detailed connections of the operator's talking set and of her order-wire keys are indicated1 and also the ringing equipment is indicated as being adapted for four-party harmonic work4 ,illustration3 ,fig4 #ceb4 ,kellogg ,two-,wire ,board ,in connection with this ringing key it may be stated that the springs #g1 #h1 #i1 and #aj are individually operated by the pressure of one of the ringing key buttons1 while the spring #ag1 connected with the sleeve side of the calling plug1 is always operated simultaneously with the operation of any one of the other springs4 ,as a result the proper ringing circuit is established1 it being understood that the upper contacts of the springs #g1 #h1 #i1 and #aj lead to the terminals of their respective ringing generators1 the other terminals of which are grounded4 ,the circuit is1 therefore1 from the generator1 through the ringing key1 out through the tip side of the line1 back over the sleeve side of the line1 and to ground through the spring #ag1 resistance #aa1 and the battery1 which is one of the cord-circuit batteries4 ,the object of this coil #aa and the battery connection through it to the ringing-key spring is to prevent the falling back of the cut-off relay when the ringing key is operated4 ,this will be clear when it is remembered that the cut-off relay is energized by battery current fed over the sleeve strand of the cord1 and obviously1 since it is necessary when the ringing key is operated to cut off the supply wire back of the key1 this would de-energize the cut-off relay when the ringing key was depressed1 and the falling back of the cut-off relay contacts would make it impossible to ring because the sleeve side of the line would be cut off4 ,the battery supply through the resistance #aa is1 therefore1 substituted on the sleeve strand of the cord for the battery supply through the normal connection4 ,busy ,test4 ,the busy test depends on all of the test rings being at zero potential on an idle line and at a higher potential on a busy line4 ,obviously1 when the line is not switched1 the test rings are at zero potential on account of a ground through the cut-off relay4 ,when1 however1 a plug is inserted in either the answering or multiple jacks1 the test rings will all be raised in potential due to being connected with the live side of the battery through the sleeve strand of the cord4 ,conditions on the line external to the central office cannot make an idle line test busy because1 owing to the presence of the cut-off relay1 the sleeve contacts of all the jacks are disconnected from the line when it is idle4 ,the test circuit from the tip of the calling plug to ground at the operator's set passes through the tip strand of the cord1 thence through a pair of normally closed extra contacts on the supervisory relay #d1 thence in series through all the ringing key springs #aj1 #i1 #h1 and #g1 thence through an extra pair of springs #ab and #ac on the listening key--closed only when the listening key is operated--and thence to ground through a retardation coil #ad4 ,no battery or other source of potential exists in this circuit between ground and the tip of the calling plug and1 therefore1 the tip is normally at ground potential4 ,the sleeve ring of the jack being at ground potential if the line is idle1 no current will flow and no click will be produced in testing such a line4 ,if1 however1 the line is busy1 the test ring will be at a higher potential and1 therefore1 current will flow from the tip of the calling plug to ground over the path just traced1 and this will cause a rise in potential at the terminal of the condenser #ae and a momentary flow of current through the tertiary winding #af of the operator's induction coil2 hence the click4 ,illustration3 ,,switch ,,room ,,of ,,citizens' ,,telephone ,,company1 ,,grand ,,rapids1 ,,mich4 ,one of the ,earliest ,large ,automatic ,offices4 ,obviously the testing circuit from the tip of the calling plug to ground at the operator's set is only useful during the time when the calling plug is not in a jack1 and as the tip strand of the calling plug has to do double duty in testing and in serving as a part of the talking circuit1 the arrangement is made that the testing circuit will be automatically broken and the talking circuit through the tip strand automatically completed when the plug is inserted into a jack in establishing a connection4 ,this is accomplished by means of the extra contact on the relay #d1 which relay1 it will be remembered1 is held energized when its corresponding plug is inserted in a jack4 ,during the time when the plug is not inserted1 this relay is not energized and the test circuit is completed through the back contact of its right-hand armature4 ,when connection is made at the jack1 this relay becomes energized and the tip strand of the cord circuit is made complete by the right-hand lever being pulled against the front contact of this relay4 ,the keys shown to the right of the operator's set are order-wire keys4 ,summary of ,operation4 ,we may give a brief summary of the operation of this system as shown in ,fig4 #ceb4 ,the left-hand station calls and the line relay pulls up1 lighting the lamp4 ,the operator inserts an answering plug in the answering jack1 thus energizing the cut-off relay which operates to cut off the line relay and to complete the connection between the jacks and the external line4 ,the act of plugging in by the operator also raises the potential of all the test rings so as to guard the line against intrusion by other callers4 ,the supervisory lamp #e remains unlighted because1 although the relay #b is operated1 the relay #a is also operated1 due to the calling subscriber's receiver being off its hook4 ,the operator throws her listening key1 communicates with the subscriber1 and1 learning that the right-hand station is wanted1 proceeds to test that line4 ,if the line is idle1 she will get no click1 because the tip of her calling plug and the tested ring will be at the same ground potential4 ,she then plugs in and presses the proper ringing-key button to send out the proper frequency to ring the particular subscriber on the line--if there be more than one--the current from the battery through the coil #aa and spring #ag serving during this operation to hold up the cut-off relay4 ,as soon as the operator plugs in with the calling plug1 the supervisory lamp #f lights1 assuming that the called subscriber had not already removed his receiver from its hook1 due to the fact that the relay #d is energized and the relay #c is not4 ,as soon as the called subscriber responds1 the relay #c becomes energized and the supervisory lamp goes out4 ,if the line called for had been busy by virtue of being plugged at another section1 the tip of the operator's plug in testing would have found the test ring raised to a potential above the ground1 and1 as a consequence1 current would have flowed from the tip of this plug through the back contact of the right-hand lever of relay #d1 thence through the ringing key springs and the auxiliary listening-key springs to ground through the retardation coil #ad4 ,this would have produced a click by causing a momentary flow of current through the tertiary winding #af of the operator's set4 ,wiring of ,line ,circuit4 ,the more complete wiring diagram of a single subscriber's line1 ,fig4 #cec1 shows the placing in the circuits of the terminals and jumper wires of the main distributing frame and of the intermediate distributing frame1 and also shows how the pilot lamps and night-alarm circuits are associated with a group of lines4 ,the main distributing frame occupies the same relative position in this line circuit as in the ,western ,electric1 being located in the main line circuit outside of all the switchboard apparatus4 ,the intermediate distributing frame occupies a different relative position from that in the ,western ,electric line4 ,it will be recalled by reference to ,fig4 #cdh that the line lamp and the answering jack were permanently associated with the line and cut-off relays1 such mutations of arrangement as were possible at the intermediate distributing frame serving only to vary the connection between the multiple of a line and one of the various groups of apparatus consisting of an answering jack and line lamp and associated relays4 ,in the ,kellogg arrangement1 ,fig4 #cec1 the line and cut-off relays1 instead of being permanently associated with the answering jack and line lamp1 are permanently associated with the multiple jacks1 no changes1 of which the intermediate or main frames are capable1 being able to alter the relation between a group of multiple jacks and its associated line and cut-off relays4 ,in this ,kellogg arrangement the intermediate distributing frame may only alter the connection of an answering jack and line lamp with the multiple and its permanently associated relays4 ,the pilot and night alarm arrangements of ,fig4 #cec should be obvious from the description already given of other similar systems4 ,illustration3 ,fig4 #cec4 ,kellogg ,two-,wire ,line ,circuit ,dean ,multiple ,board4 ,in ,fig4 #ced are shown the circuits of the multiple switchboard of the ,dean ,electric ,company4 ,the subscriber's station equipment shown at ,station ,a and ,station ,b will be recognized as the ,wheatstone-bridge circuit of the ,dean ,company4 ,line ,circuit4 ,the line circuit is easily understood in view of what has been said concerning the ,western ,electric line circuit1 the line relay #a being single wound and between the live side of the battery and the ring side of the line4 ,the cut-off relay #b is operated whenever a plug is inserted in a jack and serves to sever the connection of the line with the normal line signaling apparatus4 ,cord ,circuit4 ,the cord circuit is of the four-relay type1 but employs three conductors instead of two1 as in the two-wire system4 ,the relay #c1 being in series between the battery and the sleeve contact on the plug1 is energized whenever a plug is inserted in the jack1 its winding being placed in series with the cut-off relay of the line with which the plug is connected4 ,this completes the circuit through the associated supervisory lamp unless the relay #d is energized1 the local lamp circuit being controlled by the back contact of relay #d and the front contact of relay #c4 ,it is through the two windings of the relay #d that current is fed to the subscriber's station1 and1 therefore1 the armature of this relay is responsive to the movements of the subscriber's hook4 ,as the relay #c holds the supervisory lamp circuit closed as long as a plug is inserted in a jack of the line1 it follows that during a connection the relay #d will have entire control of the supervisory lamp4 ,listening ,key4 ,the listening key1 as usual1 serves to connect the operator's set across the talking strands of the cord circuit1 and the action of this in connection with the operator's set needs no further explanation4 ,ringing ,keys4 ,the ringing-key arrangement illustrated is adapted for use with harmonic ringing1 the single springs #e1 #f1 #g1 and #h each being controlled by a separate button and serving to select the particular frequency that is to be sent to line4 ,the two springs #i and #aj always act to open the cord circuit back of the ringing keys1 whenever any one of the selective buttons is depressed1 in order to prevent interference by ringing current with the other operations of the circuit4 ,two views of these ringing keys are shown in ,figs4 #cee and #cef4 ,fig4 #cef is an end view of the entire set4 ,in ,fig4 #cee the listening key is shown at the extreme right and the four selective buttons at the left4 ,when a button is released it rises far enough to cause the disengagement of the contacts1 but remains partially depressed to serve as an indication that it was last used4 ,the group of springs at the extreme left of ,fig4 #cee are the ones represented at #i and #aj in ,fig4 #ced and by the anvils with which those springs co-operate4 ,illustration3 ,fig4 #ced4 ,dean ,multiple ,board ,circuits ,test4 ,the test in this ,dean system is simple1 and1 like the ,western ,electric and ,kellogg systems1 it depends on the raising of the potential of the test thimbles of all the line jacks of a line when a connection is made with that line by a plug at any position4 ,when an operator makes a test by applying the tip of the calling plug to the test thimble of a busy line1 current passes from the test thimble through the tip strand of the cord to ground through the left-hand winding of the calling supervisory relay #d4 ,the drop of potential through this winding causes the tip strand of the cord to be raised to a higher potential than it was before1 and as a result the upper plate of the condenser #aa is thus altered in potential and this change in potential across the condenser results in a click in the operator's ear4 ,illustration3 ,fig4 #cee4 ,dean ,party ,line ,ringing ,key ,illustration3 ,fig4 #cef4 ,dean ,party ,line ,ringing ,key ,stromberg-,carlson ,multiple ,board4 ,line ,circuit4 ,in ,fig4 #ceg is shown the multiple common-battery switchboard circuits employed by the ,stromberg-,carlson ,telephone ,manufacturing ,company4 ,the subscriber's line circuits shown in this drawing are of the three-wire type and1 with the exception of the subscriber's station1 are the same as already described for the ,western ,electric ,company's system4 ,cord ,circuit4 ,the cord circuit employed is of the two-conductor type1 the plugs being so constructed as to connect the ring and thimble contacts of the jack when inserted4 ,this cord circuit is somewhat similar to that employed by the ,kellogg ,switchboard and ,supply ,company1 shown in ,fig4 #ceb1 except that only one battery is employed1 and that certain functions of this circuit are performed mechanically by the inter-action of the armatures of the relays4 ,supervisory ,signals4 ,when the answering plug is inserted in a jack1 in response to a call1 the current passing to the subscriber's station and also through the cut-off relay must flow through the relay #a1 thus energizing it4 ,as the calling subscriber's receiver is at this time removed from the hook switch1 the path for current will be completed through the tip of the jack1 thence through the tip of the plug1 through relay #b to ground1 causing relay #b to be operated and to break the circuit of the answering supervisory lamp4 ,the two relays #a and #b are so associated mechanically that the armature of #a controls the armature of #b in such a manner as to normally hold the circuit of the answering supervisory lamp open4 ,but1 however1 when the plug is inserted in a jack1 relay #a is operated and allows the operation of relay #b to be controlled by the hook switch at the subscriber's station4 ,the supervisory relay #c associated with the calling cord is operated when the calling plug is placed in a jack1 and this relay normally holds the armature of relay #d in an operated position in a similar manner as the armature of relay #a controlled that of relay #b4 ,supervisory relay #d is under the control of the hook switch at the called subscriber's station4 ,test4 ,in this circuit1 as in several previously described1 when a plug is inserted in a jack of a line1 the thimble contacts of the jacks associated with that line are raised to a higher potential than that which they normally have4 ,the operator in testing a busy line1 of course having previously moved the listening key to the listening position1 closes a path from the test thimble of the jack1 through the tip of the calling plug1 through the contacts of the relay #d1 the inside springs of the listening key1 thence through a winding of the induction coil associated with her set to ground4 ,the circuit thus established allows current to flow from the test thimble of the jack through the winding of her induction coil to ground1 causing a click in her telephone receiver4 ,the arrangement of the ringing circuit does not differ materially from that already described for other systems and1 therefore1 needs no further explanation4 ,illustration3 ,fig4 #ceg4 ,stromberg-,carlson ,multiple ,board ,circuits ,multiple ,switchboard ,apparatus4 ,coming now to a discussion of the details of apparatus employed in multiple switchboards1 it may be stated that much of the apparatus used in the simpler types is capable of doing duty in multiple switchboards1 although1 of course1 modification in detail is often necessary to make the apparatus fit the particular demands of the system in which it is to be used4 ,jacks4 ,probably the most important piece of apparatus in the multiple switchboard is the jack1 its importance being increased by the fact that such very large numbers of them are sometimes necessary4 ,switchboards having hundreds of thousands of jacks are not uncommon4 ,the multiple jacks are nearly always mounted in strips of twenty and the answering jacks usually in strips of ten1 the length of the jack strip being the same in each case in the same board and1 therefore1 giving twice as wide a spacing in the answering as in the multiple jacks4 ,the distance between centers in the multiple jacks varies from a quarter of an inch--which is perhaps the extreme minimum--to half an inch1 beyond which larger limit there seems to be no need of going in any case4 ,it is customary that the jack strip shall be made of the same total thickness as the distance between the centers of two of its jacks1 and from this it follows that the strips when piled one upon the other give the same vertical distance between jack centers as the horizontal distance4 ,in ,fig4 #ceh is shown a strip of multiple and a strip of answering jacks of ,western ,electric make1 this being the type employed in the ,no4 #a standard switchboards for large exchanges4 ,in ,fig4 #cei are shown the multiple and answering jacks employed in the ,no4 #aj ,western ,electric switchboard4 ,the multiple jacks in the ,no4 #a switchboard are mounted on #c_/h-;inch centers1 the jacks having three branch terminal contacts4 ,the multiple jacks of the ,no4 #aj switchboard indicated in ,fig4 #cei are mounted on #a_/b-;inch centers1 each jack having five contacts as indicated by the requirement of the circuits in ,fig4 #cdi4 ,in ,fig4 #cfj are shown the answering and multiple jacks of the ,kellogg ,switchboard and ,supply ,company's two-wire system4 ,the extreme simplicity of these is particularly well shown in the cut of the answering jack1 and these figures also show clearly the customary method of numbering jacks4 ,in very large multiple boards it has been the practice of the ,kellogg ,company to space the multiple jacks on #c_/aj-;inch centers1 and in their smaller multiple work1 they employ the #a_/b-;inch spacing4 ,with the #c_/aj-;inch spacing that company has been able to build boards having a capacity of #ah1jjj lines1 that many jacks being placed within the reach of each operator4 ,in all modern multiple switchboards the test thimble or sleeve contacts are drawn up from sheet brass or ,german silver into tubular form and inserted in properly spaced borings in strips of hard rubber forming the faces of the jacks4 ,these strips sometimes are reinforced by brass strips on their under sides4 ,the springs forming the other terminals of the jack are mounted in milled slots in another strip of hard rubber mounted in the rear of and parallel to the front strip and rigidly attached thereto by a suitable metal framework4 ,in this way desired rigidity and high insulation between the various parts is secured4 ,illustration3 ,fig4 #ceh4 ,answering and ,multiple ,jacks for ,no4 #a ,board ,lamp ,jacks4 ,the lamp jacks employed in multiple work need no further description in view of what has been said in connection with lamp jacks for simple common-battery boards4 ,the lamp jack spacing is always the same as the answering jack spacing1 so that the lamps will come in the same vertical alignment as their corresponding answering jacks when the lamp strips and answering jack strips are mounted in alternate layers4 ,illustration3 ,fig4 #cei4 ,answering and ,multiple ,jacks for ,no4 #aj ,board ,illustration3 ,fig4 #cfj4 ,answering and ,multiple ,jacks for ,kellogg ,two-,wire ,board ,relays4 ,next in order of importance in the matter of individual parts for multiple switchboards is the relay4 ,the necessity for reliability of action in these is apparent1 and this means that they must not only be well constructed1 but that they must be protected from dust and moisture and must have contact points of such a nature as not to corrode even in the presence of considerable sparking and of the most adverse atmospheric conditions4 ,economy of space is also a factor and has led to the almost universal adoption of the single-magnet type of relay for line and cut-off as well as supervisory purposes4 ,illustration3 ,fig4 #cfa4 ,type of ,line ,relay ,illustration3 ,fig4 #cfb4 ,type of ,cut-,off ,relay ,the ,western ,electric ,company employs different types of relays for line1 cut-off1 and supervisory purposes4 ,this is contrary to the practice of most of the other companies who make the same general type of relay serve for all of these purposes4 ,a good idea of the type of ,western ,electric line relay1 as employed in its ,no4 #a board1 may be had from ,fig4 #cfa4 ,as is seen this is of the tilting armature type1 the armature rocking back and forth on a knife-edge contact at its base1 the part on which it rests being of iron and of such form as to practically complete1 with the armature and core1 the magnetic circuit4 ,the cut-off relay1 ,fig4 #cfb1 is of an entirely different type4 ,the armature in this is loosely suspended by means of a flexible spring underneath two ,l-shaped polar extensions1 one extending up from the rear end of the core and the other from the front end4 ,when energized this armature is pulled away from the core by these ,l-shaped pieces and imparts its motion through a hard-rubber pin to the upper pair of springs so as to effect the necessary changes in the circuit4 ,illustration3 ,fig4 #cfc4 ,western ,electric ,combined ,line and ,cut-off ,relay ,illustration3 ,fig4 #cfd4 ,western ,electric ,supervisory ,relay ,illustration3 ,fig4 #cfe4 ,line ,relay ,no4 #aj ,board ,much economy in space and in wiring is secured in the type of switchboards employing cut-off as well as line relays by mounting the two relays together and in making of them1 in fact1 a unitary piece of apparatus4 ,since the line relay is always associated with the cut-off relay of the same line and with no other1 it is obvious that this unitary arrangement effects a great saving in wiring and also secures a great advantage in the matter of convenience of inspection4 ,such a combined cut-off and line relay1 employed in the ,western ,electric ,no4 #a relay board1 is shown in ,fig4 #cfc4 ,these are mounted in banks of ten pairs1 a common dust cap of sheet iron covering the entire group4 ,the ,western ,electric supervisory relay1 ,fig4 #cfd1 is of the tilting armature type and is copper clad4 ,the dust cap in this case fits on with a bayonet joint as clearly indicated4 ,in ,fig4 #cfe is shown the line relay employed in the ,western ,electric ,no4 #aj board4 ,illustration3 ,fig4 #cff4 ,kellogg ,line and ,cut-off ,relays ,illustration3 ,fig4 #cfg4 ,strip of ,kellogg ,line and ,cut-,off ,relays ,the ,kellogg ,company employs the type of relay of which the magnetic circuit was illustrated in ,fig4 #ie4 ,in its multiple boards it commonly mounts the line and cut-off relays together1 as shown in ,fig4 #cff4 ,a single1 soft iron shell is used to cover both of these1 thus serving as a dust shield and also as a magnetic shield to prevent cross-talk between adjacent relays--an important feature1 since it will be remembered the cut-off relays are left permanently connected with the talking circuit4 ,fig4 #cfg1 which shows a strip of twenty such pairs of relays1 from five of which the covers have been removed1 is an excellent detail view of the general practice in this respect2 obviously1 a very large number of such relays may be mounted in a comparatively small space4 ,the mounting strip shown in this cut is of heavy rolled iron and is provided with openings through which the connection terminals--shown more clearly in ,fig4 #cff--project4 ,on the back of this mounting strip all the wiring is done and much of this wiring--that connecting adjacent terminals on the back of the relay strip--is made by means of thin copper wires without insulation1 the wires being so short as to support themselves without danger of crossing with other wires4 ,when these wires are adjacent to ground or battery wires they may be protected by sleeving1 so as to prevent crosses4 ,illustration3 ,fig4 #cfh4 ,monarch ,relay ,an interesting feature in relay construction is found in the relay of the ,monarch ,telephone ,manufacturing ,company shown in ,figs4 #cfh and #cfi4 ,the assembled relay and its mounting strip and cap are shown in ,fig4 #cfh4 ,this relay is so constructed that by the lifting of a single latch not only the armature but the coil may be bodily removed1 as shown in ,fig4 #cfi1 in which the latch is shown in its raised position4 ,as seen1 the armature has an ,l-shaped projection which serves to operate the contact springs lying on the iron plate above the coil4 ,the simplicity of this device is attractive1 and it is of convenience not only from the standpoint of easy repairs but also from the standpoint of factory assembly1 since by manufacturing standard coils with different characters of windings and standard groups of springs1 it is possible to produce without special manufacture almost any combination of relay4 ,illustration3 ,fig4 #cfi4 ,monarch ,relay ,assembly4 ,the arrangement of the key and jack equipment in complete multiple switchboard sections is clearly shown in ,fig4 #cgj1 which shows a single three-position section of one of the small multiple switchboards of the ,kellogg ,switchboard and ,supply ,company4 ,the arrangement of keys and plugs on the key shelf is substantially the same as in simple common-battery boards4 ,as in the simple switchboards the supervisory lamps are usually mounted on the hinged key shelf immediately in the rear of the listening and ringing keys and with such spacing as to lie immediately in front of the plugs to which they correspond4 ,the reason for mounting the supervisory lamps on the key shelf is to make them easy of access in case of the necessity of lamp renewals or repairs on the wiring4 ,the space at the bottom of the vertical panels1 containing the jacks1 is left blank1 as this space is obstructed by the standing plugs in front of it4 ,above the plugs1 however1 are seen the alternate strips of line lamps and answering jacks1 the lamps in each case being directly below the corresponding answering jacks4 ,above the line lamps and answering jacks in the two positions at the right there are blank strips into which additional line lamps and jacks may be placed in case the future needs of the system demand it4 ,the space above these is the multiple jack space1 and it is evident from the small number of multiple jacks in this little switchboard that the present equipment of the board is small4 ,it is also evident from the amount of blank space left for future installations of multiple jacks that a considerable growth is expected4 ,thus1 while there are but four banks of #ajj multiple jacks1 or #djj in all1 there is room in the multiple for #cjj banks of #ajj multiple jacks1 or #c1jjj in all4 ,the method of grouping the jacks in banks of #ajj and of providing for their future growth is clearly indicated in this figure4 ,the next section at the right of the one shown would contain a duplicate set of multiple jacks and also an additional equipment of answering jacks and lamps4 ,illustration3 ,a ,,multiple ,,manual ,,switching ,,board ,,for ,,toll ,,connections ,,in ,,an ,,automatic ,,system ,multiple ,jacks are ,provided for ,each ,line through ,which ,toll ,connections are ,handled ,directly4 ,illustration3 ,fig4 #cgj4 ,small ,multiple ,board ,section ,for ordinary local service no operator would sit at the left-hand position of the section shown1 that being the end position1 since the operator there would not be able easily to reach the extreme right-hand portion of the third position and would have nothing to reach at her left4 ,this end position in this particular board illustrated is provided with toll-line equipment1 a practice not uncommon in small multiple boards4 ,to prevent confusion let us assume that the multiple jack space contains its full equipment of #c1jjj jacks on each section4 ,the operator in the center position of the section shown could easily reach any one of the jacks on that section4 ,the operator at the third position could reach any jack on the second and third position of her section1 but could not well reach multiple jacks in the first position4 ,she would1 however1 have a duplicate of the multiple jacks in this first position in the section at her right1 i4 e41 in the fourth position1 and it makes no difference on what portion of the switchboard she plugs into the multiple so long as she plugs into a jack of the right line4 ,,chapter ,,xxvii ,,trunking ,,in ,,multi-,,office ,,systems ,it has been stated that a single exchange may involve a number of offices1 in which case it is termed a multi-office exchange4 ,in a multi-office exchange1 switchboards are necessary at each office in which the subscribers' lines of the corresponding office district terminate4 ,means for intercommunication between the subscribers in one office and those in any other office are afforded by inter-office trunks extended between each office and each of the other offices4 ,if the character of the community is such that each of the offices has so few lines as to make the simple switchboard suffice for its local connections1 then the trunking between the offices may be carried out in exactly the same way as explained between the various simple switchboards in a transfer system1 the only difference being that the trunks are long enough to reach from one office to another instead of being short and entirely local to a single office4 ,such a condition of affairs would only be found in cases where several small communities were grouped closely enough together to make them operate as a single exchange district1 and that is rather unusual4 ,the subject of inter-office trunking so far as manual switchboards are concerned is1 therefore1 confined mainly to trunking between a number of offices each equipped with a manual multiple switchboard4 ,necessity for ,multi-,office ,exchanges4 ,before taking up the details of the methods and circuits employed in trunking in multi-office systems1 it may be well to discuss briefly why the multi-office exchange is a necessity1 and why it would not be just as well to serve all of the subscribers in a large city from a single huge switchboard in which all of the subscribers' lines would terminate4 ,it cannot be denied1 when other things are equal1 that it is better to have only one operator involved in any connection which means less labor and less liability of error4 ,the reasons1 however1 why this is not feasible in really large exchanges are several4 ,the main one is that of the larger investment required4 ,considering the investment first from the standpoint of the subscriber's line1 it is quite clear that the average length of subscriber's line will be very much greater in a given community if all of the lines are run to a single office1 than will be the case if the exchange district is divided into smaller office districts and the lines run merely from the subscribers to the nearest office4 ,there is a direct and very large gain in this respect1 in the multi-office system over the single office system in large cities1 but this is not a net gain1 since there is an offsetting investment necessary in the trunk lines between the offices1 which of course are separate from the subscribers' lines4 ,approaching the matter from the standpoint of switchboard construction and operation1 another strong reason becomes apparent for the employment of more than one office in large exchange districts4 ,both the difficulties of operation and the expense of construction and maintenance increase very rapidly when switchboards grow beyond a certain rather well-defined limit4 ,obviously1 the limitation of the multiple switchboard as to size involves the number of multiple jacks that it is feasible to place on a section4 ,multiple switchboards have been constructed in this country in which the sections had a capacity of #ah1jjj jacks4 ,schemes have been proposed and put into effect with varying success1 for doubling and quadrupling the capacity of multiple switchboards1 one of these being the so-called divided multiple board devised by the late ,milo ,g4 ,kellogg1 and once used in ,cleveland1 ,ohio1 and ,st4 ,louis1 ,missouri4 ,each of these boards had an ultimate capacity of #bd1jjj lines1 and each has been replaced by a straight multiple board of smaller capacity4 ,in general1 the present practice in ,america does not sanction the building of multiple boards of more than about #aj1jjj lines capacity1 and as an example of this it may be cited that the largest standard section manufactured for the ,bell companies has an ultimate capacity of #i1fjj lines4 ,european engineers have shown a tendency towards the opposite practice1 and an example of the extreme in this case is the multiple switchboard manufactured by the ,ericsson ,company1 and installed in ,stockholm1 in which the jacks have been reduced to such small dimensions as to permit an ultimate capacity of #fj1jjj lines4 ,the reasons governing the decision of ,american engineers in establishing the practice of employing no multiple switchboards of greater capacity than about #aj1jjj lines1 briefly outlined1 are as follows3 ,the building of switchboards with larger capacity1 while perfectly possible1 makes necessary either a very small jack or some added complexity1 such as that of the divided multiple switchboard1 either of which is considered objectionable4 ,extremely small jacks and large multiples introduce difficulties as to the durability of the jacks and the plugs1 and also they tend to slow down the work of operators and to introduce errors4 ,they also introduce the necessity of a smaller gauge of wire through the multiple than it has been found desirable to employ4 ,considered from the standpoint of expense1 it is evident that as a multiple switchboard increases in number of lines1 its size increases in two dimensions1 i4 e41 in length of board and height of section1 and this element of expense1 therefore1 is a function of the square of the number of lines4 ,the matter of insurance1 both with respect to the risk as to property loss and the risk as to breakdown of the service1 also points distinctly in the direction of a plurality of offices rather than one4 ,both from the standpoint of risk against fire and other hazards1 which might damage the physical property1 and of risk against interruption to service due to a breakdown of the switchboard itself1 or a failure of its sources of current1 or an accident to the cable approaches1 the single office practice is like putting all one's eggs in one basket4 ,another factor that has contributed to the adoption of smaller switchboard capacities is the fact that in the very large cities even a #dj1jjj line multiple switchboard would still not remove the necessity of multi-office exchanges with the consequent certainty that a large proportion of the calls would have to be trunked anyway4 ,undoubtedly1 one of the reasons for the difference between ,american and ,european practice is the better results that ,american operating companies have been able to secure in the handling of calls at the incoming end of trunks4 ,this is due1 no doubt1 in part to the differences in social and economic conditions under which exchanges are operated in this country and abroad1 and also in part to the characteristics of the ,english tongue when compared to some of the other tongues in the matter of ease with which numbers may be spoken4 ,in ,america it has been found possible to so perfect the operation of trunking under proper operating conditions and with good equipment as to relieve multi-office practice of many of the disadvantages which have been urged against it4 ,classification4 ,broadly speaking there are two general methods that may be employed in trunking between exchanges4 ,the first and simplest of these methods is to employ so-called two-way trunks4 ,these1 as their name indicates1 may be used for completing connections between offices in either direction1 that is1 whether the call originates at one end or the other4 ,the other way is by the use of one-way trunks1 wherein each trunk carries traffic in one direction only4 ,where such is the case1 one end of the trunk is always used for connecting with the calling subscriber's line and is termed the outgoing end1 and the other end is always used in completing the connection with the called subscriber's line1 and is referred to as the incoming end4 ,traffic in the other direction is handled by another set of trunks differing from the first set only in that their outgoing and incoming ends are reversed4 ,as has already been pointed out1 a system of trunks employing two-way trunks is called a single-track system1 and a system involving two sets of one-way trunks is called a double-track system4 ,it is to be noted that the terms outgoing and incoming1 as applied to the ends of trunks and also as applied to traffic1 always refer to the direction in which the trunk handles traffic or the direction in which the traffic is flowing with respect to the particular office under consideration at the time4 ,thus an incoming trunk at one office is an outgoing trunk at the other4 ,two-,way ,trunks4 ,two-way trunks are nearly always employed where the traffic is very small and they are nearly always operated by having the ,a-operator plug directly into the jack at her end of the trunk and displaying a signal at the other end by ringing over the trunk as she would over an ordinary subscriber's line4 ,the operator at the distant exchange answers as she would on an ordinary line1 by plugging into the jack of that trunk1 and receives her orders over the trunk either from the originating operator or from the subscriber1 and then completes the connection with the called subscriber4 ,such trunks are often referred to as ring-down trunks1 and their equipment consists in a drop and jack at each end4 ,in case there is a multiple board at either or both of the offices1 then the equipment at each end of the trunk would consist of a drop and answering jack1 together with the full quota of multiple jacks4 ,it is readily seen that this mode of operation is slow1 as the work that each operator has to do is the same as that in connecting two local subscribers1 plus the time that it takes for the operators to communicate with each other over the trunk4 ,one-,way ,trunks4 ,where one-way trunks are employed in the double-track system1 the trunks1 assuming that they connect multiple boards1 are provided with multiple jacks only at their outgoing ends1 so that any operator may reach them for an outgoing connection1 and at their incoming ends they terminate each in a single plug and in suitable signals and ringing keys1 the purpose of which will be explained later4 ,over such trunks there is no verbal communication between the operators1 the instructions passing between the operators over separate order-wire circuits4 ,this is done in order that the trunk may be available as much as possible for actual conversation between the subscribers4 ,it may be stated at this point that the duration of the period from the time when a trunk is appropriated by the operators for the making of a certain connection until the time when the trunk is finally released and made available for another connection is called the holding time1 and this holding time includes not only the period while the subscribers are in actual conversation over it1 but also the periods while the operators are making the connection and afterwards while they are taking it down4 ,it may be said1 therefore1 that the purpose of employing separate order wires for communication between the operators is to make the holding time on the trunks as small as possible and1 therefore1 for the purpose of enabling a given trunk to take part in as many connections in a given time as possible4 ,in outline the operation of a one-way trunk between common-battery1 manual1 multiple switchboards is1 with modifications that will be pointed out afterwards1 as follows3 ,when a subscriber's line signal is displayed at one office1 the operator in attendance at that position answers and finding that the call is for a subscriber in another office1 she presses an order-wire key and thereby connects her telephone set directly with that of a ,b-operator at the proper other office4 ,unless she finds that other operators are talking over the order wire1 she merely states the number of the called subscriber1 and the ,b-operator whose telephone set is permanently connected with that order wire merely repeats the number of the called subscriber and follows this by designating the number of the trunk which the ,a-operator is to employ in making the connection4 ,the ,a-operator1 thereupon1 immediately and without testing1 inserts the calling plug of the pair used in answering the call into the trunk jack designated by the ,b-operator2 the ,b-operator simultaneously tests the multiple jack of the called subscriber and1 if she finds it not busy1 inserts the plug of the designated trunk into the multiple jack of the called subscriber and rings his bell by pressing the ringing key associated with the trunk cord used4 ,the work on the part of the ,a-operator in connecting with the outgoing end of the trunk and on the part of the ,b-operator in connecting the incoming end of the trunk with the line goes on simultaneously1 and it makes no difference which of these operators completes the connection first4 ,it is the common practice of the ,bell operating companies in this country to employ what is called automatic or machine ringing in connection with the ,b-operator's work4 ,when the ,b-operator presses the ringing key associated with the incoming trunk cord1 she pays no further attention to it1 and she has no supervisory lamp to inform her as to whether or not the subscriber has answered4 ,the ringing key is held down1 after its depression by the operator1 either by an electromagnet or by a magnet-controlled latch1 and the ringing of the subscriber's bell continues at periodic intervals as controlled by the ringing commutator associated with the ringing machine4 ,when the subscriber answers1 however1 the closure of his line circuit results in such an operation of the magnet associated with the ringing key as to release the ringing key and thus to automatically discontinue the ringing current4 ,when a connection is established between two subscribers through such a trunk the supervision of the connection falls entirely upon the ,a-operator who established it4 ,this means that the calling supervisory lamp at the ,a-operator's position is controlled over the trunk from the station of the called subscriber1 the answering supervisory lamp being1 of course1 under the control of the calling subscriber as in the case of a local connection4 ,it is1 therefore1 the ,a-operator who always initiates the taking down of a trunk connection1 and when1 in response to the lighting of the two lamps1 she withdraws her calling plug from the trunk jack1 the supervisory lamp associated with the incoming end of the trunk at the other office is lighted1 and the ,b-operator obeys it by pulling down the plug4 ,if1 upon testing the multiple jack of the called subscriber's line1 the ,b-operator finds the line to be busy1 she at once inserts the trunk plug into a so-called busy-back jack1 which is merely a jack whose terminals are permanently connected to a circuit that is intermittently opened and closed1 and which also has impressed upon it an alternating current of such a nature as to produce the familiar buzz-buzz in a telephone receiver4 ,the opening and closing of this circuit causes the calling supervisory lamp of the ,a-operator to flash at periodic intervals just as if the called subscriber had raised and lowered his receiver1 but more regularly4 ,this is the indication to the ,a-operator that the line called for is busy4 ,the buzzing sound is repeated back through the cord circuit of the ,a-operator to the calling subscriber and is a notification to him that the line is busy4 ,sometimes1 as is practiced in ,new ,york ,city1 for instance1 the buzzing feature is omitted1 and the only indication that the calling subscriber receives that the called-for line is busy is being told so by the ,a-operator4 ,this may be considered a special feature and it is employed in ,new ,york because there the custom exists of telling a calling subscriber1 when the line he has called for has been found busy1 that the party will be secured for him and that he1 the calling subscriber1 will be called1 if he desires4 ,a modification of this busy-back feature that has been employed in ,boston1 and perhaps in other places1 is to associate with the busy-back jack at the ,b-operator's position a phonograph which1 like a parrot1 keeps repeating ,line busy--please call again4 ,where this is done the calling subscriber1 if he understands what the phonograph says1 is supposed to hang up his receiver1 at which time the ,a-operator takes down the connection and the ,b-operator follows in response to the notification of her supervisory lamp4 ,the phonograph busy-back scheme1 while ingenious1 has not been a success and has generally been abandoned4 ,as a rule the independent operating companies in this country have not employed automatic ringing1 and in this case the ,b-operators have been required to operate their ringing keys and to watch for the response of the called subscriber4 ,in order to arrange for this1 another supervisory lamp1 termed the ringing lamp1 is associated with each incoming trunk plug1 the going out of this lamp being a notification to the ,b-operator to discontinue ringing4 ,western ,electric ,trunk ,circuits4 ,the principles involved in inter-office trunking with automatic ringing1 are well illustrated in the trunk circuit employed by the ,western ,electric ,company in connection with its ,no4 #a relay boards4 ,the dotted dividing line through the center of ,fig4 #cga represents the separating space between two offices4 ,the calling subscriber's line in the first office is shown at the extreme left and the called subscriber's line in the second office is shown at the extreme right4 ,both of these lines are standard multiple switchboard lines of the form already discussed4 ,the equipment illustrated in the first office is that of an ,a-board1 the cord circuit shown being that of the regular ,a-operator4 ,the outgoing trunk jacks connecting with the trunk leading to the other office are1 it will be understood1 multipled through the ,a-sections of the board and contain no relay equipment1 but the test rings are connected to ground through a resistance coil #a1 which takes the place of the cut-off relay winding of a regular line so far as test conditions and supervisory relay operation are concerned4 ,the equipment illustrated in the second office is that of a ,b-board1 it being understood that the called subscriber's line is multipled through both the ,a- and ,b-boards at that office4 ,the part of the equipment that is at this point unfamiliar to the reader is1 therefore1 the cord circuit at the ,b-operator's board4 ,this includes1 broadly speaking1 the means3 7#a7 for furnishing battery current to the called subscriber2 7#b7 for accomplishing the ringing of the called subscriber and for automatically stopping the ringing when he shall respond2 7#c7 for performing the ordinary switching functions in connection with the relays of the called subscriber's line in just the same way that an ,a-operator's cord carries out these functions2 and 7#d7 for causing the operation of the calling supervisory relay of the ,a-operator's cord circuit in just the same manner1 under control of the connected called subscriber1 as if that subscriber's line had been connected directly to the ,a-operator's cord circuit4 ,illustration3 ,fig4 #cga4 ,inter-,office ,connection--,western ,electric ,system ,the operation of these devices in the ,b-operator's cord circuit may be best understood by following the establishment of the connection4 ,assuming that the calling subscriber in the first office desires a connection with the subscriber's line shown in the second office1 and that the ,a-operator at the first office has answered the call1 she will then communicate by order wire with the ,b-operator at the second office1 stating the number of the called subscriber and receiving from that operator in return the number of the trunk to be employed4 ,the two operators will then proceed simultaneously to establish the connection1 the ,a-operator inserting the calling plug into the outgoing trunk jack1 and the ,b-operator inserting the trunk plug into the multiple jack of the called subscriber's line after testing4 ,we will assume at first that the called subscriber's line is found idle and that both of the operators complete their respective portions of the work at the same time and we will consider first the condition of the calling supervisory relay at the ,a-operator's position4 ,the circuit of the calling supervisory lamp will have been closed through the resistance coil #a connected with the outgoing trunk jacks and the lamp will be lighted because1 as will be shown1 it is not yet shunted out by the operation of its associated supervisory relay4 ,tracing the circuit of the calling supervisory relay of the ,a-operator's circuit1 it will be found to pass from the live side of the battery to the ring side of the trunk circuit through one winding of the repeating coil of the ,b-operator's cord2 beyond this the circuit is open1 since no path exists through the condenser #b bridged across the trunk circuit or through the normally open contacts of the relay #c connected in the talking circuit of the trunk4 ,the association of this relay #c with the repeating coil and the battery of the trunk is seen to be just the same as that of a supervisory relay in the ,a-operator's cord1 and it is clear1 therefore1 that this relay #c will not be energized until the called subscriber has responded4 ,when it is energized it will complete the path to ground through the ,a-operator's calling supervisory relay and operate to shunt out the ,a-operator's calling supervisory lamp in just the same manner as if the ,a-operator's calling plug had been connected directly with the line of the calling subscriber4 ,in other words1 the called subscriber in the second office controls the relay #c1 which1 in turn1 controls the calling supervisory relay of the ,a-operator1 which1 in turn1 shunts out its lamp4 ,the connection being completed between the two subscribers1 the ,b-operator depresses one or the other of the ringing keys #e or #f1 according to which party on the line is called1 assuming that it is a two-party line4 ,it will be noticed that the springs of these ringing keys are not serially arranged in the talking circuit1 but the cutting off of the trunk circuit back of the ringing keys is accomplished by the set of springs shown just at the left of the ringing keys1 which set of springs #g is operated whenever either one of the ringing keys is depressed4 ,an auxiliary pair of contacts1 shown just below the group of springs #g1 is also operated mechanically whenever either one of the ringing keys is depressed1 and this serves to close one of two normally open points in the circuit of the ringing-key holding magnet #h4 ,this holding magnet #h is so arranged with respect to the contacts of the ringing key that whenever any one of them is depressed by the operator1 it will be held depressed as long as the magnet is energized just the same as if the operator kept her finger on the key4 ,the other normally open point in the circuit of the holding magnet #h is at the lower pair of contacts of the test and holding relay #i4 ,this relay is operated whenever the trunk plug is inserted in the jack of a called line1 regardless of the position of the subscriber's equipment on that line4 ,the circuit may be traced from the live side of the battery through the trunk disconnect lamp #d1 coil #i1 sleeve strand of cord1 and to ground through the cut-off relay of the line4 ,the insertion of the trunk plug into the jack thus leaves the completion of the holding-magnet circuit dependent only upon the auxiliary contact on the ringing key1 and1 therefore1 as soon as the operator presses either one of these keys1 the clutch magnet is energized and the key is held down1 so that ringing current continues to flow at regular intervals to the called subscriber's station4 ,the ringing current issues from the generator #aj1 but the supply circuit from it is periodically interrupted by the commutator #aa geared to the ringing-machine shaft4 ,this periodically interrupted ringing current passes to the ringing-key contacts through the coil of the ringing cut-off relay #ab1 and thence to the subscriber's line4 ,the ringing current is1 however1 insufficient to cause the operation of this relay #ab as long as the high resistance and impedance of the subscriber's bell and condenser are in the circuit4 ,it is1 however1 sufficiently sensitive to be operated by this ringing current when the subscriber responds and thus substitutes the comparatively low resistance and impedance path of his talking apparatus for the previous path through his bell4 ,the pulling up of the ringing cut-off relay #ab breaks a third normally closed contact in the circuit of the holding coil #h1 de-energizing that coil and releasing the ringing key1 thus cutting off ringing current4 ,there is a third brush on the commutator #aa connected with the live side of the central battery1 and this is merely for the purpose of assuring the energizing of the ringing cut-off relay #ab1 should the subscriber respond during the interval while the commutator #aa held the ringing current cut off4 ,the relay #ab may thus be energized either from the battery1 if the subscriber responds during a period of silence of his ringer1 or from the generator #aj1 if the subscriber responds during a period while his bell is sounding2 in either case the ringing current will be promptly cut off by the release of the ringing key4 ,the trunk operator's disconnect lamp is shown at #d1 and it is to be remembered that this lamp is lighted only when the ,a-operator takes down the connection at her end1 and also that this lamp is entirely out of the control of the subscribers1 the conditions which determine its illumination being dependent on the positions of the operators' plugs at the two ends of the trunk4 ,with both plugs up1 the lamp #d will receive current1 but will be shunted to prevent its illumination4 ,the path over which it receives this current may be traced from battery through the lamp #d1 thence through the coil of the relay #i and the cut-off relay of the called subscriber's line4 ,this current would be sufficient to illuminate the lamp1 but the lamp is shunted by a circuit which may be traced from the live side of battery through the contact of the relay #ac1 closed at the time1 and through the coil of the trunk cut-off relay coil #ad4 ,the resistance of this coil is so proportioned to the other parts of the circuit as to prevent the illumination of the lamp just exactly as in the case of the shunting resistances of the lamps in the ,a-operator's cord4 ,it will be seen1 therefore1 that the supply of current to the trunk disconnect lamp is dependent on the trunk plug being inserted into the jack of the subscriber's line and that the shunting out of this lamp is dependent on the energization of the relay #ac4 ,this relay #ac is energized as long as the ,a-operator's plug is inserted into the outgoing trunk jack1 the path of the energizing circuit being traced from the live side of the battery at the second office through the right-hand winding of this relay1 thence over the tip side of the trunk to ground at the first office4 ,from this it follows that as long as both plugs are up1 the disconnect lamp will receive current but will be shunted out1 and as soon as the ,a-operator pulls down the connection1 the relay #ac will be de-energized and will thus remove the shunt from about the lamp1 allowing its illumination4 ,the left-hand winding of the relay #ac performs no operating function1 but is merely to maintain the balance of the talking circuit1 it being bridged during the connection from the ring side of the trunk to ground in order to balance the bridge connection of the right-hand coil from the live side of battery to the tip side of the trunk circuit4 ,the relay #ad1 already referred to as forming a shunt for the trunk disconnect lamp1 has for its function the keeping of the talking circuit through the trunk open until such time as the relay #ac operates1 this being purely an insurance against unnecessary ringing of a subscriber in case the ,a-operator should by mistake plug into the wrong trunk4 ,it is not1 therefore1 until the ,a-operator has plugged into the trunk and the relay #ac has been operated to cause the energization of the relay #ad that the ringing of the called subscriber can occur1 regardless of what the ,b-operator may have done4 ,the relay #i has an additional function to that of helping to control the circuit of the ringing-key holding magnet4 ,this is the holding of the test circuit complete until the operator has tested and made a connection and then automatically opening it4 ,the test circuit of the ,b-operator's trunk may be traced1 at the time of testing1 from the thimble of the multiple jack under test1 through the tip of the cord1 thence through the uppermost pair of contacts of the relay #i to ground through a winding of the ,b-operator's induction coil4 ,after the test has been made and the plug inserted1 the relay #i1 which is operated by the insertion of the plug1 acts to open this test circuit and at the same time complete the tip side of the cord circuit4 ,in the upper portion of ,fig4 #cga the order-wire connections1 by which the ,a-operator and the ,b-operator communicate1 are indicated4 ,it must be remembered in connection with these that the ,a-operator only has control of this connection1 the ,b-operator being compelled necessarily to hear whatever the ,a-operators have to say when the ,a-operators come in on the circuit4 ,illustration3 ,fig4 #cgb4 ,incoming ,trunk ,circuit ,the incoming trunk circuit employed by the ,western ,electric ,company for four-party line ringing is shown in ,fig4 #cgb1 it being necessarily somewhat modified from that shown in ,fig4 #cga1 which is adapted for two-party line ringing only4 ,in addition to the provision of the four-party line ringing keys1 by which positive or negative pulsating current is received over either limb of the line1 and to the provision of the regular alternating current ringing key for ringing on single party lines1 it is necessary in the ringing cut-off relay to provide for keeping the alternating and the pulsating ringing currents entirely separate4 ,for this reason1 the ringing cut-off relay #ab is provided with two windings1 that at the right being in the path of the alternating ringing currents that are supplied to the alternating current key1 and that at the left being in the ground return path for all of the pulsating ringing currents supplied to the pulsating keys4 ,with this explanation it is believed that this circuit will be understood from what has been said in connection with ,fig4 #cga4 ,the operation of the holding coil #h is the same in each case1 the holding magnet in ,fig4 #cgb serving to hold depressed any one of the five ringing keys that may have been used in calling the subscriber4 ,illustration3 ,,automatic ,,equipment1 ,,main ,,office1 ,,berkeley1 ,,california ,a ,feature of ,interest ,here is ,that the ,cement ,floor is ,treated with a ,filler and ,painted1 with ,no ,other ,covering4 ,illustration3 ,fig4 #cgc4 ,western ,electric ,trunk ,ringing ,key ,the standard four-party line1 trunk ringing key of the ,western ,electric ,company is shown in ,fig4 #cgc4 ,in this the various keys operate not by pressure but rather by being pulled by the finger of the operator in such a way as to subject the key shaft to a twisting movement4 ,the holding magnet lies on the side opposite to that shown in the figure and extends along the full length of the set of keys1 each key shaft being provided with an armature which is held by this magnet until the magnet is de-energized by the action of the ringing cut-off relay4 ,illustration3 ,fig4 #cgd4 ,trunk ,relay ,illustration3 ,fig4 #cge4 ,trunk ,relay ,the standard trunk relays employed by the ,western ,electric ,company in connection with the circuits just described are shown in ,figs4 #cgd and #cge4 ,in each case the dust-cap or shield is also shown4 ,the relay of ,fig4 #cgd is similar to the regular cut-off relay and is the one used for relays #i and #ad of ,figs4 #cga and #cgb4 ,the relay of ,fig4 #cge is somewhat similar to the subscriber's line relay in that it has a tilting armature1 and is the one used at #ac in ,figs4 #cga and #cgb4 ,the trunk relay #c in ,figs4 #cga and #cgb is the same as the ,a-operator's supervisory relays already discussed4 ,it has been stated that under certain circumstances ,b-operator's trunk circuits devoid of ringing keys1 and consequently of all keys1 may be employed4 ,this1 so far as the practice of the ,bell companies is concerned1 is true only in offices where there are no party lines1 or where1 as in many of the ,chicago offices1 the party lines are worked on the jack per station basis4 ,in jack per station working1 the selection of the station on a party line is determined by the jack on which the plug is put1 rather than by a ringing key1 and hence the keyless trunk may be employed4 ,illustration3 ,fig4 #cgf4 ,keyless ,trunk ,a keyless trunk as used in ,new ,york is shown in ,fig4 #cgf4 ,this has no manually operated keys whatever1 and the relay #ag1 when it is operated1 establishes connection between the ringing generator and the conductors of the trunk plug4 ,the relays #c1 #ac1 and #ab operate in a manner identical with those bearing corresponding numbers in ,fig4 #cga4 ,as soon as the trunk operator plugs into the multiple jack of the called subscriber1 the relay #af will operate for the same reason that the relay #i operated in connection with ,fig4 #cga4 ,the trunk disconnect lamp will receive current1 but if the operator has already established connection with the other end of the trunk1 this lamp will not be lighted because shunted by the relay #ag1 due to the pulling up of the armature of the relay #ac4 ,the relay #ae plays no part in the operation so far described1 because of the fact that its winding is short-circuited by its own contacts and those of relay #ab1 when the latter is not energized4 ,as a result of the operation of the relay #ag1 ringing current is sent to line1 the supply circuit including the coil of the relay #ab4 ,as soon as the subscriber responds to this ringing current1 the armature of the relay #ab is pulled up1 thus breaking the shunt about the relay #ae1 which1 therefore1 starts to operate in series with the relay #ag1 but as its armatures assume their attracted position1 the relay #ag is cut out of the circuit1 the coil of the relay #ae being substituted for that of the relay #ag in the shunt path around the lamp #d4 ,the relay #ag falls back and cuts off the ringing current4 ,the relay #ae now occupies the place with respect to the shunt around the lamp #d that the relay #ag formerly did1 the continuity of this shunt being determined by the energization of the relay #ac4 ,when the ,a-operator at the distant exchange withdraws the calling plug from the trunk jack1 this relay #ac becomes de-energized1 breaking the shunt about the lamp #d and permitting the display of that lamp as a signal to the operator to take down the connection4 ,it may be asked why the falling back of relay #ae will not again energize relay #ag and thus cause a false ring on the called subscriber4 ,this will not occur because both the relays #ae and #ag depend for their energization on the closure of the contacts of the relay #ac1 and when this falls back the relay #ag cannot again be energized even though the relay #ae assumes its normal position4 ,kellogg ,trunk ,circuits4 ,the provision for proper working of trunk circuits in connection with the two-wire multiple switchboards is not an altogether easy matter1 owing particularly to the smaller number of wires available in the plug circuits4 ,it has been worked out in a highly ingenious way1 however1 by the ,kellogg ,company1 and a diagram of their incoming trunk circuit1 together with the associated circuits involved in an inter-office connection1 is shown in ,fig4 #cgg4 ,illustration3 ,fig4 #cgg4 ,inter-,office ,connection--,kellogg ,system ,this figure illustrates a connection from a regular two-wire multiple subscriber's line in one office1 through an ,a-operator's cord circuit there1 to the outgoing trunk jacks at that office1 thence through the incoming trunk circuit at the other office to the regular two-wire multiple subscriber's line at that second office4 ,the portion of this diagram to be particularly considered is that of the ,b-operator's cord circuit4 ,the trunk circuit terminates in the multipled outgoing trunk jacks at the first office1 the trunk extending between offices consisting1 of course1 of but two wires4 ,we will first consider the control of the calling supervisory lamp in the ,a-operator's cord circuit1 it being remembered that this control must be from the called subscriber's station4 ,it will be noticed that the left-hand armature of the relay #a serves normally to bridge the winding of relay #b across the cord circuit around the condenser #c4 ,when1 however1 the relay #a pulls up1 the coil of relay #d is substituted in this bridge connection across the trunk4 ,the relay #b has a very high resistance winding--about #ae1jjj ohms--and this resistance is so great that the tip supervisory relay of the ,a-operator's cord will not pull up through it4 ,as a result1 when this relay is bridged across the trunk circuit1 the tip relay on the calling side of the ,a-operator's cord circuit is de-energized1 just as if the trunk circuit were open1 and this results in the lighting of the ,a-operator's calling supervisory lamp4 ,the winding of the relay #d1 however1 is of low resistance--about #ej ohms--and when this is substituted for the high-resistance winding of the relay #b1 the tip relay on the calling side of the ,a-operator's cord is energized1 resulting in the extinguishing of the calling supervisory lamp4 ,the illumination of the ,a-operator's calling supervisory lamp depends1 therefore1 on whether the high-resistance relay #b1 or the low-resistance relay #d1 is bridged across the trunk1 and this depends on whether the relay #a is energized or not4 ,the relay #a1 being bridged from the tip side of the trunk circuit to ground and serving as the means of supply of battery current to the called subscriber1 is operated whenever the called subscriber's receiver is removed from its hook4 ,therefore1 the called subscriber's hook controls the operation of this relay #a1 which1 in turn1 controls the conditions which cause the illumination or darkness of the calling supervisory lamp at the distant office4 ,assuming that the ,a-operator has received and answered a call1 and has communicated with the ,b-operator1 telling her the number of the called subscriber1 and has received1 in turn1 the number of the trunk to be used1 and that both operators have put up the connection1 then it will be clear from what has been said that the calling supervisory lamp of the ,a-operator will be lighted until the called subscriber removes his receiver from its hook1 because the tip relay in the ,a-operator's cord circuit will not pull up through the #ae1jjj-ohm resistance winding of the relay #b4 ,as soon as the subscriber responds1 however1 the relay #a will be operated by the current which supplies his transmitter4 ,this will substitute the low-resistance winding of the relay #d for the high-resistance winding of the relay #b1 and this will permit the energizing of the tip supervisory relay of the ,a-operator and put out the calling supervisory lamp at her position4 ,as in the ,western ,electric circuit1 therefore1 the control of the ,a-operator's calling supervisory lamp is from the called subscriber's station and is relayed back over the trunk to the originating office4 ,in this circuit1 manual instead of automatic ringing is employed1 therefore1 unlike the ,western ,electric circuit1 means must be provided for notifying the ,b-operator when the calling subscriber has answered4 ,this is done by placing at the ,b-operator's position a ringing lamp associated with each trunk cord1 which is illuminated when the ,b-operator places the plug of the incoming trunk into the multiple jack of the subscriber's line1 and remains illuminated until the subscriber has answered4 ,this is accomplished in the following manner3 when the operator plugs into the jack of the line called1 relay #e is energized but is immediately de-energized by the disconnecting of the circuit of this relay from the sleeve conductor of the cord when the ringing key is depressed1 the selection of the ringing key being determined by the particular party on the line desired4 ,these ringing keys have associated with them a set of springs #i1 which springs are operated when any one of the ringing keys is depressed4 ,thus1 with a ringing key depressed and the relay #e de-energized1 the ringing lamp will be illuminated by means of a circuit as follows3 from the live side of the battery1 through the ringing lamp #ab1 through the back contact and armature of the relay #f1 through the armature and contact of relay #d1 then through the armature and front contact of relay #b--which at this time is the relay bridged across the trunk and1 therefore1 energized--and thence through the back contact and armature of relay #e to ground4 ,when the subscriber removes his receiver from the hook1 the relay #a will become energized as previously described1 and will1 therefore1 operate relay #f to break the circuit of the ringing lamp4 ,the circuit thus established by the operation of relay #a is as follows3 from the live side of battery1 through the winding of relay #f1 through the armature and contact of relay #a1 through the armature and contact of relay #d1 through the armature and front contact of relay #b1 thence through the armature and back contact of relay #e to ground4 ,as soon as the ,b-operator notes that the ringing lamp has gone out1 she knows that no further ringing is required on that line1 thus allowing the operation of relay #e and accomplishing the locking out of the ringing lamp during the remainder of that connection4 ,the relay #f1 after having once pulled up1 remains locked up through the rear contact of the left-hand armature of relay #e and ground1 until the plug is removed from the jack4 ,at the end of the conversation1 when the ,a-operator has disconnected her cord circuit on the illumination of the supervisory signals1 both relays #b and #d will be in an unoperated condition and will provide a circuit for illuminating the disconnect lamp associated with the ,b-operator's cord4 ,this circuit may be traced as follows3 from battery through the disconnect lamp1 through the armatures and contacts of relays #b and #d1 thence through the front contact and armature of relay #e to ground1 thus illuminating the disconnect lamp4 ,the ringing lamp will not be re-illuminated at this time1 due to the fact that it has been previously locked out by relay #f4 ,the operator then removes the plug from the jack of the line called1 and the apparatus in the trunk circuit is restored to normal condition4 ,in the circuit shown only keys are provided for ringing two parties4 ,this circuit1 however1 is not confined to the use of two-party lines1 but may be extended to four parties by simply duplicating the ringing keys and by connecting them with the proper current for selectively ringing the other stations4 ,the method of determining as to whether the called line is free or busy is similar to that previously described for the ,a-operator's cord circuit when making a local connection1 and differs only in the fact that in the case of the trunk cord the test circuit is controlled through the contacts of a relay1 whereas in the case of the ,a-operator's cord1 the test circuit was controlled through the contacts of the listening key4 ,the function of the resistance #aj and the battery connected thereto is the same as has been previously described4 ,the general make-up of trunking switchboard sections is not greatly different from that of the ordinary switchboard sections where no trunking is involved4 ,in small exchanges where ring-down trunks are employed1 the trunk line equipment is merely added to the regular jack and drop equipment of the switchboard4 ,in common-battery multiple switchboards the ,a-boards differ in no respect from the standard single office multiple boards1 except that immediately above the answering jacks and below the multiple there are arranged in suitable numbers the jacks of the outgoing trunks4 ,where the offices are comparatively small1 the incoming trunk portions of the ,b-boards are usually merely a continuance of the ,a-boards1 the subscriber's multiple being continuous with and differing in no respect from that on the ,a-sections4 ,instead of the usual pairs of ,a-operators' plugs1 cords1 and supervisory equipment1 there are on the key and plug shelves of these ,b-sections the incoming trunk plugs and their associated equipment4 ,in large offices it is customary to make the ,b-board entirely separate from the ,a-board1 although the general characteristics of construction remain the same4 ,the reason for separate ,a- and ,b-switchboards in large exchanges is to provide for independent growth of each without the growth of either interfering with the other4 ,a portion of an incoming trunk1 or ,b-board1 is shown in ,fig4 #cgh4 ,the multiple is as usual1 and1 of course1 there are no outgoing trunk jacks nor regular cord pairs4 ,instead the key and plug shelves are provided with the incoming-trunk plug equipments1 thirty of these being about the usual quota assigned to each operator's position4 ,in multi-office exchanges1 employing many central offices1 such1 for instance1 as those in ,new ,york or ,chicago1 it is frequently found that nearly all of the calls that originate in one office are for subscribers whose lines terminate in some other office4 ,in other words1 the number of calls that have to be trunked to other offices is greatly in excess of the number of calls that may be handled through the multiple of the ,a-board in which they originate4 ,it is not infrequent to have the percentage of trunked calls run as high as #ge per cent of the total number of calls originating in any one office1 and in some of the offices in the larger cities this percentage runs higher than #ij per cent4 ,illustration3 ,fig4 #cgh4 ,section of ,trunk ,switchboard ,illustration3 ,fig4 #cgi4 ,section of ,partial ,multiple ,switchboard ,this fact has brought up for consideration the problem as to whether1 when the nature of the traffic is such that only a very small portion of the calls can be handled in the office where they originate1 it is worth while to employ the multiple terminals for the subscribers' lines on the ,a-boards4 ,in other words1 if so great a proportion as #ij per cent of the calls have to be trunked any way1 is it worth while to provide the great expense of a full multiple on all the sections of the ,a-board in order to make it possible to handle the remaining #aj per cent of the calls directly by the ,a-operators8 ,as a result of this consideration it has been generally conceded that where such a very great percentage of trunking was necessary1 the full multiple of the subscribers' lines on each section was not warranted1 and what is known as the partial multiple board has come into existence in large manual exchanges4 ,in these the regular subscribers' multiple is entirely omitted from the ,a-board1 all subscribers' calls being handled through outgoing trunk jacks connected by trunks to ,b-boards in the same as well as other offices4 ,in these partial multiple ,a-boards1 the answering jacks are multipled a few times1 usually twice1 so that calls on each line may be answered from more than one position4 ,this multipling of answering jacks does not in any way take the place of the regular multipling in full multiple boards1 since in no case are the calls completed through the multiple jacks4 ,it is done merely for the purpose of contributing to team work between the operators4 ,a portion of such a partial multiple ,a-board is shown in ,fig4 #cgi4 ,this view shows slightly more than one section1 and the regular answering jacks and lamps may be seen at the bottom of the jack space just above the plugs4 ,above these are placed the outgoing trunk jacks1 those that are in use being indicated with white designation strips4 ,above the outgoing trunk jacks are placed the multiples of the answering jacks1 these not being provided with lamps4 ,the partial multiple ,a-section of ,fig4 #cgi is a portion of the switchboard equipment of the same office to which the trunking section shown in ,fig4 #cgh belongs4 ,that this is a large multiple board may be gathered from the number of multiple jacks in the trunking section1 #h1djj being installed with room for #aj1ejj4 ,that the board is a portion of an equipment belonging to an exchange of enormous proportions may be gathered from the number of outgoing trunk jacks shown in the ,a-board1 and in the great number of order-wire keys shown between each of the sets of regular cord-circuit keys4 ,the switchboards illustrated in these two figures are those of one of the large offices of the ,new ,york ,telephone ,company on ,manhattan ,island1 and the photographs were taken especially for this work by the ,western ,electric ,company4 ,cable ,color ,code4 ,a great part of the wiring of switchboards requires to be done with insulated wires grouped into cables4 ,in the wiring of manual switchboards as described in the seven preceding chapters1 and of automatic and automanual systems and of private branch-exchange and intercommunicating systems described in succeeding chapters1 cables formed as follows are widely used3 ,tinned soft copper wires1 usually of ,no4 #bb or ,no4 #bd ,b4 ,s4 gauge1 are insulated1 first with two coverings of silk1 then with one covering of cotton4 ,the outer 7cotton7 insulation of each wire is made of white or of dyed threads4 ,if dyed1 the color either is solid red1 black1 blue1 orange1 green1 brown1 or slate1 or it is striped1 by combining one of those colors with white or a remaining color4 ,the object of coloring the wires is to enable them to be identified by sight instead of by electrical testing4 ,wires so insulated are twisted into pairs1 choosing the colors of the line and mate according to a predetermined plan4 ,an assortment of these pairs then is laid up spirally to form the cable core1 over which are placed certain wrappings and an outer braid4 ,a widely used form of switchboard cable has paper and lead foil wrappings over the core1 and the outer cotton braid finally is treated with a fire-resisting paint4 ,,standard ,,color ,,code ,,for ,,cables ---------------------------------------- ------------------------ ,,mate ,,line ,,wire ---------------------------------------- ----- ,white ,red ,black ,red-,white ,black-,white ---------------------------------------- -------------------- ,blue #a #ba #da #fa #ha ,orange #b #bb #db #fb #hb ,green #c #bc #dc #fc #hc ,brown #d #bd #dd #fd #hd ,slate #e #be #de #fe #he ,blue-,white #f #bf #df #ff #hf ,blue-,orange #g #bg #dg #fg #hg ,blue-,green #h #bh #dh #fh #hh ,blue-,brown #i #bi #di #fi #hi ,blue-,slate #aj #cj #ej #gj #ij ,orange-,white #aa #ca #ea #ga #ia ,orange-,green #ab #cb #eb #gb #ib ,orange-,brown #ac #cc #ec #gc #ic ,orange-,slate #ad #cd #ed #gd #id ,green-,white #ae #ce #ee #ge #ie ,green-,brown #af #cf #ef #gf #if ,green-,slate #ag #cg #eg #gg #ig ,brown-,white #ah #ch #eh #gh #ih ,brown-,slate #ai #ci #ei #gi #ii ,slate-,white #bj #dj #fj #hj #ajj ---------------------------------------- -------------------- ,the numerals represent the pair numbers in the cable4 ,the wires of spare pairs usually are designated by solid red with white mate for first spare pair1 and solid black with white mate for second spare pair4 ,individual spare wires usually are colored red-white for first individual spare1 and black-white for second individual spare4 ,,chapter ,,xxviii ,,fundamental ,,considerations ,,of ,,automatic ,,systems ,definition4 ,the term automatic1 as applied to telephone systems1 has come to refer to those systems in which machines at the central office1 under the guidance of the subscribers1 do the work that is done by operators in manual systems4 ,in all automatic telephone systems1 the work of connecting and disconnecting the lines1 of ringing the called subscriber1 even though he must be selected from among those on a party line1 of refusing to connect with a line that is already in use1 and informing the calling subscriber that such line is busy1 of making connections to trunk lines and through them to lines in other offices and doing the same sort of things there1 of counting and recording the successful calls made by a subscriber1 rejecting the unsuccessful1 and nearly all the thousand and one other acts necessary in telephone service1 are performed without the presence of any guiding intelligence at the central office4 ,the fundamental object of the automatic system is to do away with the central-office operator4 ,in order that each subscriber may control the making of his own connections there is added to his station equipment a call transmitting device by the manipulation of which he causes the central-office mechanisms to establish the connections he desires4 ,we think that the automatic system is one of the most astonishing developments of human ingenuity4 ,the workers in this development are worthy of particular notice4 ,from occupying a position in popular regard in common with long-haired men and short-haired women they have recently appeared as sane1 reasonable men with the courage of their convictions and1 better yet1 with the ability to make their convictions come true4 ,the scoffers have remained to pray4 ,arguments ,against ,automatic ,idea4 ,naturally there has been a bitter fight against the automatic4 ,those who have opposed it have contended3 ,first3 that it is too complicated and1 therefore1 could be neither reliable or economical4 ,second3 that it is too expensive1 and that the necessary first cost could not be justified4 ,third3 that it is too inflexible and could not adapt itself to special kinds of service4 ,fourth3 that it is all wrong from the subscribers' point of view as the public will not tolerate doing its own operating4 ,complexity4 ,this first objection as to complexity1 and consequent alleged unreliability and lack of economy should be carefully analyzed4 ,it too often happens that a new invention is cast into outer darkness by those whose opinions carry weight by such words as it cannot work2 it is too complicated4 ,fortunately for the world1 the patience and fortitude which men must possess before they can produce meritorious1 though intricate inventions1 are usually sufficient to prevent their being crushed by any such offhand condemnation1 and the test of time and service is allowed to become the real criterion4 ,it would be difficult to find an art that has gone forward as rapidly as telephony4 ,within its short life of a little over thirty years it has grown from the phase of trifling with a mere toy to an affair of momentous importance to civilization4 ,there has been a tendency1 particularly marked during recent years1 toward greater complexity2 and probably every complicated new system or piece of apparatus has been roundly condemned1 by those versed in the art as it was1 as being unable to survive on account of its complication4 ,to illustrate3 ,a prominent telephone man1 in arguing against the nickel-in-the-slot method of charging for telephone service once said1 partly in jest1 ,the ,lord never intended telephone service to be given in that way4 ,this1 while a little off the point1 is akin to the sweeping aside of new telephone systems on the sole ground that they are complicated4 ,these are not real reasons1 but rather convenient ways of disposing of vexing problems with a minimum amount of labor4 ,important questions lying at the very root of the development of a great industry may not be put aside permanently in this offhand way4 ,the ,lord has never1 so far as we know1 indicated just what his intentions were in the matter of nickel service2 and no one has ever shown yet just what degree of complexity will prevent a telephone system from working4 ,it is safe to say that1 if other things are equal1 the simpler a machine is1 the better2 but simplicity1 though desirable1 is not all-important4 ,complexity is warranted if it can show enough advantages4 ,if one takes a narrow view of the development of things mechanical and electrical1 he will say that the trend is toward simplicity4 ,the mechanic in designing a machine to perform certain functions tries to make it as simple as possible4 ,he designs and re-designs1 making one part do the work of two and contriving schemes for reducing the complexity of action and form of each remaining part4 ,his whole trend is away from complication1 and this is as it should be4 ,other things being equal1 the simpler the better4 ,a broad view1 however1 will show that the arts are becoming more and more complicated4 ,take the implements of the art of writing3 ,the typewriter is vastly more complicated than the pen1 whether of steel or quill1 yet most of the writing of today is done on the typewriter1 and is done better and more economically4 ,the art of printing affords even more striking examples4 ,in telephony1 while every effort has been made to simplify the component parts of the system1 the system itself has ever developed from the simple toward the complex4 ,the adoption of the multiple switchboard1 of automatic ringing1 of selective ringing on party lines1 of measured-service appliances1 and of automatic systems have all constituted steps in this direction4 ,the adoption of more complicated devices and systems in telephony has nearly always followed a demand for the performance by the machinery of the system of additional or different functions4 ,as in animal and plant life1 so in mechanics--the higher the organism functionally the more complex it becomes physically4 ,greater intricacy in apparatus and in methods is warranted when it is found desirable to make the machine perform added functions4 ,once the functions are determined upon1 then the whole trend of the development of the machine for carrying them out should be toward simplicity4 ,when the machine has reached its highest stage of development some one proposes that it be required to do something that has hitherto been done manually1 or by a separate machine1 or not at all4 ,with this added function a vast added complication may come1 after which1 if it develops that the new function may with economy be performed by the machine1 the process of simplification again begins1 the whole design finally taking on an indefinable elegance which appears only when each part is so made as to be best adapted in composition1 form1 and strength to the work it is to perform4 ,achievements in the past teach us that a machine may be made to do almost anything automatically if only the time1 patience1 skill1 and money be brought to bear4 ,this is also true of a telephone system4 ,the primal question to decide is1 what functions the system is to perform within itself1 automatically1 and what is to be done manually or with manual aid4 ,sometimes great complications are brought into the system in an attempt to do something which may very easily and cheaply be done by hand4 ,cases might be pointed out in which fortunes and life-works have been wasted in perfecting machines for which there was no real economic need4 ,it is needless to cite cases where the reverse is true4 ,the matter of wisely choosing the functions of the system is of fundamental importance4 ,in choosing these the question of complication is only one of many factors to be considered4 ,one of the strongest arguments against intricacy in telephone apparatus is its greater initial cost1 its greater cost of maintenance1 and its liability to get out of order4 ,greater complexity of apparatus usually means greater first cost1 but it does not necessarily mean greater cost of up-keep or lessened reliability4 ,a dollar watch is more simple than an expensive one4 ,the one1 however1 does its work passably and is thrown away in a year or so2 the other does its work marvelously well and may last generations1 being handed down from father to son4 ,merely reducing the number of parts in a machine does not necessarily mean greater reliability4 ,frequently the attempt to make one part do several diverse things results in such a sacrifice in the simplicity of action of that part as to cause undue strain1 or wear1 or unreliable action4 ,better results may be attained by adding parts1 so that each may have a comparatively simple thing to do4 ,illustration3 ,,western ,,electric ,,company ,,typical ,,charging ,,outfit ,,at ,,dawson1 ,,georgia ,the stage of development of an art is a factor in determining the degree of complexity that may be allowed in the machinery of that art4 ,a linotype machine1 if constructed by miracle several hundred years ago1 would have been of no value to the printer's art then4 ,the skill was not available to operate and maintain it1 nor was the need of the public sufficiently developed to make it of use4 ,similarly the automatic telephone exchange would have been of little value thirty years ago4 ,the knowledge of telephone men was not sufficiently developed to maintain it1 telephone users were not sufficiently numerous to warrant it1 and the public was not sufficiently trained to use it4 ,industries1 like human beings1 must learn to creep before they can walk4 ,another factor which must be considered in determining the allowable degree of complexity in a telephone system is the character of the labor available to care for and manage it4 ,usually the conditions which make for unskilled labor also lend themselves to the use of comparatively simple systems4 ,thus1 in a small village remote from large cities the complexity inherent in a common-battery multiple switchboard would be objectionable4 ,the village would probably not afford a man adequately skilled to care for it1 and the size of the exchange would not warrant the expense of keeping such a man4 ,fortunately no such switchboard is needed4 ,a far simpler device1 the plain magneto switchboard--so simple that the girl who manipulates it may also often care for its troubles--is admirably adapted to the purpose4 ,so it is with the automatic telephone system2 even its most enthusiastic advocate would be foolish indeed to contend that for all places and purposes it was superior to the manual4 ,these remarks are far from being intended as a plea for complex telephone apparatus and systems2 every device1 every machine1 and every system should be of the simplest possible nature consistent with the functions it has to perform4 ,they are rather a protest against the broadcast condemnation of complex apparatus and systems just because they are complicated1 and without regard to other factors4 ,such condemnation is detrimental to the progress of telephony4 ,where would the printing art be today if the linotype1 the cylinder press1 and other modern printing machinery of marvelous intricacy had been put aside on account of the fact that they were more complicated than the printing machinery of our forefathers8 ,that the automatic telephone system is complex1 exceedingly complex1 cannot be denied1 but experience has amply proven that its complexity does not prevent it from giving reliable service1 nor from being maintained at a reasonable cost4 ,expense4 ,the second argument against the automatic--that it is too expensive--is one that must be analyzed before it means anything4 ,it is true that for small and medium-sized exchanges the total first cost of the central office and subscribers' station equipment1 is greater than that for manual exchanges of corresponding sizes4 ,the prices at which various sizes of automatic exchange equipments may be purchased vary1 however1 almost in direct proportion to the number of lines1 whereas in manual equipment the price per line increases very rapidly as the number of lines increases4 ,from this it follows that for very large exchanges the cost of automatic apparatus becomes as low1 and may be even lower than for manual4 ,roughly speaking the cost of telephones and central-office equipment for small exchanges is about twice as great for automatic as for manual1 and for very large exchanges1 of about #aj1jjj lines1 the cost of the two for switchboards and telephones is about equal4 ,for all except the largest exchanges1 therefore1 the greater first cost of automatic apparatus must be put down as one of the factors to be weighed in making the choice between automatic and manual1 this factor being less and less objectionable as the size of the equipment increases and finally disappearing altogether for very large equipments4 ,greater first cost is1 of course1 warranted if the fixed charges on the greater investment are more than offset by the economy resulting4 ,the automatic screw machine1 for instance1 costs many times more than the hand screw machine1 but it has largely displaced the hand machine nevertheless4 ,flexibility4 ,the third argument against the automatic telephone system--its flexibility--is one that only time and experience has been able to answer4 ,enough time has elapsed and enough experience has been gained1 however1 to disprove the validity of this argument4 ,in fact1 the great flexibility of the automatic system has been one of its surprising developments4 ,no sooner has the statement been made that the automatic system could not do a certain thing than forthwith it has done it4 ,it was once quite clear that the automatic system was not practicable for party-line selective ringing2 yet today many automatic systems are working successfully with this feature2 the selection between the parties on a line being accomplished with just as great certainty as in manual systems4 ,again it has seemed quite obvious that the automatic system could not hope to cope with the reverting call problem1 i4 e41 enabling a subscriber on a party line to call back to reach another subscriber on the same line2 yet today the automatic system may do this in a way that is perhaps even more satisfactory than the way in which it is done in multiple manual switchboards4 ,it is true that the automatic system has not done away with the toll operator and it probably never will be advantageous to require it to do so for the simple reason that the work of the toll operator in recording the connections and in bringing together the subscribers is a matter that requires not only accuracy but judgment1 and the latter1 of course1 no machine can supply4 ,it is probable also that the private branch-exchange operator will survive in automatic systems4 ,this is not because the automatic system cannot readily perform the switching duties1 but the private branch-exchange operator has other duties than the mere building up and taking down of connections4 ,she is1 as it were1 a door-keeper guarding the telephone door of a business establishment2 like the toll operator she must be possessed of judgment and of courtesy in large degree1 neither of which can be supplied by machinery4 ,in respect to toll service and private branch-exchange service where1 as just stated1 operators are required on account of the nature of the service1 the automatic system has again shown its adaptability and flexibility4 ,it has shown its capability of working in harmony with manual switchboards1 of whatever nature1 and there is a growing tendency to apply automatic devices and automatic principles of operation to manual switchboards1 whether toll or private branch or other kinds1 even though the services of an operator are required1 the idea being to do by machinery that portion of the work which a machine is able to do better or more economically than a human being4 ,attitude of ,public4 ,the attitude of the public toward the automatic is one that is still open to discussion2 at least there is still much discussion on it4 ,a few years ago it did seem reasonable to suppose that the general telephone user would prefer to get his connection by merely asking for it rather than to make it himself by spelling it out on the dial of his telephone instrument4 ,we have studied this point carefully in a good many different communities and it is our opinion that the public finds no fault with being required to make its own connections4 ,to our minds it is proven beyond question that either the method employed in the automatic or that in the manual system is satisfactory to the public as long as good service results1 and it is beyond question that the public may get this with either4 ,subscriber's ,station ,equipment4 ,the added complexity of the mechanism at the subscriber's station is in our opinion the most valid objection that can be urged against the automatic system as it exists today4 ,this objection has1 however1 been much reduced by the greater simplicity and greater excellence of material and workmanship that is employed in the controlling devices in modern automatic systems4 ,however1 the automatic system must always suffer in comparison with the manual in respect of simplicity of the subscriber's equipment4 ,the simplest conceivable thing to meet all of the requirements of telephone service at a subscriber's station is the modern common-battery manual telephone4 ,the automatic telephone differs from this only in the addition of the mechanism for enabling the subscriber to control the central-office apparatus in the making of calls4 ,from the standpoint of maintenance1 simplicity at the subscriber's station is1 of course1 to be striven for since the proper care of complex devices scattered all over a community is a much more serious matter than where the devices are centered at one point1 as in the central office4 ,nevertheless1 as pointed out1 complexity is not fatal1 and it is possible1 as has been proven1 to so design and construct the required apparatus in connection with the subscribers' telephones as to make them subject to an amount of trouble that is not serious4 ,comparative ,costs4 ,a comparison of the total costs of owning1 operating1 and maintaining manual and automatic systems usually results in favor of the automatic1 except in small exchanges4 ,this seems to be the consensus of opinion among those who have studied the matter deeply4 ,although the automatic usually requires a larger investment1 and consequently a larger annual charge for interest and depreciation1 assuming the same rates for each case1 and although the automatic requires a somewhat higher degree of skill to maintain it and to keep it working properly than the manual1 the elimination of operators or the reduction in their number and the consequent saving of salaries and contributory expenses together with other items of saving that will be mentioned serves to throw the balance in favor of the automatic4 ,the ease with which the automatic system lends itself to inter-office trunking makes feasible a greater subdivision of exchange districts into office districts and particularly makes it economical1 where such would not be warranted in manual working4 ,all this tends toward a reduction in average length of subscribers' lines and it seems probable that this possibility will be worked upon in the future1 more than it has been in the past1 to effect a considerable saving in the cost of the wire plant1 which is the part of a telephone plant that shows least and costs most4 ,automatic vs4 ,manual4 ,taking it all in all the question of automatic versus manual may not and can not be disposed of by a consideration of any single one of the alleged features of superiority or inferiority of either4 ,each must be looked at as a practical way of giving telephone service1 and a decision can be reached only by a careful weighing of all the factors which contribute to economy1 reliability1 and general desirability from the standpoint of the public4 ,public sentiment must neither be overlooked nor taken lightly1 since1 in the final analysis1 it is the public that must be satisfied4 ,methods of ,operation4 ,in all of the automatic telephone systems that have achieved any success whatever1 the selection of the desired subscriber's line by the calling subscriber is accomplished by means of step-by-step mechanism at the central office1 controlled by impulses sent or caused to be sent by the acts of the subscriber4 ,strowger ,system4 ,in the so-called ,strowger system1 manufactured by the ,automatic ,electric ,company of ,chicago1 the subscriber1 in calling1 manipulates a dial by which the central-office switching mechanism is made to build up the connection he wants4 ,the dial is moved as many times as there are digits in the called subscriber's number and each movement sends a series of impulses to the central office corresponding in number respectively to the digits in the called subscriber's number4 ,during each pause1 except the last one1 between these series of impulses1 the central-office mechanism operates to shift the control of the calling subscriber's line from one set of switching apparatus at the central office to another4 ,in case a four-digit number is being selected first1 the movement of the dial by the calling subscriber will correspond to the thousands digit of the number being called1 and the resulting movement of the central-office apparatus will continue the calling subscriber's line through a trunk to a piece of apparatus capable of further extending his line toward the line terminals of the thousand subscribers whose numbers begin with the digit chosen4 ,the next movement of the dial corresponding to the hundreds digit of the called number will operate this piece of apparatus to again extend the calling subscriber's line through another trunk to apparatus representing the particular hundred in which the called subscriber's number is4 ,the third movement of the dial corresponding to the tens digit will pick out the group of ten containing the called subscriber's line1 and the fourth movement corresponding to the units digit will pick out and connect with the particular line called4 ,lorimer ,system4 ,in the ,lorimer automatic system invented by the ,lorimer ,brothers1 and now being manufactured by the ,canadian ,machine ,telephone ,company of ,toronto1 ,canada1 the subscriber sets up the number he desires complete by moving four levers on his telephone so that the desired number appears visibly before him4 ,he then turns a handle and the central-office apparatus1 under the control of the electrical conditions thus set up by the subscriber1 establishes the connection4 ,in this system1 unlike the ,strowger system1 the controlling impulses are not caused by the movement of the subscriber's apparatus in returning to its normal position after being set by the subscriber4 ,instead1 the conditions established at the subscriber's station by the subscriber in setting up the desired number1 merely determine the point in the series of impulses corresponding to each digit at which the stepping impulses local to the central office shall cease1 and in this way the proper number of impulses in the series corresponding to each digit is determined4 ,magnet- vs4 ,power-,driven ,switches4 ,these two systems differ radically in another respect4 ,in the ,strowger system it is the electrical impulses initiated at the subscriber's apparatus that actually cause the movement of the switching parts at the central office1 these impulses energizing electromagnets which move the central-office switching devices a step at a time the desired number of steps4 ,in the ,lorimer system the switches are all power-driven and the impulses under the control of the subscriber's instrument merely serve to control the application of this power to the various switching mechanisms4 ,these details will be more fully dealt with in subsequent chapters4 ,multiple vs4 ,trunking4 ,it has been shown in the preceding portion of this work that the tendency in manual switchboard practice has been away from trunking between the various sections or positions of a board1 and toward the multiple idea of operating1 wherein each operator is able to complete the connection with any line in the same office without resorting to trunks or to the aid of other operators4 ,strangely enough the reverse has been true in the development of the automatic system4 ,as long as the inventors tried to follow the most successful practice in manual working1 failure resulted4 ,the automatic systems of today are essentially trunking systems and while they all involve multiple connections in greater or less degree1 all of them depend fundamentally upon the extending of the calling line by separate lengths until it finally reaches and connects with the called line4 ,grouping of ,subscribers4 ,in this connection we wish to point out here two very essential features without which1 so far as we are aware1 no automatic telephone system has been able to operate successfully4 ,the first of these is the division of the total number of lines in any office of the exchange into comparatively small groups and the employment of correspondingly small switch units for each group4 ,many of the early automatic systems that were proposed involved the idea of having each switch capable in itself of making connection with any line in the entire office4 ,as long as the number of lines was small--one hundred or thereabouts--this might be all right1 but where the lines number in the thousands1 it is readily seen that the switches would be of prohibitive size and cost4 ,trunking between ,groups4 ,this feature made necessary the employment of trunk connections between groups4 ,by means of these the lines are extended a step at a time1 first entering a large group of groups1 containing the desired subscriber2 then entering the smaller group containing that subscriber2 and lastly entering into connection with the line itself4 ,the carrying out of this idea was greatly complicated by the necessity of providing for many simultaneous connections through the switchboard4 ,it was comparatively easy to accomplish the extension of one line through a series of links or trunks to another line1 but it was not so easy to do this and still leave it possible for any other line to pick out and connect with any other idle line without interference with the first connection4 ,a number of parallel paths must be provided for each possible connection4 ,groups of trunks are1 therefore1 provided instead of single trunks between common points to be connected4 ,the subscriber who operates his instrument in making a call knows nothing of this and it is1 of course1 impossible for him to give any thought to the matter as to which one of the possible paths he shall choose4 ,it was by a realization of these facts that the failures of the past were turned into the successes of the present4 ,the subscriber by setting his signal transmitter was made to govern the action of the central-office apparatus in the selection of the proper group of trunks4 ,the group being selected1 the central-office apparatus was made to act at once automatically to pick out and connect with the first idle trunk of such group4 ,thus1 we may say that the subscriber by the act performed on his signal transmitter1 voluntarily chooses the group of trunks1 and immediately thereafter the central-office apparatus without the volition of the subscriber picks out the first idle one of this group of trunks so chosen4 ,this fundamental idea1 so far as we are aware1 underlies all of the successful automatic telephone-exchange systems4 ,it provides for the possibility of many simultaneous connections through the switchboard1 and it provides against the simultaneous appropriation of the same path by two or more calling subscribers and thus assures against interference in the choice of the paths4 ,outline of ,action4 ,in order to illustrate this point we may briefly outline the action of the ,strowger automatic system in the making of a connection4 ,assume that the calling subscriber desires a connection with a subscriber whose line bears the number #i1efg4 ,the subscriber in making the call will1 by the first movement of his dial1 transmit nine impulses over his line4 ,this will cause the selective apparatus at the central office1 that is at the time associated with the calling subscriber's line1 to move its selecting fingers opposite a group of terminals representing the ends of a group of trunk lines leading to apparatus employed in connecting with the ninth thousand of the subscribers' lines4 ,while the calling subscriber is getting ready to transmit the next digit1 the automatic apparatus1 without his volition1 starts to pick out the first idle one of the group of trunks so chosen4 ,having found this it connects with it and the calling subscriber's line is thus extended to another selective apparatus capable of performing the same sort of function in choosing the proper hundreds group4 ,in the next movement of his dial the calling subscriber will send five impulses4 ,this will cause the last chosen selective switch to move its selective fingers opposite a group of terminals representing the ends of a group of trunks each leading to a switch that is capable of making connection with any one of the lines in the fifth hundred of the ninth thousand4 ,again during the pause by the subscriber1 the switch that chose this group of trunks will start automatically to pick out and connect with the first idle one of them1 and will thus extend the line to a selective switch that is capable of reaching the desired line1 since it has access to all of the lines in the chosen hundred4 ,the third movement of the dial sends six impulses and this causes this last chosen switch to move opposite the sixth group of ten terminals1 so that there has now been chosen the nine hundred and fifty-sixth group of ten lines4 ,the final movement of the dial sends seven impulses and the last mentioned switch connects with the seventh line terminal in the group of ten previously chosen and the connection is complete1 assuming that the called line was not already engaged4 ,if it had been found busy1 the final switch would have been prevented from connecting with it by the electrical condition of certain of its contacts and the busy signal would have been transmitted back to the calling subscriber4 ,fundamental ,idea4 ,this idea of subdividing the subscribers' lines in an automatic exchange1 of providing different groups of trunks so arranged as to afford by combination a number of possible parallel paths between any two lines1 of having the calling subscriber select1 by the manipulation of his instrument1 the proper group of trunks any one of which might be used to establish the connection he desires1 and of having the central-office apparatus act automatically to choose and connect with an idle one in this chosen group1 should be firmly grasped4 ,it appears1 as we have said1 in every successful automatic system capable of serving more than one small group of lines1 and until it was evolved automatic telephony was not a success4 ,testing4 ,as each trunk is chosen and connected with1 conditions are established1 by means not unlike the busy test in multiple manual switchboards1 which will guard that trunk and its associated apparatus against appropriation by any other line or apparatus as long as it is held in use4 ,likewise1 as soon as any subscriber's line is put into use1 either by virtue of a call being originated on it1 or by virtue of its being connected with as a called line1 conditions are automatically established which guard it against being connected with any other line as long as it is busy4 ,these guarding conditions of both trunks and lines1 as in the manual board1 are established by making certain contacts1 associated with the trunks or lines1 assume a certain electrical condition when busy that is different from their electrical condition when idle2 but unlike the manual switchboard this different electrical condition does not act to cause a click in any one's ear1 but rather to energize or de-energize certain electromagnets which will establish or fail to establish the connection according to whether it is proper or improper to do so4 ,local and ,inter-,office ,trunks4 ,the groups of trunks that are used in building up connections between subscribers' lines may be local to the central office1 or they may extend between different offices4 ,the action of the two kinds of trunks1 local or inter-office1 is broadly the same4 ,,chapter ,,xxix ,,the ,,automatic ,,electric ,,company',s ,,system ,almost wherever automatic telephony is to be found--and its use is extensive and rapidly growing--the so-called ,strowger system is employed4 ,it is so named because it is the outgrowth of the work of ,almon ,b4 ,strowger1 an early inventor in the automatic telephone art4 ,that the system should bear the name of ,strowger1 however1 gives too great prominence to his work and too little to that of the engineers of the ,automatic ,electric ,company under the leadership of ,alexander ,e4 ,keith4 ,principles of ,selecting ,switch4 ,the underlying features of this automatic system have already been referred to in the abstract4 ,a better grasp of its principles may1 however1 be had by considering a concrete example of its most important piece of apparatus--the selecting switch4 ,the bare skeleton of such a switch1 sufficient only to illustrate the salient point in its mode of operation1 is shown in ,fig4 #chj4 ,the essential elements of this are a vertical shaft capable of both longitudinal and rotary motion2 a pawl and ratchet mechanism actuated by a magnet for moving the shaft vertically a step at a time2 another pawl and ratchet mechanism actuated by another magnet for rotating the shaft a step at a time2 an arm carrying wiper contacts on its outer end1 mounted on and moving with the shaft2 and a bank of contacts arranged on the inner surface of a section of a cylinder adapted to be engaged by the wiper contacts on this movable arm4 ,these various elements are indicated in the merest outline and with much distortion in ,fig4 #chj1 which is intended to illustrate the principles of operation rather than the details as they actually are in the system4 ,in the upper left-hand corner of this figure1 the magnet shown will1 if energized by impulses of current1 attract and release its armature and1 in doing so1 cause the pawl controlled by this magnet to move the vertical shaft of the switch up a step at a time1 as many steps as there are impulses of current4 ,the vertical movement of this shaft will carry the wiper arm1 attached to the lower end of the shaft1 up the same number of steps and1 in doing so1 will bring the contacts of this wiper arm opposite1 but not engaging1 the corresponding row of stationary contacts in the semi-cylindrical bank4 ,likewise1 through the ratchet cylinder on the intermediate portion of the shaft1 the magnet shown at the right-hand portion of this figure will1 when energized by a succession of electrical impulses1 rotate the shaft a step at a time1 as many steps as there are impulses4 ,this will thus cause the contacts of the wiper arm to move over the successive contacts in the row opposite to which the wiper had been carried in its vertical movement4 ,illustration3 ,fig4 #chj4 ,principles of ,automatic ,switch ,at the lower left-hand corner of this figure1 there is shown a pair of keys either one of which1 when operated1 will complete the circuit of the magnet to which it is connected1 this circuit including a common battery4 ,in a certain rough way this pair of key switches in the lower left-hand corner of the drawing may be taken as representing the call-transmitting apparatus at the subscriber's station1 and the two wires extending therefrom may be taken as representing the line wires connecting that subscriber's station to the central office2 but the student must avoid interpreting them as actual representations of the subscriber's station calling apparatus or the subscriber's line since their counterparts are not to be found in the system as it really exists4 ,here again accuracy has been sacrificed for ease in setting forth a feature of operation4 ,still referring to ,fig4 #chj1 it will be seen that the bank contacts consist of ten rows1 each having ten pairs of contacts4 ,assume again1 for the sake of simplicity1 that the exchange under consideration has one hundred subscribers and that each pair of bank contacts represents the terminals of one subscriber's line4 ,assume further that the key switches in the lower left-hand corner of the figure are being manipulated by a subscriber at that station and that he wishes to obtain a connection with line ,no4 #fg4 ,by pressing and releasing the left-hand key six times1 he will cause six separate impulses of current to flow through the upper left-hand magnet and this will cause the switch shaft to move up six steps and bring the wiper arm opposite the sixth row of bank contacts4 ,if he now presses and releases his right-hand key seven times1 he will1 through the action of the right-hand magnet1 rotate the shaft seven steps1 thus bringing the wipers into contact with the seventh contact of the sixth row and thus into contact with the desired line4 ,as the wiper contacts on the switch arm form the terminals of the calling subscriber's line1 it will be apparent that the calling subscriber is now connected through his switch with the line of subscriber ,no4 #fg4 ,as stated1 each of the pairs of bank contacts are connected with the line of a subscriber2 the line1 ,fig4 #chj1 is shown so connected to the forty-first pair of contacts1 that is to the first contact in the fourth row4 ,the selecting switch shown in ,fig4 #chj would be for the sole use of the subscriber on the line ,no4 #da4 ,each of the other subscribers would have a similar switch for his own exclusive use4 ,since any of the switches must be capable of reaching line ,no4 #fg1 for instance1 when moved up six rows and around seven1 it follows that the sixty-seventh pair of contacts in each bank of the entire one hundred switches must also be connected together and to line ,no4 #fg4 ,the same is1 of course1 true of all the contacts corresponding to any other number4 ,multiple connections are thus involved between the corresponding contacts of the banks1 in much the same way as in the corresponding jacks in the multiple of a manual switchboard4 ,as a result of this multiple connection of the bank contacts1 any subscriber may move the wiper arm of his selecting switch into connection with the line of any other subscriber4 ,the ,up-and-,around ,movement4 ,the elemental idea to be grasped by the discussion so far1 is the so-called up-and-around method of action of the selecting switches employed in this system4 ,this preliminary discussion may be carried a step further by saying that the arrangement is such that when a subscriber presses both his keys and grounds both of the limbs of his line1 such a condition is brought about as will cause all holding pawls to be withdrawn from the shaft1 and thus allow it to return to its normal position with respect to both its vertical and rotary movements4 ,no attempt has been made in ,fig4 #chj to show how this is accomplished4 ,function of ,line ,switch4 ,such a system as has been briefly outlined in the foregoing would require a separate selecting switch for each subscriber's line and would be limited to use in exchanges having not more than one hundred lines4 ,in the modern system of the ,automatic ,electric ,company1 the requirement that each subscriber shall have a selective switch1 individual to his own line1 has been eliminated by introducing what is called an individual line switch by means of which any one of a group of subscribers' lines1 making a call1 automatically appropriates one of a smaller group of selecting switches and makes it its own only while the connection exists4 ,subdivision of ,subscribers' ,lines4 ,the limitation as to the size of the exchange has been overcome1 without increasing the number of bank contacts in any selecting switch1 by dividing the subscribers' lines into groups of one hundred and causing selecting switches automatically to extend the calling subscriber's line first into a group of groups corresponding1 for instance1 to the thousand containing the called subscriber's line1 and then into the particular group containing the line1 and lastly1 to connect with the individual line in that group4 ,underlying ,feature of ,trunking ,system4 ,it will be remembered that in the chapter on fundamental principles of automatic systems1 it was stated that the subscriber1 by means of the signal transmitter at his station1 was made to govern the action of the central-office apparatus in the selection of a proper group of trunks2 and the group being selected1 the central-office apparatus was made to act automatically to pick out and connect with the first idle trunk of such group4 ,this selection by the subscriber of a group followed by the automatic selection from among that group forms the basis of the trunking system4 ,it is impossible1 by means of any simple diagram1 to show a complete scheme of trunking employed1 but ,fig4 #cha will give a fundamental conception of it4 ,this figure shows how a single calling line1 indicated at the bottom1 may find access into any particular line in an office having a capacity for ten thousand4 ,names of ,selecting ,switches4 ,selecting switches of the up-and-around type are the means by which the calling line selects and connects with the trunk lines required in building up the connection1 and finally selects and connects with the line of the called subscriber4 ,where such a switch is employed for the purpose of selecting a trunk1 it is called a selector switch4 ,it is a first selector when it serves to pick out a major group of lines1 i4 e41 a group containing a particular thousand lines or1 in a multi-office system1 a group represented by a complete central office4 ,it is a second selector when it serves to make the next subdivision of groups2 a third selector if further subdivision of groups is necessary2 and finally it is a connector when it is employed to pick out and connect with the particular line in the final group of one hundred lines to which the connection has been brought by the selectors4 ,in a single office of #aj1jjj-line capacity1 therefore1 we would have first and second selectors and connectors1 the first selectors picking out the thousands1 the second selectors the hundreds1 and the connectors the individual line4 ,in a multi-office system we may have first1 second1 and third selectors and connectors1 the first selector picking out the office1 the second selector the thousands in that office1 the third selector the hundreds1 and the connector the individual lines4 ,the ,line ,switch4 ,in addition to the selectors and connectors there are line switches1 which are comparatively simple1 one individual to each line4 ,each of these has the function1 purely automatic1 of always connecting a line1 as soon as a call is originated on it1 to some one of a smaller group of first selectors available to that line4 ,this idea may be better grasped when it is understood that1 in the earlier systems of the ,automatic ,electric ,company1 there was a first selector permanently associated with each line4 ,by the addition of the comparatively simple line switch1 a saving of about ninety per cent of the first selectors was effected1 since the number of first selectors was thereby reduced from a number equal to the number of lines in a group to a number equal to the number of simultaneous connections resulting from calls originating in that group4 ,in other words1 by the line switch1 the number of first selectors is determined by the traffic rather than by the number of lines4 ,scheme of ,trunking4 ,with this understanding as to the names and broader functions of the things involved1 ,fig4 #cha may now be understood4 ,the line switch of the single line1 as indicated here1 has only the power of selection among three trunks1 but it is to be understood that in actual practice1 it would have access to a greater number1 usually ten4 ,so1 also1 throughout this diagram we have shown the apparatus and trunks arranged in groups of three instead of in groups of ten1 only the first three thousands groups being indicated and the first three hundreds groups in each thousand4 ,again only three levels instead of ten are indicated for each selecting switch1 it being understood that in the diagram the various levels are represented by concentric arcs of circles1 and the trunk contacts by dots on these arcs4 ,line-,switch ,action4 ,when the subscriber1 whose line is shown at the bottom of the figure1 begins to make a call1 the line switch acts to connect his line with one of the first selector trunks available to it4 ,this selection is entirely preliminary and1 except to start it1 is in no way under the control of the calling subscriber4 ,the calling line now has under its control a first selector which1 for the time being1 becomes individual to it4 ,let it be assumed that the line switch found the first of the first selector trunks already appropriated by some other switch1 but that the second one of these trunks was found idle4 ,this trunk being appropriated by the line switch places the center one of the first selectors shown under the control of the subscriber's line4 ,this first selector then acts in response to the first set of selective impulses sent out by his signal transmitter4 ,illustration3 ,,dean ,,harmonic ,,converter ,dry ,cell ,type for ,magneto ,exchange4 ,the ,dean ,electric ,co4 ,illustration3 ,fig4 #cha4 ,scheme of ,trunking ,first ,selector ,action4 ,we will assume that the calling subscriber desires to connect with ,no4 #cbac4 ,the first movement of the subscriber's signal transmitter will send1 therefore1 three impulses over the line4 ,these impulses will act on the vertical magnet of the first selector switch to move it up three steps4 ,on this level of the contact bank of this switch all of the contacts will represent second selector trunks leading to the third thousand group4 ,the other ends of these trunks will terminate in the wipers and also in the controlling magnets of second selectors serving this thousand4 ,this function on the part of the first selector controlled by the act of the subscriber will have thus selected a group of trunks leading to the third thousand1 but the subscriber has nothing to do with which one of the trunks of this group will actually be used4 ,immediately following the vertical movement of the first selector switch the rotary movement of this switch will start and will continue until the wipers of that switch have found contacts of an idle trunk leading to a second selector4 ,assuming that the first trunk was the one found idle1 the first selector wipers would pause on the first pair of contacts in the third level of its bank1 and the trunk chosen may be seen leading from that contact off to the group of second selectors belonging to the third thousand4 ,for clearness1 the chosen trunks in this assumed connection are shown heavier than the others4 ,second ,selector ,action4 ,the next movement of the dial by the subscriber in establishing his desired connection will send two impulses1 it being desired to choose the second hundred in the third thousand4 ,the first selector will have become inoperative before this second series of impulses is sent and1 therefore1 only the second selector will respond4 ,its vertical magnet acting under the influence of these two impulses will step up its wiper contacts opposite the second row of bank contacts1 and the subscriber will thus have chosen the group of trunks leading to the second hundred in the third thousand4 ,here1 again1 the automatic operation of picking out the first idle one of this chosen group of trunks will take place without the volition of the subscriber1 and it will be assumed that the first two trunks on this level of the second selector were found already engaged and that the third was therefore chosen4 ,the connection continues1 as indicated by heavy lines in ,fig4 #cha1 to the third one of the connectors in the second hundred of the third thousand4 ,any one of these connectors would have accomplished the purpose but this is assumed to be the first one found idle by the second selector4 ,connector ,action4 ,the third movement of the subscriber's dial will send but one impulse1 this corresponding to the first group of ten in the second hundred in the third thousand4 ,this impulse will move the connector shaft up to the first level of bank contacts2 and from now on the action of the connector differs radically from that of the selectors4 ,the connector is not searching for an idle trunk in the group but for a particular line and1 therefore1 having chosen the group of ten lines in the desired hundred1 the connector switch waits for further guidance from the subscriber4 ,this comes in the form of the final set of impulses sent by the subscriber's signal transmitter which1 in this case1 will be three in number1 corresponding to the final digit in the number of the called subscriber4 ,this series of impulses will control the rotary movement of the connector wipers which will move along the first level and stop on the third one4 ,the process is seen to be one of successive selection1 first of a large group1 then of a smaller1 again of a smaller1 and finally of an individual4 ,if the line is found not busy1 the connection between the two subscribers is complete and the called subscriber's bell will be rung4 ,if it is found busy1 however1 the connector will refuse to connect and will drop back to its normal position1 sending a busy signal back to the calling subscriber4 ,the details of ringing and the busy-back operation may only be understood by a discussion of drawings1 subsequently to be referred to4 ,two-,wire and ,three-,wire ,systems4 ,in most of the systems of the ,automatic ,electric ,company in use today the impulses by which the subscriber controls the central-office apparatus flow over one side of the line or the other and return by ground4 ,the metallic circuit is used for talking and for ringing the called subscriber's bell1 while ground return circuits1 on one side of the line or the other1 are used for sending all the switch controlling impulses4 ,recently this company has perfected a system wherein no ground is required at the subscriber's station and no ground return path is used for any purpose between the subscriber and the central office4 ,this later system is known as the two-wire system1 and in contra-distinction to it1 the earlier and most used system has been referred to as the three-wire4 ,it is not meant by this that the line circuits actually have three wires but that each line employs three conductors1 the two wires of the line and the earth4 ,the three-wire system will be referred to and described in detail1 and from it the principles of the two-wire system will be readily understood4 ,illustration3 ,fig4 #chb4 ,automatic ,wall ,set ,illustration3 ,fig4 #chc4 ,automatic ,desk ,stand ,subscriber's ,station ,apparatus4 ,the detailed operation of the three-wire system may be best understood by considering the subscriber's station apparatus first4 ,the general appearance of the wall set is shown in ,fig4 #chb1 and of the desk set in ,fig4 #chc4 ,these instruments embody the usual talking and call-receiving apparatus of a common-battery telephone and in addition to this1 the signal transmitter1 which is the thing especially to be considered now4 ,the diagrammatic illustration of the signal transmitter and of the relation that its parts bear to the other elements of the telephone set is shown in ,fig4 #chd4 ,it has already been stated that the subscriber manipulates the signal transmitter by rotating the dial on the face of the instrument4 ,a clearer idea of this dial and of the finger stop for it may be obtained from ,figs4 #chb and #chc4 ,illustration3 ,fig4 #chd4 ,circuits of ,telephone ,set ,operation4 ,to make a call for a given number the subscriber removes his receiver from its hook1 then places his forefinger in the hole opposite the number corresponding to the first digit of the desired number4 ,by means of the grip thus secured1 he rotates the dial until its movement is stopped by the impact of the finger against the stop4 ,the dial is then released and in its return movement it sends the number of impulses corresponding to the first digit in the called number4 ,a similar movement is made for each digit4 ,in ,fig4 #chd is given a phantom view of the dial1 in order to show more clearly the relation of the mechanical parts and contacts controlled by it4 ,for a correct idea of its mechanical action it must be understood that the shaft #a1 the lever #b1 and the interrupter segment #c are all rigidly fastened to the dial and move with it4 ,a coiled spring always tends to move the dial and these associated parts back to their normal positions when released by the subscriber1 and a centrifugal governor1 not shown1 limits the speed of the return movement4 ,the subscriber's hook switch is mechanically interlocked with the dial so as to prevent the dial being moved from its normal position until the hook is in its raised position4 ,this interlocking function involves also the pivoted dog #d4 ,normally the lower end of this dog lies in the path of the pin #e carried on the lever #b1 and thus the shaft1 dial1 and segment are prevented from any considerable movement when the receiver is on the hook4 ,however1 when the receiver is removed from its hook1 the upwardly projecting arm from the hook engages a projection on the dog #d and moves the dog out of the path of the pin #e4 ,thus the dial is free to be rotated by the subscriber4 ,the pin #f is mounted in a stationary position and serves to limit the backward movement of the dial by the lever #b striking against it4 ,ground ,springs3--,five groups of contact springs must be considered1 some of which are controlled wholly by the position of the switch hook1 others jointly by the position of the switch hook and the dial1 others by the movement of the dial itself1 and still others by the pressure of the subscriber's finger on a button4 ,the first of these groups consists of the springs #g and #h1 the function of which is to control the continuity of the ground connection at the subscriber's station4 ,the arrangement of these two springs is such that the ground connection will be broken until the subscriber's receiver is removed from its hook4 ,as soon as the receiver is raised1 these springs come together in an obvious manner1 the dog #d being lifted out of the way by the action of the hook4 ,the ledge on the lower portion of the spring #g serves as a rest for the insulated arm of the dog #d to prevent this dog1 which is spring actuated1 from returning and locking the dial until after the receiver has been hung up4 ,bell and ,transmitter ,springs3--,the second group is that embracing the springs #i1 #aj1 #aa1 and #ab4 ,the springs #aj and #aa are controlled by the lower projection from the switch hook1 the spring #aa engaging the spring #ab only when the hook is down4 ,the spring #aj engages the spring #i only when the hook lever is up and not then unless the dial is in its normal position4 ,while the hook is raised1 therefore1 the springs #i and #aj break contact whenever the dial is moved and make contact again when it returns to its normal position4 ,the springs #aa and #ab control the circuit through the subscriber's bell while the springs #i and #aj control the continuity of the circuit from one side of the line to the other so as to isolate the limbs from each other while the signal transmitter is sending its impulses to the central office4 ,impulse ,springs3--,the third group embraces springs #ac1 #ad1 and #ae and these are the ones by which the central-office switches are controlled in building up a connection4 ,something of the prevailing nomenclature which has grown up about the automatic system may be introduced at this point4 ,the movements of the selecting switches at the central office are referred to as vertical and rotary for obvious reasons4 ,on account of this the magnet which causes the vertical movement is referred to as the vertical magnet and that which accomplishes the rotary movement as the rotary magnet4 ,it happens that in all cases the selecting impulses sent by the subscriber's station1 corresponding respectively to the number of digits in the called subscriber's number1 are sent over one side of the line and in nearly all cases these selecting impulses actuate the vertical movements of the selecting switches4 ,for this reason the particular limb of the line over which the selecting impulses are sent is called the vertical limb4 ,the other limb of the line is the one over which the single impulse is sent after each group of selecting impulses1 and it is this impulse in every case which causes the selector switch to start rotating in its hunt for an idle trunk4 ,this side of the line is1 therefore1 called rotary4 ,for the same reasons the impulses over the vertical side of the line are called vertical impulses and those over the rotary side1 rotary impulses4 ,the naming of the limbs of the line and of the current impulses vertical and rotary may appear odd but it is1 to say the least1 convenient and expressive4 ,coming back to the functions of the third group of springs1 #ac1 #ad1 and #ae1 #ae may be called the vertical spring since it sends vertical impulses2 #ac1 the rotary spring since it sends rotary impulses2 and #ad1 the ground spring since1 when the hook is up1 it is connected with the ground4 ,on the segment #c there are ten projections or cams #af which1 when the dial is moved1 engage a projection of the spring #ae4 ,when the dial is being pulled by the subscriber's finger1 these cams engage the spring #ae in such a way as to move it away from the ground spring and no electrical contact is made4 ,on the return of the dial1 however1 these cams engage the projection on the spring #ae in the opposite way and the passing of each cam forces this vertical spring into engagement with the ground spring4 ,it will readily be seen1 therefore1 by a consideration of the spacing of these cams on the segment and the finger holes in the dial that the number of cams which pass the vertical spring #ae will correspond to the number on the hole used by the subscriber in moving the dial4 ,near the upper right-hand corner of the segment #c1 as shown in ,fig4 #chd1 there is another projection or cam #ag1 the function of which is to engage the rotary spring #ac and press it into contact with the ground spring4 ,thus1 the first thing that happens in the movement of the dial is for the projection #ag to ride over the hump on the rotary spring and press the contact once into engagement with the ground spring2 and likewise1 the last thing that happens on the return movement of the dial is for the rotary spring to be connected once to the ground spring after the last vertical impulse has been sent4 ,if both the rotary and vertical sides of the line are connected with the live side of the central-office battery1 it follows that every contact between the vertical and the ground spring or between the rotary and the ground spring will allow an impulse of current to flow over the vertical or the rotary side of the line4 ,we may summarize the action of these impulse springs by saying that whenever the dial is moved from its normal position1 there is1 at the beginning of this movement1 a single rotary impulse over the rotary side of the line2 and that while the dial returns1 there is a series of vertical impulses over the vertical side of the line2 and just before the dial reaches its normal position1 after the sending of the last vertical impulse1 there is another impulse over the rotary side of the line4 ,the mechanical arrangements of the interrupter segment #c and its associated parts have been greatly distorted in ,fig4 #chd in order to make clear their mode of operation4 ,this drawing has been worked out with great care1 with this in mind1 at a sacrifice of accuracy in regard to the actual structural details4 ,ringing ,springs3--,the fourth group of springs in the subscriber's telephone is the ringing group and embraces the springs #ah1 #ai1 and #bj4 ,the springs #ai and #bj are normally closed and maintain the continuity of the talking circuit4 ,when1 however1 the button attached to the spring #ai--which button may be seen projecting from the instrument shown in ,fig4 #chb1 and from the base of the one shown in ,fig4 #chc--;is pressed1 the continuity of the talking circuit is interrupted and the vertical side of the line is connected with the ground4 ,it is by this operation1 after the connection has been made with the desired subscriber's line1 that the central-office apparatus acts to send ringing current out on that line4 ,release ,springs3--,the fifth set of springs is the one shown at the left-hand side of ,fig4 #chd1 embracing springs #ba1 #bb1 and #bc4 ,the long curved spring #ba is engaged by the projecting lug on the switch hook when it rises so as to press this spring away from the other two4 ,on the return movement of the hook1 however1 this spring is pressed to the left so as to bring all three of them into contact1 and this1 it will be seen1 grounds both limbs of the line at the subscriber's station4 ,this combination cannot be effected by any of the other springs at any stage of their operation1 and it is the one which results in the energization of such a combination of relays and magnets at the central office as will release all parts involved in the connection and allow them to return to their normal positions ready for another call4 ,salient ,points4 ,if the following things are borne in mind about the operation of the subscriber's station apparatus1 an understanding of the central-office operations will be facilitated4 ,first1 the selective impulses always flow over the vertical side of the line2 they are always preceded and always followed by a single impulse over the rotary side of the line4 ,the ringing button grounds the vertical side of the line and the release springs ground both sides of the line simultaneously4 ,the ,line ,switch4 ,the first thing to be considered in connection with the central-office apparatus is the line switch4 ,this1 it will be remembered1 is the device introduced into each subscriber's line at the central office for the purpose of effecting a reduction of the number of first selectors required at the central office1 and also for bringing about certain important functional results in connection with trunking between central and sub-offices4 ,the function of the line switch in connection with the subscriber's line1 however1 is purely that of reducing the number of first selectors4 ,the line switches of one hundred lines are all associated to form a single unit of apparatus1 which1 besides the individual line switches1 includes certain other apparatus common to those lines4 ,such a group of one hundred line switches and associated common apparatus is called a line-switch unit1 or frequently1 a ,keith unit4 ,confusion is likely to arise in the mind of the reader between the individual line switch and the line-switch unit1 and to avoid this we will refer to the piece of apparatus individual to the line as the line switch1 and to the complete unit formed of one hundred of these devices as a line-switch unit4 ,line and ,trunk ,contacts4 ,each line switch has its own bank of contacts arranged in the arc of a circle1 and in this same arc are also placed the contacts of each of the ten individual trunks which it is possible for that line to appropriate4 ,the contacts individual to the subscriber's line in the line switch are all multipled together1 the arrangement being such that if a wedge or plunger is inserted at any point1 the line contacts will be squeezed out of their normal position so as to engage the contacts of the trunk corresponding to the particular position in the arc at which the wedge or plunger is inserted4 ,a small plunger individual to each line is so arranged that it may be thrust in between the contact springs in the line-switch bank in such manner as to connect any one of the trunks with the line terminals represented in that row1 the particular trunk so connected depending on the portion of the arc toward which the plunger is pointed at the time it is thrust in the contacts4 ,these banks of lines and trunk contacts are horizontally arranged1 and piled in vertical columns of twenty-five line switches each4 ,the ten trunk contacts are multipled vertically through the line-switch banks1 so that the same ten trunks are available to each of the twenty-five lines4 ,we thus have1 in effect1 an old style1 ,western ,union1 cross-bar switchboard1 the line contacts being represented in horizontal rows and the trunk contacts in vertical rows1 the connection between any line and any trunk being completed by inserting a plunger at the point of intersection of the horizontal and the vertical rows corresponding to that line and trunk4 ,trunk ,selection4 ,the plungers by which the lines and trunks are connected are1 as has been said1 individual to the line1 and all of the twenty-five plungers in a vertical row are mounted in such manner as to be normally held in the same vertical plane1 and this vertical plane is made to oscillate back and forth by an oscillating shaft so as always to point the plungers toward a vertical row of trunk contacts that represent a trunk that is not in use at the time4 ,the to-and-fro movement of this oscillating shaft1 called the master bar1 is controlled by a master switch and the function of this master switch is always to keep the plungers pointed toward the row of contacts of an idle trunk4 ,the thrusting movement of the individual plungers into the contact bank is controlled by magnets individual to the line and under control of the subscriber in initiating a call4 ,as soon as the plunger of a line has been thus thrust into the contact bank so as to connect the terminals of that line with a given trunk1 the plunger is no longer controlled by the master bar and remains stationary4 ,the master bar then at once moves all of the other plungers that are not in use so that they will point to the terminals of another trunk that is not in use4 ,the plungers of all the line switches in a group of twenty-five are1 therefore1 subject to the oscillating movements of the master bar when the line is not connected to a first selector trunk4 ,as soon as a call is originated on a line1 the corresponding plunger is forced into the bank and is held stationary in maintaining the connection to a first selector trunk1 and all of the other plungers not so engaged1 move on so as to be ready to engage another idle trunk4 ,trunk ,ratio4 ,the assignment of ten trunks to twenty-five lines would be a greater ratio of trunks than ordinary traffic conditions require4 ,this ratio of trunks to lines is1 however1 readily varied by multipling the trunk contacts of several twenty-five line groups together4 ,thus1 ten trunks may be made available to one hundred subscribers' lines by multipling the trunks of four twenty-five line switch groups together4 ,in this case the four master bars corresponding to the four groups of twenty-five line switches are all mechanically connected together so as to move in unison under the control of a single master switch4 ,if more than ten and less than twenty-one trunks are assigned to one hundred lines1 then each set of ten trunks is multipled to the trunk contacts of fifty line switches1 the two master bars of these switches being connected together and controlled by a common master switch4 ,structure of ,line ,switch4 ,the details of the parts of a line switch that are individual to the line are shown in ,fig4 #che1 the line and trunk contact bank being shown in the lower portion of this figure and also in a separate view in the detached figure at the right4 ,a detailed group of several such line switches with the oscillating master bar is shown in ,fig4 #chf4 ,this figure shows quite clearly the relative arrangement of the line and trunk contact banks1 the plungers for each bank1 and the master bar4 ,illustration3 ,fig4 #che4 ,line ,switch ,in practice1 four groups of twenty-five line switches each are mounted on a single framework and the group of one hundred line switches1 together with certain other portions of the apparatus that will be referred to later1 form a line-switch unit4 ,a front view of such a unit is shown in ,fig4 #chg4 ,in order to give access to all portions of the wiring and apparatus1 the framework supporting each column of fifty line switches is hinged so as to open up the interior of the device as a whole4 ,a line-switch unit thus opened out is shown in ,fig4 #chh4 ,illustration3 ,fig4 #chf4 ,portion of ,line-,switch ,unit ,circuit ,operation4 ,the mode of operation of the line switch may be best understood in connection with ,fig4 #chi1 which shows in a schematic way the parts of a line switch that are individual to a subscriber's line1 and also those that are common to a group of fifty or one hundred lines4 ,those portions of ,fig4 #chi which are individual to the line are shown below the dotted line extending across the page4 ,the task of understanding the line switch will be made somewhat easier if ,figs4 #che and #chi are considered together4 ,the individual parts of the line switch are shown in the same relation to each other in these two figures with the exception that the bank of line and trunk springs in the lower right-hand corner of ,fig4 #chi have been turned around edgewise so as to make an understanding of their circuit connections possible4 ,illustration3 ,fig4 #chg4 ,line-,switch ,unit ,illustration3 ,fig4 #chh4 ,line-,switch ,unit ,illustration3 ,fig4 #chi4 ,circuits of ,line-,switch ,unit ,the vertical and rotary sides of the subscriber's line are shown entering at the lower left-hand corner of this figure1 and they pass to the springs of the contact bank4 ,immediately adjacent to these springs are the trunk contacts from which the vertical and the rotary limbs of the first selector trunk proceed4 ,the plunger is indicated at #a1 it being in the form of a wheel of insulating material4 ,it is carried on the rod #b pivoted on a lever #c1 which1 in turn1 is pivoted at #d in a stationary portion of the framework4 ,a spring #e1 secured to the underside of the lever #c and projecting to the left beyond the pivot #d of this lever1 serves always to press the right-hand portion of the lever #c forward in such direction as to tend to thrust it into the contact bank4 ,the plunger is normally held out of the contact bank by means of the latch #f carried on the armature #g of the trip magnet4 ,when the trip magnet is energized it pulls the armature #g to the left and thus releases the plunger and allows it to enter the contact bank4 ,illustration3 ,,power ,,switchboard ,,for ,,medium-,,sized ,,office ,mercury ,arc ,rectifier ,panel and ,transformer at ,right4 ,the master bar is shown at #h1 and a feather on this bar engages a notch in the segment attached to the rear end of the plunger rod #b4 ,this master bar is common to all of the plunger rods and by its oscillatory movement1 under the influence of the master switch1 it always keeps all of the idle plunger bars pointed toward the contacts of an idle trunk4 ,as soon1 however1 as the trip magnet is operated to cause the insertion of a plunger into the contact bank1 the feather on the master bar is disengaged by the notch in the segment of the plunger rod1 and the plunger rod is1 therefore1 no longer subject to the oscillating movement of the master bar4 ,when the release magnet is energized1 it attracts its armature #i and this lifts the armature #g of the trip magnet so that the latch #f rides on top of the left-hand end of the lever #c4 ,then1 when the release magnet is de-energized1 the spring #e1 which was put under tension by the latch1 moves the entire structure of levers back to its normal position1 withdrawing the plunger from the bank of contacts4 ,the notch on the edge of the segment of the plunger rod1 when thus released1 will probably not strike the feather on the master bar1 and the plunger rod will thus not come under the control of the master bar until the master bar has moved1 in its oscillation1 so that the feather registers with the notch1 after which this bar will move with all the others4 ,if1 while the plunger is waiting to be picked up by the master bar1 the same subscriber should call again1 his line will be connected with the same trunk as before4 ,there is no danger in this1 however1 that the trunk will be found busy1 because the master bar will not have occupied a position which would make it possible for any of the lines to appropriate this trunk during the intervening time4 ,master ,switch4 ,associated with each master bar there is a master switch which determines the position in which the master bar shall stop in order that the idle plungers may be pointed always to the contacts of an idle trunk4 ,the arm #aj of this switch is attached to the master bar and oscillates with it and serves to connect the segment #aa successively with the contacts #ab1 which are connected respectively to the third1 or release wire of each first selector trunk4 ,in the figure the arm #aj is shown resting on the sixth contact of the switch and this sixth contact is connected to a spring #ac in the line-switch contact bank that has not yet been referred to4 ,as soon as the plunger is inserted into the contact bank1 the spring #ad will be pressed into engagement with the spring #ac1 and this spring #ad is connected with the live side of the battery through the release magnet winding4 ,the contact strip #aa on the master switch is thus connected through the release magnet to the battery and from this current flows through the left-hand winding of the master-switch relay4 ,this energizes this relay and causes the closure of the circuit of the locking magnet which magnet unlocks the master bar to permit its further rotation4 ,the unlocking of the master bar brings the spring #ae into engagement with #af and thus energizes the master magnet1 the armature of which vibrates back and forth after the manner of an electric-bell armature1 and steps the wheel #ag around4 ,the wheel #ag is mechanically connected to the master bar so that each complete revolution of the wheel will cause one complete oscillation of the master bar4 ,the master bar will thus be moved so as to cause all the idle plungers to sweep through an arc and this movement will stop as soon as the master-switch arm #aj connects the arc #aa with one of the contacts #ab that is not connected to the live side of the battery through the springs #ac and #ad of some other line switch4 ,it is by this means that the plungers of the line switches are always kept pointing at the contacts of an idle trunk4 ,the way in which this feature has been worked out must demand admiration and accounts for the marvelous quickness of this line switch4 ,the fact that the plungers are pointed in the right direction before the time comes for their use1 leaves only the simple thrusting motion of the plunger to accomplish the desired connection immediately upon the initiation of a call by the subscriber4 ,locking ,segment4 ,it will be understood that the locking segment #ah and the master-switch contact finger #aj are both rigidly connected with the master bar #h and move with it1 the locking segment #ah serving always to determine accurately the angular position at which the master bar and the master-switch arm are brought to rest4 ,bridge ,cut-,off4 ,one important feature of automatic switching1 particularly as exemplified in the system of the ,automatic ,electric ,company1 is the disconnection1 after its use1 of each operating magnet of each piece of apparatus involved in making a connection4 ,since these operating magnets are always bridged across the line at the time of their operation and then cut off after they have performed their function1 this feature may be referred to as the bridge cut-off4 ,guarding ,functions4 ,still another feature of importance is the means for guarding a line or a piece of apparatus that has already been appropriated or made busy1 so that it will not be appropriated or connected with for use in some other connection4 ,for this latter purpose contacts and wires are associated with each piece of apparatus1 which are multipled to similar contacts on other pieces of apparatus in much the same way and for a similar purpose that the test thimbles in a multiple switchboard are multipled together4 ,such wires and contacts in the ,automatic ,electric ,company's apparatus are called private wires and contacts4 ,the bridge cut-off and guarding functions are provided for in the line switch by a bridge cut-off relay shown in ,fig4 #chi and also in ,fig4 #che1 it being the upper one of the individual line relays in each of those figures4 ,this bridge cut-off relay is operated as soon as the plunger of the line is thrust into the bank2 the contacts #ai and #bj1 closed by the plunger1 serving to complete the circuit of this relay4 ,to make clear the bridge cut-off feature it will be noted that the trip magnet of a line switch is connected in a circuit traced from the rotary side of the line through the contacts #ba and #bb of the bridge cut-off relay1 thence through the coil of the trip magnet to the common wire leading to the spring #bc of the master-bar locking device and thence to the live side of the battery4 ,obviously1 therefore1 as soon as the bridge cut-off relay operates1 the trip magnet becomes inoperative and can cause no further action of the line switch because its circuit is broken between the springs #ba and #bb4 ,the private or guarding feature is taken care of by the action of the plunger in closing contacts #ai and #bj1 since the private wire leading to the bridge cut-off relay is1 as has already been stated1 connected to ground when these contacts are closed4 ,this private wire leads off and is multipled to the private contacts on all the connectors that have the ability to reach this line1 and the fact that this wire is grounded by the line switch as soon as it becomes busy1 establishes such conditions at all of the connectors that they will refuse to connect with this line as long as it is busy1 in a way that will be pointed out later on4 ,relation of ,line ,switch and ,connectors4 ,the vertical and rotary wires of the subscriber's line are shown leading off to the connector banks at the left-hand side of ,fig4 #chi1 and one side of this connection passes through the contacts #bd and #be of the bridge cut-off relay on the line switch4 ,it is through this path that a connection from some other line through a connector to this line is established and it is seen that this path is held open until the bridge cut-off relay of the line switch is operated4 ,for such a connection to this line the bridge cut-off relay of the line switch is operated over the private wire leading from the connector1 and the operation of the bridge cut-off relay at this time serves to render inoperative the line switch1 so that it will not perform its usual functions should the called subscriber start to make a call after his line had been seized4 ,summary of ,line-,switch ,operation4 ,to summarize the operation of a line switch when a call is originated on its line1 the first movement of the calling subscriber's dial will ground the rotary side of the line and operate the trip magnet4 ,this will cause the plunger to be inserted into the bank1 and thus extend the line to the first selector trunk through the closing of the right-hand set of springs shown in the lower right-hand corner of ,fig4 #chi4 ,the insertion of the plunger will also connect the battery through the left-hand winding of the master-switch relay and1 by the sequence of operations which follows1 cause the master bar to move all of the idle plungers so as to again point them to an idle trunk4 ,the closure of contacts #ai and #bj by the plunger causes the operation of the bridge cut-off relay which opens the circuit of the trip magnet1 rendering it inoperative2 and also establishes ground potential on all the private wire contacts of that line in the banks of the connectors1 so as to guard the line and its associated apparatus against intrusion by others4 ,the line is cut through1 therefore1 to a first selector and all of the line-switch apparatus is completely cut off from the talking circuit4 ,it must be remembered that all of the actions of the line switch1 which it has taken so long to describe1 occur practically instantaneously and as a result of the first part of the first movement of the subscriber's dial4 ,the line switch has done its work and gone out of business before the selective impulses of the first digit begin to take place4 ,selecting ,switches4 ,the first selector is now in control of the calling subscriber4 ,the circuits and elements of the first selector switch are shown in ,fig4 #cij4 ,the general mechanical structure of the first selectors1 second selectors1 and connectors1 is the same and may be referred to briefly here4 ,fig4 #cia shows a rear view of a first selector2 ,fig4 #cib1 a side view of a second selector2 and ,fig4 #cic1 a front view of a connector4 ,the arrangement of the vertical and rotary magnets1 of the selector shafts1 and of the contact banks are identical in all three of these pieces of apparatus and all these switches work on the up-and-around principle referred to in connection with ,fig4 #chj4 ,it is thought that with the general structure shown in ,figs4 #cia1 #cib1 and #cic in mind1 the actual operation may be understood much more readily from ,fig4 #cij4 ,four magnets--the vertical1 the rotary1 the private1 and the release--produce the switching movements of the machine4 ,these magnets are controlled by various combinations brought upon the circuits by three relays--the vertical1 the rotary1 and the back release4 ,the fourth relay shown1 called the off-normal1 is purely for signaling purposes1 as will be described4 ,side ,switch4 ,another important element of the selecting switches is the so-called side switch which might better be called a pilot switch--but we are not responsible for its name4 ,this side switch has for its function the changing of the control of the subscriber's line to successive portions of the selector mechanism1 rendering inoperative those portions that have already performed their functions and that1 therefore1 are no longer needed4 ,this switch may be seen best in ,fig4 #cib just above the upper bank of contacts4 ,it is shown in ,fig4 #cij greatly distorted mechanically so as to better illustrate its electrical functions4 ,illustration3 ,fig4 #cij4 ,circuits of ,first ,selector ,the contact levers #a1 #b1 #c1 and #d of the side switch are carried upon the arm #e which is pivoted at #f4 ,all of these contact levers1 therefore1 move about #f as an axis4 ,the side switch has three positions and it is shown1 in ,fig4 #cij1 in the first one of these4 ,when the private magnet armature is attracted and released once1 the escapement carried by it permits the spring #g to move the arm #e so as to bring the wipers of the side switch into its second position2 the second pulling up and release of the private magnet armature will cause the movement of the side switch wipers into the third position4 ,it is to be noted that the escapement which releases the side switch arm may be moved either by the private or by the rotary magnet1 since the armature of the latter has a finger which engages the private magnet armature4 ,illustration3 ,fig4 #cia4 ,rear ,view of ,first ,selector ,functions of ,side ,switch4 ,the functions of the side switch may be briefly outlined in connection with the first selector1 as an example4 ,in the first position it extends the control of the subscriber's signal transmitter through the first selector trunk and line relays to the vertical and private magnets so that these magnets will be responsive to the selecting impulses corresponding to the first digit4 ,in its second position it brings about such a condition of affairs that the rotary magnet will be brought into play and automatically move the wipers over the bank contacts in search of an idle trunk4 ,in its third position1 both the vertical and rotary relays are cut off and the line is cut straight through to the second selector trunk1 and only those parts of the first selector apparatus are left in an operative state which have to do with the private or guarding circuits and with the release4 ,similar functions are performed by the side switch in connection with the other selecting switches4 ,illustration3 ,fig4 #cib4 ,side ,view of ,second ,selector ,release ,mechanism4 ,another one of the features of the switch that needs to be considered before a detailed understanding of its operation may be had1 is the mechanical relation of the holding and the release dog4 ,this dog is shown at #h and1 in the language of the art1 is called the double dog4 ,as will be seen1 it has two retaining fingers1 one adapted to engage the vertical ratchet and the other1 the rotary ratchet on the selector shaft4 ,this double dog is pivoted at #i and is interlinked in a peculiar way with the armature of the vertical magnet1 the armature of the release magnet1 and the arm of the side switch4 ,the function of this double dog is to hold the shaft in whatever vertical position it is moved by the vertical magnet and then1 when the rotary magnet begins to operate1 to hold the shaft in its proper angular position4 ,it will be noted that the fixed dog #aj is ineffective when the shaft is in its normal angular position4 ,but as soon as the shaft is rotated1 this fixed dog #aj becomes the real holding pawl so far as the vertical movement is concerned4 ,the double dog #h is normally held out of engagement with the vertical and the rotary ratchets by virtue of the link connection1 shown at #aa1 between the release magnet armature and the rear end of the double dog4 ,on the previous release of the switch the attraction of the release magnet armature permitted the link #aa to hook over the end of the dog #h and thus1 on its return movement1 to pull this dog out of engagement with its ratchets4 ,this movement also resulted in pushing on the link #ab which is pivoted to the side switch arm #e1 and thus the return movement of the release magnet is made to restore the side switch to its normal position4 ,in order that the double dog may be made effective when it is required1 and in order that the side switch may be free to move under the influence of the private magnet1 the double dog is released from its connection with the release magnet armature by the first movement of the vertical magnet in a manner which is clear from the drawing4 ,first ,selector ,operation4 ,in discussing the details of operation of the various selectors it will be found convenient to divide the discussion according to the position of the side switch4 ,this will bring about a logical arrangement because it is really the side switch which determines by its position the sequence of operation4 ,illustration3 ,fig4 #cic4 ,front ,view of ,connector ,first ,position of ,side ,switch4 ,this is the position shown in ,fig4 #cij1 and is the normal position4 ,the vertical and the rotary lines extending from the calling subscriber are continued by the levers #a and #b of the side switch through the vertical and the rotary relay coils1 respectively1 to the live side of battery4 ,the lever #d of the side switch in this position connects to ground the circuit leading from the line switch through the release trunk1 and the winding of the off-normal relay4 ,this winding is thus put in series with the release magnet of the line switch1 but on account of high resistance of the off-normal relay no operation of the release magnet is caused4 ,this will1 however1 permit such current to flow through the release circuit as will energize the sensitive off-normal relay and cause it to attract its armature and light the off-normal lamp4 ,if this lamp remains lighted more than a brief period of time1 it will attract notice and will indicate that the corresponding selector has been appropriated by a line switch and that for some reason the selector has gone no further4 ,this lamp1 therefore1 is an aid in preventing the continuance of this abnormal condition4 ,the first thing that happens after the line switch has connected the calling subscriber with the first selector is a succession of impulses over the vertical side of the line1 this being the set of impulses corresponding in number to the thousands digit or to the office1 if there is more than one4 ,it will be understood that here we are considering a single office of ten-thousand-line capacity or thereabouts1 and that1 therefore1 this first set of impulses corresponds to the thousands digit in the called subscriber's line4 ,each one of these impulses will flow from the battery through the vertical relay and each movement of this relay armature will close the circuit of the vertical magnet and cause the shaft of the selector to be stepped up to the proper level4 ,immediately following the first series of selecting impulses from the subscriber's station1 a single impulse follows over the rotary side of the line4 ,this gives the rotary relay armature one impulse and this in turn closes the circuit of the private magnet once4 ,the single movement of the private magnet armature allows the escapement finger on the arm #e to move one step and this brings the side switch contacts into the second position4 ,second ,position of ,side ,switch4 ,in this position lever #d of the side switch places a ground on the wire leading through the rotary magnet to a source of interrupted battery current4 ,the impulses which thus flow through the rotary magnet occur at a frequency dependent upon the battery interrupter and this is at a rate of approximately fifteen impulses per second4 ,the rotary magnet will step the selector shaft rapidly around until something occurs to stop these impulses4 ,this something is the finding by the private wiper of an ungrounded private contact in the bank1 since all of the contacts corresponding to busy trunks are grounded1 as will be explained4 ,the action of the private magnet enters into this operation in the following way3 ,a circuit may be traced from the battery through the private magnet to the third side switch wiper when in its second position1 thence through the back release relay to the private wiper4 ,if the wiper is at the time on the private bank contact of a busy trunk1 it will find that contact grounded and the private magnet will be energized4 ,the energizing of this magnet will not1 however1 cause the release of the side switch4 ,it must be energized and de-energized4 ,the private magnet armature will1 therefore1 be operated by the finger of the rotary magnet armature on the first rotary step4 ,the private magnet will be energized and hold its armature operated if the private wiper finds a ground on the first bank contact and will stay energized as long as the private wiper is passing over private contacts of busy trunks4 ,its armature will not be allowed to fall back during the passage of the wiper from one trunk to another1 because during that interval the finger of the rotary magnet will hold it operated4 ,as soon1 however1 as the private wiper reaches the private bank contact of an idle trunk1 no ground will be found and the circuit of the private magnet will be left open4 ,when the impulse through the rotary magnet ceases1 the private magnet armature will fall back and the side switch will be released to its third position4 ,third ,position of ,side ,switch4 ,the first thing to be noted in this position is that the calling line is cut straight through to the second selector trunk1 the connection being clean with no magnets bridged across or tapped off4 ,the third wiper of the side switch1 when in its third position1 is grounded and this connects the release wire of the second selector trunk1 on which the switch wipers rest1 through the private wiper1 the winding of the back release magnet1 and the third wiper of the side switch to ground4 ,this establishes a path for the subsequent release current through the back release magnet2 and1 of equal importance1 it places a ground on the private bank contact of that trunk so that the private wiper of any other switch will be prevented from stopping on the contacts of this trunk in the same manner that the wiper of this switch was prevented from stopping on other trunks that were already in use4 ,the fourth lever on the side switch1 when in its third position1 serves merely to close the circuit of the rotary off-normal lamp4 ,this lamp is for the purpose of calling attention to any first selector switch that has been brought into connection with some second selector trunk and which1 for some reason1 has failed in its release4 ,these off-normal lamps are so arranged that they may be switched off manually to avoid burning them during the hours of heaviest traffic4 ,at night they afford a ready means of testing for switches that have been left off-normal1 since the manual switches controlling these lamps may then be closed1 and any lamps which burn will show that the switches corresponding to them are off-normal4 ,simple tests then suffice to show whether they are properly or improperly in their off-normal position4 ,release of the ,first ,selector4 ,as will be shown later1 the normal way of releasing the switches is from the connector back over the release wire4 ,it is sufficient to say at this point that when the proper time for release comes1 an impulse of current will come back over the second selector trunk release wire through the private wiper1 to the back release relay magnet1 and thence to ground through the third wiper of the side switch which is in its third position4 ,it may be asked why the back release magnet was not energized during the previous operations described1 when current passed through it4 ,the reason for this is that in those previous operations the private magnet was always included in series in the circuit and on account of the high resistance of the private magnet1 sufficient current did not pass through the back release magnet to energize it4 ,when the back release relay is energized1 it closes the circuit of the release magnet and thus1 through the link #aa1 draws the double dog away from its engagement with the shaft ratchets and at the same time1 through the link #ab1 restores the side switch to its normal position4 ,whenever the release magnet is operated it acts as a relay to close a pair of contacts associated with it and thus to momentarily ground the release wire of the first selector trunk extending back to the line switch4 ,referring to ,fig4 #chi1 it will be seen that this path leads through the contacts #ac and #ad and the release magnet to the battery4 ,it is by this means that the line switch is released1 the release impulse being relayed back from the first selector4 ,second ,selector ,operation4 ,for the purpose of considering the action of the second selector1 we will go back to the point where the first selector had connected with a second selector trunk and where its side switch had moved into its third position4 ,in this condition1 it will be remembered1 the trunk line was cut through to a second selector trunk and all first selector apparatus cleared from the talking circuit4 ,the second selector chosen is one corresponding to the thousands group as determined by the first digit of the called subscriber's number4 ,the circuits of a second selector are shown in ,fig4 #cid and it must be borne in mind that the mechanical arrangements for producing the vertical and the rotary movement of the shaft and for operating the side switch are practically the same as those of the first selector4 ,as in the first selector1 the sequence of operation is controlled by the successive positions of the side switch1 the first position permitting the selection of the hundreds corresponding to the vertical impulses1 the second position allowing the selector to search for an idle trunk in that hundred1 and the third position cutting the trunk through and clearing the circuit of obstructing apparatus4 ,first ,position of ,side ,switch4 ,the first thing that happens when the subscriber begins to move his dial in the transmission of the second series of selecting impulses is the sending of a preliminary impulse over the rotary side of the line4 ,this1 in the case of the second selector1 energizes the rotary relay which1 in turn1 energizes the private magnet2 but the private magnet in the case of the second selector can do nothing toward the release of the side switch because the projection #e'1 on the side switch arm #e1 meets a projection on the rear of the selector shaft which thus prevents the movement of the side switch arm #e until the selector shaft has been moved out of its normal position4 ,immediately after the establishment of the connection to the selector1 the second set of selecting impulses comes in over the vertical wire from the subscriber's station4 ,these impulses1 corresponding in number to the hundreds digit1 will energize the vertical relay and cause it1 in turn1 to energize the vertical magnet1 stepping up the selector shaft to the level corresponding to the hundred sought4 ,the single rotary impulse1 which follows just before the subscriber's dial reaches its normal position1 will energize the rotary relay of the second selector4 ,this1 in turn1 energizes the private magnet which makes a single movement of its armature and allows the escapement finger on the side switch arm to move one step and bring the side switch contacts into the second position4 ,illustration3 ,fig4 #cid4 ,circuits of ,second ,selector ,second ,position of ,side ,switch4 ,no detailed discussion of this is necessary1 since1 with the side switch in its second position1 the actions which occur in causing the wipers of the second selector to seek and connect with an idle trunk line1 are exactly the same as in the case of the first selector4 ,when the second selector wipers finally reach a resting place on the bank contacts1 the private magnet armature1 operated during the hunting process1 is released and the side switch is thus shifted into the third position4 ,third ,position of ,side ,switch4 ,the moving of the side switch into its final position brings about the same state of affairs with respect to the second selector that already exists with respect to the first selector4 ,the trunk line is cut straight through and all bridge circuits or by-paths from it are cut off4 ,the same guarding conditions are established to prevent other lines or other pieces of apparatus from making connections that will interfere with the one being established1 and the same provisions are made for working the back release when the proper impulse comes from the connector1 and for passing this back release impulse on to the first selector in the same way that the first selector passes it on to the line switch4 ,the line of the calling subscriber has now been extended to a connector1 and that connector is one of a group--usually ten--which alone has the ability to reach the particular hundred lines containing the line of the desired subscriber4 ,the selection has1 therefore1 been narrowed down from one in ten thousand to one in one hundred4 ,the ,connector--,its ,functions4 ,it has already been stated that the connector is of the same general type of apparatus as the first and the second selectors4 ,unlike the first and the second selectors1 however1 the connector is required to make a double selection under the guidance of the subscriber4 ,the first selector makes a single selection of a group under the guidance of the subscriber and then an automatic selection in that group not controlled by the subscriber4 ,so it is with the second selector4 ,the connector1 however1 makes a selection of a group of ten under the guidance of the subscriber and then1 again under the guidance of the subscriber1 it picks out a particular one of that group4 ,the connector also has other functions in relation to the ringing of the called subscriber and the giving of a busy signal to the calling subscriber in case the line wanted is found busy4 ,it has still other functions in that the talking current1 which is finally supplied to connected subscribers1 is supplied through paths furnished by it4 ,location of the ,connectors4 ,connectors are the only ones of the selecting switches that are in any sense individual to the subscribers' lines4 ,none of them is individual to a subscriber's line1 but it may be said that a group of ten connectors is individual to a group of one hundred subscribers' lines4 ,since each group of one hundred lines has a group of connectors of its own and since each one hundred lines also has a line-switch unit of its own1 and since the lines of this group must be multipled through the bank contacts of the connectors of this individual group and through the bank contacts of the line switches of this particular unit1 it follows that on account of the wiring problems involved there is good reason for mounting the connectors in close proximity to the line switches representing the same group of lines4 ,some help in the grasping of this thought may result if it be remembered that the line switch is1 so to speak1 the point of entry of a call and that the connector is the point of exit1 and1 in order to reduce the amount of wiring and to economize space1 the point of exit and the point of entry are made as close together as possible4 ,the relative locations and grouping of the line switches and connectors are clearly shown in ,fig4 #cie1 which is a rear view of the same line-switch unit that was illustrated in ,figs4 #chg and #chh4 ,illustration3 ,,gas ,,engine ,,and ,,power ,,board ,citizens' ,telephone ,co41 ,racine1 ,wis4 ,the ,dean ,electric ,co4 ,operation of the ,connector4 ,the circuits of the connector are shown in ,fig4 #cif4 ,in addition to the features that have been pointed out in the first and the second selectors1 all of which are to be found1 with some modifications1 perhaps1 in the connector1 there must be considered the features in the connector of busy-signal operation1 of ringing the called subscriber1 of battery supply to both subscribers1 and of the trunk release operation4 ,these may be best understood by tracing through the operations of the connector from the time it is picked up by a second selector until the connection is finally completed1 or until the busy signal has been given in case completion was found impossible4 ,as in the first and the second selectors1 the sequence of operations is determined by the position of the side switch4 ,illustration3 ,fig4 #cie4 ,connector ,side of ,line-,switch ,unit ,illustration3 ,fig4 #cif4 ,circuits of ,connector ,first ,position of ,side ,switch4 ,the connector in a ten-thousand-line system is the recipient of the impulses resulting from the third and fourth movements of the subscriber's dial4 ,considering the third movement of the subscriber's dial1 the first impulse resulting from it comes over the rotary side of the line and results in the rotary relay attracting its armature once4 ,this results in a single impulse through the private magnet which1 however1 does nothing because the projection #e' strikes against a projection on the selector shaft4 ,these two projections interfere only when the selector shaft is in its normal position4 ,then follows the series of impulses from the subscriber's station corresponding to the tens digit in the called subscriber's number4 ,these pass over the vertical side of the line and through the vertical relay1 energizing that relay a corresponding number of times4 ,the vertical magnet1 as in the case of the first and the second selectors1 is included in the circuit controlled by the vertical relay and this results in the connector shaft being stepped up to the level corresponding to the particular tens group containing the called subscriber's number4 ,it will be noted that the impulses from the vertical side of the line1 which cause this selection1 pass through one winding #ac of the calling battery supply relay4 ,this relay is operated by these vertical selecting impulses1 but in this position of the side switch the closure of its local circuits accomplishes nothing4 ,immediately after the tens group of selecting impulses over the vertical side of the line1 there follows a single rotary impulse from the subscriber's station which1 as in the case of the first and the second selectors1 energizes the rotary relay and causes it to give one impulse to the private magnet4 ,this impulse is now able1 since the shaft has moved from its normal position1 to release the side switch arm one notch1 and the side switch1 therefore1 moves into its second position4 ,second ,position of ,side ,switch4 ,it is principally in this second position of the side switch that the connector selecting function differs from that of the first and the second selector4 ,there is no trunk to be hunted1 but rather the rotary movement of the connector wipers must be made in response to the impulses1 from the subscriber's station1 which correspond to the units digit in the selected number4 ,the first impulse corresponding to the fourth movement of the subscriber's dial is a rotary one1 and1 as usual1 it passes through the rotary relay winding and this1 in turn1 gives an impulse to the private magnet4 ,the private magnet at this time has already released the side switch arm to its second position1 but it is unable to release it further because of a feather on the wiper shaft--which projects just far enough to engage the lug #e'1 when the shaft is in its normal angular position--thus preventing the side switch arm from moving farther than its second position4 ,then follows over the vertical side of the line the last set of selecting impulses corresponding to the units digit4 ,this1 as before1 energizes the vertical relay1 but in the second position of the side switch1 it is to be noted1 that the vertical relay no longer controls the vertical magnet2 the side switch has shifted the control of the vertical relay to the rotary magnet4 ,the rotary magnet is1 therefore1 energized a number of times corresponding to the last digit in the called number and the wipers of the connectors are thus brought to the contacts of the line sought--their final goal4 ,at this point many things may happen1 and the things that do happen depend on whether the called subscriber's line is idle or busy4 ,called-,line ,busy3--,it will first be assumed that the called line is busy4 ,the testing operation at the connectors occurs in the second position of the side switch4 ,if the called line is busy1 it will be either because it is connected to by some other connector or because it has itself made a call4 ,in the former case the private contacts of that line in the banks of all the connectors serving that hundreds group of lines will be grounded through the private wiper of some other connector4 ,that this is so1 may be seen by tracing the circuit from the private wiper on the shaft to the third side switch wiper which is grounded in the third position2 the other connector that has already engaged the line will1 of course1 have its side switch in its final1 or third position4 ,again1 if the line called is busy1 because a call has already been made from this line to some other line1 the private contacts on the connectors corresponding to the line will be grounded1 as will be seen by tracing from the private bank contacts1 which are shown in ,fig4 #cif1 through the private wire to the line switch1 which is shown in ,fig4 #chi1 and from thence to ground through the springs #ai and #bj1 which are brought together when the line switch is operated4 ,in any event1 therefore1 the determining condition of a busy line is that its private bank contacts on all connectors of its group shall be grounded4 ,under the present assumed condition1 therefore1 the connector wipers1 which have been brought to the bank contacts of the desired line1 will find a ground at the private bank contact4 ,the connector shaft stops for an instant on the contacts of this busy line and immediately there follows over the rotary side of the line the inevitable single rotary impulse4 ,this energizes the rotary relay and this1 as usual1 energizes the private magnet4 ,remembering now that the connector side switch is in its second position and that the private wiper of the connector has found a ground1 we may trace back from the private wiper through the third side switch wiper to its second contact2 thence through the contact springs #ad and #ae1 closed by the private magnet2 thence through the release magnet2 thence through the contact springs #af and #ag of the calling battery supply relay to the live side of the battery4 ,this calling battery supply relay will1 at this time1 have its core energized because the coil #ah is in series with the rotary relay coil which1 as just stated1 was energized by the last rotary impulse4 ,this series of operations has led to the energizing of the release magnet1 and1 as a result1 the double dog of the connector is pulled out of the connector shaft ratchets and the shaft and the side switch are restored to their normal position4 ,busy-,back ,signal3--,the connector has dropped back to normal in all respects4 ,the calling subscriber1 not knowing this1 presses his ringing button4 ,this grounds the vertical side of the line at his station and operates the vertical relay at the connector4 ,this steps the shaft of the connector up one step and causes the closure of the contacts #ai and #bj at the top of the connector shaft4 ,this establishes a connection to a circuit carrying periodically interrupted battery current on which an inductive hum is placed4 ,this circuit may be traced from this source through the springs #bj and #ai to the first wiper of the side switch1 thence through the normally closed contacts of the ringing relay to the rotary side of the line1 and the varying potential to which this path is subjected produces an inductive flow back to the calling subscriber's telephone1 and gives him the necessary signal which consists of a hum or buzzing noise with which all users of automatic systems soon become familiar4 ,release on ,busy ,connection3--,the connector1 since its last release1 has been stepped up one notch and must again be released4 ,when the subscriber hangs up his receiver after receiving the busy signal1 he grounds both sides of his line momentarily by the action of the springs #ba1 #bb1 and #bc of ,fig4 #chd4 ,this operates the rotary and the vertical relays on the connector simultaneously and brings together for the first time the springs #ba and #bb of ,fig4 #cif4 ,this establishes a connection from the battery through the springs #af and #ag on the calling battery supply relay1 thence through the release magnet of the connector1 thence through the springs #bb and #ba of the vertical and the rotary relay1 thence through the release trunk back to the second selector4 ,from here the circuit passes through the private wiper of that selector and the back release relay to ground through the third side switch wiper which is in the third position4 ,considering this circuit in respect to its action on the connector it is obvious that it energizes the release magnet on the connector which restores the connector to normal as before4 ,at the second selector this circuit passed through the back release relay1 which closed a circuit through the release magnet and through the back release relay contacts1 thence back over the second selector release trunk to the back release relay of the first selector1 and through the third wiper of the side switch on that selector to ground1 since that side switch also is in its third position4 ,the current through this circuit energizes the release magnet of the second selector and restores it to its normal position and also energizes the back release relay of the first selector4 ,this1 in turn1 closes the circuit from the battery through the release magnet of the first selector and contacts of the back release relay to ground4 ,this works the release magnet of the first selector and restores that selector to normal4 ,the contacts on the first selector release magnet1 shown in ,fig4 #cij1 are closed by the action of the release magnet and this closes the path from ground back through the first selector release wire1 and through the contacts #ac and #ad of the line switch1 through the line switch release magnet to battery1 and this restores the line switch to normal4 ,the reason for the term back release will now be apparent4 ,the release operation at the connector is relayed back to the second selector2 that of the second selector back to the first selector2 and that of the first selector back to the line switch4 ,until this plan was adopted1 the release magnet of each selector and connector involved in a connection was left bridged across the talking circuit so as to be available for release2 and it sometimes occurred that a first selector would be released before a second selector or connector1 which latter switches would thus be left off-normal until rescued by an attendant4 ,the back release plan makes it impossible for the connection necessary for the release of a switch to be torn down until the release is actually accomplished4 ,called ,line ,found ,idle3--,it will be remembered that1 before the digression necessary to trace through the operations occurring upon the finding of a busy line1 the connector wipers had been brought1 by the influence of the calling subscriber's impulses1 into engagement with the contacts of the desired line2 that the connector side switch was in its second position2 and that the final rotary impulse following the last series of selecting impulses had not been sent4 ,the condition now to be assumed is that the called subscriber's line is free and the private wiper1 therefore1 has found and rests on an ungrounded private bank contact4 ,the final rotary impulse which immediately follows will operate the rotary relay and this1 in turn1 will operate the private magnet4 ,this happened under the assumed condition that the line was busy1 but in that case the release magnet was also operated at the same time and restored all conditions to normal4 ,under the present condition the operation of the private magnet will perform its usual function and move the side switch of the connector into its third position4 ,third ,position of ,side ,switch4 ,when the side switch of the connector moves to its third position1 it1 as usual1 cuts the talking circuit straight through from the vertical and the rotary sides of the trunk leading from the previous selector to the outgoing terminal of the subscriber's line1 which may be traced upon ,fig4 #cif back through the line switch1 shown in ,fig4 #chi4 ,several things are to be noted about the talking circuit so established3 ,first1 the inclusion of the condensers in the vertical and the rotary sides of the connector circuit4 ,the purpose of this will be referred to later4 ,second1 the inclusion in this circuit at the connector of a pair of normally closed contacts in the ringing relay4 ,it may be said in passing that the ringing relay corresponds exactly in function to a ringing key in a manual switchboard4 ,third1 the talking circuit leading from the connector to the called subscriber's line passes on one side through the springs #bd and #be of the bridge cut-off relay of the line switch1 which is shown in ,fig4 #chi4 ,these springs are normally open and would prevent the completion of the talking circuit but for the fact that the bridge cut-off relay of the line switch is energized over the private wire leading to the connector bank and then through the connector wiper to the third side switch wiper which1 at this time1 is in its third position4 ,the talking circuit is thus complete4 ,the operation of this bridge cut-off relay on the line switch has not only completed the talking circuit but it has also opened the circuit of the trip magnet of the line switch so as to prevent the operation of the trip magnet by the subscriber on that line in case he should attempt to make a call during the interval between the time when his line was connected with by the connector and the time when he answers the call4 ,the third wiper of the connector side switch when moved into its third position1 puts the ground on all of the private bank contacts of the line chosen and thus guards that line against connection by others1 as already described4 ,it also operates the bridge cut-off relay of the line switch as just mentioned4 ,the fourth wiper of the side switch1 when moved into its third position1 establishes such a connection as will place the ringing relay under the control of the vertical relay4 ,this may be seen by tracing from ground to the vertical relay springs #bc and #bd1 thence through the normally closed upper pair of contacts on the private magnet1 thence through the fourth wiper on the side switch to its third contact1 thence through the ringing relay magnet1 and through the springs #af and #ag of the calling battery supply relay and to battery4 ,the calling battery supply relay winding being in series with the vertical relay winding1 the two operate together and close the two normally open points in the ringing relay circuit4 ,this ringing relay acts as an ordinary ringing key and connects the generator to the called subscriber's line in an obvious manner1 at the same time opening the talking circuit back of the ringing relay in order to prevent the ringing current chattering the relays in the circuit back of it4 ,all that remains now is for the called subscriber to respond4 ,when he does he closes the metallic circuit of the line through his talking apparatus4 ,battery ,supply to ,connected ,subscriber4 ,throughout the whole process of building up a connection1 it will be remembered that both sides of the calling line are connected through the respective vertical and rotary relays involved in building up the connection with the live side of the battery4 ,at the time when the connection is finally established and the called subscriber rung1 both sides of the calling line are connected through various relay windings to the live side of the battery4 ,such a condition leaves both sides of the line at the same potential and1 therefore1 there is no tendency for current to flow through the calling subscriber's talking apparatus1 even though it is connected across the circuit of the line4 ,it remains1 therefore1 to be seen how these conditions are so changed after the building up of a connection as to supply the calling subscriber with talking current4 ,the calling subscriber can get no current until the called subscriber responds4 ,when the connection is first made with the called subscriber's line1 battery connection to his line is made from the live side of battery through the normally closed contacts of the calling battery supply relay1 thence through the winding #be of the called battery supply relay to the vertical side of the called line4 ,the grounded side of the battery is connected to the rotary side of his line through the third wiper of the connector and the coil #bf of the called battery supply relay4 ,as a result1 this subscriber receives proper talking current through the coils #be and #bf1 and this relay is operated by the flow of this current4 ,the operation of this called battery supply relay merely shifts the connection of the rotary side of the calling subscriber's line from its normal battery connection1 to ground1 and thus the battery is placed straight across the calling subscriber's line so as to supply talking current4 ,this supply circuit to the calling subscriber may be traced from the live side of the battery through the winding #ac of the calling battery supply relay and the winding of the vertical relay to the vertical side of the line1 and from the grounded side of battery through the third side switch wiper in its third position to the now closed pair of contacts in the called battery supply relay through the coil #ah of the calling battery supply relay and the coil of the rotary relay to the rotary side of the line4 ,it will be noted that the system of battery supply is that of the standard condenser and retardation coil scheme largely employed in manual practice2 and that aside from the coils through which the battery current is supplied to the connected subscribers1 there are no taps from1 or bridges across1 the two sides of the talking circuit4 ,release after ,conversation4 ,it remains now only to secure the disconnection of the subscribers after they are through talking4 ,when the calling subscriber hangs up1 the whole disconnection is brought about1 all of the apparatus1 including connector1 selectors1 and line switch1 returning to normal4 ,this is done by the back release system and is accomplished in almost the same way as has already been described in connection with the disconnect after an unsuccessful call4 ,there is this difference1 however3 after an unsuccessful call when the line called for was found busy1 the release was made while the connector side switch was in its normal position4 ,in the present case1 the release must be made with the connector side switch in its third position and with the talking battery bridged across the metallic circuit rather than connected between each limb of the line and ground4 ,it must be remembered that the calling battery supply relay1 while traversed by current during the conversation1 is not magnetically energized because1 with the current flowing through the metallic circuit of the line1 the two windings exert a differential effect4 ,as soon1 however1 as the calling subscriber hangs up his receiver1 this differential action ceases1 due to the grounding of both sides of the line at the subscriber's station4 ,this relay1 therefore1 operates and cuts off battery from the called battery supply relay and this1 in turn1 releases its armature and thus changes the connection of the rotary side of the calling line from ground to live side of the battery4 ,the normal condition of the battery connection now being restored1 both the vertical and the rotary relays at the connector become operated1 due to the ground on both sides of the line at the subscriber's station1 and this1 as we have seen1 is the condition which brings about the operation of the connector release magnet1 and the relaying back of the disconnect impulse successively through the selectors to the line switch4 ,multi-,office ,system4 ,in exchanges involving more than one office1 the same general principles and mode of operation already outlined apply4 ,if the total number of subscribers in the multi-office exchange is to be less than ten thousand1 then four digit numbers suffice1 and the first movement of the dial may be made to select the office into which the connection is to go1 the subscribers' lines being so numbered with respect to the offices that each office will contain only certain thousands4 ,the choosing of the thousand by the calling subscriber1 therefore1 takes care in itself of the choice of offices4 ,where1 however1 a multi-office exchange is to provide for connections among a greater number of lines than ten thousand and less than one hundred thousand1 then it will take five movements of the dial to make the selection--the five movements corresponding either to the five digits in a number or to the name of an office1 as indicated on the dial1 and the four digits of a smaller number4 ,the lines may all carry five digit numbers or1 what is considered better practice1 may be designated by an office name followed by a four digit number4 ,in this latter case the numbers of the subscribers' lines will in each case be contained in one or more of the tens of thousands groups1 no number having more than four digits4 ,and the first movement of the dial1 whether the name or number plan be adopted1 will select an office2 or1 looking at it another way1 will select a group of ten thousand and this being done1 the next four successive movements of the dial will select the numbers in that ten thousand in just the some way that has been already described4 ,certain difficulties arise1 however1 in multi-office working due to the fact that the three-wire trunks between offices would in most cases be objectionable4 ,as long as the trunks extend between the various groups of apparatus in the same office1 it is cheaper to provide three wires for each of them than it is to make any additional complication in the apparatus4 ,where the trunking is done between offices1 however1 the system may be so modified as to work over two wire inter-office trunks4 ,the ,trunk ,repeater4 ,the purpose of the trunk repeater is to enable the inter-office trunking to be done over two wires4 ,it may be said that the trunk repeater is a device placed in the outgoing trunk circuit at the office in which a call originates1 which will do over the two wires of the trunk leading from it to the distant office just the same thing that the subscriber's signal transmitter does over the two wires of the subscriber's lines4 ,it has certain other functions in regard to feeding the battery for talking purposes back to the calling subscriber's line1 taking the place in this respect of the calling battery feed relay in the connector in a single office exchange4 ,illustration3 ,fig4 #cig4 ,circuits of ,trunk ,repeater ,the circuits of a trunk repeater are shown in ,fig4 #cig4 ,in considering it1 it must be understood that the three wires entering the figure at the left are the vertical1 rotary1 and release wires of a second selector trunk leading from the first selector banks in the same office4 ,the two wires leading from the right of the figure are those extending to the distant office1 and terminate there in second selectors4 ,the vertical and the rotary sides of this trunk as shown at the left will receive the impulses from the subscriber's station coming through the line switch and the first selector1 as usual4 ,the vertical impulses will pass through the winding of the vertical relay and through the winding #a of the calling battery supply relay and thence to battery1 the same as on a connector4 ,these impulses will work the armatures of both of these relays in unison4 ,the movements of the vertical relay armature in response to these impulses will cause corresponding impulses to flow over a circuit which may be traced from ground1 through the springs #c and #b of the vertical relay1 the springs #d and #e of the bridged relay #f and thence to the vertical side of the trunk and to the distant office1 where it passes into a second selector and through its vertical relay to battery4 ,thus the vertical impulses are passed on over the two-wire trunk to the second selector at the distant office4 ,it becomes necessary1 however1 to prevent these impulses from passing back through the winding of the bridge relay #f and this is done by means of the sluggish relay #g4 ,this relay receives local battery impulses in unison with those sent over the trunk by the vertical relay1 these being supplied from the battery at the local office through the contacts #h and #i of the calling battery supply relay1 which works in unison with the vertical relay4 ,these rapidly recurring impulses are too fast for the sluggish relay #g to follow4 ,and this relay merely pulls up its armature and cuts off both sides of the trunk leading back to the first selector4 ,the rotary impulses are repeated to the rotary side of the two-wire trunk in a similar way4 ,considering now the operation of the trunk repeater in the reverse direction1 the action of the bridging relay #f is of vital importance4 ,normally both sides of trunk line are connected to the live side of the battery and1 therefore1 there is no difference of potential between them and no tendency to operate the bridged relay4 ,when the connection has been fully established to the subscriber at the distant office1 and that subscriber has responded1 the action of his battery supply relay will1 as before stated1 change the connection of the rotary side of the line from battery to ground1 and thus bridge the battery at the distant exchange across the trunk4 ,this action will pull up the bridged relay #f at the trunk repeater and will perform exactly the same function with respect to the connection of the battery with the calling subscriber's line4 ,in other words1 it will change the connection of the rotary side of the calling line from battery to ground1 thus establishing the necessary difference in potential to give the calling subscriber the necessary current for transmission purposes4 ,the disconnect feature is about the same as already described4 ,when the calling subscriber hangs up his receiver both the vertical and rotary relays of the trunk repeater operate1 which places the ground on both sides of the two-wire trunk to the distant office1 which is the condition for releasing all of the apparatus there4 ,for the purpose of convenience the simplified diagram of ,fig4 #cih has been prepared1 which shows the complete connection from a calling subscriber to a called subscriber in a multi-office exchange1 wherein the first movement of the dial is employed to establish the connection to the proper office and the four succeeding movements to make a selection among ten thousand lines in that office4 ,this circuit1 therefore1 employs at the first office the line switch1 the first selector1 and the trunk repeater2 and at the second office the second selector1 third selector1 connector1 and line switch4 ,the third selector is omitted from ,fig4 #cih1 but this will cause no confusion1 since it is exactly like the second selector4 ,the circuits shown are exactly like those previously described but in drawing them the main idea has been to simplify the connections to the greatest possible extent at a sacrifice in the clearness with which the mechanical inter-relation of parts is shown4 ,no correct understanding of the circuits of an automatic system is possible without a clear idea of the mechanical functions performed by the different parts1 and1 therefore1 we have described what are apparently the more complex circuit drawings first4 ,it is believed that the student1 in attempting to gain an understanding of this marvel of mechanical and electrical intricacy1 will find his task less burdensome if he will refer freely to both the simplified circuit drawing of ,fig4 #cih and the more complex ones preceding it4 ,by doing so he will often be enabled to clear up a doubtful circuit point from the simpler diagram and a doubtful mechanical point from those diagrams which represent more clearly the mechanical relation of parts4 ,illustration3 ,fig4 #cih4 ,connection between a ,calling and a ,called ,subscriber in an ,automatic ,system ,automatic ,sub-,offices4 ,obviously1 the system of trunking employed in automatic exchanges lends itself with great facility to the subdivision of an exchange into a large number of comparatively small office districts and the establishment of branch offices or sub-offices at the centers of these districts4 ,the trunking between large offices has already been described4 ,an attractive feature of the automatic system is the establishment of so-called sub-stations or sub-offices4 ,where there is1 in an outlying district1 a distinct group of subscribers whose lines may readily be centered at a common point within that district and where the number of such subscribers and lines is insufficient to establish a fully equipped office1 it is possible to establish a so-called sub-station or sub-office connected with the main office of that district by trunk lines4 ,at this sub-office there are placed only line switches and connectors4 ,when a call is originated on one of these sub-office lines1 the line switch acts instantly to connect that line with one of the trunks leading to the main office of that district1 at which this trunk terminates in a first selector4 ,from there on1 the connection is the same as that in a system in which no sub-offices are employed4 ,calls coming into this sub-office over trunk lines from the main office are received on the connectors at the sub-office and the connection is made with the sub-office line by the connector in the usual manner4 ,this arrangement1 it is seen1 amounts merely to a stretching of the connector trunks for a given group of lines so that they will reach out from a main office to a sub-office1 it being more economical to lengthen the smaller number of trunks and by so doing to decrease in length the larger number of subscribers' lines4 ,the ,rotary ,connector4 ,for certain purposes it becomes desirable in automatic work to employ a special form of connector which will have in itself a certain ability to make automatic selection of one of a group of previously chosen trunks in much the same manner as the first and second selectors automatically choose the first idle one of a group of trunks4 ,such a use is demanded in private branch-exchange working where a given business establishment1 for instance1 has a plurality of lines connecting its own private switchboard with the central office4 ,the directory number of all these lines is1 for convenience1 made the same1 and it is important1 therefore1 that when a person attempts to make a connection with this establishment1 he will not fail to get his connection simply because the first one of these lines happens to be busy4 ,for such use a given horizontal row of connector terminals or a part of such a row is assigned to the lines leading to the private branch exchange and the connector is so modified as to have a certain discretionary power of its own4 ,as a result1 when the common number of all these lines is called1 the connector will choose the first one1 if it is not already engaged by some other connector1 but if it is1 it will pass on to the next1 and so on until an idle one is found4 ,it is only when the connector has hunted through the entire group of lines and found them all busy that it will refuse to connect and will give the busy signal to the calling subscriber4 ,party ,lines4 ,the description of this system as given above has been confined entirely to direct line working2 however1 party lines may be and are frequently employed4 ,the circuits and apparatus used with direct lines are1 with slight modifications1 applicable to use with party lines4 ,the harmonic method of ringing is employed and the stations are so arranged with respect to the connectors that those requiring the same frequency for ringing the bells are in groups served by the same set of connectors4 ,illustration3 ,,power ,,machinery ,citizens' ,telephone ,company1 ,racine1 ,wis4 ,the ,dean ,electric ,co4 ,the party lines are operated on the principle commonly known in manual practice as the jack per station arrangement4 ,each party line will1 therefore1 have sets of terminals appearing in separate hundreds2 the connectors associated with each of these hundreds being so arranged as to impress the proper frequency of ringing current on the line4 ,from the subscribers' standpoint the operation is the same as for direct lines1 as the particular hundreds digit of a number serves to select one of a group of connectors capable of connecting the proper ringing current to the line4 ,to avoid confusion1 which would be caused by a subscriber on a party line attempting to make a call when the line is already in use by some other subscriber1 the subscribers' stations are so arranged that when the line is in use all other stations on the line are locked out4 ,illustration3 ,fig4 #cii4 ,wall ,set for ,two-,wire ,system ,the ,two-,wire ,automatic ,system4 ,the two-wire system that has recently been introduced by the ,automatic ,electric ,company brings about the very important result of accomplishing all of the automatic switching over metallic circuit lines without the use of ground or common returns4 ,the system is thus relieved of the disturbing influences to which the three-wire system is sometimes subjected1 due to differences in earth potential between various portions of the system1 which may add to or subtract from the battery potential and alter the net potential available between two distant points4 ,the introduction of this system has also made possible certain other incidental features of advantage1 one of which is a great simplification and reduction in size of the subscriber's station signal-transmitting apparatus4 ,with the doing away of the ground as a return circuit1 it becomes impossible to send vertical impulses over one side of the line and to follow them by single rotary impulses over the other side of the line4 ,yet it becomes necessary to distinguish between the pure selective impulses and those impulses which dictate a change of function at the central office4 ,the plan has1 therefore1 been adopted of accomplishing the selection in each case by short and rapidly recurring impulses and of accomplishing those functions formerly brought about by the single impulse over the rotary side of the line by a pause between the respective series of selective impulses4 ,this is accomplished at the central office by replacing the vertical and the rotary relays of the three-wire system by a quick-acting and a sluggish relay1 respectively2 the quick-acting relay performing the functions previously carried out by the vertical relay1 and the sluggish relay acting only during the pauses between the successive series of quick impulses to do the things formerly done by the rotary relay4 ,this has resulted in a delightful simplification of subscriber's apparatus1 since it is now necessary only to provide a device which will connect the two sides of the line together the required number of times in quick succession and then allow a pause with the circuit closed while the subscriber is getting ready to transmit another set of impulses corresponding to another digit4 ,the calling device has no mechanical function co-acting with any of the other parts of the telephone and may be considered as a separate mechanical device electrically connected with the line4 ,the transmitting device is not much larger than a large watch and a good idea of it may be had from ,fig4 #cii1 which shows the latest wall set1 and ,fig4 #djj1 which shows the latest desk set of the ,automatic ,electric ,company4 ,we regret the fact that this company has made the request that the complete details of their two-wire system be not published at this time4 ,illustration3 ,fig4 #djj4 ,desk ,stand for ,two-,wire ,system ,,chapter ,,xxx ,,the ,,lorimer ,,automatic ,,system ,the ,lorimer automatic telephone system has not been commercially used in this country but is in commercial operation in a few places in ,canada4 ,it is interesting from several points of view4 ,it was invented1 built1 and installed by the ,lorimer ,brothers--,hoyt1 ,george ,william1 and ,egbert--of ,brantford1 ,ontario4 ,these young men without previous telephonic training and1 according to their statements1 without ever having seen the inside of a telephone office1 conceived and developed this system and put it in practical operation4 ,with the struggles and efforts of these young men in accomplishing this feat we have some familiarity1 and it impresses us as one of the most remarkable inventive achievements that has come to our attention1 regardless of whatever the merits or demerits of the system may be4 ,the ,lorimer system is interesting also from the fact that1 in most cases1 it represents the mechanical rather than the electrical way of doing things4 ,the switches are power driven and electrically controlled rather than electrically driven and electrically controlled1 as in the system of the ,automatic ,electric ,company4 ,the subscriber's station apparatus consists of the usual receiver1 speech transmitter1 call bell1 and hook switch1 and in addition a signal transmitter arranged to be manipulated by the subscriber so as to control the operation of the central-office apparatus in connecting with any desired line in the system4 ,the central-office apparatus is designed throughout upon the principle of switching by means of power-driven switches which are under the control of the signal transmitters of the calling subscriber's station4 ,the switches employed in making a connection are all so arranged with respect to constantly rotating shafts that the movable member of such switches may be connected to the shafts by means of electromagnets controlled directly or indirectly by relays1 which1 in turn1 are brought under the control of the signal transmitters4 ,the circuits are so designed in many instances that the changes necessary for the different steps are brought about by the movement of the switches themselves1 thus permitting the use of circuits which are rather simple4 ,the switches employed are all of a rotary type2 the co-ordinate selection1 which is accomplished in the ,automatic ,electric ,company's system by a vertical and rotary movement1 being brought about in this system by the independent rotation of two switches4 ,subscriber's ,station ,equipment4 ,a subscriber's desk-stand set1 except the call bell1 is shown in ,fig4 #dja1 and a wall set complete in ,fig4 #djb4 ,in both of these illustrations may be seen the familiar transmitter1 receiver1 and hook switch1 and in the wall set1 the call bell4 ,the portion of these telephone sets which is unfamiliar at present is the part which is enclosed in the enlarged base of the desk stand and the protruding device below the speech transmitter in the wall set--the signal transmitter referred to earlier in the chapter4 ,the small push button and small plate through which the number may be seen directly below the transmitter in ,fig4 #djb1 are for the purpose of registering calls4 ,illustration3 ,fig4 #dja4 ,lorimer ,automatic ,desk ,stand ,the signal transmitter is a device whose function is to record mechanically the number of the subscriber's station with which connection is desired1 and to transmit that record to the central office by a system of electrical impulses over the line conductors4 ,instead of operating by its own initiative1 the signal transmitter is adapted to respond to central-office control in transmitting electrically the number which has been recorded mechanically upon it4 ,the signal transmitter shown removed from the base of the desk stand at the left in ,fig4 #djc comprises in part four sets of contact pins having ten pins in each set1 one set for each of the digits of a four-digit number4 ,there are also several additional contact pins for signaling and auxiliary controlling purposes4 ,all of these contact pins are arranged upon the circumference of a circle and a movable brush mounted upon a shaft at the center of the circle is adapted to be rotated by a clock spring and to make contact with each of the pins successively4 ,the call is started1 after the number desired has been set on the dial1 by giving the crank at the right of the signal transmitter a complete turn and thus winding the spring4 ,the shaft carrying the signal transmitter brush carries also an escapement wheel1 the pallet of which is directly controlled by an electromagnet4 ,illustration3 ,fig4 #djb4 ,lorimer ,automatic ,wall ,set ,the four dials with the numerals printed on them are attached to four levers1 respectively1 and are moved by their levers opposite windows1 near the top of the casing4 ,through each of these windows a single numeral may be seen on the corresponding one of the dials4 ,the dials may be adjusted so that the four numerals seen will read from left to right to correspond to the number of the line with which connection is desired4 ,the setting of the dials so that the number desired shows at the small circular opening results in connecting the earth or a common return conductor to one pin of each set of ten pins1 the pin grounded in each set depending upon the numerical value of the digit for which the dial is set4 ,the circle of contact pins is set in an insulating disk1 the signal transmitting brush operates upon the pins on one side of the disk1 and electrical fingers attached to the dials operate upon the pins on the other side of the disk4 ,the escapement wheel is a single toothed disk attached directly to the shaft which carries the signal brush and its pallet is attached rigidly to the magnet armature4 ,illustration3 ,fig4 #djc4 ,desk ,stand with ,signal ,transmitter ,removed ,once a call has been turned in1 the entire subscriber's station equipment is locked beyond power of the subscriber to tamper with it in any way1 rendering it impossible either to defeat the call which has been started or to prevent the subscriber's station as a whole from returning completely to normal position and thus restoring itself for regular service4 ,the key shown just below the signal transmitter in the case of the desk stand1 and at the right in the wall set1 is for the purpose of operating a relay at the central office which1 in turn1 connects ringing current to the line of the subscriber with which connection has been made1 and thus actuates the call bell4 ,as the number set up at the signal transmitter remains in full view until reset for some other number1 it is easily checked by inspection and also lessens the labor involved in making a second call for the same line1 which is frequently necessary when the line is found busy the first time called4 ,central-,office ,apparatus4 ,the subscriber's lines are divided into groups of one hundred lines each at the central office1 each group being served by a single unit of central-office apparatus4 ,in a central-office unit there is sectional apparatus which appears but once for the unit of one hundred lines2 divisional apparatus which appears a number of times for each unit1 depending upon the traffic2 and line apparatus which appears one hundred times for each unit or once for each line4 ,the sectional apparatus comprises devices whose duties are1 first1 to detect a calling line1 and second1 to assign to the calling line a set of idle divisional apparatus which serves to perform the necessary switching functions and complete the connection4 ,the sets of divisional apparatus1 or1 as called in this system1 divisions1 are common to a section and are employed in a manner similar to the connecting cords of a manual switchboard4 ,the number of these divisions provided for each section is1 therefore1 determined by the number of simultaneous connections resulting from calls originating in the section4 ,it has been the custom in building this apparatus to provide each section with seven divisions or connective elements4 ,the line apparatus comprises one relay1 having a single winding1 and two pairs of contacts operated by its armature4 ,this device is substantially the well known cut-off relay almost universally employed in common-battery systems4 ,the fixed multiple contacts of the lines in the switching banks of the connecting apparatus are considered as pertaining to the various pieces of apparatus on which they are found rather than to their respective lines4 ,a good idea may be obtained of the arrangement of the sectional and divisional apparatus by referring to ,fig4 #djd1 which is one unit of a thousand-line equipment4 ,the apparatus in the vertical row at the extreme left of the illustration is the sectional apparatus1 while the remaining seven vertical rows of apparatus are the divisions4 ,the ,section4 ,the sectional apparatus for each unit consists of three separate devices called for convenience a decimal indicator1 a division starter1 and a decimal-register controller4 ,all of these devices are normally motionless when idle4 ,the energization of the decimal indicator1 in response to the inauguration of a call at a subscriber's station1 results immediately in an action of the division starter which starts a division to connect with the line calling4 ,it results also in the starting of the decimal-register controller1 the remaining unit of sectional apparatus4 ,it is thus seen that upon the starting of a call by a subscriber1 all of the sectional apparatus belonging to his one hundred lines immediately becomes active1 the division starter acting to start a division1 the decimal indicator becoming energized to indicate the tens group in which the call has appeared1 and the decimal-register controller becoming active to adjust the decimal register of the division assigned by the division starter4 ,the division starter having assigned a division for the exclusive use of this particular call1 passes to a position from which it may start a similar idle division when the next call is received4 ,the decimal register controller makes its half revolution for the call and comes to rest1 awaiting a subsequent call1 and the decimal indicator continues energized but only momentarily1 since it is released by the action of the cut-off relay when the call is taken in charge by the divisional connective devices4 ,calls may follow each other rapidly1 the connective devices being entirely independent of each other after having been assigned to the respective calling lines4 ,as has been described1 the decimal indicator starts the division starter and the decimal-register controller in quick succession4 ,the division starter1 shown at the extreme bottom of the left-hand row of ,fig4 #djd1 is a cylinder switch of the same general type as used throughout this system4 ,in it the terminals of a switch in each division appear as fixed contact points in a circle over which move the brushes of the division starter4 ,the decimal-register controller has the duties of transmitting to the divisional apparatus a series of current impulses corresponding in number to the numerical value of the tens digit of the calling line4 ,this is effected by providing before a movable brush ten contacts from which the brush may receive current4 ,these contacts are normally not connected to battery1 so that the brush in passing over them does not receive current from them2 however1 when the brush has reached the contact corresponding in number to the tens digit of the calling line1 a relay associated with the decimal-register controller charges the contacts with the potential of the main battery1 and each of the remaining contacts passed over by the brush sends a current impulse to a device designed to indicate on the division selected for the call the tens digit of the calling line4 ,the ,connective ,division4 ,the connective division1 seven of which are shown in ,fig4 #djd1 is an assemblage of switches comprising1 as a whole1 a set suitable for a complete connection from calling to called subscriber4 ,each connective division in the unit illustrated is completely equipped to care for a called number of three digits1 i4 e41 each division will connect its calling line with any one of one thousand lines which may be called4 ,by a system of interconnecting between divisions1 each division may be equipped with interconnecting apparatus so as to make it possible to complete a call with any one of ten thousand lines4 ,each connecting division of a ten-thousand-line exchange comprises six major switches4 ,of the six major switches1 one is termed a secondary connector1 another an interconnector1 and the four remaining are termed the primary portion of the division4 ,illustration3 ,fig4 #djd4 ,unit of ,switching ,apparatus ,before taking up the operation of the switches1 the mechanical nature of the switches themselves will be described4 ,the switches are built with a contact bank cylindrical in form and with internal movable brushes traveling in a rotary manner in circular paths upon horizontal rows of contacts fixed in the cylindrical banks4 ,for driving these brushes a constantly rotating main power-driven shaft is provided4 ,between each shaft and the rotating brushes of each major switch is an electric clutch1 which1 by the movement of an armature1 causes the brushes of the switch to partake of the motion of the shaft and by the return of the armature to come again to rest4 ,the motion of the brushes of the major switches1 or cylinder switches1 as they are frequently called because of their form1 is constantly in the same direction4 ,they have a normal position upon a set of the cylinder contacts4 ,they leave their normal position and take any predetermined position as controlled by the magnets of the clutch1 and1 having served the transient purpose1 they return to their normal position by traversing the remainder of their complete revolution and stopping in their position of rest or idleness4 ,the mechanical construction of each of the cylinder switches is such that it may disengage its clutch and bring its brushes to rest only with the brushes in some one of a number of predetermined positions4 ,the locations of the brushes in these positions of rest1 or stop positions1 as they are called1 may differ with the different cylinder switches1 according to the nature of the duty required of the switch1 and the total number of stop positions also may vary4 ,the primary and secondary connectors1 the interconnector selectors1 and the interconnectors each have eleven stop positions2 the rotary switch has eight stop positions2 the signal-transmitter controller has but two4 ,in the six cylinder switches making up a connective division and required for any conversation1 in a ten-thousand-line exchange some of the switches are set to positions which are determined by the control of the calling subscriber and represent by their selective positions the value of some digit of the calling or called subscriber's number4 ,others are switches controlling the call in its progress and controlling the switches responsive to the call4 ,these latter switches take positions independent of the numbers4 ,in addition to the major switches1 there are upon each division four minor switches termed registers4 ,each consists of an arc of fixed contacts accompanied by a set of brushes which sweep over the contacts4 ,instead of being driven by an electromagnet1 the register brushes are placed under tension of a spring which tends at all times to draw them forward4 ,they are then restrained by an escapement device similar to a pallet escapement in a clock1 the pallet being controlled by the register's magnets4 ,when a series of impulses are received by the register magnets1 the pallet is actuated a corresponding number of times and the register brushes are permitted to move forward under tension of their powerful propelling spring4 ,each register is associated with a major switch1 and the register brushes are engaged by a cam upon the associated major switch1 and are restored to normal position against the tension of their propelling spring1 the force of restoration being obtained from the main shaft4 ,the electrical clutches which connect and disconnect the movable brushes of the major switches from the main driving shaft are controlled in all instances by circuits local to the central office4 ,in some instances these circuits include relay contacts and are controlled by a relay4 ,in other instances they are formed solely through switch contacts4 ,in all cases the control1 when from a distance1 is received upon relays suitable for being controlled by the small currents which are adapted to flow over long lines4 ,in all instances the power for moving a brush is derived from the main shaft and only the control of the movement is derived from electromagnets1 relays1 or other electric sources4 ,in many instances the clutch circuit is closed through contacts of its own switch and1 therefore1 may be closed only when its switch is in some predetermined position4 ,all of the switches are mechanically powerful and designed particularly to sustain the wear of long-continued and oft-repeated usage4 ,this is true also of the moving parts which carry the brushes and of the journals sustaining those parts4 ,the ,switches of the ,connective ,division4 ,the six major switches of the connecting division are as follows3 ,the ,primary ,connector3--,the function of this switch is to connect the conductors of the calling line with the switching devices of the connective division4 ,associated with this switch is a register termed the decimal register4 ,the one hundred lines of the section are terminated in fixed multiple contacts in the cylinder switch of the primary connector4 ,the calling line is selected and connected with by adjusting the decimal register to a position corresponding to the calling line's tens digit and adjusting the brushes of the cylinder switch to a position corresponding to the calling line's unit digit4 ,the ,rotary ,switch3--,this is a master switch1 or pilot switch1 consisting of a cylinder switch without register4 ,its duty is the control of other switches and the completion of circuits formed in part through other switches4 ,it is the pilot switch and the switch of initiative and control for the entire connective division4 ,signal-,transmitter ,controller3--,the primary function of this switch is the generation of signaling impulses of two classes4 ,impulses of the first class pass over central-office circuits only and are effective upon magnets of the divers major and minor switches2 impulses of the second class pass over a line conductor of the calling line and are effective upon the signal transmitter at the subscriber's station4 ,the impulses sent out over the line to the subscriber's station cause the brush to pass over the contacts and thereby indicate the numerical values of the various digits set by the dials4 ,this switch also enters in an important manner into the circuits involved in the testing of the called line for the busy condition4 ,it is controlled by the rotary switch4 ,interconnector ,selector3--,in an exchange using four digits in the numbers1 the register of the interconnector selector is adjusted in each call to a position corresponding to the numerical value of the thousands digit of the called number4 ,the cylinder switch then acts to select an idle trunk4 ,the switch is controlled by the rotary switch in connection with the signal transmitter controller4 ,interconnector3--,this switch is similar to the interconnector selector in design and in function4 ,it is a cylinder switch with register4 ,the register is adjusted in each call to a position corresponding to the numerical value of the hundreds digit of the number called and the cylinder switch then operates to select an idle trunk4 ,the switch is controlled by the rotary switch in connection with the signal transmitter controller4 ,secondary ,connector3--,this switch contains in its cylinder bank of contacts the multiple points of one hundred subscribers' lines and its function is to connect the conductors of the called line to the conductors of the connective division4 ,this is accomplished by adjusting the register to correspond to the value of the tens digit of the line desired and by adjusting the cylinder brushes to correspond to the value of the units digit of the line4 ,the switch is controlled by the rotary switch in connection with the signal-transmitter controller4 ,operation4 ,a brief description of the progress of a call from its institution to the complete connection and subsequent disconnection begins with the adjustment of the dial indicators of the telephone set and the turning of the crank of the signal transmitter one revolution4 ,this act1 performed by the calling subscriber1 connects one of the line conductors to earth4 ,immediately the decimal indicator associated with the section in which the calling line terminates is energized and starts the division starter4 ,the division starter instantly starts the rotary switch of an idle division4 ,the rotary switch now starts the decimal-register controller and connects to it the decimal register of the primary connector of the division selected4 ,all of the above acts in the central office occur practically simultaneously4 ,the impulses generated by the controller are effective upon the decimal register of the started division and1 therefore1 adjust that register to a position corresponding to the tens value of the calling line4 ,the rotary switch now disconnects the tens register and starts the cylinder brushes of the primary connector which automatically stop when they encounter the calling line4 ,at this instant the cut-off relay of the line is energized and the decimal indicator is released4 ,the call now is clear of all sectional apparatus and another call may come through immediately1 being assigned in charge of another idle division4 ,the total time in which any call is in charge of the sectional apparatus1 i4 e41 the total time from the grounding of the line conductor at the sub-station until the line has been connected with by the primary connector of some division of that section and the sectional apparatus has been released by the operation of the cut-off relay1 approximates two-fifths of a second4 ,the next operation initiated by the rotary switch is the starting of the signal-transmitter controller of the connective division1 which1 in turn1 adjusts the register of the interconnector selector to a position corresponding to the thousands digit of the number of the called line as indicated by the signal transmitter at the calling station4 ,this selects an interconnector serving the lines of the selected thousand4 ,this initial selection being completed the rotary switch readjusts the circuits of the connective division in such manner that in the further progress of the signal-transmitter controller1 its impulses will be effective upon the register of the selected interconnector4 ,in this manner1 the register of the interconnector1 which may be upon the same connective division as the rotary switch handling the call1 or which may be the interconnector of some other division1 as determined by the number of the called subscriber1 is adjusted to a position corresponding to the second or hundreds digit of the number called4 ,the cylinder switch of the interconnector then selects and appropriates an idle trunk extending to a secondary connector upon some connective division serving the hundred selected4 ,the rotary switch again shifts the circuits of the connective division in such manner that the signal-transmitter controller is effective upon the secondary connector1 both register and cylinder1 and adjusts the register and cylinder1 respectively1 with their brushes in contact with the tens and units digits1 respectively1 of the number of the called line4 ,the conductors of the called line now are connected through the secondary connector1 the interconnector1 and the interconnector selector to the rotary switch2 the conductors of the calling line are connected through the primary connector to the rotary switch2 thus completely connecting the lines except at the rotary switch4 ,to effect the connecting together of the two lines1 both rotary switch and signal-transmitter controller must pass forward into their next positions1 the connection when thus effected being made through conductors containing a repeating coil and main battery connection for supplying talking current to the two lines and containing also ringing and supervisory relays4 ,the called line is tested to determine if busy during the short interval in which the rotary switch takes a short step to connect the calling and the called lines4 ,in this step of the rotary switch the busy-test relay is connected to the guard wire or busy-test wire of the called line1 and if that line be busy1 the relay interferes with the control exercised by the rotary switch upon the signal-transmitter controller1 and the controller is prevented from taking the step required to connect the line4 ,thus1 when a busy line is encountered1 the final step of the rotary switch is taken to set up the conversation conditions1 but the signal-transmitter controller does not take its final step2 by this failure of the signal-transmitter controller due to the action of the busy-test relay1 the calling line is not connected to the called line but is connected to a busy-back tone generator instead4 ,whether the line encountered be busy or idle1 the connective division remains in its condition as then adjusted until the subscriber hangs his receiver upon the hook switch to obtain disconnection4 ,the ringing of the bell of the called station is done directly by the calling subscriber in pressing the ringing key4 ,the disconnection is effected1 when the receiver of the calling line is hung up1 by the supervisory relay in the central office1 whose winding is included in the line circuit1 and whose contacts act directly to start the rotary switch4 ,in disconnecting1 the rotary switch starts the primary and the secondary connectors and thus instantly releases both the calling and the called lines4 ,thereafter the rotary switch in passing from position to position restores switch after switch of the connective division to normal and finally itself returns to normal in preparation for its assignment to service in answering a subsequent call4 ,,chapter ,,xxxi ,,the ,,automanual ,,system ,two systems of telephony are now in common use in this country--the manual system and the automatic4 ,with the growth of the automatic1 and the gradually ripening conviction1 which is now fully matured in the minds of most telephone engineers1 that automatic switching is practical1 there has been a growing tendency toward doing automatically many of the things that had previously been done manually4 ,one of the results of this tendency has been the production of the automanual system1 the invention of ,edward ,e4 ,clement1 an engineer and patent attorney1 of ,washington1 ,d4 ,c4 ,in connection with ,mr4 ,clement's name1 as inventor1 must be mentioned that of ,charles ,h4 ,north1 whose excellent work as a designer and manufacturer has contributed much toward the present excellence of this highly interesting system4 ,characteristics of ,system4 ,the name automanual is coined from the two words1 automatic and manual1 and is intended to suggest the idea that the system partakes in part of the features of the automatic system and in part of those of the manual system4 ,we regret that neither space nor the professional relation which we have had with the development of this system will permit us to make public an extended and detailed description of its apparatus and circuits4 ,only the general features of the system may1 therefore1 be dealt with4 ,illustration3 ,,power ,,apparatus ,,for ,,common-,,battery ,,manual ,,office ,,of ,,medium ,,size ,the underlying idea of the automanual system is to relieve the subscriber of all work in connection with the building up of his connection1 except the asking for it2 to complicate the subscriber's station equipment in no way1 it being left the same as in the common-battery manual system2 to do away with manual apparatus1 such as jacks1 cords and plugs1 at the central office1 and to substitute for it automatic switching apparatus which will be guided in its movements1 not by the subscriber1 but by a very much smaller number of operators than would be necessary to manipulate a manual switchboard4 ,general ,features of ,operation4 ,a broad view of the operation of the system is this4 ,the subscriber desiring to make a call takes down his receiver1 and this causes a lamp to light in front of an operator4 ,the operator presses a button and is in telephonic communication with the subscriber4 ,receiving the number desired1 the operator sets it up on a keyboard in just about the same way that a typist will set up the letters of a short word on a typewriting machine4 ,the setting up of the number on the keyboard being accomplished1 the proper condition of control of the associated automatic apparatus at the central office is established and the operator has no further connection with the call4 ,the automatic switching apparatus guided by the conditions set up on the operator's keyboard proceeds to make the proper selection of trunks and to establish the proper connections through them to build up a talking circuit between the calling subscriber and the called and to ring the called subscriber's bell1 or1 if his line is found busy1 the apparatus refuses to connect with it and sends a busy signal back to the calling subscriber4 ,the operator performs no work in disconnecting the subscribers1 that being automatically taken care of when they hang up their receivers at the close of the conversation4 ,from the foregoing it will be seen that there is this fundamental difference between the automatic and the automanual--the automatic system dispenses entirely with the central-office operator for all ordinary switching functions2 the automanual employs operators but attempts to so facilitate their work that they may handle very many more calls than would be possible in a manual system1 and at the same time secures the advantages of secrecy which the automatic system secures to its subscribers4 ,subscriber's ,apparatus4 ,one of the main points in the controversy concerning automatic versus manual systems is whether or not it is desirable to have the subscriber ask for his connection or to have him make certain simple movements with his fingers which will lead to his securing it4 ,the developers of the automanual system have taken the position that the most desirable way1 so far as the subscriber is concerned1 is to let him ask for it4 ,it is probable that this point will not be a deciding one in the choice of future systems1 since it already seems to be proven that the subscribers in automatic systems are willing to go through the necessary movements to mechanically set up the call4 ,the advantage which the automanual system shares with the manual1 however1 in the greater simplicity of its subscriber's station apparatus1 cannot be gainsaid4 ,illustration3 ,fig4 #dje4 ,operators' ,key ,tables ,illustration3 ,fig4 #djf4 ,top ,view of ,key ,table ,operator's ,equipment4 ,the general form of the operator's equipment is shown in ,fig4 #dje4 ,a closer view of the top of one of the key tables is shown in ,fig4 #djf4 ,as will be seen1 the equipment on each operator's position consists of three separate sets of push-button keys closely resembling in external appearance the keys of a typewriter or adding machine4 ,immediately above each set of keys are the signal lamps belonging to that set4 ,the operator's keys are arranged in strips of ten1 placed across rather than lengthwise on the key shelf4 ,one of these strips is shown in ,fig4 #djg4 ,there are as many strips of keys in each set as there are digits in the subscribers' numbers1 i4 e41 three in a system having a capacity of less than one thousand2 four in a system of less than ten thousand2 and so on4 ,in addition to the number keys of each set is a partial row of keys1 including what is called a starting key and also keys for making the party-line selection4 ,illustration3 ,fig4 #djg4 ,strip of ,selecting ,keys ,illustration3 ,fig4 #djh4 ,wiring of ,key ,shelf ,the simplicity of the operator's key equipment is one of its attractive features4 ,fig4 #djh shows one of the key shelves opened so as to expose to view all of the apparatus and wiring that is placed before the operator4 ,the reason for providing more than one key set on each operator's position is1 that after a call has been set up on one key set1 a few seconds is required before the automatic apparatus controlled by the key set can do its work and release the key set ready for another call4 ,the provision of more than one key set makes it possible for the operator to start setting up another call on another key set without waiting for the first to be released by the automatic apparatus4 ,illustration3 ,fig4 #dji4 ,switch ,room of ,automanual ,central ,office ,automatic ,switching ,equipment4 ,a general view of the arrangement of automatic switches in an exchange established by the ,north ,electric ,company at ,ashtabula1 ,ohio1 is shown in ,fig4 #dji4 ,the desk in the foreground is that of the wire chief4 ,this automatic apparatus consists largely of relays and automatic selecting switches4 ,the switches are of the step-by-step type1 having vertical and rotary movements1 and an idea of one of them1 minus its contact banks1 is given in ,fig4 #daj4 ,the control of the automatic switches by the operator's key sets is through the medium of a power-driven1 impulse-sending machine4 ,from this machine impulses are taken corresponding to the numbers of the keys depressed4 ,illustration3 ,fig4 #daj4 ,selecting ,switch ,automatic ,distribution of ,calls4 ,a feature of great interest in this system is the manner in which the incoming calls are distributed among the operators4 ,from each key set an operator's trunk is extended to what is called a secondary selector switch1 through which it may be connected to a primary selector trunk and calling line4 ,when a subscriber calls by taking down his receiver1 his line relay pulls up and causes a primary selector switch to connect his line with an idle local trunk or link circuit1 at the same time starting up a secondary selector switch which immediately connects the primary trunk and the calling line to an operator's idle key set4 ,if an operator is at the time engaged in setting up a call on a key set1 or if that key set is still acting to control the sending of impulses to the automatic switches1 it may be said to be busy1 and it is not selected by this preliminary selecting apparatus in response to an incoming call4 ,as soon1 however1 as the necessary impulses have been taken from the key set by the automatic apparatus1 that key set is released and is again ready to receive a call4 ,in this way the calls come before each operator only as that operator is able and ready to receive them4 ,setting up a ,connection4 ,as soon as the key-set lamp lights1 in response to such an incoming call1 the operator presses a listening button1 receives the number from the subscriber1 and depresses the corresponding number buttons on that key set1 thereby determining the numbers in each of the series of impulses to be sent to the selector and the connector switches to make the desired connection4 ,the operator repeats this number to the calling subscriber as she sets it up1 and then presses the starting button1 whereupon her work is done so far as that call is concerned4 ,if1 upon repeating the call to the subscriber1 the operator finds that she is in error1 she may change the number set up at any time before she has pressed the starting button4 ,building up a ,connection4 ,the keys so set up determine the number of impulses that will be transmitted by the impulse-sending machine to the selector and the connector switches4 ,these switches1 impelled by these impulses1 establish the connection if the line called for is not already connected to4 ,if a party-line station is called for1 the proper station on it will be selectively rung as determined by the party-line key depressed by the operator4 ,if the line is found busy1 the connector switch refuses to make the connection and places a busy-back signal on the calling line4 ,speed in ,handling ,calls4 ,this necessarily brief outline gives an idea only of the more striking features of the automanual system4 ,a study of the rapidity with which calls may be handled in actual practice shows remarkable results as compared with manual methods of operating4 ,the operators set up the number keys corresponding to a called number with the same rapidity that the keys of a typewriter are pressed in spelling a word4 ,in fact1 even greater speed is possible1 since it is noticed that the operators frequently will depress all of the keys of a number at once1 as by a single striking movement of the fingers4 ,the rapidity with which this is done defies accurate timing by a stop watch in the hands of an expert4 ,it is practically true1 therefore1 that the time consumed by the operator in handling any one call is that which is taken in getting the number from the subscriber and in repeating it back to him4 ,,table ,,xi ,total ,time ,consumed by ,operator in ,handling ,calls on ,automanual ,system ---------------------------------------- ------------------------- ,first #ajj ,calls ---------------------------------------- ------------------------- ,longest ,individual ,period #ab4dj seconds ,average five longest ,individual ,periods #g4dd seconds ,average ten longest ,individual ,periods #f4cd seconds ,shortest ,individual ,period #a4fj seconds ,average five shortest ,individual ,periods #a4ib seconds ,average ten shortest ,individual ,periods #a4if seconds ,average ,entire #ajj ,calls #c4cif seconds ,hourly ,rate at which calls were being handled #ajfj ---------------------------------------- ------------------------- ---------------------------------------- ------------------------- ,second #ajj ,calls ---------------------------------------- ------------------------- ,longest ,individual ,period #g4fj seconds ,average five longest ,individual ,periods #e4eb seconds ,average ten longest ,individual ,periods #e4cd seconds ,shortest ,individual ,period #b4jj seconds ,average five shortest ,individual ,periods #b4jd seconds ,average ten shortest ,individual ,periods #b4ah seconds ,average ,entire #ajj ,calls #c4cgd seconds ,hourly ,rate at which calls were being handled #ajfg ---------------------------------------- ------------------------- ---------------------------------------- ------------------------- ,third #ajj ,calls ---------------------------------------- ------------------------- ,longest ,individual ,period #e4dj seconds ,average five longest ,individual ,periods #e4cb seconds ,average ten longest ,individual ,periods #d4dd seconds ,shortest ,individual ,period #a4fj seconds ,average five shortest ,individual ,periods #a4fe seconds ,average ten shortest ,individual ,periods #a4hj seconds ,average ,entire #ajj ,calls #c4afj seconds ,hourly ,rate at which calls were being handled #aaci ---------------------------------------- ------------------------- ,owing to the difficulty of securing accurate traffic data by means of a stop watch1 an automatic1 electrical timing device1 capable of registering seconds and hundredths of a second1 has been used in studying the performance of this system in regular operation at ,ashtabula ,harbor4 ,the operators were not informed that the records were being taken1 and the data tabulated represents the work of two operators in handling regular subscribers' calls4 ,the figures in ,table ,,xi are given by ,c4 ,h4 ,north as representing the total time consumed by the operator from the time her line lamp was lighted until her work in connection with the call was finished1 and it included1 therefore1 the pressing of the listening button1 the receiving of the number from the subscriber1 repeating it back to him1 setting up the connection on the keys1 and pressing the starting key4 ,it will be seen that the average time for each #ajj calls is quite uniform and is slightly over three seconds4 ,the considerable variation in the individual calls1 ranging from a maximum of #ab4dj seconds down to a minimum of #a4fj seconds1 is due almost entirely to the difference between the subscribers in the speed with which they can give their numbers4 ,these figures indicate that1 in each of the tests1 calls were being handled at the rate of more than one thousand per hour by each operator4 ,the test of the subscriber's waiting time1 i4 e41 the time that he waited for the operator to answer1 for one hundred calls made without the knowledge of the operator1 showed the results as given in ,table ,,xii1 in which a split second stop watch was used in making the observations4 ,,table ,,xii ,subscribers' ,waiting ,time ---------------------------------------- ------------------ ,number of ,calls ,tested #ajj ,longest ,individual ,period #e4bj seconds ,average #e ,longest ,individual ,periods #d4fd seconds ,average #aj ,longest ,individual ,periods #c4hj seconds ,shortest ,individual ,period #a4jj seconds ,average #e ,shortest ,individual ,periods #a4bh seconds ,average #aj ,shortest ,individual ,periods #a4cd seconds ,average ,entire #ajj ,calls #b4jg seconds ---------------------------------------- ------------------ ,the length of time which the subscriber has to wait before receiving an answer from the operator is1 of course1 one of the factors that enters into the giving of good telephone service1 and the times shown by this test are considerably shorter than ordinarily maintained in manual practice4 ,the waiting time of the subscriber is not1 of course1 a part of the time that is consumed by the operator1 and the real economy so far as the operator's time is concerned is shown in the tests recorded in ,table ,,xi4 ,,chapter ,,xxxii ,,power ,,plants ,the power plant is an organization of devices to furnish to a telephone system the several kinds of current1 at proper pressures1 for the performance of the several general electrical tasks within the exchange4 ,kinds of ,currents ,employed4 ,sources of both direct and alternating current are required and a single exchange may employ these for one or more of the following purposes3 ,direct ,current4 ,current which flows always in one direction whether steady or varying1 is referred to as direct current1 and may be required for transmitters1 for relays1 for line1 supervisory1 and auxiliary signals1 for busy tests1 for automatic switches1 for call registers1 for telegraphy1 and in the form of pulsating current for the ringing of biased bells4 ,alternating ,current4 ,sources of alternating current are required for the ringing of bells1 for busy-back and other automatic signals to subscribers1 for howler signals to attract the attention of subscribers who have left their receivers off their hooks1 and for signaling over composite lines4 ,types of ,power ,plants4 ,clearly the requirements for current supply differ greatly for magneto and common-battery systems4 ,there is1 however1 no great difference between the power plants required for the automatic and the manual common-battery systems4 ,in the simplest form of telephone system--two magneto telephones on a private line--the power plant at each station consists of two elements3 one1 the magneto generator1 which is a translating device for turning hand power into alternating current for ringing the bell of the distant station2 and the other1 a primary battery which furnishes current to energize the transmitter4 ,in such a system1 therefore1 each telephone has its own power plant4 ,the term power plant1 however1 as commonly employed in telephone work1 refers more particularly to the organization of devices at the central office for furnishing the required kinds of current1 and it is to power plants in this sense that this chapter is devoted4 ,magneto ,systems4 ,if magneto lines be connected to a switchboard1 the current for throwing the drop at the switchboard is furnished by the subscriber's generator1 and the current for energizing the subscriber's transmitter is furnished by the local battery at his station2 but sources of current must be provided for enabling the central-office operator to signal or talk to the subscribers4 ,these are about the only needs for which current must be furnished in an ordinary magneto central office4 ,if a multiple board is employed1 direct current is also needed for the purpose of the busy test and also for operating the drop restoring circuits1 if the electrical method of restoring the drops is employed4 ,common-,battery ,systems4 ,in common-battery systems the requirements are very much more extensive4 ,the subscribers' telephones have no power plants of their own1 but are provided with a common source of direct current located at the central office for supplying the talking current1 and for operating the central-office signals1 and the operators are provided with one or more common sources of alternating or pulsating current for ringing the subscribers' bells4 ,common-battery equipment requires the use of currents of different kinds for a greater number of auxiliary purposes than does magneto equipment4 ,these facts make the power plant in a common-battery office much more important than in a magneto office4 ,operators' ,transmitter ,supply4 ,in a small magneto exchange1 the transmitter current may be had from primary batteries1 a separate battery being employed for each operator's set4 ,when there are more than three or four operators1 however1 it is usual1 even in magneto offices1 to obtain the transmitter current from a common storage battery4 ,a storage battery has the fortunate quality of very low internal resistance1 therefore a number of operators' transmitters may be actuated by one source without introducing cross-talk4 ,in other words1 a storage battery is a current-furnishing device of good regulation1 the variation of consumption in one circuit leading from it causing slight variation in the currents of other circuits leading from it4 ,if this were not so1 cross-talk would exist between the telephones of the operators' positions connected to the same battery4 ,this regulating quality enables the multiple feeding of telephone circuits to be carried further than the mere supplying of operators' sets and is the quality which makes possible the successful use of a storage battery as the single source of transmitter current for common-battery central-office equipment4 ,in furnishing a plurality of operators' transmitters from a common battery1 the importance of low resistance and inductance in the portion of the path that is common to all of the circuits must not be overlooked4 ,not only is a battery of extremely low resistance required1 but also conductors leading from it that are common to two or more of the circuits should be of very low resistance and consequently large in cross-section and as short as possible4 ,in common-battery offices there is obviously no need of employing a separate battery for the operators' transmitters1 since they may readily be supplied from the common storage battery which supplies direct current to the subscribers' lines4 ,ringing-,current ,supply4 ,magneto ,generators4 ,as a central-office equipment is required to ring many subscribers' bells1 only the small ones find it convenient to ring them by means of hand-operated magneto generators4 ,small magneto switchboards are usually equipped so that each operator is provided with a hand-generator1 but even where such is the case some source of ringing current not manually operated is desirable4 ,in larger switchboards the hand generators are entirely dispensed with4 ,the magneto generator may be driven by a belt from any convenient constantly moving pulley1 and the early telephone exchanges were often equipped with such generators having better bearings and more current capacity than those in magneto telephones4 ,these were adapted to be run constantly from some source of power1 delivering ringing current to the operators' keyboards at from #af to #bj cycles per second4 ,pole ,changers4 ,vibrating pole changers were also used in the early exchanges1 but passed out of use1 partly because of poor design1 but more because of the absence of good forms of primary batteries for vibrating them and for furnishing the direct currents to be transformed into alternating line current for ringing the bells4 ,the pole changer was redesigned after the beginning of the great spread of telephony in the ,united ,states in #ahic4 ,today it is firmly established as an element of good telephone practice4 ,fig4 #daa illustrates the principle upon which one of the well-known pole changers--the ,warner--operates4 ,in this #a is an electromagnet supplied by a constant-current battery #b to keep the vibratory system continually in motion4 ,this motor magnet and its battery work in a local circuit and cause vibration in exactly the same manner as the armature of an ordinary electric door bell is caused to vibrate4 ,the battery from which the ringing current is derived is indicated at #c1 and the poles of this are connected1 respectively1 to the vibrating contacts #d and #e4 ,these contacts are merely the moving members of a pole changing switch1 and a study of the action will readily show that when these moving parts engage the right-hand contacts1 current will flow to the line supposed to be connected to the terminals #f and #g in one direction1 while1 when these parts engage the left-hand contacts1 current will flow to the line in the reverse direction4 ,the circuit of the condenser shown is controlled by the armature of the relay #h4 ,the winding of this relay is put directly in the circuit of the main battery #c1 so that whenever current is drawn from this battery to ring a distant bell1 this relay will be operated and will bridge the condenser across the circuit of the line4 ,the purpose of the condenser is to make the impulses flowing from the pole changer less abrupt1 and the reason for having its bridged circuit normally broken is to prevent a waste of current from the battery #c1 due to the energy which would otherwise be consumed by the condenser if it were left permanently across the line4 ,illustration3 ,fig4 #daa4 ,warner ,pole ,changer ,illustration3 ,fig4 #dab4 ,pole ,changers for ,harmonic ,ringing ,pole changers for ringing bells of harmonic party lines are required to produce alternating currents of practically constant frequencies4 ,the ideal arrangement is to cause the direct currents from a storage battery to be alternated by means of the pole changers1 and then transformed into higher voltages required for ringing purposes1 the transformer also serving to smooth the current wave1 making it more suitable for ringing purposes4 ,in ,fig4 #dab such an arrangement1 adapted to develop currents for harmonic ringing on party lines1 is shown4 ,the regular common battery of the central office is indicated at #a1 #b being an auxiliary battery of dry cells1 the purpose of which will be presently referred to4 ,at the right of the battery #a there is shown the calling plug with its associated party-line ringing keys adapted to impress the several frequencies on the subscribers' lines4 ,the method by which the current from the main storage battery passes through the motor magnets of the several vibrators1 and by which the primary currents through the transformers are made to alternate at the respective frequencies of these vibrators1 will be obvious from the drawing4 ,it is also clear that the secondary currents developed in these transformers are led to the several ringing keys so as to be available for connection with the subscribers' lines at the will of the operator4 ,the condensers are bridged across the primary windings of the transformers for the purpose of aiding in smoothing out the current waves4 ,the use of the auxiliary battery #b and the retardation coil #c in the main supply lead is for the purpose of preventing the pulsating currents drawn from the main battery #a from making the battery noisy4 ,these two batteries have like poles connected to the supply lead1 and the auxiliary battery furnishes no current to the system except when the electromotive force of the impulse flowing from the main battery is choked down by the impedance coil and the deficiency is then momentarily supplied for each wave by the auxiliary battery4 ,this is the method developed by the ,dean ,electric ,company for preventing the pole-changer system from causing disturbances on lines supplied from the same main battery4 ,illustration3 ,fig4 #dac4 ,multi-,cyclic ,generator ,set ,ringing ,dynamos4 ,alternating and pulsating currents for ringing purposes are also largely furnished from alternating-current dynamos similar to those used in commercial power and lighting work1 but specially designed to produce ringing currents of proper frequency and voltage4 ,these are usually driven by electric motors deriving their current either from the commercial supply mains or from the central-office battery4 ,in large exchanges harmonic ringers are usually operated by alternating-current generators driven by motors1 a separate dynamo being provided to furnish the current of each frequency4 ,fig4 #dac shows a set of four such generators directly connected to a common motor4 ,as no source of commercial power for driving such generators is absolutely uniform1 and since the frequency of the ringing current must remain very close to a constant predetermined rate1 some means must be employed for holding the generators at a constant speed of revolution1 and this is done by means of a governor shown at the right-hand end of the shaft in ,fig4 #dac4 ,the principle of this governor is shown in ,fig4 #dad4 ,a weighted spring acts1 by centrifugal force1 to make a contact against an adjustable screw1 when the speed of the shaft rises a predetermined amount4 ,this spring and its contact are connected to two collector rings #a and #b on the motor shaft1 and connection is made with these by the brushes #c and #d4 ,the closing of the governor contact serves1 therefore1 merely to short-circuit the resistance #e1 which is normally included in the shunt field of the motor4 ,this governor is based on the principle that weakening the field increases the speed4 ,it acts to insert the resistance in series with the field winding when the speed falls1 and this1 in turn1 results in restoring the speed to normal4 ,illustration3 ,fig4 #dad4 ,governor for ,harmonic ,ringing ,generators ,auxiliary ,signaling ,currents4 ,alternating currents1 such as those employed for busy signals to subscribers in automatic systems1 those for causing loud tones in receivers which have been left off the hook switch1 and those for producing loud tones in calling receivers connected to composite lines1 all need to be of much higher frequency than alternating current for ringing bells4 ,the simplest way of producing such tones is by means of an interrupter like that of a vibrating bell2 but this is not the most reliable way and it is usual to produce busy or busy-back currents by rotating commutators to interrupt a steady current at the required rate4 ,as the usual busy-back signal is a series of recurrent tones about one-half second long1 interspersed with periods of silence1 the rapidly commuted direct current is required to be further commuted at a slow rate1 and this is conveniently done by associating a high-speed commutator with a low-speed one4 ,such an arrangement may be seen at the left-hand end of the multicyclic alternating machine shown in ,fig4 #dac4 ,this commuting device is usually associated with the ringing machine because that is the one thing about a central office that is available for imparting continuous rotary motion4 ,primary ,sources4 ,most telephone power plants consume commercial electric power and deliver special electric current4 ,usually some translating device1 such as a motor-generator or a mercury-arc rectifier1 is employed to transform the commercial current into the specialized current required for the immediate uses of the exchange4 ,charging from ,direct-,current ,mains4 ,in some cases commercial direct current is used to charge the storage batteries without the intervention of the translating devices1 resistances being used in series with the battery to regulate the amount of current4 ,commercial direct current usually is available at pressures from #aaj volts and upward1 while telephone power plants contain storage batteries rarely of pressures higher than #ej volts4 ,to charge a #ej-volt storage battery direct from #aaj-volt mains results in the loss of about half the energy purchased1 this lost energy being set free in the form of heat generated in the resistance devices4 ,notwithstanding this1 it is sometimes economical to charge directly from the commercial direct-current power mains1 but only in small offices where the total amount of current consumed is not large and where the greatest simplicity in equipment is desirable4 ,it is better1 however1 in nearly all cases1 to convert the purchased power from the received voltage to the required voltage by some form of translating device1 such as a rotary converter or a mercury-arc rectifier4 ,rotary ,converters4 ,broadly speaking1 a rotary converter consists of a motor adapted to the voltage and kind of current received1 mechanically coupled to a generator adapted to produce current of the required kind and voltage4 ,the harmonic ringing machine shown in ,fig4 #dac is an example of this1 this particular one being adapted to receive direct current at ordinary commercial pressure and to deliver four different alternating currents of suitable pressures and frequencies4 ,it is to be understood1 however1 that the conversion may be from direct current to direct current1 from alternating to direct1 or from direct to alternating4 ,such a device where the motor is a separate and distinct machine from the generator or generators is called a motor-generator4 ,it is usual to connect the motors and the generators together directly by a coupling having some flexibility1 as shown in ,fig4 #dac1 so as to prevent undue friction in the bearings4 ,illustration3 ,,the ,,power ,,and ,,wire ,,chief',s ,,room ,,of ,,the ,,exchange ,,at ,,webb ,,city1 ,,missouri ,as an alternative to the converting device made up of a motor coupled to a generator1 both motor and generator windings may be combined on the same core and rotate within the same field4 ,such a rotary converter has been called a dynamotor4 ,as a rule the dynamotor is only suitable for small power-plant work4 ,it has the following objectionable features3 7a7 ,it is difficult to regulate its output1 since the same field serves for both the motor and the dynamo windings4 ,for this reason its main use is as a ringing machine where the regulation of the output is not an important factor4 7b7 ,furthermore1 the fact that the motor and dynamo armature windings are on the same core makes it difficult to guard against breakdowns of the insulation between the two windings1 especially when the driving current is of high voltage4 ,charging ,dynamos4 ,the dynamo for charging the storage battery is1 of course1 a direct-current machine and may be a part of a motor generator or it may derive its power from some other than an electric motor1 such as a gas or steam engine4 ,it should be able to develop a voltage slightly above that of the voltage of the storage battery when at its maximum charge1 so as always to be able to deliver current to the charging battery regardless of the state of charge4 ,a #cj-volt generator1 for example1 can charge eleven cells in series economically2 a #fj-volt generator can charge twenty-five cells in series economically4 ,battery-charging generators are controlled as to their output by varying a resistance in series with their fields4 ,such machines are usually shunt-wound4 ,sometimes they are compound-wound1 but compounding is less important in telephone generators than in some other uses4 ,a feature of great importance in the design of charging generators is smoothness of current4 ,if it were possible to design generators to produce absolutely even or smooth current1 the storage battery would not be such an essential feature to common-battery exchanges1 because then the generator might deliver its current directly to the bus bars of the office without any storage-battery connection and without causing noise on the lines4 ,such generators have been built in small units4 ,even if these smooth current generators were commercially developed to a degree to produce absolutely no noise on the lines1 the storage battery would still be used1 since its action as a reservoir for electrical energy is important4 ,it not only dispenses with the necessity of running the generators continuously1 but it also affords a safeguard against breakdowns which is one of its important uses4 ,the ability to carry the load of a central office directly on the charging generator without the use of a storage battery is of no importance except in an emergency which takes the storage battery wholly out of service4 ,since the beginning of common-battery working such emergencies have happened a negligible number of times4 ,far more communities have lacked telephone service because of accidents beyond human control than because of storage-battery failures4 ,in power plants serving large offices1 the demand upon the storage battery is great enough to require large plate areas in each cell4 ,the internal resistance1 therefore1 is small and considerable fluctuations may exist in the charging current without their being heard in the talking circuits4 ,the amount of noise to be heard depends also on the type of charging generator4 ,increasing the number of armature coils and commutator segments increases the smoothness of the charging current4 ,the shape of the generator pole pieces is also a factor in securing such smoothness4 ,if1 with a given machine and storage battery1 the talking circuits are disturbed by the charging current1 relief may be obtained by inserting a large impedance in the charging circuit4 ,this impedance requires to be of low resistance1 because whatever heat is developed in it is lost energy4 ,this means that the best conditions exist when the resistance is low and the inductance large4 ,these conditions are satisfied by using in the impedance coil many turns of large wire and an ample iron core4 ,dynamotors are not generally suitable for charging purposes4 ,not only is the difficulty in regulating their output a disadvantage1 but the fact that the primary and secondary windings are so closely associated on the armature core makes them carry into the charging current1 not only the commutator noises of the generator end1 but of the motor end as well4 ,mercury-,arc ,rectifiers4 ,in common-battery offices serving a few hundred lines1 and where the commercial supply is alternating current1 it is good practice to transform it into direct-battery charging current by means of a mercury-arc rectifier4 ,it is a device broadly similar to the mercury-arc lamp produced by ,peter ,cooper ,hewitt4 ,it contains no moving parts and operates at high efficiency without introducing noises into the telephone lines4 ,it requires little care and has good length of life4 ,illustration3 ,fig4 #dae4 ,mercury-,arc ,rectifier ,circuits ,the circuit of a mercury-arc rectifier charging outfit is shown in ,fig4 #dae4 ,the mercury-arc rectifier proper consists of a glass bulb containing vacuum and a small amount of mercury4 ,when its terminals are connected1 as indicated--the two anodes across an alternating-current source and the cathode with a circuit that is to be supplied with direct current--this device has the peculiarity of action that current will flow alternately from the two anodes always to the cathode and never from it4 ,the cathode1 therefore1 becomes a source of positive potential and1 as such1 is used in charging the storage battery through the series reactance coil and the compensating reactances1 as indicated4 ,the line transformer shown at the upper portion of ,fig4 #dae1 is the one for converting the high-potential alternating current to the comparatively low-potential current required for the action of the rectifier4 ,the transformer below this has a one-to-one ratio1 and is called the insulating transformer4 ,its purpose is to safeguard the telephone apparatus and circuits against abnormal potentials from the line1 and also to prevent the ground1 which is commonly placed on the neutral wire of transformers on commercial lighting circuits1 from interfering with the ground that is commonly placed on the positive pole of the central-office battery4 ,provision ,against ,breakdown4 ,in order to provide against breakdown of service1 a well-designed telephone power plant should have available more than one primary source of power and more than one charging unit and ringing unit4 ,duplicate ,primary ,sources4 ,in large cities where the commercial power service is highly developed and a breakdown of the generating station is practically impossible1 it is customary to depend on that service alone4 ,in order to insure against loss of power due to an accident to portions of the distributing system1 it is the common custom to run two entirely separate power leads into the office1 coming1 if possible1 from different parts of the system so that a breakdown on one section will not deprive the telephone exchange of primary power4 ,in smaller places where the commercial service is not so reliable1 it is usual to provide1 in addition to the commercial electric-power service1 an independent source of power in the form of a gas or steam engine4 ,this may be run as a regular source1 the commercial service being employed as an emergency or vice versa1 as economy may dictate4 ,in providing a gas engine for driving charging dynamos1 it is important to obtain one having as good regulation as possible1 in order to obtain a charging current of practically constant voltage4 ,duplicate ,charging ,machines4 ,the storage batteries of telephone exchanges are usually provided of sufficient capacity to supply the direct-current needs of the office for twenty-four hours after a full charge has been given them4 ,this in itself is a strong safeguard against breakdown4 ,in addition to this the charging machines should be in duplicate1 so that a burnt-out armature or other damage to one of the charging units will not disable the plant4 ,duplicate ,ringing ,machines4 ,it is equally important that the ringing machines1 whether of the rotary or vibrating type1 be in duplicate4 ,for large exchanges the ringing machines are usually dynamos1 and it is not unusual to have one of these driven from the commercial power mains and the other from the storage battery4 ,with this arrangement complete failure of all sources of primary power would still leave the exchange operative as long as sufficient charge remains in the storage battery4 ,capacity of ,power ,units4 ,in designing telephone switchboards it is the common practice to so design the frameworks that the space for multiple jacks is in excess of that required for the original installation4 ,in a like manner1 the power plant is also designed with a view of being readily increased in capacity to an amount sufficient to provide current for the ultimate number of subscribers' lines for which the switchboard is designed4 ,the motor generators1 or whatever means are provided for charging the storage batteries1 are usually installed of sufficient size to care for the ultimate requirements of the office4 ,the ringing machines are also provided for the ultimate equipment4 ,however1 in the case of the storage battery1 it is common practice to provide the battery tanks of sufficient size to care for the ultimate capacity1 while the plates are installed for a capacity only slightly in excess of that required for the original installation4 ,as the equipment of subscribers' lines is increased1 additional plates may1 therefore1 be added to the cells without replacing the storage battery as a whole1 and without making extraordinary provisions to prevent the interruption of service4 ,it is also customary to provide charging and supply leads from the storage battery of carrying capacity sufficient for the ultimate requirements of the office4 ,storage ,battery4 ,the storage battery is the power plant element which has made common-battery systems possible4 ,the common-battery system is the element which has made the present wide development of telephony possible4 ,a storage-battery cell is an electro-chemical device in which a chemical state is changed by the passage of current through the cell1 this state tending to revert when a current is allowed to flow in the opposite direction4 ,a storage cell consists of two conductors in a solution1 the nature and the relation of these three elements being such that when a direct current is made to pass from one conductor to the other through the solution1 the compelled chemical change is proportional to the product of the current and its duration4 ,when the two conductors are joined by a path over which current may flow1 a current does flow in the opposite direction to that which charged the cell4 ,all storage batteries so far in extensive use in telephone systems are composed of lead plates in a solution of sulphuric acid in water called the electrolyte4 ,in charging1 the current tends to oxidize the lead of one plate and de-oxidize the other4 ,in discharging1 the tendency is toward equilibrium4 ,the containers1 employed in telephone work1 for the plates and electrolyte are either of glass or wood with a lead lining1 the glass jars being used for the smaller sized plates of small capacity cells1 while the lead-lined wooden tanks are employed with the larger capacity cells4 ,the potential of a cell is slightly over two volts and is independent of the shape or size of the plates for a given type of battery4 ,the storage capacity of a cell is determined by the size and the number of plates4 ,therefore1 by increasing the number of plates and the areas of their surfaces1 the ampere-hour capacity of the cell is correspondingly increased4 ,the desired potential of the battery is obtained by connecting the proper number of cells in series4 ,storage-battery cells used in telephone work vary from #b plates having an area of #ab square inches each1 to cells having over #ej plates1 each plate having an area of #bdj square inches4 ,the ampere-hour capacity of these batteries varies from #f ampere hours to #d1jjj ampere hours1 respectively1 when used at an average #h-;hour discharge rate4 ,in ,fig4 #daf is illustrated a storage cell employing a glass container and having fifteen plates4 ,each plate is #aa inches high and #aj-a_/b inches wide1 with an area1 therefore1 of #aae4e square inches4 ,such a cell has a normal capacity of #efj ampere hours4 ,the type illustrated is one made by the ,electric ,storage ,battery ,company of ,philadelphia1 ,pa4,a ,illustration3 ,fig4 #daf4 ,storage ,cell ,installation4 ,in installing the glass jars it is customary to place them in trays partially filled with sand4 ,they are1 however1 at times installed on insulators so designed as to prevent moisture from causing leakage between the cells4 ,the cells using wooden tanks are placed on glass or porcelain insulators1 and the tanks are placed with enough clearance between them to prevent the lead lining of adjacent tanks from being in contact and thereby short-circuiting the cells4 ,after the positive and the negative plates have been installed in the tanks1 their respective terminals are connected to bus bars1 these bus bars being1 for the small types of battery1 lead-covered clamping bolts1 while in the larger types reinforced lead bus bars are employed1 to which the plates are securely joined by a process called lead burning4 ,this process consists in melting a portion of the bus bar and the terminal lug of the plate by a flame of very high temperature1 thus fusing each individual plate to the proper bus bar4 ,the plates of adjacent cells are connected to the same bus bar1 thus eliminating the necessity of any other connection between the cells4 ,initial ,charge4 ,as soon as the plates have been installed in the tanks and welded to the bus bars1 the cell should be filled with electrolyte having a specific gravity of #a4ahj to #a4aij to one-half inch above the tops of the plates and then the charge should be immediately started at about the normal rate4 ,in the case of a battery consisting of cells of large capacity1 it is customary to place the electrolyte in the cells as nearly simultaneously as possible rather than to completely fill the cells in consecutive order4 ,when the electrolyte is placed in the cells simultaneously1 the charge is started at a very much reduced rate before the cells are completely filled1 the rate being increased as the cells are filled1 the normal rate of charge being reached when the cells are completely filled4 ,readings should be taken hourly of the specific gravity and temperature of the electrolyte1 voltage of the cells1 and amperage of charging current4 ,a record or log should be kept of the specific gravity and voltage of each of the cells of the battery regularly during the life of the battery and it is well to commence this record with the initial charge4 ,the initial charge should be maintained for at least ten hours after the time when the voltage and specific gravity have reached a maximum4 ,if for any reason it is impractical to continue the initial charge uninterrupted1 the first period of charging should be at least from twelve to fifteen hours4 ,however1 every effort should be made to have the initial charge continuous1 as an interruption tends to increase the time necessary for the initial charge1 and if the time be too long between the periods of the initial charge1 the efficiency and capacity of the cells are liable to be affected4 ,in case of a large battery1 precaution should be taken to insure that the ventilation is exceptionally good1 because if it is not good the temperature is liable to increase considerably and thereby cause an undue amount of evaporation from the cells4 ,the object of the temperature readings taken during the charge is to enable corrections to be made to the specific gravity readings as obtained by the hydrometer1 in order that the correct specific gravity may be ascertained4 ,this correction is made by adding 4#jja specific gravity for each three degrees in temperature above #gj deg4 ,fahrenheit1 or subtracting the same amount for each three degrees below #gj deg4 ,fahrenheit4 ,at the time the cells begin to gas they should be gone over carefully to see that they gas evenly1 and also to detect and remedy early in the charging period any defects which may exist4 ,if there is any doubt in regard to the time at which the cells reach a maximum voltage and specific gravity1 the charge should be continued sufficiently long before the last ten hours of the charge are commenced to eliminate any such doubt1 as in many cases poor efficiency and low capacity of a cell later in its life may be traced to an insufficient initial charge4 ,operation4 ,after the battery has been put in commission the periodic charges should be carefully watched1 as excessive charging causes disintegration and decreases the life and capacity of the battery2 while1 on the other hand1 undercharging will result in sulphating of the plates and decrease of capacity1 and1 if the undercharge be great1 will result in a disintegration of the plates4 ,it is1 therefore1 essential that the battery be charged regularly and at the rate specified for the particular battery in question4 ,in order to minimize the chance of either continuously overcharging or undercharging the battery1 the charges are divided into two classes1 namely1 regular charges and overcharges4 ,the regular charges are the periodic charges for the purpose of restoring the capacity of the battery after discharge4 ,the overcharges1 which should occur once a week or once in every two weeks1 according to the use of the battery1 are for the purpose of insuring that all cells have received their proper charge1 for reducing such sulphating as may have occurred on cells undercharged1 and for keeping the plates1 in general1 in a healthy condition4 ,the specific gravity of the electrolyte1 the voltage of the battery1 and the amount of gasing observed are all indications of the amount of charge which the battery has received and should all be considered when practicable4 ,either the specific gravity or voltage may be used as the routine method of determining the proper charge1 but1 however1 if the proper charge is determined by the voltage readings1 this should be frequently checked by the specific gravity1 and vice versa4 ,during the charging and discharging of a battery the level of the electrolyte in the cells will fall4 ,as the portion of the electrolyte which is evaporated is mainly water1 the electrolyte may be readily restored to its normal level by adding distilled water or carefully collected rain water4 ,pilot ,cell4 ,as the specific gravity of all the cells of a battery1 after having once been properly adjusted1 will vary the same in all the cells during use1 it has been found satisfactory to use one cell1 commonly termed the pilot cell1 for taking the regular specific gravity readings and only reading the specific gravity of all the cells occasionally or on the overcharge4 ,this cell must be representative of all the cells of the battery1 and if the battery is so subdivided in use that several sets of cells are liable to receive different usage1 a pilot cell should be selected for each group4 ,overcharge4 ,if the battery is charged daily1 it should receive an overcharge once a week1 or if charged less frequently1 an overcharge should be given at least once every two weeks4 ,in making an overcharge this should be done at a constant rate and at a rate specified for the battery4 ,during the overcharge the voltage of the battery and the specific gravity of the pilot cell should be taken every fifteen minutes from the time the gasing begins4 ,the charge should be continued until five consecutive1 specific-gravity readings are practically the same4 ,the voltage of the battery should not increase during the last hour of the charge4 ,as the principal object of the overcharge is to insure that all of the cells have received the proper charge1 it must1 therefore1 be continued long enough to not only properly charge the most efficient cells1 but also to properly charge those which are lower in efficiency4 ,the longer the interval between overcharges1 the greater will be the variation between the cells and1 therefore1 it is necessary to continue the overcharge longer when the interval between overcharges is as great as two weeks4 ,before the overcharge is made the cells should be carefully inspected for short circuits and other abnormal conditions4 ,these inspections may best be made by submerging an electric lamp in the cell1 if the cell be of wood1 or of allowing it to shine through from the outside1 if it be of glass4 ,by this means any foreign material may be readily detected and removed before serious damage is caused4 ,in making these inspections it must be borne in mind that whatever tools or implements are used must be non-metallic and of some insulating material4 ,regular ,charge4 ,regular charges are the periodic charges for restoring the capacity of the battery1 and should be made as frequently as the use of the battery demands4 ,the voltage of the cells is a good guide for determining when the battery should be recharged4 ,the voltage of a cell should never be allowed to drop below #a4h volts1 and it is usually considered better practice to recharge when the battery has reached #a4i volts4 ,if a battery is to remain idle for even a short time1 it should be left in a completely charged condition4 ,the regular charges for cells completely equipped with plates should be continued until the specific gravity of the pilot cell has risen to five points below the maximum attained on the preceding overcharge1 or1 if only partially equipped with plates1 until it has risen to three points below the previous maximum4 ,the voltage per cell at this time should be from 4#je volts to 4#a volts below that obtained on the previous overcharge4 ,at this time all the cells should be gasing1 but not as freely as on an overcharge4 ,low ,cells4 ,an unhealthy condition in a cell usually manifests itself in one of the following ways3 ,falling off in specific gravity or voltage relative to the rest of the cells1 lack of gasing when charged1 and color of the plates1 either noticeably lighter or darker than those of other cells of the battery4 ,when any of the above conditions are found in a cell1 the cell should receive immediate attention1 as a delay may mean serious trouble4 ,the cell should be thoroughly inspected to determine if a short-circuit exists1 either caused by some foreign substance1 by an excess of sediment in the bottom of the tank1 or by portions of the plates themselves4 ,if such a condition is found1 the cause should be immediately removed and1 if the defect has been of short duration1 the next overcharge will probably restore it to normal condition4 ,if the defect has existed for some time1 it is often necessary to give the cell a separate charge4 ,this may be done by connecting it directly to the charging generator with temporary leads and thus bring it back to its normal condition4 ,it is sometimes found necessary to replace the cell in order to restore the battery to its normal condition4 ,sediment4 ,the cells of the battery should be carefully watched to prevent the sediment which collects in the bottom of the jar or tank during use from reaching the bottom of the plates1 thereby causing short circuits between them4 ,when the sediment in the cell has reached within one-half inch of the bottom of the plates1 it should be removed at once4 ,with small cells using glass jars this can most easily be done directly after an overcharge by carefully drawing off the electrolyte without disturbing the sediment and then removing it from the jar4 ,the plates and electrolyte should be replaced in the jar as soon as convenient to prevent the plates from becoming dry4 ,if the plates are large and in wooden tanks1 the sediment can most easily be removed by means of a scoop made especially for the purpose4 ,the preferable time to clean the tanks is just before an overcharge4 ,replacing ,batteries4 ,there comes a time in the life of nearly every central-office equipment when the storage battery must be completely renewed4 ,this is due to the fact that the life of even the best of storage batteries is not as great as the life of the average switchboard equipment4 ,it may also be due to the necessity for greater capacity than can be secured with the existing battery tanks1 usually caused by underestimating the traffic the office will be required to handle4 ,again1 it is sometimes necessary to make extensive alterations in an existing battery1 perhaps due to the necessity for changing its location4 ,to change a battery one cell at a time1 keeping the others in commission meanwhile1 has often been done1 but it is always expensive and unsatisfactory and is likely to shorten the life of the battery1 due to improper and irregular forming of the plates during the initial charge4 ,the advent of the electric automobile industry has brought with it a convenient means for overcoming this difficulty4 ,portable storage cells for automobile use are available in almost every locality and may often be rented at small cost4 ,a sufficient number of such cells may be temporarily installed1 enough of them being placed in multiple to give the necessary output4 ,by floating a temporary battery so formed across the charging mains and running the generators continuously1 a temporary source of current supply may be had at small expense for running the exchange during the period required for alterations4 ,usually a time of low traffic is chosen for making the changes1 such as from ,saturday evening to ,monday morning4 ,very large central-office batteries1 serving as many as #f1jjj lines1 have thus been taken out of service and replaced without interfering with the traffic and with the use of but a comparatively few portable cells4 ,one precaution has to be observed in such work1 and that is not to subject the portable cells to too great an overcharge1 due to the great excess of generator over battery capacity4 ,this is easily avoided by watching the ammeters to see that the input is not in too great excess of the output1 and if necessary1 by frequently stopping the machines to avoid this4 ,power ,switchboard4 ,the clearing-house of the telephone power plant is the power board4 ,in most cases1 it carries switches1 meters1 and protective devices4 ,switches4 ,the switches most essential are those for opening and closing the motor and the generator circuits of the charging sets and with these usually are associated the starting rheostats of the motors and the field rheostats of the generators4 ,the starting rheostats are adapted to allow resistance to be removed from the motor armature circuit1 allowing the armature to gain speed and increase its counter-electromotive force without overheating4 ,the accepted type has means for opening the driving circuit automatically in case its voltage should fall1 thus preventing a temporary interruption of driving current from damaging the motor armature on its return to normal voltage4 ,illustration3 ,fig4 #dag4 ,power-,plant ,circuits ,meters4 ,the meters usually are voltmeters and ammeters1 the former being adapted to read the several voltages of direct currents in the power plant4 ,an important one to be known is the voltage of the generator before beginning a battery charge1 so that the generator may not be thrown on the storage battery while generating a voltage less than that of the battery4 ,if this were done1 the battery would discharge through the generator armature4 ,the voltmeter enables the voltage of the charging generator to be kept above that of the battery1 as the latter rises during charge4 ,it enables the performance of several cells of the battery to be observed4 ,a convenient way is to connect the terminals of the several cells to jacks on the power board and to terminate the voltmeter in a plug4 ,the ammeter1 with suitable connections1 enables the battery-charge rate to be kept normal and the battery discharge to be observed4 ,in order to economize power1 it is best to charge the battery during the hours of heavy load4 ,the generator output then divides1 the switchboard taking what the load requires1 the battery receiving the remainder4 ,in systems requiring the terminal voltage of the equipment to be kept constant within close limits1 either it is necessary to use two batteries--never drawing current from a battery during charge--or to provide means of compensating for the rise of voltage while the battery is under charge4 ,the latter is the more modern method and is done either by using fewer cells when the voltage per cell is higher or by inserting counter-electromotive force cells in the discharge leads1 opposing the discharge by more or fewer cells as the voltage of the battery is higher or lower4 ,in either method1 switches on the power board enable the insertion and removal of the necessary end cells or counter-electromotive force cells4 ,protective ,devices4 ,the protective devices required on a power board are principally circuit-breakers and fuses4 ,circuit-breakers are adapted to open motor and generator circuits when their currents are too great1 too small1 or in the wrong direction4 ,fuses are adapted to open circuits when the currents in them are too great4 ,the best type is that in which the operation of the fuses sounds or shows an alarm1 or both4 ,power-,plant ,circuits4 ,the circuit arrangement of central-office power plants is subject to wide variation according to conditions4 ,the type of telephone switchboard equipment1 whether magneto or common-battery1 automatic or manual1 will1 of course1 largely affect the circuit arrangement of the power plant4 ,fig4 #dag shows a typical example of good practice in this respect for use with a common-battery manual switchboard equipment4 ,besides showing the switches for handling the various machines and the charge-and-discharge leads from the storage battery1 this diagram shows how current from the storage battery is delivered to various parts of the central-office equipment4 ,footnote ,a3 ,the instructions given later in this chapter are for batteries of this make1 although they are applicable in many respects to all types commonly used in telephone work4 ,,chapter ,,xxxiii ,,housing ,,central-,,office ,,equipment ,the ,central-,office ,building4 ,proper arrangement of the central-office equipment depends largely upon the design of the central-office building4 ,the problem involved should not be solved by the architect alone4 ,the most careful co-operation between the engineer and the architect is necessary in order that the various parts of the telephonic equipment may be properly related1 and that the wires connecting them with each other and with the outside lines be disposed of with due regard to safety1 economy1 and convenience4 ,so many factors enter into the design of a central-office building that it is impossible to lay down more than the most general rules4 ,the attainment of an ideal is often impossible1 because of the fact that the building is usually in congested districts1 and its very shape and size must be governed by the lot on which it is built1 and by the immediate surroundings4 ,frequently1 also1 the building must be used for other purposes than those of a telephone office1 so that the several purposes must be considered in its design4 ,again1 old buildings1 designed for other purposes1 must sometimes be altered to meet the requirements of a telephone office1 and this is perhaps the most difficult problem of all4 ,the exterior of the building is a matter that may be largely decided by the architect and owner after the general character of the building has been determined4 ,one important feature1 however1 and one that has been overlooked in many cases that we know of1 is to so arrange the building that switchboard sections and other bulky portions of the apparatus1 which are necessarily assembled at the factory rather than on the site1 may be brought into the building without tearing down the walls4 ,fire ,hazard4 ,the apparatus to be housed in a central-office building often represents a cost running into the hundreds of thousands of dollars2 but whether of large or small first cost1 it is evident that its destruction might incur a very much greater loss than that represented by its replacement value4 ,in guarding the central-office equipment against destruction by fire or other causes1 the telephone company is concerned to a very much greater extent than the mere cost of the physical property2 since it is guarding the thing which makes it possible to do business4 ,while the cost of the central office and its contents may be small in comparison with the total investment in outside plant and other portions of the equipment1 it is yet true that these larger portions of the investment become useless with the loss of the central office4 ,there is another consideration1 and that is the moral obligation of the operating company to the public4 ,a complete breakdown of telephone service for any considerable period of time in a large city is in the nature of a public calamity4 ,for these reasons the safeguarding of the central office against damage by fire and water should be in all cases a feature of fundamental importance1 and should influence not only the character of the building itself1 but in many cases the choice of its location4 ,size of ,building4 ,it goes without saying that the building must be large enough to accommodate the switchboards and other apparatus that is required to be installed4 ,the requirement does not end here1 however4 ,telephone exchange systems have1 with few exceptions1 grown very much faster than was expected when they were originally installed4 ,many buildings have had to be abandoned because outgrown4 ,in planning the building1 therefore1 the engineer should always have in mind its ultimate requirements4 ,it is not always necessary that the building shall be made large enough at the outset to take care of the ultimate requirements1 but where this is not done1 the way should be left clear for adding to it when necessity demands4 ,illustration3 ,,ringing ,,and ,,charging ,,machines ,,and ,,power ,,board ,plaza ,office1 ,new ,york ,telephone ,co4 ,strength of ,building4 ,the major portion of telephone central-office apparatus1 whether automatic or manual1 is not of such weight as to demand excessive strength in the floors and walls of buildings4 ,exceptions to this may be found in the storage battery1 in the power machinery1 especially where subject to vibration1 and in certain cases in the cable runs4 ,after the ultimate size of the equipment has been determined1 the engineer and the architect should confer on this point1 particularly with reference to the heavier portions of the apparatus1 to make sure that adequate strength is provided4 ,the approximate weights of all parts of central-office equipments may readily be ascertained from the manufacturers4 ,provision for ,employes4 ,in manual offices particularly it has been found to be not only humane1 but economical to provide adequate quarters for the employes1 both in the operating rooms and places where they actually perform their work1 and in the places where they may assemble for recreation and rest4 ,the work of the telephone operator1 particularly in large cities1 is of such a nature as often to demand frequent periods of rest4 ,this is true not only on account of the nervous strain on the operator1 but also on account of the necessity1 brought about by the demands of economy1 for varying the number of operators in accordance with the traffic load4 ,these features accentuate the demand for proper rooms where recreation1 rest1 and nourishment may be had4 ,provision for ,cable ,runways4 ,in very small offices no special structural provision need be made in the design of the building itself for the entrance of the outside cables1 and for the disposal of the cables and wires leading between various portions of the apparatus4 ,for large offices1 however1 this must necessarily enter as an important feature in the structure of the building itself4 ,it is important that the cables be arranged systematically and in such a way that they will be protected against injury and at the same time be accessible either for repairs or replacement1 or for the addition of new cables to provide for growth4 ,disorderly arrangement of the wires or cables results in disorder indeed1 with increased maintenance cost1 uneconomical use of space1 inaccessibility1 liability to injury1 and general unsightliness4 ,the carrying of cables from the basement to the upper floors or between floors elsewhere must be provided for in a way that will not be wasteful of space1 and arrangements must be made for supporting the cables in their vertical runs4 ,in the aggregate their weight may be great1 and furthermore each individual cable must be so supported that its sheath will not be subject to undue strain4 ,another factor which must be considered in vertical cable runs is the guarding against such runs forming natural flues through which flames or heated gases would pass1 in the event of even an unimportant fire at their lower ends4 ,arrangement of ,apparatus in ,small ,manual ,offices4 ,where a common-battery multiple switchboard equipment is used1 at least three principal rooms should be provided--one for the multiple switchboard proper2 one for the terminal and power apparatus1 including the distributing frames1 racks1 and power machinery2 and the third for the storage battery4 ,these should adjoin each other for purposes of convenience and of economy in wiring4 ,illustration3 ,fig4 #dah4 ,typical ,small ,office ,floor ,plan ,floor ,plans for ,small ,manual ,offices4 ,as was pointed out1 there are several plans of disposing of the main and intermediate distributing frames and the line and cut-off relay racks4 ,the one most practiced is to mount the relay rack alongside the main and intermediate distributing frame in the terminal room4 ,a typical floor plan of such an arrangement for a small office1 employing as a maximum five sections of multiple switchboards1 is shown in ,fig4 #dah4 ,this is an ideal arrangement well adapted for a rectangular floor space and on that account may often be put into effect4 ,it should be noted that the switchboard grows from left to right1 and that alternative arrangements are shown for disposing of those sections beyond the second4 ,the cable turning section through which the multiple and answering jacks are led to the terminal frames is placed as close as possible to the terminal frames4 ,this results in a considerable saving in cable4 ,an interesting feature of this floor plan is the arrangement of unitary sections of main and intermediate frames and relay racks1 representing recent practice of the ,western ,electric ,company4 ,the iron work of the three racks is built in sections and these are structurally connected across so that the first section of the main frame1 the intermediate frame1 and the relay rack form one unit1 the structural iron work which ties them together forming the runway for the cables between them4 ,but two of these units1 including two sections of each frame1 are shown installed1 the provision for growth being indicated by dotted lines4 ,the battery room in this case provides for the disposal of the battery cells in two tiers4 ,this room is merely partitioned off from the distributing or terminal room4 ,where this is done the partition walls should be plastered on both sides so as to prevent1 as far as possible1 the entrance of any battery fumes into the apparatus rooms4 ,the wire chief's desk1 as will be noted1 is located in such a position as to give easy access from it not only to the distributing frames and relay rack1 but to the power apparatus as well4 ,combined ,main and ,intermediate ,frames4 ,for use in small exchanges1 the ,western ,electric ,company has recently put on the market a combined main and intermediate distributing frame4 ,this is constructed about the same as an ordinary main frame1 the protectors being on one side and the line and intermediate frame terminals on the other4 ,the lower half of the terminals on each vertical bay is devoted to the outside line terminals and the upper half is devoted to intermediate frame terminals4 ,this arrangement is indicated in the elevation in ,fig4 #dai4 ,with the use of this combined main and intermediate frame1 the floor plan of ,fig4 #dah may be modified1 as shown in ,fig4 #dbj4 ,illustration3 ,fig4 #dai4 ,combined ,main and ,intermediate ,frames ,illustration3 ,fig4 #dbj4 ,small ,office ,floor ,plan ,illustration3 ,fig4 #dba4 ,terminal ,apparatus--,small ,office ,in ,fig4 #dba is given an excellent idea of terminal-room apparatus carried out in accordance with the more usual plan of employing separate main and intermediate distributing frames4 ,at the extreme right of this figure the protector side of the main frame is shown4 ,it will be understood that the line cables terminate on the horizontal terminal strips on the other side of this frame and are connected through the horizontal and vertical runways of the frame to the protector terminals4 ,the intermediate frame is shown in the central portion of the figure1 the side toward the left containing the answering-jack terminals1 and the side toward the right the multiple jack terminals1 these latter being arranged horizontally4 ,this horizontal and vertical arrangement of the terminals on the main and intermediate distributing frames has been the distinguishing feature between the ,bell and ,independent practice1 the ,bell ,companies adhering to the horizontal and vertical arrangement1 while the ,independent ,companies have employed the vertical arrangement on both sides4 ,we are informed that in the future the new smaller installations of the ,bell ,companies will be made largely with the vertical arrangement on both sides4 ,at the left of ,fig4 #dba is shown the relay rack in two sections of two bays each4 ,this illustration also gives a good idea of the common practice in disposing of the cables between the frames in iron runways just below the ceiling of the terminal room4 ,types of ,line ,circuits4 ,the design of the terminal-room floor plan will depend largely on the arrangement of apparatus in the subscribers' line circuits with respect to the distributing frames and relay racks4 ,the ,bell practice in this respect has already been referred to and is illustrated in ,fig4 #cdh4 ,in this the line and cut-off relays are permanently associated with the answering jacks and lamps1 resulting in the answering-jack equipment being subject to change with respect to the multiple and the line through the jumpers of the intermediate frame4 ,the practice of the ,kellogg ,company1 on the other hand1 has been illustrated in ,fig4 #cec1 and in this the line and cut-off relays are permanently associated with the multiple and with the line1 only the answering jacks and lamps being subject to change through the jumper wires on the intermediate frame4 ,this latter arrangement has led to a very desirable parallel arrangement of the two distributing frames and the relay rack4 ,these are made of equal length so as to correspond bay for bay1 and are placed side by side with only enough space between them for the passage of workmen--the relay rack lying between the main and intermediate frames4 ,in this scheme all the multiple and answering-jack cables run from the intermediate distributing frame1 and the cabling between the intermediate frame and the relay rack and between the relay rack and the main frame is run straight across from one rack to the other4 ,this results in a great saving of cable within the terminal room1 over that arrangement wherein the cabling from one frame to another is necessarily led along the length of the frame to its end and then passes through a single runway to the end of the other frame4 ,large ,manual ,offices4 ,for purposes of illustrating the practice in housing the apparatus in very large offices equipped with manual switchboards1 we have chosen the ,chelsea office of the ,new ,york ,telephone ,company as an excellent example of modern practice4 ,illustration3 ,fig4 #dbb4 ,floor ,plan1 ,operating ,room1 ,chelsea ,office1 ,new ,york ,city ,the ground plan of the building is ,u-shaped1 in order to provide the necessary light over the rather large floor areas4 ,the plan of the operating floor--the sixth floor of the building--is shown in ,fig4 #dbb4 ,as will be seen1 this constitutes a single operating room1 the ,a-board being located in the right wing and the ,b-board in the left4 ,the point from which both boards grow is near the center of the front of the building1 the boards coming together at this point in a common cable turning section4 ,the disposal of the various desks for the manager1 chief operator1 and monitors is indicated4 ,those switchboard sections which are shown in full lines are the ones at present installed1 the provision for growth being indicated in dotted lines4 ,illustration3 ,fig4 #dbc4 ,terminal ,room and ,operators' ,quarters1 ,chelsea ,office1 ,new ,york ,city ,the fifth floor is devoted to the terminal room and operators' quarters1 the terminal room occupying the left-hand wing and the major portion of the front of the building1 and the operators' quarters the right-hand wing4 ,the line and the trunk cables come up from the basement of the building at the extreme left1 being supported directly on the outside wall of the building4 ,arriving at the fifth floor1 they turn horizontally and are led under a false flooring provided with trap doors1 to the protector side of the main frame4 ,the disposal of the cables between the various frames will be more readily understood by reference to the following photographs4 ,a general view of a portion of the ,a-board of the ,chelsea office is shown in ,fig4 #dbd1 this view being taken from a point in the left-hand wing looking toward the front4 ,in ,fig4 #dbe is shown a closer view of a smaller portion of the board4 ,fig4 #dbf gives an excellent idea of the rear of this switchboard and of the disposal of the cables and wires4 ,the main mass of cables at the top are those of the multiple4 ,immediately below these may be seen the outgoing trunk cables4 ,the forms of the answering-jack cables lie below these and are not so readily seen1 but the cables leading from these forms are led down to the runway at the bottom of the sections1 and thence along the length of the board to the intermediate distributing frame on the floor below4 ,the layer of cables1 supported on the iron rack immediately above the answering-jack cable runway1 shown at the extreme bottom of the view1 are those containing the wires leading from the repeating coils to the cord circuits4 ,an interesting feature of this board is the provisions for protection against injury by fire and water4 ,on top of the boards throughout their entire length there is laid a heavy tarpaulin curtain with straps terminating in handles hanging down from its edges4 ,these may be seen in ,fig4 #dbf and also in ,fig4 #dbe4 ,the idea of this is that if the board is exposed to a water hazard1 as in the case of fire1 the board may be completely covered1 front and rear1 with this tarpaulin curtain1 by merely pulling the straps4 ,the entire force--both operators and repairmen--is drilled to assure the carrying out of this plan4 ,the rear of the boards is adapted to be enclosed by wooden curtains1 similar to those employed in roll-top desks4 ,these are all raised in the rear view of ,fig4 #dbf1 the housing for the rolled-up curtain being shown at the extreme top of the sections4 ,in order to guard the multiple cables and the multiple jacks against fire which might originate in the cord-circuit wiring1 a heavy asbestos partition is placed immediately above the cord racks and is clearly shown in ,fig4 #dbf4 ,illustration3 ,fig4 #dbd4 ,subscribers' ,board4 ,chelsea ,office1 ,new ,york ,city ,illustration3 ,fig4 #dbe4 ,subscribers' ,board4 ,chelsea ,office1 ,new ,york ,city ,illustration3 ,fig4 #dbf4 ,rear ,view ,chelsea ,switchboard ,illustration3 ,fig4 #dbg4 ,terminal and ,power ,apparatus4 ,chelsea ,office ,a view of the terminal and power room is shown in ,fig4 #dbg4 ,in the upper left-hand corner the cables may be seen in their passage downward from the cable turning section between the ,a- and ,b-boards4 ,the large group of cables shown at the extreme left is the ,a-board multiple4 ,this passes down and then along the horizontal shelves of the intermediate frame1 which is the frame in the extreme left of this view4 ,the ,b-board multiple comes down through another opening in the floor1 and as is shown1 after passing under the ,a-board multiple joins it in the same vertical run from which it passes to the intermediate frame4 ,the cord-circuit cables lead down through the same opening as that occupied by the ,a-board multiple and pass off to the right-hand one of the racks shown1 which contains the repeating coils4 ,the cables leading from the opening in the ceiling to the right-hand side of the intermediate distributing frame are the answering-jack cables1 and from the terminals on this side of this frame other cables pass in smaller groups to the relay terminals on the relay racks which lie between the intermediate frame and the coil rack4 ,the power board is shown at the extreme right4 ,the fuse panel at the left of the power board contains in its lower portion fuses for the battery supply leads to the operator's position and to private-branch exchanges1 and in its upper portion lamps and fuses for the ringing generator circuits for the various operators' positions and also for private-branch exchanges4 ,at the lower left-hand portion of this view is shown the battery cabinet4 ,it is the practice of the ,new ,york ,telephone ,company not to employ separate battery rooms1 but to locate its storage batteries directly in the terminal room and to enclose them1 as shown1 in a wooden cabinet with glass panels1 which is ventilated by means of a lead pipe extending to a flue in the wall4 ,one unit of charging machines1 consisting of motor and generator1 is shown in the immediate foreground4 ,a duplicate of this unit is employed but is not shown in this view4 ,the various ringing and message register machines are shown beyond the charging machines4 ,three of these smaller machines are for supplying ringing current and the remainder are for supplying #cj-volt direct current for operating the message registers4 ,one of the machines of each set is wound to run from the main storage battery in case of a failure of the general lighting service from which the current for operating is normally drawn4 ,illustration3 ,fig4 #dbh4 ,terminal ,apparatus4 ,chelsea ,office ,illustration3 ,fig4 #dbi4 ,floor ,plan1 ,automatic ,office1 ,lansing1 ,michigan ,another view of the terminal-room apparatus is given in ,fig4 #dbh4 ,this is taken from the point marked ,b on the floor plan of ,fig4 #dbc4 ,at the right may be seen the message registers on which the calls of the subscribers in this office are counted as a basis for the bills for their service4 ,at the extreme left is shown the private-line test board4 ,through this board run all of the lines leased for private use1 and also all of the order wire or call lines passing through this office4 ,the purpose of such an arrangement is to facilitate the testing of such line wires4 ,at the right of this private-line test board is shown a four-position wire chief's desk1 upon which are provided facilities for making all of the tests inside and outside4 ,illustration3 ,fig4 #dcj4 ,line-,switch ,units ,illustration3 ,fig4 #dca4 ,automatic ,apparatus at ,lansing ,office ,the main frame is shown at the right of ,fig4 #dbh1 just to the right of a gallery from which a step-ladder leads4 ,the left-hand side of this frame is the line or protector side1 but the portion toward the observer in this picture is unequipped4 ,these equipped protector strips carry #djj pairs of terminals each1 and the consequent length of these strips makes necessary the gallery shown1 in order that all of them may be readily accessible4 ,illustration3 ,fig4 #dcb4 ,main ,distributing ,frame1 ,lansing ,office ,illustration3 ,fig4 #dcc4 ,line ,switches ,illustration3 ,,power ,,plant ,,for ,,automatic ,,switchboard ,,equipment ,bay ,cities ,home ,telephone ,company1 ,berkeley1 ,cal4 ,illustration3 ,fig4 #dcd4 ,secondary ,line ,switches and ,first ,selectors ,automatic ,offices4 ,there is no great difference in the amount of floor space required in central offices employing automatic and manual equipment4 ,whatever difference there is1 is likely to be in favor of the automatic4 ,the fact that no such rigid requirement exists in the arrangement of automatic apparatus1 as that which makes it necessary to place the sections of a multiple board all in one row1 makes it possible to utilize the available space more economically with automatic than with manual equipment4 ,illustration3 ,fig4 #dce4 ,second ,selectors ,illustration3 ,fig4 #dcf4 ,toll ,distributing ,frame and ,harmonic ,converters ,in manual practice it is necessary to place the distributing frames and power apparatus in a separate room from that containing the switchboard1 but in an automatic exchange no such necessity exists2 in fact1 so far as the distributing-frame equipment is concerned1 it is considered desirable to have it located in the same room as the automatic switches4 ,the battery room in an automatic exchange should be entirely separate from the operating room1 since the fumes from the battery would be fatal to the proper working of the automatic switches4 ,typical ,automatic ,office4 ,the floor-plan and views of a medium-sized automatic office at ,lansing1 ,michigan1 have been chosen as representing typical practice4 ,the floor plan is shown in ,fig4 #dbi4 ,the apparatus indicated in full lines represents the present equipment1 and that in dotted lines the space that will be required by the expected future equipment4 ,in ,fig4 #dcj is shown a group of five line-switch units1 representing a total of five hundred lines4 ,the length of such a unit is practically fourteen feet and the breadth over all about twenty-two inches4 ,fig4 #dca shows a general view of this ,lansing office1 taken from a point of view indicated at ,a on the floor plan of ,fig4 #dbi4 ,fig4 #dcb shows the main distributing frame1 which is of ordinary type2 ,fig4 #dcc shows a closer view of some of the primary line switches2 ,fig4 #dcd is a view of the secondary line switches and first selectors1 the latter being on the right2 ,fig4 #dce is a view of the frequency selectors and second selectors1 the former being used in connection with party-line work2 and ,fig4 #dcf is a view of the toll distributing frame and harmonic converters for party-line ringing4 ,a general view of the main switching room in the ,grant ,avenue office of the ,home ,telephone ,company of ,san ,francisco is given in ,fig4 #dcg1 this being taken before the work of installation had been fully completed4 ,the present capacity of the equipment is #f1jjj and the ultimate #ab1jjj lines4 ,this office is one of a number of similar ones recently installed for the ,home ,telephone ,company in ,san ,francisco1 the combination of which forms by far the largest automatic exchange yet installed4 ,the scope of the plans is such as to enable #abe1jjj subscribers to be served without any change in the fundamental design1 and by means merely of addition in equipment and lines as demanded by the future subscriptions for telephone service4 ,illustration3 ,fig4 #dcg4 ,grant ,avenue ,office--,san ,francisco ,,chapter ,,xxxiv ,,private ,,branch ,,exchanges ,definitions4 ,a telephone exchange devoted to the purely local uses of a private establishment such as a store1 factory1 or business office1 is a private exchange4 ,if1 in addition to being used for such local communication1 it serves also for communication with the subscribers of a city exchange1 it becomes in effect a branch of the city exchange and1 therefore1 a private branch exchange4 ,the term ,p4 ,b4 ,x4 has become a part of the telephone man's vocabulary as an abbreviation for private branch exchange4 ,private exchanges for purely local use require no separate treatment as any of the types of switching equipments for interconnecting the lines for communication1 that have been or that will be described herein1 may be used4 ,the problem becomes a special one1 however1 when communication must also be had with the subscribers of a public exchange1 since then trunking is involved in which the conditions differ materially from those encountered in trunking between the several offices in a multi-office exchange4 ,for such communication one or more trunk lines are led from the private branch office usually to the nearest central office of the public exchange and such trunks are called private branch-exchange trunks4 ,they are the paths for communication between the private exchange and the public exchange4 ,for establishing the connections either between the local lines themselves or between the local lines and the trunks1 and for performing other duties that will be referred to1 one or more private branch-exchange operators are employed at the switchboard of the private establishment4 ,the private branch exchange may operate in conjunction with a manual or an automatic public exchange1 but whether manual or automatic1 the private exchange is usually manually operated1 although it is quite possible to make a private branch exchange that is wholly automatic and will1 therefore1 involve no operator at all4 ,functions of the ,private ,branch-,exchange ,operator4 ,it is possible1 as just stated1 entirely to dispense with the private branch-exchange operator so far as the mere connection and disconnection of the lines is concerned4 ,but the real function of the private branch-exchange operator is a broader one than this and it is for this reason that even in connection with automatic public exchanges1 operators are desirable at the private branches4 ,the private branch-exchange operator is1 as it were1 the doorkeeper of the telephone entrance to the private establishment4 ,she is the person first met by the public in entering this telephone door4 ,there is the same reason1 therefore1 why she should be intelligent1 courteous1 and obliging as that the ordinary doorkeeper should possess these characteristics4 ,as to incoming traffic to a private branch exchange1 an intelligent operator may do much toward directing the calls to the proper department or person1 even though the person calling may have little idea as to whom he desires to reach4 ,this saves the time of the person who makes the call as well as that of the people at the private branch stations1 since it prevents their being unnecessarily called4 ,the functions of the private branch-exchange operator are no less important with respect to outgoing calls4 ,it is the duty of the operator to obtain connections through the city exchange for the private branch subscriber1 who merely asks for a certain connection and hangs up his receiver to await her call when she shall have obtained it4 ,this saving of time of busy people by having the branch-exchange operator make their calls for them has one attending disadvantage1 which is that the person in the city exchange who is called does not1 when he answers his telephone1 find the real party with whom he is to converse1 but has to wait until that party responds to the private branch operator's call4 ,this is akin to asking a person to call at one's office and then being out when he gets there4 ,this drawback is greatly accentuated where both the parties that are to be involved in the connection are people high in authority in certain establishments at private branch exchanges4 ,some business houses have made the rule that the private branch operator shall not connect with their lines until she has actually heard the voice of the proper party at the other end4 ,when two subscribers in two different private branch exchanges where this rule is enforced1 attempt to get into communication with each other1 the possibilities of trouble are obvious4 ,all that may be said on this matter is that the person who calls another by telephone should extend that person the same courtesies that he would had he called him in person to his office2 and that a person who is called by telephone by another should meet him with the same consideration as if he had received a personal call at his office or home4 ,the arbitrary ruling made by some corporations and persons1 which results always in the other fellow's doing the waiting1 is not ethically correct nor is it good policy4 ,private ,branch ,switchboards4 ,private branch switchboards may be of common-battery or magneto types regardless of whether they work in conjunction with main office equipments having common-battery or magneto equipments4 ,usually a magneto private branch exchange works in conjunction with a magneto main office1 but this is not always true4 ,there are cases where the private branch equipment of modern common-battery type works in conjunction with main office equipment of the magneto type2 and in some of these cases the private branch exchange has a much larger number of subscribers than the main office4 ,this is likely to be true in large summer resort hotels located in small and otherwise unimportant rural districts4 ,in one such case within our knowledge the private branch exchange has a larger number of stations than the total census population of the town1 resulting in an apparent telephone development considerably greater than one hundred per cent4 ,magneto ,type4 ,where both the private branch and the main office equipments are of the magneto type1 the private branch requirements are met by a simple magneto switchboard of the requisite size1 and the trunking conditions are met by ring-down trunks extending to the main office4 ,in this case the supervision is that of the ordinary clearing-out drop type1 the operators working together as best they may4 ,common-,battery ,type4 ,the cases where the private branch board is of common-battery type and the main office of magneto type are comparatively so few that they need not be treated here4 ,where they do occur they demand special treatment because the main portion of the traffic over the trunk lines to the city or town central office is likely to be toll traffic through that office over long-distance lines4 ,the principal reason why the equipment of the town offices under such conditions is magneto rather than common battery is that the traffic conditions are those of short season and heavy toll1 and common-battery switching equipment at the main office has no especial advantages for toll work4 ,illustration3 ,fig4 #dch4 ,desk ,type1 ,private ,branch ,board ,for small private branch exchanges the desk type of switch board1 shown in ,fig4 #dch1 is largely used4 ,the operator frequently has other work to do and the desk is1 therefore1 a convenience4 ,in larger private exchanges1 such as those requiring more than one operator1 some form of upright cabinet is employed1 and if1 as sometimes occurs1 the branch exchange is of such size as to demand a multiple board1 then the general form of the board does not differ materially from the standard types of multiple board employed in regular central office work4 ,the most common private branch-exchange condition is that of a common-battery branch working into a common-battery main office4 ,in such the main point to be considered is that of supervision of trunk-line connections4 ,cord ,type4 ,for the larger sizes of branch exchange switchboards1 the switching apparatus is practically the same as that of ordinary manual switchboards wherein the connections are made between the various lines by means of pairs of cords and plugs4 ,the private branch-exchange trunk lines usually terminate on the private branch board in jacks but in some cases plug-ended trunks are used4 ,illustration3 ,fig4 #dci4 ,key ,type1 ,private ,branch ,board ,the line signals may consist in mechanical visual signals or in lamps1 the choice between these depending largely on the source of battery supply at the branch exchange1 a matter which will be considered later4 ,the trunk-line signals at the private branch board are usually ordinary drops which are thrown when the main-exchange operator rings on the line as she would on an ordinary subscriber's line4 ,frequently1 however1 lamp signals are used for this purpose1 being operated by locking relays energized when the main-office operator rings or1 in some cases1 operated at the time when the main-office operator plugs into the trunk-line jack4 ,illustration3 ,fig4 #ddj4 ,circuits1 ,key-,type ,board ,key ,type4 ,for small private branch-exchange switchboards1 a type employing no cords and plugs has come into great favor during recent years4 ,instead of connecting the lines by jacks and plugs1 they are connected by means of keys closely resembling ordinary ringing and listening keys4 ,such a switchboard is shown in ,fig4 #dci1 this having a capacity of three trunks1 seven local lines1 and the equivalent of five cord circuits4 ,the drops associated with the three trunks may be seen in the upper left-hand side of the face of the switchboard4 ,immediately below these in three vertical rows are the keys which are used in connecting the trunks with the cord circuits or connecting bus wires4 ,at the right of the drop associated with the trunks are seven visual signals1 these being the calling signals of the local lines4 ,the seven vertical rows of keys1 immediately to the right of the three trunk-line rows1 are the line keys4 ,the throwing of any one of these keys and of a trunk-line key in the same horizontal row in the same direction will connect a line with a trunk through the corresponding bus wires1 leaving one of the supervisory visual signals1 shown at the extreme top of the board1 connected with the circuit4 ,the keys in a single row at the right are those by means of which the operator may bridge her talking set across any of the cord circuits4 ,the circuits of this particular board are shown in ,fig4 #ddj4 ,this is equipped for common-battery working1 the battery feed wires being shown at the left4 ,supervision of ,private ,branch ,connections4 ,at the main office where common-battery equipment is used1 the private branch trunks terminate before the ,a-operators exactly in the same way as ordinary subscribers' lines1 i4 e41 each in an answering jack and lamp at one position and in a multiple jack on each section4 ,it goes without saying1 therefore1 that the handling of a private branch call1 either incoming or outgoing1 should be done by the ,a-operator in the same manner as a call on an ordinary subscriber's line1 and that the supervision of the connection should impose no special duties on the ,a-operator4 ,there has been much discussion1 and no final agreement1 as to the proper method of controlling the supervisory lamp at the main office of a cord that is1 at the time1 connected to a private branch trunk4 ,three general methods have been practiced3 ,the first method is to have the private branch subscriber directly control the supervisory lamp at the main office without producing any effect upon the private branch supervisory signal2 this latter signal being displayed only after the connection has been taken down at the main office and in response to the withdrawal of the main office plug from the private branch jack4 ,this is good practice so far as the main-office discipline is concerned but it results in a considerable disadvantage to both the city and private branch subscribers in that it is impossible for the private branch subscriber1 when connected to the other1 to re-signal the private branch operator without the connection being first taken down4 ,the second method is to have the private branch subscriber control both the supervisory signal at the private branch board and at the main board4 ,this has the disadvantage of bringing both operators in on the circuit when the private branch subscriber signals4 ,the third method1 and one that seems best1 is to place the supervisory lamp of the private branch board alone under the control of the private branch subscriber1 so that he may attract the attention of the private branch operator without disturbing the supervisory signal at the main office4 ,the supervisory signal at the main office in this case is displayed only when the private branch operator takes down the connection4 ,this practice results in a method of operation at the main office that involves no special action on the part of the ,a-operator4 ,she takes down the connection only when the main-office subscriber has hung up his telephone and the private branch subscriber has disconnected from the trunk4 ,whatever method is employed1 private branch disconnection is usually slow1 and for this reason many operating companies instruct the ,a-operators to disconnect on the lighting of the supervisory lamp of the city subscriber4 ,with ,automatic ,offices4 ,private branch exchanges most used in connection with automatic offices employ manual switchboards1 with the cord circuits of which is associated a signal transmitting device by which the operator instead of the subscriber may manipulate the automatic apparatus of the public exchange by impulses sent over the private branch-exchange trunk lines4 ,the subscriber's equipment at the private branch stations may be either automatic or manual4 ,frequently the same private branch exchange will contain both kinds4 ,with the manual sub-station equipment the operation is exactly the same as in a private branch of a manual exchange1 except that the private branch operator by means of her dial makes the central-office connection instead of telling the main-office operator to do so for her4 ,with automatic sub-station equipment at the private branch the subscribers1 by removing their receivers from their hooks1 call the attention of the private branch operator1 who may receive their orders and make the desired central-office connection for them1 or who may plug their lines through to the central office and allow the subscribers to make the connection themselves with their own dials4 ,in automatic equipment of the common-battery type1 some change always takes place in the calling line at the time the called subscriber answers4 ,in the three-wire system during the time of calling1 both wires are of the same polarity with respect to earth4 ,at the time of the answering of the called subscriber1 the two wires assume different polarities1 one being positive to the other4 ,such a change is sufficient for the actuation of devices local to the private exchange switchboard and may be interpreted through the calling supervisory signal in such a way as to allow it to glow during calling and not to glow after the called subscriber has answered4 ,in the two-wire automatic system a similar change can be arranged for1 with similar advantageous results4 ,secrecy4 ,in private exchanges operating in connection with automatic central offices1 the secret feature of individual lines may or may not be carried into the private exchange equipment4 ,some patrons of automatic exchanges set a high value on the absence of any operator in a connection and transact business over such lines which they would not transact at all over manual lines or would not transact in the same way over manual lines4 ,to some such patrons1 the presence of a private exchange operator1 even though employed and supervised by themselves1 seems to be a disadvantage4 ,to meet such a feeling1 it is not difficult to arrange the circuits of a private exchange switchboard so that the operator may listen in upon a cord circuit at any time and overhear what is being said upon it so long as two subscribers are not in communication on that cord circuit4 ,that is1 she may answer a call and may speak to the calling person at any time she wishes until the called person answers4 ,when he does answer and conversation can take place1 some device operates to disconnect her listening circuit from the cord circuit1 not to be connected again until at least one of the subscribers has hung up his receiver4 ,with private exchange apparatus so arranged1 the secrecy of the system is complete4 ,battery ,supply4 ,there are three available methods of supplying direct current for talking and signaling purposes to private branch exchanges1 each of which represents good practice under certain conditions4 ,first1 by means of pairs of wires extended from the central-office battery2 second1 by means of a local storage battery at the private branch exchange charged over wires from the central office2 and third1 by means of a local storage battery at the private exchange charged from a local source4 ,the choice of these three methods depends always on the local conditions and it is a desirable feature1 to be employed by large operating companies1 to have all private branch-exchange switchboards provided with simple convertible features contained within the switchboard for adapting it to any one of these methods of supplying current4 ,if a direct-current power circuit is available at the private branch exchange1 it may be used for charging the local storage battery by inserting mere resistance devices in the charging leads4 ,if the local power circuit carries alternating current1 a converting device of some sort must be used and for this purpose1 if the exchange is large enough to warrant it1 a mercury rectifier is an economical and simple device4 ,the supply of current to private branch exchanges over wires leading to the central-office battery has the disadvantage of requiring one or several pairs of wires in the cables carrying the trunk wires4 ,no special wires are run1 regular pairs in the paper insulated line or trunk cables being admirably suited for the purpose4 ,sufficient conductivity may be provided by placing several such pairs in multiple4 ,if the amount of current required by the private exchange warrants it1 pairs of charging wires from the central office may be fewer if a battery is charged over them than if they are used direct to the bus bars of the private exchange switchboard4 ,if they are used in the latter way1 and this is simpler for reasons of maintenance1 some means must be provided to prevent the considerable resistance of the supply wires from introducing cross-talk into the circuit of the private exchange4 ,this is accomplished by bridging a considerable capacity across the supply pairs at the private exchange--ten to twelve microfarads usually suffice4 ,this point has already been referred to and illustrated in connection with ,fig4 #ada4 ,the number of pairs of wires1 or1 in other words1 the amount of copper in the battery lead between the central office and the private branch-exchange switchboard needs to be properly determined not only to eliminate cross-talk when the proper condensers are used with them1 but to furnish the proper difference of potential at the private exchange bus bars1 so that the line and supervisory signals will receive the proper current4 ,it is a convenience in installing and maintaining private exchange switchboards of this kind to prepare tables showing the number of pairs of ,no4 #ai gauge and ,no4 #bb gauge wires required for a private exchange at a given distance from its central office and of a probable amount of traffic4 ,the traffic may be expressed in the maximum number of pairs of cords which will be in use at one time4 ,with this fact and the distance1 the number of pairs of wires required may be determined4 ,ringing ,current4 ,the ringing current may be provided in two ways3 over pairs of wires from the city-office ringing machines or by means of a local hand generator1 or both4 ,a key should enable either of these sources of ringing current to be chosen at will4 ,marking of ,apparatus4 ,all apparatus should be marked with permanent and clear labels4 ,that private exchange switchboard is best at which an almost uninformed operator could sit and operate it at once4 ,it is not difficult to lay out a scheme of labels which will enable such a board to be operated without any detailed instructions being given4 ,desirable ,features4 ,the board should contain means of connecting certain of the local private exchange lines to the central-office trunks when the board is unattended4 ,also1 it is desirable that it should contain means whereby any local private exchange line may be connected to the trunk so that its station will act as an ordinary subscriber's station4 ,whether the trunks of the private exchange lead to a manual or an automatic equipment1 it often is desired to connect a local line through in that way1 either so that the calling person may make his calls without the knowledge of the private exchange operator1 because he wishes to make a large number of calls in succession1 or because for some other reason he prefers to transact his business directly with or through the exchange than to entrust it to his operator4 ,,chapter ,,xxxv ,,intercommunicating ,,systems ,definition4 ,the term intercommunicating has been given to a specialized type of telephone system wherein the line belonging to each station is extended to each of the other stations1 resulting in all lines extending to all stations4 ,each station is provided with apparatus by means of which the telephone user there may connect his own telephone with the line of the station with which he wishes to communicate1 enabling him to signal and talk with the person at that station4 ,limitations4 ,the idea is simple4 ,each person does his own switching directly1 and no operator is required4 ,it is easy to see1 however1 that the system has limitations4 ,the amount of line wire necessary in order to run each line to each station is relatively great1 and becomes prohibitive except in exchanges involving a very small number of subscribers1 none of which is remote from the others4 ,again1 the amount of switching apparatus required becomes prohibitive for any but a small number of stations4 ,as a result1 twenty-five or thirty stations are considered the usual practical limit for intercommunicating systems4 ,types4 ,an intercommunicating system may be either magneto or common-battery1 according to whether it uses magneto or common-battery telephones4 ,the former is the simpler2 the latter is the more generally used4 ,illustration3 ,,western ,,electric ,,company ,,battery ,,room ,,at ,,monmouth1 ,,illinois ,simple ,magneto ,system4 ,the schematic circuit arrangement of an excellent form of magneto intercommunicating system is given in ,fig4 #dda4 ,in this1 five metallic circuit lines are led to as many stations1 an ordinary two-contact open jack being tapped off of each line at each station4 ,a magneto bell of the bridging type is permanently bridged across each line at the station to which that line belongs4 ,the telephone at each station is an ordinary bridging magneto set except that its bell is1 in each case1 connected to the line as just stated4 ,each telephone is connected through a flexible cord to a two-contact plug adapted to fit into any of the jacks at the same station4 ,the operation is almost obvious4 ,if a person at ,station ,a desires to call ,station ,e1 he inserts his plug into the jack of line ,e at his station and turns his generator crank4 ,the bell of ,station ,e rings regardless of where the plug of that station may be4 ,the person at ,station ,e responds by inserting his own plug in the jack of line ,e1 after which the two parties are enabled to converse over a metallic circuit4 ,it makes no difference whether the persons1 after talking1 leave these plugs in the jacks or take them out1 since the position of the plug does not alter the relation of the bell with the line4 ,illustration3 ,fig4 #dda4 ,magneto ,intercommunicating ,system ,this system has the advantage of great simplicity and of being about as fool proof as possible4 ,it is1 however1 not quite as convenient to use as the later common-battery systems which require no turning of a generator crank4 ,common-,battery ,systems4 ,in the more popular common-battery systems two general plans of operation are in vogue1 one employing a plug and jacks at each station for switching the home instrument into circuit with any line1 and the other employing merely push buttons for doing the same thing4 ,these may be referred to as the plug type and the push-button type1 respectively4 ,illustration3 ,fig4 #ddb4 ,plug ,type of ,common-,battery ,intercommunicating ,system ,kellogg ,plug ,type4 ,the circuits of a plug type of intercommunicating system1 as manufactured by the ,kellogg ,company1 are shown in ,fig4 #ddb4 ,while only three stations are shown1 the method of connecting more will be obvious4 ,this system requires as many pairs of wires running to all stations as there are stations1 and in addition1 two common wires for ringing purposes4 ,the talking battery feed is through retardation coils to each line4 ,when all the hooks are down1 each call bell is connected between the lower common wire and the tip side of the talking circuit individual to the corresponding station4 ,the ringing buttons at each station are connected between the tip of the plug at that station and the upper common wire4 ,as a result1 when a person at one station desires to call another1 it is only necessary for him to insert his plug in the jack of the desired station and press his ringing button2 the circuit being traced from one pole of the ringing battery through the upper common ringing wire1 ringing key of the station making the call1 tip of plug1 tip conductor of called station's line1 bell of called station1 and back to the ringing battery through the lower common ringing wire4 ,illustration3 ,fig4 #ddc4 ,push-,button ,wall ,set ,kellogg ,push-,button ,type4 ,fig4 #ddc shows a ,kellogg wall-type intercommunicating set employing the push-button method of selecting1 and ,fig4 #ddd shows the internal arrangement of this set4 ,illustration3 ,fig4 #ddd4 ,push-,button ,wall ,set ,western ,electric ,system4 ,the method of operation of the push-button key employed in the intercommunicating system of the ,western ,electric ,company is well shown in ,fig4 #dde4 ,when the button is depressed all the way down1 as shown in the center cut of ,fig4 #dde1 which represents the ringing position of the key1 contact is made with the line wires of the station called1 and ringing current is placed on the line4 ,when the pressure is released1 the button assumes an intermediate position1 as shown in the right-hand cut1 which represents the talking position of the key and in which the ringing contacts #a and #b are open1 but contact with the line for talking purposes is maintained4 ,the key is automatically held in this intermediate position by locking plate #c until this plate is actuated by the operation of another button which releases the key so that it assumes its normal position as shown in the left-hand cut4 ,when a button is depressed to call a station1 it first connects the called station's line to the calling station through the two pairs of contacts #d and #e and then connects the ringing battery to that line by causing the spring #a to engage the contact #b4 ,the ringing current then passes through the bell at the called station1 through the back contacts of the switch hook at that station1 over one side of the line1 and through the way-down contact #a of the button at the calling station1 thence over the other side of the battery line back to the ringing battery1 operating the bell at the called station4 ,illustration3 ,fig4 #dde4 ,push-,button ,action1 ,western ,electric ,system ,the circuits of the ,western ,electric system are similar to those of ,fig4 #ddb1 but adapted1 of course1 to the push-button arrangement of switches4 ,two batteries are employed1 one for ringing and the other for talking1 talking current being fed to the lines through retardation coils to prevent interference or cross-talk from other stations which might be connected together at the same time4 ,monarch ,system4 ,as the making of connections in an intercommunicating system is entirely in the hands of the user1 it is desirable that the operation be simple and that carelessness on the part of the user result in as few evil effects as possible4 ,for instance1 the leaving of the receiver off its hook will1 in many systems1 result in such a drain on the battery as to greatly shorten its life4 ,the system of the ,monarch ,company has certain distinctive features in this respect4 ,it is of the push-button type and as in the system just discussed1 one pressure of the finger on one button clears the station of previous connections1 rings the station called1 and establishes a talking connection between the caller's telephone and the line desired4 ,in addition to this1 the system is designed to eliminate battery waste by so arranging the circuits that the battery current does not flow through either called or calling instrument until a complete connection is made--the calling button down at one station1 the home button down at the called station1 and both receivers off the hook4 ,it does not hurt the batteries1 therefore1 if one neglects to hang up his receiver4 ,illustration3 ,fig4 #ddf4 ,push-,button ,wall ,set ,illustration3 ,fig4 #ddg4 ,push-,button ,action1 ,monarch ,system ,three views of the wall set of this system are shown in ,fig4 #ddf1 which illustrates how both the door and the containing box are separately hinged for easy access to the apparatus and connecting rack4 ,as in the ,western ,electric and ,kellogg push-button systems1 each push-button key has three positions1 as shown in ,fig4 #ddg4 ,the first button shows all the springs open1 the normal position of the key4 ,the second button is in the half-way or talking position with all the springs1 except the ringing spring1 in contact4 ,the third button shows the springs all in contact1 the condition which exists when ringing a station4 ,the mechanical construction of the key is shown in ,fig4 #ddh4 ,each button has a separate frame upon which the springs are mounted4 ,any one of the frames with its group of contact springs may be removed without interfering with either the electrical or the mechanical operation of the others4 ,this is a convenient feature1 making possible the installation of as few stations as are needed at first1 and the subsequent addition of buttons as other stations are added4 ,illustration3 ,fig4 #ddh4 ,push-,button ,keys ,the restoring feature is a horizontal metal carriage1 in construction very much like a ladder--one round pressing against each key frame1 due to the tension on the carriage exerted by a single flat spring4 ,the plunger of each button is equipped with a shoulder1 which normally is above the round of the ladder4 ,when the button is operated1 this shoulder presses against a round of the carriage forcing it over far enough so that the shoulder can slip by4 ,the upper surface of the shoulder is flat1 and on passing below the pin1 allows the carriage to slip back into its normal position and the pin rests on the top of the shoulder holding the plunger down4 ,this position places the talking springs in contact4 ,the ringing springs are open until the plunger is pressed all the way down1 then the ringing contact is made4 ,when the pressure is released1 the plunger comes back to the half-way or talking position1 leaving the ringing contacts open again4 ,when another button is pressed1 the same operation takes place and1 by virtue of the carriage being temporarily displaced1 the original key is left free to spring back to its normal position4 ,each station is provided with a button for each other station and a home button4 ,the salient feature of the system is that before a connection may be established1 the button at the calling station corresponding to the station called and also the home button of the station called must be depressed1 if it is not already down4 ,the home key at any station1 when depressed1 transposes the sides of the line with respect to the talking apparatus4 ,the home key also has a spring which changes the normal connection of the line at that station from the negative to the positive side of the talking battery4 ,unless1 therefore1 a connection between two stations is made through the calling key at one station and the home key at the other1 no current can flow even though both receivers are off their hooks1 because in that case no connection will exist with the positive side of the battery4 ,this relation is shown in ,fig4 #ddi1 which gives a simplified circuit arrangement for two connected stations4 ,illustration3 ,fig4 #ddi4 ,monarch ,intercommunicating ,system ,referring to ,fig4 #ddi1 when the station called depresses the home button the talking circuit is then completed after the hook switch is raised4 ,this is because the talking battery is controlled by the home key4 ,conductors from both the negative and the positive sides of the battery enter this key4 ,in the normal position of the springs1 the negative side of the battery is in contact with the master spring in the home key and through these springs the negative battery is applied to all the calling keys1 and from there on to the hook switch4 ,when1 however1 the home button is operated1 the spring which carries the negative battery to the home key is opened1 and the spring which carries the positive battery is closed4 ,this puts the positive battery on at the hook switch instead of the negative battery1 as in its normal condition4 ,in this system it is seen that a separate pair of line wires is used for each station1 and in addition to these1 two common pairs are run to all stations1 one for ringing and one for talking battery connections4 ,for ,private ,branch ,exchanges4 ,so far the intercommunicating system has been discussed only with respect to its use in small isolated plants4 ,it has a field of usefulness in connection with city exchange work1 as it may be made to serve admirably as a private branch exchange4 ,where this is done1 one or more trunk lines leading to an office of the city exchange are run through the intercommunicating system exactly as a local line in that system1 being tapped to a jack or push button at every station4 ,a person at any one of the stations may originate a call to the main office by inserting his plug in the trunk jack1 or pushing his trunk push button4 ,also any station1 within hearing or sight of the trunk-line signal from the main office1 may answer a main-office call in the same way4 ,in order that the convenience of a private branch exchange may be fully realized1 however1 it is customary to provide an attendant's station at which is placed the drop or bell on which the incoming trunk signal is received4 ,the duty of this attendant during business hours is to answer trunk calls from the main office and finding out what party is desired1 call up the proper station on the intercommunicating system4 ,the party at that station may then connect himself with the trunk4 ,the practice of the ,dean ,company1 for instance1 is as follows in regard to trunking between intercommunicating systems and main offices with common-battery equipment4 ,the attendant's station telephone cabinet contains1 besides the push-button keys for local and trunk connections1 a drop signal and release key1 together with relays in each trunk circuit4 ,the latter are used to hold the trunks until the desired party responds4 ,the main-exchange trunk lines1 besides terminating at the attendant's station1 are wired through the complete intercommunicating system so that any intercommunicating telephone can be connected direct to the central office by depressing the trunk key1 which is provided with a button of distinctive color4 ,the pressing of the trunk key allows the telephone to take its current from the main-office storage battery and to operate the main-office line and supervisory signals direct1 without making it necessary to call on the attendant to set up the connection4 ,illustration3 ,fig4 #dej4 ,junction ,box ,illustration3 ,fig4 #dea4 ,typical ,arrangement of ,intercommunicating ,system ,incoming calls from the common-battery main office to the intercommunicating system are all handled by the attendant4 ,the main-office operator signals the intercommunicating system by ringing1 the same as for a regular subscriber's line4 ,this will operate a drop in the attendant's station cabinet1 and through an armature contact1 give a signal on a low-pitched buzzer4 ,this alarm buzzer operates only when the main exchange is ringing and1 therefore1 does not require that the drop shutter be restored immediately4 ,an extra key may be provided for an extension night-alarm bell1 for use where the attendant also does work in a room separate from that containing the attendant's station telephone equipment4 ,the attendant operator answers the main-line signal by pressing the proper trunk button1 as designated by the operated drop on the attendant's cabinet4 ,the answering of the trunk connects a locking relay across the circuit so that the attendant may call the desired party on the intercommunicating system without having to hold the trunk manually4 ,the party desired is then notified which trunk to use and the attendant operator hangs up her receiver1 no further attention being necessary on her part4 ,the trunk-holding relay is automatically released when the desired party 7with the telephone receiver off the hook7 depresses the proper trunk button1 thus clearing the trunk line of all bridged apparatus and making the talking circuit the same as in the regular type of private branch-exchange switchboard4 ,the most convenient way of installing the wires of an intercommunicating system is to run a cable containing the proper number of pairs to provide for the ultimate number of stations to all the stations1 tapping off from the conductors in the cable to the jacks or push buttons at each station4 ,these tap connections are best made by means of junction boxes which contain terminals for all the conductors4 ,such a junction box1 with the through cable and the tap cable in place1 is illustrated in ,fig4 #dej4 ,a schematic lay-out of the various parts of a ,dean intercommunicating system1 provided with an attendant's station and with trunks to a city office1 is given in ,fig4 #dea4 ,,chapter ,,xxxvi ,,long-,,distance ,,switching ,definitions4 ,telephone messages between communities are called long-distance messages4 ,they are also called toll messages4 ,almost all long-distance traffic is handled by message-rate 7measured-service7 methods of charge4 ,all measured-service messages are toll messages1 whether they are completed within a given community or between communities4 ,the term long-distance1 therefore1 is more descriptive than the term toll4 ,the subject of local and long-distance measured service is treated exhaustively in a chapter of its own4 ,some telephone-exchange operating companies call their own inter-city business toll1 and use the term long-distance for business carried between exchanges for them by another company4 ,the distinction seems to be unwarranted4 ,use of ,repeating ,coil4 ,most long-distance lines are magneto circuits4 ,if they are switched to grounded circuits1 repeating coils need to be inserted4 ,toll switching equipments contain means of inserting repeating coils in the connecting cords when required4 ,their use reduces the volume of transmitted speech1 but often is essential even in connecting metallic circuit lines1 as a quiet local metallic circuit may have a ground upon it which will cause excessive noises when a quiet long-distance line is connected to it4 ,switching through ,local ,board4 ,in the simplest form of long-distance switching1 the lines terminate in switchboards with local lines and may be connected with each other and with the local lines through the regular cord circuits1 if the equipment be of the magneto type4 ,the waystations on such a line are equipped with magneto generators4 ,these waystations may signal each other by bell ringing2 the central office may call any waystation by ringing the proper signal and may supervise in a way all traffic on such lines by noting the calls for other stations than the supervising exchange4 ,operators' ,orders4 ,by ,call ,circuits4 ,where the long-distance traffic between two communities is large1 economy requires that the sending of signals by ringing over the line1 waiting for an answer1 and then reciting the details of the call1 be improved upon4 ,if the traffic is large and the distance between communities small1 call circuits are established in the same way as between the switchboards in several manual central offices of an exchange4 ,the long-distance operator handling the originating call passes the necessary details to the distant operator by telephone over the call circuit4 ,such circuits also are known as order circuits4 ,they are accessible to originating operators at keys and are connected directly and permanently to the telephone sets of receiving operators4 ,one call circuit can handle the orders for a large number of actual conversation circuits4 ,the operator at the receiving end designates the conversation circuit which shall be used1 the originating operator following that instruction4 ,by ,telegraph4 ,where traffic and distance are large1 conversation lines cost more than in the case last assumed4 ,it then is of greater importance to use all the possible talking circuits for actual conversations in order that the revenue may be as high as possible4 ,a phantom circuit good enough for call circuit purposes would be good enough for actual commercial messages1 therefore1 it is customary to furnish such originating and receiving operators with ,morse telegraph sets4 ,the lines are obtained by applying composite apparatus to the conversation circuits4 ,two ,morse circuits can be had from each long-distance line without impairing any quality of that line except the ability to ring over it4 ,as one ,morse circuit can carry information enough between two operators to enable them to keep many telephone circuits busy1 they do not need to ring upon the composited lines1 so that nothing is lost while revenue is gained4 ,two-,number ,calls4 ,in cases where the traffic between communities is large1 where the rate is small1 and where the conversations are short and more on the general order of local calls1 it is usual to handle the switches exactly as local calls are trunked between central offices of the same exchange4 ,that is1 the subscriber's operator who answers the call trunks it1 by the assistance of a call circuit and an incoming trunk operator4 ,the subscriber's operator records only the numbers of the calling and called subscribers4 ,no long-distance operators at all assist in these connections4 ,they are known as two-number calls4 ,the calling subscriber remains at his telephone until the conversation is finished4 ,particular-,party-,calls4 ,in cases where the traffic is smaller1 and where the rate is large1 it is customary to handle the calls through long-distance operators4 ,the ticket records the particular party wished1 and the calls are named particular party calls4 ,in such connections the calling patron is allowed to hang up his receiver1 after his call is recorded1 and is called again when his correspondent is found and is ready to talk4 ,this makes all calls for conversations outgoing ones4 ,only recording operators receive calls from patrons4 ,line operators make calls to patrons4 ,trunking4 ,long-distance lines entering a city usually terminate in one office only1 no matter how many offices the local exchange may have4 ,it is possible to terminate these long-distance lines on a position of the multiple switchboard for local lines4 ,for a variety of reasons this is not practiced except in special cases4 ,the usual method is to terminate them in a special long-distance board and to provide trunk lines from this board to the one or more local switchboards of the exchange4 ,in common-battery systems these toll trunks are so arranged that the called local subscriber receives transmitter current from the office nearest to him1 yet is able to show the long-distance operator the position of his switch hook and is able to be called by the long-distance operator without the intervention of the switching operator in the local office1 even though two repeating coils may be in the trunk circuit4 ,through ,ringing4 ,there is a distinct traffic advantage in having the ringing of the subscriber under the control of the long-distance operator4 ,the latter may call for the subscriber by stating her wish over the call circuit associated with the long-distance trunk4 ,the connection having been made by the switching operator1 the long-distance operator may withhold ringing the subscriber's bell until all is in readiness for the conversation4 ,high-,voltage ,toll ,trunks4 ,in some systems1 the long-distance trunks are further specialized by being enabled to furnish transmitter current to subscribers at a higher voltage than is used in local conversations4 ,with a given construction of transmitters there is a critical maximum current which can be carried by the granular carbon of the instrument without excessive heating1 consequent noises1 and permanent damage4 ,the shortest lines and the longest lines of an exchange district being served by a source of current common to all1 the standard potential of this source must be such as to give the longest lines current enough without giving the shortest lines too much4 ,the very longest local lines1 however1 do not receive current enough from the standard potential to give maximum efficiency when talking over long distances1 though they get enough for local conversations4 ,by providing a battery with a voltage twice that used for local conversations and connecting it into the current supply element of the toll trunk through non-inductive resistances1 not too much current may be given to the shortest lines and considerably more than normal current to the longest lines4 ,ticket ,passing4 ,when only one operator is necessary in a town1 her duty being to switch both local and long-distance lines1 she may write her own tickets and execute them entire4 ,in larger communities with larger long-distance traffic1 the duties need to be specialized4 ,the subscribers' wants as to long-distance connections are given by themselves to recording long-distance operators1 who write them on tickets and pass these to operators who get the parties together4 ,the problem of ticket-passing becomes important and many mechanical carriers have been tried1 culminating in the system which utilizes vacuum tubes4 ,this is in some ways similar to vacuum or compressed-air tube systems for carrying cash in retail stores4 ,the ticket is carried1 however1 without any enclosing case and the tubes are flat instead of round1 i4 e41 they are rectangular in section4 ,by suitable means a vacuum is maintained in a large common tube having a tap to a box-like valve at each line operator's position4 ,a ticket tube connects this valve with a distributing table at or near which the tickets are written4 ,the tickets are of uniform size and are so made as to enable a flap to be bent up easily along one edge4 ,the distributing operator has merely to insert the ticket1 bent edge foremost1 in the open end of the tube1 whereupon the air pressure behind it will drive it through to its destination1 near by or far away4 ,the tickets travel thirty feet a second4 ,the tube may be bent into almost any required form4 ,the ticket1 on arriving at a line operator's position1 slides between two springs1 breaking a shunt around a relay and allowing the latter to light the lamp4 ,waystations4 ,waystations on long-distance lines may be equipped in several ways4 ,most of them have magneto sets and can ring each other4 ,some are equipped with common-battery sets and get all current for signaling and transmission from a terminal central office4 ,in the latter case1 there is the advantage that the ringers are in series with condensers1 assisting greatly in tests for fault locations4 ,such tests are hindered by the presence of ringer bridges across the line1 as in magneto practice4 ,condensers can be inserted in series with ringers of magneto sets if the testing advantage is valued highly enough4 ,a disadvantage of the use of common-battery sets in waystations on long-distance lines is the lessened transmission volume of the stations farthest from the current source4 ,center ,checking4 ,an operating advantage of common-battery sets on long-distance lines is that all calls are forced to be answered by the terminal station4 ,waystations can not call each other1 as they have no calling means4 ,with magneto sets1 waystation agents sometimes call each other direct and neglect to record the call and to remit its price4 ,when they can not call each other direct1 the revenues of the company increase4 ,a traffic method which requires all calls from waystations to be made to a central switching office is called a center-checking system4 ,it is so called because all checking for stations so switched is done at the central point instead of each waystation keeping its own records of calls sent and received4 ,in such practice it is usual to bill each station once a month for the messages it sent4 ,where center checking is not practiced1 the agent makes a report and sends a remittance4 ,center checking comes about naturally for waystations having no ringing equipment4 ,center checking originated long before the invention of common-battery systems4 ,it requires merely that no waystation shall have a generator which can ring a bell4 ,the method most widely used is to equip the waystations with magneto generators which produce direct currents only2 such a generator cannot operate a polarized ringer4 ,it is not usual to produce the direct current by actually rectifying the alternating current1 but merely by omitting half the impulses1 sending to the line only alternate half-cycles of the current generated4 ,any drop or relay adapted to respond to regular ringing current will respond to this modified form of generator4 ,,chapter ,,xxxvii ,,telephone ,,traffic ,the term traffic1 with reference to telephone service1 has come to mean the gross transaction of communication between telephone users4 ,this traffic may be expressed in whatever terms are found convenient for the particular phase considered4 ,unit of ,traffic4 ,with reference to payment for local telephone service1 the conversation is the unit of traffic4 ,in the daily operations of telephone systems there are fewer conversations than there are connections and fewer connections than there are calls1 because lines are found busy and all calls to subscribers are not answered4 ,for these reasons1 in traffic inquiries which have to do with the amount of business which subscribers attempt to transact1 the total traffic in a given time usually is considered as so many calls originated by the subscribers in the community4 ,from this condition arises the term originating calls4 ,for the reason that the purpose of the switching equipment in a central office is to make connections1 the abilities of operators and of equipments frequently are measured in terms of connections per hour or per other unit of time4 ,for the reason that in charging for service all unavailing calls are omitted1 the conversation is the unit of traffic4 ,traffic ,variations4 ,telephone-exchange traffic is subject to such general variations as are noted in the way a compass needle points north1 the migrations of birds1 the blowing of the trade winds1 and other natural phenomena4 ,there are variations in traffic which occur each day1 others which change with the seasons1 and still others which are related to holidays and other special commercial and social events4 ,for instance1 the day before ,thanksgiving ,day1 in many regions1 is the busiest telephone traffic day in the year4 ,illustration3 ,,western ,,electric ,,motor-,,generator ,,charging ,,set ,the daily variations in telephone traffic are closely related to commercial activities and certain general features of this daily variation are common to all telephone systems everywhere4 ,fig4 #deb is a typical graphic record of the traffic of a telephone exchange and represents what happens in almost every town or city4 ,the total calls in this figure are not given as absolute units but would vary to adapt the figure to a particular case4 ,the figure shows principally that the traffic in the night is light2 that it rises to its maximum height somewhere between #aj o'clock ,a4,m4 and noon2 that though it is never as high again during that day1 the afternoon peak is over #hj per cent as great2 and that two minor peaks appear about the dinner hour and after evening entertainments4 ,illustration3 ,fig4 #deb4 ,load ,curve ,busy-,hour ,ratio4 ,if the story told by ,fig4 #deb were to be turned into a table of calls per hour1 the busiest hour of the day would be found to correspond to the highest portion of the figure1 and in that busiest hour of the day1 if a number of selected days were to be compared1 would be found a very constant traffic4 ,the number of calls made1 or the number of connections completed1 in that particular hour1 day by day1 would be found to be much the same4 ,the ratio of the number of units in that hour to the number of units in that entire day would be found to be practically the same ratio day by day4 ,this ratio of busy hour to total day would be found to be much more nearly constant than the gross number of calls per hour or per day4 ,in a large1 busy city1 about one-eighth of the total daily calls are in some one hour2 in a smaller1 less active city1 probably one-tenth are so congested4 ,this is reasonable when one remembers that in the larger city the active business of the day begins later and ends earlier4 ,importance of ,traffic ,study4 ,a knowledge of the amount of traffic in an exchange1 and its distribution as to time and as to the divisions of the exchange1 is important for a number of reasons4 ,traffic knowledge is essential in order that the equipment may be designed and placed in the proper way and the total load distributed properly on that apparatus and its operators4 ,for example1 in an office equipped with a manual multiple switchboard1 the length of the switchboard is governed entirely by the number of operators who must work before it4 ,it is mechanically possible to make a switchboard for ten thousand lines only #ae feet long1 seating seven operators4 ,the entire multiple of ten thousand lines could appear three times in such a switchboard4 ,the seven operators could not handle the traffic we know would be originated by ten thousand lines1 with any present system of charging for service4 ,even a rough knowledge of the probable traffic would enable us to approximate the number of operators needed and to equip each position1 not only with access to the ten thousand lines to be called1 but also with just enough keyboard equipment1 serving as tools1 and just enough answering jacks1 serving as means of bringing the traffic to her4 ,it is foreknowledge of traffic which enables a switchboard to fit the task it is to perform4 ,rates of ,calling4 ,the rates of calling of different kinds of lines vary4 ,the lines of business stations originate more calls than do the lines of residences4 ,some kinds of business originate more calls than others4 ,some kinds of business have a higher rate of calling in one season than in others4 ,flat-rate lines originate more calls than do message-rate lines4 ,when a line changes from a flat rate to a message rate1 the number of originating calls per day decreases4 ,an operator's position1 handling message-rate lines only1 can serve more lines than if all of them were at flat rates4 ,the number of message-rate or coin-prepayment lines which an operator's position can care for depends not only on the traffic but on the method of charging for service1 whether by tickets or meters and upon the kind of meters2 or it depends on the method of collecting the coins4 ,in some regions1 the rate of calling1 on the introduction of a complete measured-service plan1 has been reduced to one-fourth of what it was on the flat-rate plan4 ,in manual switchboards of early types1 wherein the position of the subscriber's answering jack was fixed by his telephone number1 the inequality of traffic became a serious problem4 ,most of the subscribers who first installed telephones when the exchange was small1 retained their telephones and numbers2 as their use of the telephone grew with their business1 it was customary to find the positions answering the lower numbers much more busy than the positions answering the higher numbers1 the latter belonging to later and usually less active business places4 ,functions of ,intermediate ,distributing ,frame4 ,the intermediate distributing board was invented to meet these conditions of unequal traffic upon lines and of variations in traffic with changes of seasons and of charges4 ,the intermediate distributing board enables a line to retain its number and its position in the multiple1 but to keep its answering jack and lamp signal in any desired position4 ,if a flat-rate subscriber changes to a message rate1 his line may be moved to a message-rate position and be answered1 in company with others like it1 by an operator serving many more lines than she could serve if all of them were flat rate4 ,methods of ,traffic ,study4 ,the best way to learn traffic facts for the purposes of designing and operating equipment is to conduct systematic series of observations in all exchanges2 to record them in company with all related facts2 and to compare them from time to time1 recording the results of the comparisons4 ,then when it is required to solve a new problem1 the traffic data will enable the probable future conditions to be known with as great exactness as is possible in studies with relation to transportation or any other human activity4 ,,table ,,xiii ,calling ,rates ---------------------------------------- ---------------- ,,calls ,,per ,,day ,,with ,,different ,,kind ,,of ,,service ,,methods ,,of ,,charge ------------------------------ ,,flat ,,rate ,,message ,,rate ---------------------------------------- --------------- ,residence #h #d ,business #ab to #bj #h to #ad ,private ,exchange ,trunk #dj #be ,hotel ,exchange ,trunk #ej #cj ,apartment ,house ,trunk #cj #ah ---------------------------------------- --------------- ,there are three general ways of observing traffic4 ,a record of originating calls is known as a peg count1 because the counting formerly was done by moving a peg from place to place in a series of holes4 ,the simplest exact way is to provide each operator with a small mechanical counter1 the key of which she can depress once for each call to be counted4 ,a second way is to determine a ratio which exists1 for the particular time and place1 between the number of calls in a given period and the average number of cord circuits in use4 ,knowing this ratio1 the cord circuits can be counted1 the ratio applied1 and the probable total known4 ,the third method1 which is applicable to offices having service meters on all lines1 is to associate one master meter per position or group of lines with all the meters of that position or group1 so that each time any service meter of that position is operated1 the master meter will count one unit4 ,this method applies to either manual or automatic equipments4 ,representative ,traffic ,data4 ,for purposes of comparison1 the following are representative facts as to certain traffic conditions4 ,calling ,rates4 ,the number of calls originated per day by different kinds of lines with different methods of charge are shown in ,table ,,xiii4 ,operators' ,loads4 ,the abilities of subscribers' operators to switch these calls depend on the type of equipment used1 on the kind of management exercised1 and on the individual skill of operators4 ,with manual multiple equipment of the common-battery type1 and good management1 the numbers of originating calls per busy hour given in ,table ,,xiv can be handled by an average operator4 ,the number of calls per operator per busy hour depends upon the amount of trunking to other offices which that operator is required to do4 ,in a small city1 for example1 where all the lines are handled by one switchboard1 there is no local switching problem except to complete the connection in the multiple before each position4 ,in a large city1 where wire economy and mechanical considerations compel the lines to be handled by a number of offices with manual equipment1 some portion of the total originating load of each office must be trunked to others4 ,table ,,xiv shows that an increase of #ij per cent in the amount of out-trunking has decreased the operator's ability to less than #gj per cent of the possible maximum4 ,,table ,,xiv ,effect of ,out-,trunking on ,operator's ,capacity ---------------------------------------- --------------------------- ,,per ,,cent ,,originating ,,calls ,,capacity ,,of ,,subscribers' ,,operator',s ,,trunked ,,to ,,other ,,offices ,,position ,,in ,,calls ,,per ,,busy ,,hour ---------------------------------------- --------------------------- #j #bdj #aj #bcj #cj #bjj #ej #ahe #ge #agj #ij #afe ---------------------------------------- --------------------------- ,trunking ,factor4 ,in providing the system of trunks interconnecting the offices1 whether the equipment be manual or automatic1 it is essential to know not only how much traffic originates in each office1 but how much of it will be trunked to each other office and how many trunks will be required4 ,an interesting phase of telephone traffic studies is that it is possible to determine in advance the amount of traffic which can be completed directly in the multiple of that office and how much must be trunked elsewhere4 ,theoretical considerations would indicate that if the local multiple contains one-eighth of the total lines of the city1 one-eighth of the calls originating in that office could be completed locally and seven-eighths would be trunked out4 ,in almost all cases1 however1 it is found that more than the theoretical percentage of originating calls are for the neighborhood of that office and can be completed in the multiple4 ,this results in the determination of a factor by which the theoretical out-trunking can be multiplied to determine the probable real out-trunking4 ,in most cases1 the ratio of actual to theoretical out-trunking is #ge per cent1 or approximately that4 ,in special cases1 it may be far from #ge per cent4 ,trunk ,efficiency4 ,the capacities of trunks vary with their methods of operation and with the number of trunks in a group4 ,for example1 in the manual system where trunk operators in distant offices are instructed over call circuits and make disconnections in response to lamp signals1 such an incoming trunk operator can complete from #bej to #ejj connections per busy hour4 ,the actual ability depends upon the number of distant offices served by that operator and upon the amount of work she has to perform on each call4 ,the number of messages which can be handled by one trunk in the busy hour will depend upon the number of trunks in the group and upon the system employed4 ,it appears that the ability of trunks in this regard is higher in the automatic system than in the manual system4 ,for the latter1 ,table ,,xv gives representative facts4 ,,table ,,xv ,messages per ,trunk in ,manual ,system ---------------------------------------- ------------ ,,number ,,of ,,trunks ,,in ,,group1 ,,messages ,,per ,,trunk ,,per ,,manual ,,system ,,busy ,,hour ---------------------------------------- ------------ #e #g #aj #i #bj #ab #dj #ae #fj #ah ---------------------------------------- ------------ ,some of the reasons for the higher efficiencies of trunks in the automatic system are not well defined1 but unquestionably exist4 ,they have to do partly with the prompter answering observable in automatic systems4 ,the operation of calling being simple1 a called subscriber seems to fear that unless he answers promptly the calling party will disconnect and perhaps may call a competitor4 ,the introduction of machine-ringing on automatic lines1 where existing in competition with manual ringing on manual lines1 seems to encourage subscribers to answer even more promptly4 ,the length of conversation in automatic systems seems to be shorter than in manual systems4 ,still more important1 disconnection in automatic systems is instantaneous during all hours1 whereas in manual systems it is less prompt in the busiest and least busy hours than in the hours of intermediate congestion4 ,the practical results of trunk efficiencies in automatic systems are given in ,table ,,xvi4 ,,table ,,xvi ,messages per ,trunk in ,automatic ,system ---------------------------------------- ------------ ,,number ,,of ,,trunks ,,in ,,group1 ,,messages ,,per ,,trunk ,,per ,,automatic ,,system ,,busy ,,hour ---------------------------------------- ------------ #e #ae #aj #bb #bj #bh #dj #cb #fj #cd ---------------------------------------- ------------ ,toll ,traffic4 ,toll or long-distance traffic follows the general laws of local or exchange traffic4 ,conversations are of greater average length in long-distance traffic4 ,the long-distance line is held longer for an average conversation than is a local-exchange line4 ,the local trunks which connect long-distance lines with exchange lines for conversation are held longer than are the actual long-distance trunks between cities4 ,knowing the probable traffic to be brought to the long-distance switching center by the long-distance trunks from exchange centers1 the number of trunks required may be determined by knowing the capacity of each trunk4 ,these trunk capacities vary with the method of handling the traffic and they vary as do local trunks with the number of trunks in a group4 ,table ,,xvii illustrates this variation of capacity with sizes of groups4 ,,table ,,xvii ,messages per ,trunk in ,long-,distance ,groups ---------------------------------------- ----------- ,,number ,,of ,,long-,,distance ,,messages ,,per ,,trunk ,,per ,,trunks ,,in ,,group ,,busy ,,hour ---------------------------------------- ----------- #e #b #aj #c #bj #c4b #dj #c4e #fj #d #ajj #d4f ---------------------------------------- ----------- ,quality of ,service4 ,the quality of telephone service rendered by a particular equipment managed in a particular way depends on a great variety of elements4 ,the handling of the traffic presented by patrons is a true manufacturing problem4 ,the quality of the service rendered requires continuous testing in order that the management may know whether the service is reaching the standard2 whether the standard is high enough2 whether the cost of producing it can be reduced without lowering the quality2 and whether the patrons are getting from it as much value as they might4 ,in manual systems1 the quality of telephone service depends upon a number of elements4 ,the following are some principal ones3 #a4 ,prompt answering4 #b4 ,prompt disconnection4 #c4 ,freedom from errors in connecting with the called line4 #d4 ,promptness in connecting with the called line4 #e4 ,courtesy and the use of form4 #f4 ,freedom from failure by busy lines and failure to answer4 #g4 ,clear enunciation4 #h4 ,team work4 ,answering ,time4 ,there is an interrelation between these elements4 ,team work assists both answering and prompt disconnection4 ,the quality of telephone service can not be measured alone in terms of prompt answering4 ,formerly telephone service was boasted of as being three-second service if most of the originating calls were answered in three seconds4 ,often such prompt answering reacts to prevent prompt disconnecting4 ,patient1 systematic work is required to learn the real quality of the service4 ,as to answering1 the clearest1 truest statement concerning manual service is found by making test calls to each position1 dividing them into groups of various numbers of whole seconds each1 and comparing the percentage of these groups to the whole number of telephones to that position4 ,for example1 assume each of the calls to a given position to have been answered in ten seconds or less1 in which #ajj per cent are answered in ten seconds or less2 #hj per cent in eight seconds or less2 #fj per cent in six seconds or less4 ,it is probable that a reasonably uniform manual service will show only a small percentage answered in three seconds or under4 ,such percentages may be drawn in the form of curves1 so that at a glance one may learn efficiency in terms of prompt answering4 ,disconnecting ,time4 ,prompt disconnection was improved enormously by the introduction of relay manual boards4 ,just before the installation of relay boards in ,new ,york ,city1 the average disconnecting time was over seventeen seconds4 ,on the completion of an entire relay equipment1 the average disconnecting time was found to be under three seconds4 ,the introduction of relay manual apparatus has led subscribers to a larger traffic and to the making of calls which succeed each other very closely4 ,a most important rule is1 that disconnect signals shall be given prompt attention either by the operator who made the connection1 by an operator adjacent1 or by a monitor who may be assisting2 and another1 still more important one is1 that a flashing keyboard lamp indicating a recall shall be given precedence over all originating and all other disconnect signals4 ,accuracy and ,promptness4 ,promptness and accuracy in connecting with the called line are vital1 and yet a large percentage of errors in these elements might exist in an exchange having a very high average speed of answering the originating call4 ,indeed1 it seems quite the rule that where the effort of the management is devoted toward securing and maintaining extreme speed of original answering1 all the other elements suffer in due proportion4 ,courtesy and ,form4 ,it goes without saying that operators should be courteous2 but it is necessary to say it1 and keep saying it in the most effective form1 in order to prevent human nature under the most exasperating circumstances from lapsing a little from the standard1 however high4 ,the use of form assists both the operators and the subscribers1 because in all matters of strict routine it is much easier to secure high speed and great accuracy by making as many as possible of the operations automatic4 ,the use of the word number and other well-accepted formalities has assisted greatly in securing speed1 clear understanding1 and accurate performance4 ,the simple expedient of spelling numbers by repeating the figures in a detached form--as #a-b-e for #abe--;has taught subscribers the same expedient1 and the percentage of possible error is materially reduced by going one step further and having the operator1 in repeating1 use always the opposite form from that spoken by the calling subscriber4 ,busy and ,don't ,answer ,calls4 ,notwithstanding the old impression of the public to the contrary1 the operator has no control over the busy line and don't answer situation4 ,it is1 however1 of high importance that the management should know1 by the analysis of repeated and exhaustive tests of the service1 to what extent these troubles are degrading it4 ,in addition to improving the service by the elimination of busy reports1 there is no means of increasing revenue which is so easy and so certain as that which comes from following up the tabulated results of busy calls4 ,enunciation4 ,it must be remembered that clear enunciation for telephone purposes is a matter wholly relative1 and the ability of an operator in this regard can be determined only by a close analysis of many observations from the standpoint of a subscriber4 ,a trick of speech rather than a pleasant voice and an easy address has made the answering ability of many an operator captivating to a group of satisfied subscribers4 ,team ,work4 ,by team work is meant the ability of a group of operators1 seated side by side1 to work together as a unit in caring for the service brought to them by the answering jacks within their reach4 ,in switchboards of the construction usual today1 a call before any operator may be answered by her1 or by the operator at either the right or the left of her position4 ,in many exchanges this advantage is wholly overlooked4 ,in the period of general re-design of central-office equipments about fourteen years ago1 a switchboard was installed with mechanical visual signals and answering-jacks on a flat-top board1 and an arrangement of operators such that the signal of any call was extremely prominent and in easy reach of each one of four or possibly five operators4 ,associated with the line signals within the reach of such a group was an auxiliary lamp signal which would light when a call was made by any of the lines so terminating4 ,it was found that with this arrangement the calls were answered in a strictly even manner1 special rushes being cared for by the joint efforts of the group rather than serving to swamp the operator who happened to be in charge of the particular section affected by the rush4 ,this principle has been tried out in so many ways that it is astonishing that it is not recognized as being a vital one4 ,the whole matter is accomplished by impressing upon each operator that her duty is1 not to answer the calls of a specific number of lines before her1 but to answer1 with such promptness as is possible1 any call which is within the reach of her answering equipment4 ,observation of ,service4 ,all that is required to be known concerning the form of address and courtesy may be learned by a close observation of the operators' work by the chief operators and monitors1 and by the use of listening circuits permanently connected to the operators' sets4 ,it is naturally necessary that the use of these listening circuits by the chief operator or her assistants must not be known to the operators at the times of use1 even though they may know of the existence of such facilities4 ,with a well-designed and properly maintained automatic equipment1 the eight elements of good manual service reduce themselves to only one or two4 ,freedom from failure by busy lines and failure to answer are service-qualities independent of the kind of switching apparatus4 ,too great a percentage of busy calls for a given line indicates that the telephone facilities for calls incoming to that subscriber are inadequate4 ,the best condition would be for each subscriber to have lines enough so that none of them ever would be found busy4 ,this is the condition the telephone company tries to establish between its various offices4 ,in manual practice it is possible to keep such records as will enable the traffic department to know when the lines to a subscriber are insufficient for the traffic trying to reach him4 ,as soon as such facts are known1 they can be laid before the subscriber so that he may arrange for additional incoming lines4 ,in automatic practice this is not so simple1 as the source and destination of traffic in general is not so clearly known to the traffic department4 ,automatic recorders of busy calls are necessary to enable the facts to be tabulated4 ,,chapter ,,xxxviii ,,measured ,,service ,in the commercial relation between the public and a telephone system1 the commodity which is produced by the latter and consumed by the former is telephone service4 ,users often consider that payment is made for rental of telephone apparatus and to some persons the payment per month seems large for the rental of a mere telephone which could be bought outright for a few dollars4 ,the telephone instrument is but a small part of the physical property used by a patron of a telephone system4 ,even the entire group of property elements used by a patron in receiving telephone service represents much less than what really is his proportion of the service-rendering effort4 ,what the patron receives is service and its value during a time depends largely on how much of it he uses in that time1 and less on the number of telephones he can call4 ,the cost of telephone service varies as the amount of use4 ,it is just1 therefore1 that the selling price should vary as the amount of use4 ,rates4 ,there are two general methods of charging for telephone service and of naming rates for this charge4 ,these are called flat rates and measured-service rates4 ,the latter are also known as message rates1 because the message or conversation is the unit4 ,flat rates are those which are also known as rentals4 ,the service furnished under flat rates is also known as unlimited service1 for the reason that under it a patron pays the same amount each month and is entitled to hold as many conversations--send as many messages and make as many calls--as he wishes1 without any additional payment4 ,in the measured-service plan1 the amount of payment in a month varies in some way with the amount of use1 depending on the plan adopted4 ,the patron may pay a fixed base amount per month1 entitling him to have equipment for telephone service and to receive messages1 but being required to pay1 in addition to this base amount1 a sum which is determined by the number of messages which he sends4 ,or he may pay a base amount per month and be entitled to have the equipment1 to receive calls1 and to send a certain number of messages1 paying specifically in addition only for messages exceeding that certain number4 ,whether flat rates or measured-service rates are practiced1 the general tendency is to establish lower rates for service in homes than in business places4 ,this is another recognition of the justice of graduating the rates in accordance with the amount of use4 ,units of ,charging4 ,while both the flat-rate and the measured-rate methods of charging for unlimited and measured service are practiced in local exchanges1 long-distance service universally is sold at message rates4 ,the unit of message rates in long-distance service is time4 ,the charge for a message between two points joined by long-distance lines usually is a certain sum for a conversation three minutes long plus a certain sum for each additional minute or fraction of a minute4 ,in local service1 the message-rate time charge per message takes less account of the time unit4 ,the conversation is almost universally the unit in exchanges4 ,some managements restrict messages of multi-party lines to five minutes per conversation1 because of the desire to avoid withholding the line from other parties upon it for too long periods4 ,service sold at public stations similarly is restricted as to time1 even though the message be local to the exchange4 ,three to five minutes local conversation is sold generally for five cents in the ,united ,states4 ,the time of the average local message1 counting actual conversation time only1 is one hundred seconds4 ,toll ,service4 ,long ,haul4 ,in long-distance service1 there are two general methods of handling traffic1 as to the relations between the calling and the called stations4 ,for the greater distances1 as between cities not closely related because not belonging to one general community1 the calling patron calls a particular person and pays nothing unless he holds conversation with that person4 ,in this method1 the operator records the name of the person called for2 the name1 telephone number1 or both1 of the person calling2 the names of the towns where the message originated and ended2 the date1 the time conversation began1 and the length of time it lasted4 ,short ,haul4 ,where towns are closely related in commercial and social ways and where the traffic is large and approaches local service in character1 and yet where conversations between them are charged at different rates than are local calls within them1 a more rapid system of toll charging than that just described is of advantage4 ,in these conditions1 patrons are not sold a service which allows a particular party to be named and found1 nor is the identity of the calling person required4 ,the operator needs to know merely of these calls that they originate at a certain telephone and are for a certain other4 ,the facts she must record are fewer and her work is simpler4 ,therefore1 the cost of such switching is less than for true long-distance calls and it can be learned by careful auditing just when traffic between points becomes great enough to warrant switching them in this way4 ,such switching1 for example1 exists between ,new ,york and ,brooklyn1 between ,chicago and suburbs around it which have names of their own but really are part of the community of ,chicago1 and between ,san ,francisco and other cities which cluster around ,san ,francisco ,bay4 ,calls of the long-haul class are known as particular person or particular party calls1 while short-haul calls are known as two-number long-distance calls4 ,it is customary to handle particular party calls on long-distance switchboards and to handle two-number calls in manual systems on subscribers' switchboards exactly like local calls1 except that the two-number calls are ticketed4 ,it is customary in automatic systems to handle two-number calls by means of the regular automatic equipment plus ticketing by a suburban or two-number operator4 ,timing ,toll ,connections4 ,it formerly was customary to measure the time of long-distance conversations by noting on the ticket the time of its beginning and the time of its ending1 the operator reading the time from a clock4 ,for human and physical reasons1 such timing seems not to be considered infallible by the patron who pays the charge1 and in cases of dispute concerning overtime charges so timed1 telephone companies find it wisest to make concessions4 ,the physical cause of error in reading time from a clock is that of parallax2 that is1 the error which arises from the fact that the minute hand of a clock is some distance from the surface of the dial so that one can look under it4 ,on an ordinary clock having a large face and its minute hand pointing upward or downward1 five people standing in a row could read five different times from it at the same instant4 ,the middle person might see the minute hand pointing at #f1 indicating the time to be half-past something2 whereas1 person ,no4 #a and person ,no4 #e in the row might read the time respectively #bi and #ca minutes past something4 ,operators far to the right or to the left of a clock will get different readings1 and an operator below a clock will get different kinds of readings at different times and correct readings at few times4 ,timing ,machines3--,machines which record time directly on long-distance tickets are of value and machines which automatically compute the time elapsing during a conversation are of much greater value4 ,the calculagraph is a machine of the latter class4 ,the use of some such machine uniformly reduces controversy as to time which really elapsed4 ,parallax errors are avoided4 ,the record possesses a dignity which carries conviction4 ,illustration3 ,fig4 #dec4 ,calculagraph ,records ,calculagraph records are shown in ,fig4 #dec4 ,in the one shown in the upper portion of this figure1 the conversation began at #aj4dd ,p4,m4 ,this is shown by the right-hand dial of the three which constitute the record4 ,the minutes past #aj o'clock are shown by the hand within the dial and the hour #aj is shown by the triangular mark just outside the dial between ,x and ,,xi4 ,the duration of the conversation is shown by the middle and the left-hand dials4 ,the figures on both these dials indicate minutes4 ,the middle dial indicates roughly that the conversation lasted for a time between #j and #e minutes4 ,the left-hand dial indicates with greater exactness that the conversation lasted one and one-quarter minutes4 ,the hand of the left-hand dial makes one revolution in five minutes2 of the middle dial1 one revolution in an hour4 ,the middle dial tells how many full periods of five minutes have elapsed and the left-hand dial shows the excess over the five-minute interval4 ,the lower portion of ,fig4 #dec is a similar record beginning at the same time of day1 but lasting about five and one-half minutes4 ,as before1 the readings of the two dials are added to get the elapsed time4 ,illustration3 ,fig4 #ded4 ,relative ,position of ,hands and ,dials ,the right-hand dial1 showing merely time of day1 stands still while its hands revolve4 ,the dies which print the dials and hands of the middle and the left-hand records rotate together4 ,examining the machine1 one finds that the hands of these dials always point to zero4 ,the middle dial and hand make one complete revolution in an hour2 the left-hand dial and hand1 one in five minutes4 ,in making the records1 the dials are printed at the beginning and the hands at the end of the conversation4 ,therefore1 the hands will have moved forward during the conversation--still pointing to zero in both cases--but when printed the hands will point to some other place than they were pointing when the dials were printed4 ,in this way1 their angular distances truly indicate the lapse of time4 ,fig4 #ded shows the relative position of the hands and dials within the machine at all times4 ,it will be noted that the arrow of the left-hand dial does not point exactly to zero4 ,this is due to the fact that the dials and hands are printed by separate operations and cannot be printed simultaneously4 ,illustration3 ,,western ,,electric ,,ringing ,,machine ,another method of timing toll connections has been developed by the ,monarch ,telephone ,manufacturing ,company4 ,this employs a master clock of great accuracy1 which may be mounted on the wall anywhere in the building or another building if desired4 ,a circuit leads from this clock to a time-stamp device on the operator's key shelf1 and the clock closes this circuit every quarter minute4 ,the impulses thus sent over the circuit energize the magnet of the time stamp1 which steps a train of printing wheels around so as always to keep them set in such position as to properly print the correct time on a ticket whenever the head of the stamp is moved by the operator into contact with the ticket4 ,a large number of such stamps may be operated from the same master clock4 ,by printing the starting time of a connection below the finishing time the computation of lapsed time becomes a matter of subtraction4 ,a typical toll ticket with the beginning and ending time printed by the time stamp in the upper left-hand corner and the elapsed time recorded by hand in the upper right-hand corner is shown in ,fig4 #dee4 ,it is seen that this stamp records in the order mentioned the month1 the day1 the hour1 the minute and quarter minute1 the ,a4,m4 and ,p4,m4 division of the day1 and the year4 ,illustration3 ,fig4 #dee4 ,toll ,ticket ,used with ,monarch ,system ,an interesting feature of this system is that the same master clock may be made in a similar manner to actuate secondary clocks placed at subscribers' stations1 the impulses being sent over wires in the same cables as those containing the subscribers' lines4 ,this system1 therefore1 serves not only as a means for timing the toll tickets and operating time stamps wherever they are required in the business of the telephone company1 but also to supply a general clock and time-stamp service to the patrons of the telephone company as a by-product of the general telephone business4 ,exchange service is measured in terms of conversations without much regard to their length4 ,the payment for the service may be made at the time it is received1 as in public stations and at telephones equipped with coin prepayment devices2 or the calls from a telephone may be recorded and collection for them made at agreed intervals4 ,in the prepayment method the price per call is uniform4 ,in the deferred payment method the calls are recorded as they are made1 their number summed up at intervals1 and the amount due determined by the price per call4 ,the price per call may vary with the number of calls sold4 ,a large user may have a lower rate per call than a small user4 ,local ,service4 ,ticket ,method4 ,measured local service sometimes is recorded by means of tickets1 similarly to the described method of charging long-distance calls1 except that the time of day and the duration of conversation are not so important4 ,where local ticketing is practiced1 it is usual to write on the ticket only the number of the calling telephone and the date1 and to pass into the records only those tickets which represent actual conversations1 keeping out tickets representing calls for busy lines and calls which were not answered4 ,meter ,method4 ,the requirements of speed in good local service are opposed to the ticketing method4 ,where measured service is supplied to a substantial proportion of the lines of a large exchange1 electro-mechanical service meters are attached to the lines4 ,these service meters register as a consequence of some act on the part of the switchboard operator1 or may be caused to register by the answering of the called subscriber4 ,illustration3 ,fig4 #def4 ,connection ,meter ,in manual practice1 meters of the type shown in ,fig4 #def are associated with the lines as in ,fig4 #deg4 ,the meters are mounted separately from the switchboard1 needing only to be connected to the test-strand of the line by cabled wires4 ,if desired1 the meter may be mounted on racks in quarters especially devoted to them1 and the cases in which the racks are mounted may be kept locked4 ,in such an arrangement the meters are read from time to time through the glass doors of the cases4 ,the meters are caused to operate by pressure on the meter key ,,mk1 associated with the answering cord as in ,fig4 #deh4 ,this increases the normal potential to #cj volts4 ,when the armature of the meter has made a part of its stroke1 it closes a contact which places its #dj-ohm winding in shunt with its #ejj-ohm winding1 thus furnishing ample power for turning the meter wheels4 ,illustration3 ,fig4 #deg4 ,western ,electric ,line ,circuit and ,service ,meter ,such meters are in common use in large exchanges1 notable examples being the cities of ,new ,york and ,london4 ,in ,london1 there is a zone within which the price per call is one penny and between which and other zones the price is twopence4 ,calls within the zone either are completed by the answering operator directly in the multiple before her or are trunked to other offices in that zone4 ,calls for points outside of that zone are trunked to other offices and in giving the order the operator finds that the call circuit key lights a special signal lamp before her4 ,this reminds her that the call is at a twopence price1 so in recording it she presses the meter key twice4 ,this counts two units on the meter and the units are billed at a penny each4 ,in automatic systems it is not possible to operate a meter system in which the operator will press a key for each call to be charged1 because there is no operator4 ,in such systems--a notable example being the measured-service automatic system in ,san ,francisco--the meter registers only upon the answering of the called subscriber4 ,calls for lines found busy and calls which are not answered do not register4 ,calls for long-distance recording operators1 two-number ticket operators1 information1 complaint1 and other company departments are not registered4 ,in the ,chinatown quarter of ,san ,francisco1 where most calls begin and end in the neighborhood1 service is sold at an unlimited flat rate for neighborhood calls and at a message rate for other calls4 ,the meter system recognizes this condition and does not register calls from ,chinese subscribers for ,chinese subscribers1 though it does register calls from ,chinese subscribers to ,caucasian subscribers4 ,the nature of the system is such as to enable it to discriminate as to races1 localities1 or other peculiarities as may be desired4 ,illustration3 ,fig4 #deh4 ,western ,electric ,cord ,circuit and ,service ,meter ,key ,in the manual meter circuits of ,figs4 #deg and #deh1 the meter windings have no relation to the line conductors4 ,in the automatic arrangement just described1 there are meter windings in the line during times of calling1 but none in the line during times of conversation4 ,the balance of the line1 therefore1 is undisturbed at all times wherein balance is of any importance4 ,in both systems just described1 the meters of all lines are in their respective central offices4 ,meters for use at subscribers' stations have been devised and there is no fundamental reason why the record might not be made at the subscriber's station instead of1 or in addition to1 a central-office record4 ,experience has shown that confidence in a meter system can be secured if the meters be positive1 accurate1 and reliable4 ,the labor of reading the meters is much less when they are kept in central offices4 ,subscribers may have access to them if they wish4 ,prepayment ,method4 ,prepayment measured-service mechanisms permit a coin or token to be dropped into a machine at the subscriber's telephone at the time the conversation is held4 ,a variety of forms of telephone coin collectors are in use1 their operations being fundamentally either electrical or mechanical4 ,electrically operated coin collectors require either that the coin be dropped into the machine in order to enable the central office to be signaled in manual systems1 or the switches to be operated in automatic systems1 or they require that the coin be dropped into the machine after calling1 but before the conversation is permitted4 ,western ,electric ,company coin collectors1 shown in ,fig4 #dei1 may be operated in either way in connection with manual systems4 ,the usual way is to require the coin to be dropped before the central-office line lamp can glow4 ,the operator then rings the called subscriber and upon his answering places a sufficient potential upon the calling line to operate the polarized relay and to drop the coin into the cash box4 ,if the called subscriber does not answer or his line is busy1 potential is placed on the calling line1 moving the polarized relay in the other direction and dropping the coin into a return chute so that the subscriber may take it4 ,if it is preferred that the coin be paid only on the request of the operator1 the return feature need not be provided4 ,in both forms of operation1 the ,western ,electric coin collector is adapted to bridge its polarized relay between one limb of the line and ground during the time a coin rests on the pins1 as shown in ,fig4 #dei4 ,when no coin is on the pins--i4 e41 before calling and after the called station responds--the relay is not so bridged4 ,illustration3 ,fig4 #dei4 ,principle of ,western ,electric ,coin ,collector ,the armature of the relay responds only to a high potential and this is applied by the operator4 ,if the coin is to be taken by the company1 one polarity is sent2 if it is to be returned to the patron1 the other polarity is sent4 ,these polarities are applied to a limb of the line proper4 ,it will be recalled that pressures to actuate service meters are applied to the test-strand4 ,if wished1 keys may be arranged so as to apply #cj volts to the test-strand and the collecting potential to the line at the same operation4 ,this enables the service meter to count the tokens placed in the cash box of the coin collector1 and serves as a valuable check4 ,in automatic systems1 in one arrangement1 coin collectors are arranged so that no impulses can be sent unless a coin has been deposited1 the coin automatically passing to the cash box when the called subscriber answers1 or to the patron if it is not answered4 ,in another arrangement1 calls are made exactly as in unlimited service1 but a coin must be deposited before a conversation can be held4 ,the calling person can hear the called party speak and may speak himself but can not be heard until the coin is deposited4 ,no coin-return mechanism is required in this method4 ,coin collectors of these types usually are adapted to receive only one kind of coin1 these1 in the ,united ,states1 being either nickels or dimes4 ,for long-distance service1 where the charges vary1 it is necessary to signal to an operator just what coins are paid4 ,it is uniformly customary to send these signals by sound1 the collector being so arranged that the coins strike gongs4 ,in coin collectors of the ,gray ,telephone ,paystation ,company1 the coins strike these gongs by their own weight in falling through chutes4 ,in coin collectors of the ,baird ,electric ,company1 the power for the signals is provided by hand power1 a lever being pulled for each coin deposited4 ,both methods are in wide use4 ,,chapter ,,xxxix ,,phantom1 ,,simplex1 ,,and ,,composite ,,circuits ,definitions4 ,phantom circuits are arrangements of telephone wires whereby more working1 non-interfering telephone lines exist than there are sets of actual wires4 ,when four wires are arranged to provide three metallic circuits for telephone purposes1 two of the lines are physical circuits and one is a phantom circuit4 ,simplex and composite circuits are arrangements of wires whereby telephony and telegraphy can take place at the same time over the same wires without interference4 ,illustration3 ,fig4 #dfj4 ,phantom ,circuit ,phantom4 ,in ,fig4 #dfj four wires join two offices4 ,,rr are repeating coils1 designed for efficient transforming of both talking and ringing currents4 ,the devices marked ,a in this and the following figures are air-gap arresters4 ,currents from the telephones connected to either physical pair of wires pass1 at any instant1 in opposite directions in the two wires of the pair4 ,the phantom circuit uses one of the physical pairs as a wire of its line4 ,it does this by tapping the middle point of the line side of each of the repeating coils4 ,the impedance of the repeating-coil winding is lowered because1 all the windings being on the same core1 the phantom line currents pass from the middle to the outer connections so as to neutralize each other's influence4 ,the currents of the phantom circuit1 unlike those of the physical circuits1 are in the same direction in both wires of a pair at any instant4 ,their potentials1 therefore1 are equal and simultaneous4 ,a phantom circuit is formed most simply when both physical lines end in the same two offices4 ,if one physical line is longer than the other1 a phantom circuit may be formed as in ,fig4 #dfa1 wherein the repeating coil is inserted in the longer line where it passes through a terminal station of the shorter4 ,illustration3 ,fig4 #dfa4 ,phantom from ,two ,physical ,circuits of ,unequal ,length ,illustration3 ,fig4 #dfc4 ,two ,phantoms ,joined by ,physical ,circuit ,a circuit may be built up by adding a physical circuit to a phantom4 ,a circuit may be made up of two or more phantom circuits1 joined by physical ones4 ,in ,fig4 #dfb a phantom circuit is extended by the use of a physical circuit1 while in ,fig4 #dfc1 two phantom circuits are joined by placing between them a physical circuit4 ,illustration3 ,fig4 #dfb4 ,phantom ,extended by ,physical ,circuit ,transpositions4 ,in phantom circuits formed merely by inserting repeating coils in physical circuits and doing nothing else1 an exact balance of the sides of the phantom circuit is lacking4 ,the resistances1 insulations1 and capacities to earth of the sides may be equal1 but the exposures to adjacent telephone and telegraph circuits and to power circuits will not be equal unless the phantom circuits are transposed4 ,to transpose a set of lines of two physical wires each1 is not complicated1 though it must be done with care and in accordance with a definite1 foreknown plan4 ,transposing phantom circuits is less simple1 however1 as four wires per circuit have to be transposed1 instead of two4 ,illustration3 ,fig4 #dfd4 ,transposition of ,phantom ,circuits ,in ,fig4 #dfd1 the general spacing of transposition sections is the usual one1 #a1cjj feet1 of the ,,abcb system widely in use4 ,the pole circuit1 on pins #e and #f of the upper arm1 is transposed once each two miles4 ,the pole circuit of the second arm transposes either once or twice a mile4 ,but neither pole circuit differs in transposition from any other regular scheme except in the frequency of transposition4 ,all the other wires of each arm1 however1 are so arranged that each wire on either side of the pole circuit moves from pin to pin at section-ends1 till it has completed a cycle of changes over all four of the pins on its side4 ,in doing so1 each phantom circuit is transposed with proper regard to each of the other three on that twenty-wire line4 ,the new transposition lettering in ,fig4 #dfd is for the purpose of identifying the exact scheme of wiring each transposition pole4 ,the complication of wiring at each transposition pole is increased by the adoption of phantom circuits4 ,maintenance of all the circuits is made more costly and less easy unless the work at points of transposition is done with care and skill4 ,phantom circuits1 to be always successful1 require that the physical circuits be balanced and kept so4 ,transmission over ,phantom ,circuits4 ,under proper conditions phantom circuits are better than physical circuits1 and in this respect it may be noted that some long-distance operating companies instruct their operators always to give preference to phantom circuits1 because of the better transmission over them4 ,the use of phantom circuits is confined almost wholly to open-wire circuits2 and while the capacity of the phantom circuit is somewhat greater than that of the physical circuit1 its resistance is considerably smaller4 ,in the actual wire the phantom loop is only half the resistance of either of the physical lines from which it is made1 for it contains twice as much copper4 ,the resistance of the repeating coils1 however1 is to be added4 ,simplex4 ,simplex telegraph circuits are made from metallic circuit telephone lines1 as shown in ,fig4 #dfe4 ,the principle is identical with that of phantom telephone circuits4 ,the potentials placed on the telephone line by the telegraph operations are equal and simultaneous4 ,they cause no current to flow around the telephone loop1 only along it4 ,if all qualities of the loop are balanced1 the telephones will not overhear the telegraph impulses4 ,in the figure1 ,,aa are arresters1 as before1 ,,gg are ,morse relays2 a #b-microfarad condenser is shunted around the contact of each ,morse key ,f to quench the noises due to the sudden changes on opening the keys between dots and dashes4 ,illustration3 ,fig4 #dfe4 ,simplex ,telegraph ,circuit ,a simplex arrangement even more simple substitutes impedance coils for the repeating coils of ,fig4 #dfe4 ,the operation of the ,morse circuit is the same4 ,an advantage of such a circuit1 as shown in ,fig4 #dff1 is that the telephone circuit does not suffer from the two repeating-coil losses in series4 ,a disadvantage is1 that in ringing on such a line with a grounded generator1 the ,morse relays are caused to chatter4 ,illustration3 ,fig4 #dff4 ,simplex ,telegraph ,circuit ,the circuit of ,fig4 #dfe may be made to fit the condition of a through telephone line and a way telegraph station4 ,the midway ,morse apparatus of ,fig4 #dfg is looped in by a combination of impedance coils and condensers4 ,the plans of ,figs4 #dfe and #dff here are combined1 with the further idea of stopping direct and passing alternating currents1 as is so well accomplished by the use of condensers4 ,illustration3 ,fig4 #dfg4 ,simplex ,circuit with ,waystation ,illustration3 ,fig4 #dfh4 ,composite ,circuit ,composite4 ,composite circuits depend on another principle than that of producing equal and simultaneous potentials on the two wires of the telephone loop4 ,the opposition of impedance coils to alternating currents and of condensers to direct currents are the fundamentals4 ,the early work in this art was done by ,van ,rysselberghe1 of ,belgium4 ,in ,fig4 #dfh1 one telephone circuit forms two ,morse circuits1 two wires carrying three services4 ,each ,morse circuit will be seen to include1 serially1 two #ej-ohm impedance coils1 and to have shunts through condensers to ground4 ,the #ej-ohm coils are connected differentially1 offering low consequent impedance to ,morse impulses1 whose frequency of interruption is not great4 ,as the impedance coils are large1 have cores of considerable length1 and are wound with two separate though serially connected windings each1 their impedance to voice currents is great4 ,they act as though they were not connected differentially1 so far as voice currents are concerned4 ,because of the condensers serially in the telephone line1 voice currents can pass through it1 but direct currents can not4 ,impulses due to discharges of cores1 coils1 and capacities in the ,morse circuit could make sounds in the telephones1 but these are choked out1 or led to earth by the #cj-ohm impedance coils and the heavy ,morse condensers4 ,ringing4 ,ringing over simplex circuits is done in the way usual where no telegraph service is added4 ,both telegraphy and telephony over simplex circuits follow their usual practice in the way of calling and conversing4 ,in composite working1 however1 ringing by usual methods either is impossible because of heavy grounds and shunts1 or if it is possible to get ringing signals through at all1 the relays of the ,morse apparatus will chatter1 interfering with the proper use of the telegraph portion of the service4 ,it is customary1 therefore1 either to equip composite circuits with special signaling devices by which high-frequency currents pass over the telephone circuits1 operating relays which in turn operate local ringing signals2 or to refrain from ringing on composite circuits and to transmit orders for connections by telegraph4 ,the latter is wholly satisfactory over composite lines between points having heavy telegraph traffic1 and it is between such points as these that composite practice is most general4 ,phantoms from ,simplex and ,composite ,circuits4 ,phantom and simplex principles are identical1 and by adding the composite principle1 two simplex circuits may have a phantom superadded1 as in ,fig4 #dfi4 ,similarly1 as in ,fig4 #dgj1 two composite circuits can be phantomed4 ,this case gives seven distinct services over four wires3 three telephone loops--two physical and one phantom--and four ,morse lines4 ,illustration3 ,fig4 #dfi4 ,phantom of ,two ,simplex ,circuits ,illustration3 ,fig4 #dgj4 ,phantom of ,two ,composite ,circuits ,railway ,composite4 ,the foregoing are problems of making telegraphy a by-product of telephony4 ,with so many telegraph wires on poles over the country1 it has seemed a pity not to turn the thing around and provide for telephony as a by-product of telegraphy4 ,this has been accomplished1 and the result is called a railway composite system4 ,for the reason that the telegraph circuits are not in pairs1 accurately matched one wire against another1 and are not always uniform as to material1 it has not been possible to secure as good telephone circuits from telegraph wires as telegraph circuits from telephone wires4 ,practical results are secured by adaptation of the original principle of different frequencies4 ,a study of ,fig4 #dfh shows that over such a composite circuit the usual method of ringing from station to station over the telephone circuit by an alternating current of a frequency of about sixteen per second is practically impossible4 ,this is because of the heavy short-circuit provided by the two #cj-ohm choke coils at each of the stations1 the heavy shunt of the large condensers1 and the grounding through the #ej-ohm choke coils4 ,if high-frequency speech currents can pass over these circuits with a very small loss1 other high-frequency circuits should find a good path4 ,there are many easy ways of making such currents1 but formerly none very simple for receiving them4 ,fig4 #dga shows one simple observer of such high-frequency currents1 it being merely an adaptation of the familiar polarized ringer used in every subscriber's telephone4 ,in either position of the armature it makes contact with one or the other of two studs connected to the battery1 so that in all times of rest the relay ,a is energized4 ,when a high-frequency current passes through this polarized relay1 however1 there is enough time in which the armature is out of contact with either stud to reduce the total energy through the relay ,a and allow its armature to fall away1 ringing a vibrating bell or giving some other signal4 ,illustration3 ,fig4 #dga4 ,ringing ,device for ,composite ,circuits ,fig4 #dgb shows a form of apparatus for producing the high-frequency current necessary for signaling4 ,it is evident that if a magneto generator1 such as is used in ordinary magneto telephones1 could be made to drive its armature fast enough1 it also might furnish the high-frequency current necessary for signaling through condensers and past heavy impedances4 ,illustration3 ,fig4 #dgb4 ,ringing ,current ,device ,applying these principles of high-frequency signals sent and received to a single-wire telegraph circuit1 the arrangement shown in ,fig4 #dgc results1 this being a type of railway composite circuit4 ,the principal points of interest herein are the insertion of impedances in series with the telegraph lines1 the shunting of the telegraph relays by small condensers1 the further shunting of the whole telegraph mechanism of a station by another condenser1 and thus keeping out of the line circuit changes in current values which would be heard in the telephones if violent1 and might be inaudible if otherwise4 ,illustration3 ,fig4 #dgc4 ,railway ,composite ,circuit ,illustration3 ,,front ,,of ,,long-,,distance ,,power ,,board ,u4,s4 ,telephone ,company1 ,cleveland1 ,ohio4 ,the ,dean ,electric ,co4 ,a further interesting element is the very heavy shunting of the telephone receiver by means of an inductive coil4 ,this shunt is applied for by-path purposes so that heavy disturbing currents may be kept out of the receiver while a sufficient amount of voice current is diverted through the receiver4 ,it is well to have the inductance of this shunt made adjustable by providing a movable iron core for the shunt winding4 ,when the core is drawn out of the coil1 its impedance is diminished because the inductance is diminished4 ,this reduces the amount of disturbing noise in the receiver4 ,the core should be withdrawn as little as the amount of disturbance permits1 as this also diminishes the loudness of the received speech4 ,because the signaling over lines equipped with this form of composite working results in the ringing of a bell by means of local current1 it is of particular advantage in cases where the bell needs to ring loudly4 ,switch stations1 crossings1 and similar places where the attendant is not constantly near the telephone can be equipped with this type of composite apparatus and it so offers a valuable substitute for regular railway telegraph equipment1 with which the attendant may not be familiar4 ,the success of the local bell-ringing arrangement1 however1 depends on accurate relay adjustment and on the maintenance of a primary battery4 ,the drain on the ringing battery is greater than on the talking battery4 ,a good substitute for the bell signal on railway composite circuits is a telephone receiver responding directly to high-frequency currents over the line4 ,the receiver is designed specially for the purpose and is known as a howler4 ,its signal can be easily heard through a large room4 ,the condenser in series with it is of small capacity1 limiting the drain upon the line4 ,usually the howler is detached by the switch hook during conversation from a station4 ,railway ,composite ,set4 ,the circuit of a set utilizing such an arrangement together with other details of a complete railway composite set is shown in ,fig4 #dgd4 ,the drawing is arranged thus1 in the hope of simplifying the understanding of its principles4 ,it will be seen that the induction coil serves as an interrupter as well as for transmission4 ,all of the contacts are shown in the position they have during conversation4 ,the letters ,hc#a1 ,hc#b1 etc41 and ,kc#a1 ,kc#b1 etc41 refer to hook contacts and key contacts1 respectively1 of the numbers given4 ,the arrangements of the hook and key springs are shown at the right of the figure4 ,,rr represent impedance coils connected serially in the line and placed at terminal stations4 ,the composite telephone sets are bridged from the line to ground at any points between the terminal impedance coils4 ,the direct currents of telegraphy are prevented from passing to ground through the telephone set during conversation by the #b-microfarad condenser which is in series with the receiver4 ,they are prevented from passing to ground through the telephone set when the receiver is on the hook by a 4#je microfarad condenser in series with the howler4 ,the alternating currents of speech and interrupter signaling are kept from passing to ground at terminals by the impedance coils4 ,signals are sent from the set by pressing the key ,k4 ,this operates the vibrator by closing contacts ,kc#f and ,kc#g4 ,the howler is cut off and the receiver is short-circuited by the same operation of the key4 ,the impedance of the coil ,i is changed by moving its adjustable core4 ,illustration3 ,fig4 #dgd4 ,railway ,composite ,set ,applications4 ,a chief use of composite and simplex circuits is for ticket wire purposes4 ,these are circuits over which long-distance operators instruct each other as to connecting and disconnecting lines1 the routing of calls1 and the making of appointments4 ,one such wire will care for all the business of many long-distance trunks4 ,the public also absorbs the telegraph product of telephone lines4 ,such telegraph service is leased to brokers1 manufacturers1 merchants1 and newspapers4 ,railway companies use portable telephone adjuncts to telegraph circuits on trains for service from stations not able to support telegraph attendants1 and in a limited degree for the dispatching of trains4 ,telephone train dispatching1 however1 merits better equipment than a railway composite system affords4 ,,chapter ,,xl ,,telephone ,,train ,,dispatching,a ,it has been only within the past three few that the telephone has begun to replace the telegraph for handling train movements4 ,the telegraph and the railroads have grown up together in this country since #ahej1 and in view of the excellent results that the telegraph has given in train dispatching and of the close alliance that has always naturally existed between the railway and the telegraph1 it has been difficult for the telephone1 which came much later1 to enter the field4 ,rapid ,growth4 ,the telephone has been in general use among the railroads for many years1 but only on a few short lines has it been used for dispatching trains4 ,in these cases the ordinary magneto circuit and instruments have been employed1 differing in no respect from those used in commercial service at the present time4 ,code ringing was used and the number of stations on a circuit was limited by the same causes that limit the telephones on commercial party lines at present4 ,the present type of telephone dispatching systems1 however1 differs essentially from the systems used in commercial work1 and is1 in fact1 a highly specialized party-line system1 arranged for selective ringing and many stations4 ,the first of the present type was installed by the ,new ,york ,central and ,hudson ,river ,railroad in ,october1 #aijg1 between ,albany and ,fonda1 ,new ,york1 a distance of #dj miles4 ,this section of the road is on the main line and has four tracks controlled by block signals4 ,the ,chicago1 ,burlington1 and ,quincy ,railroad was the second to install train-dispatching circuits4 ,in ,december1 #aijg1 a portion of the main line from ,aurora to ,mendota1 ,illinois1 a distance of #df miles1 was equipped4 ,this was followed in quick succession by various other circuits ranging1 in general1 in lengths over #ajj miles4 ,at the present time there are over #bj train-dispatching circuits on the ,chicago1 ,burlington1 and ,quincy ,railroad covering #abe miles of double track1 #bh miles of multi-track1 and #a1cha miles of single track1 and connecting with #bhf stations4 ,other railroads entered this field in quick order after the initial installations1 and at the present time nearly every large railroad system in the ,united ,states is equipped with several telephone train-dispatching circuits and all of these seem to be extending their systems4 ,in #aiaj1 several railroads1 including the ,delaware1 ,lackawanna1 and ,western1 had their total mileage equipped with telephone dispatching circuits4 ,the ,atchison1 ,topeka1 and ,santa ,fe ,railroad is equipping its whole system as rapidly as possible and already is the largest user of this equipment in this country4 ,from latest information1 over #ee railroads have entered this field1 with the result that the telephone is now in use in railroad service on over #bi1jjj miles of line4 ,causes of ,its ,introduction4 ,the reasons leading to the introduction of the telephone into the dispatching field were of this nature3 ,first1 and most important1 was the enactment of ,state and ,federal ,laws limiting to nine hours the working day of railroad employes transmitting or receiving orders pertaining to the movement of trains4 ,the second1 which is directly dependent upon the first1 was the inability of the railroads to obtain the additional number of telegraph operators which were required under the provisions of the new laws4 ,it was estimated that #ae1jjj additional operators would be required to maintain service in the same fashion after the new laws went into effect in #aijg4 ,the increased annual expense occasioned by the employment of these additional operators was roughly estimated at #aj1jjj1jjj4 ,a third reason is found in the decreased efficiency of the average railway and commercial telegraph operator4 ,there is a very general complaint among the railroads today regarding this particular point1 and many of them welcome the telephone1 because1 if for no other reason1 it renders them independent of the telegrapher4 ,what has occasioned this decrease in efficiency it is not easy to say1 but there is a strong tendency to lay it1 in part1 to the attitude of the telegraphers' organization toward the student operator4 ,it is a fact1 too1 that the limits which these organizations have placed on student operators were directly responsible for the lack of available men when they were needed4 ,advantages4 ,in making this radical change1 railroad officials were most cautious1 and yet we know of no case where the introduction of the telephone has been followed by its abandonment1 the tendency having been in all cases toward further installations and more equipment of the modern type4 ,the reasons for this are clear1 for where the telephone is used it does not require a highly specialized man as station operator and consequently a much broader field is open to the railroads from which to draw operators4 ,this1 we think1 is the most far-reaching advantage4 ,the telephone method also is faster4 ,on an ordinary train-dispatching circuit it now requires from #j4a of a second to #e seconds to call any station4 ,in case a plurality of calls is desired1 the dispatcher calls one station after another1 getting the answer from one while the next is being called1 and so on4 ,by speaking into a telephone many more words may be transmitted in a given time than by ,morse telegraphy4 ,it is possible to send fifty words a minute by ,morse1 but such speed is exceptional4 ,less than half that is the rule4 ,the gain in high speed1 therefore1 which is obtained is obvious and it has been found that this is a most important feature on busy divisions4 ,it is true that in the issuance of orders1 the speed1 in telephonic train dispatching1 is limited to that required to write the words in longhand4 ,but all directions of a collateral character1 the receipt of important information1 and the instantaneous descriptions of emergency situations can be given and received at a speed limited only by that of human speech4 ,the dispatcher is also brought into a closer personal relation with the station men and trainmen1 and this feature of direct personal communication has been found to be of importance in bringing about a higher degree of co-operation and better discipline in the service4 ,telephone dispatching has features peculiar to itself which are important in improving the class of service4 ,one of these is the answer-back automatically given to the dispatcher by the waystation bell4 ,this informs the dispatcher whether or not the bell at the station rang1 and excuses by the operators that it did not1 are eliminated4 ,anyone can answer a telephone call in an emergency4 ,the station operator is frequently agent also1 and his duties often take him out of hearing of the telegraph sounder4 ,the selector bell used with the telephone can be heard for a distance of several hundred feet4 ,in addition1 it is quite likely that anyone in the neighborhood would recognize that the station was wanted and either notify the operator or answer the call4 ,in cases of emergency the train crews can get into direct communication with the dispatcher immediately1 by means of portable telephone sets which are carried on the trains4 ,it is a well-known fact that every minute a main line is blocked by a wreck can be reckoned as great loss to the railroad4 ,it is also possible to install siding telephone sets located either in booths or on poles along the right-of-way4 ,these are in general service today at sidings1 crossings1 drawbridges1 water tanks1 and such places1 where it may be essential for a train crew to reach the nearest waystation to give or receive information4 ,the advantage of these siding sets is coming more and more to be realized4 ,with the telegraph method of dispatching1 a train is ordered to pass another train at a certain siding1 let us say4 ,it reaches this point1 and to use a railroad expression1 goes into the hole4 ,now1 if anything happens to the second train whereby it is delayed1 the first train remains tied up at that siding without the possibility of either reaching the dispatcher or being reached by him4 ,with the telephone station at the siding1 which requires no operator1 this is avoided4 ,if a train finds itself waiting too long1 the conductor goes to the siding telephone and talks to the dispatcher1 possibly getting orders which will advance him many miles that would otherwise have been lost4 ,it is no longer necessary for a waystation operator to call the dispatcher4 ,when one of these operators wishes to talk to the dispatcher1 he merely takes his telephone receiver off the hook1 presses a button1 and speaks to the dispatcher4 ,with the telephone it is a simple matter to arrange for provision so that the chief dispatcher1 the superintendent1 or any other official may listen in at will upon a train circuit to observe the character of the service4 ,the fact that this can be done and that the operators know it can be done has a very strong tendency to improve the discipline4 ,the dispatchers are so relieved1 by the elimination of the strain of continuous telegraphing1 and can handle their work so much more quickly with the telephone1 that in many cases it has been found possible to increase the length of their divisions from #cj to #ej per cent4 ,railroad ,conditions4 ,one of the main reasons that delayed the telephone for so many years in its entrance to the dispatching field is that the conditions in this field are like nothing which has yet been met with in commercial telephony4 ,there was no system developed for meeting them1 although the elements were at hand4 ,a railroad is divided up into a number of divisions or dispatchers' districts of varying lengths4 ,these lengths are dependent on the density of the traffic over the division4 ,in some cases a dispatcher will handle not more than #be miles of line4 ,in other cases this district may be #cjj miles long4 ,over the length of one of these divisions the telephone circuit extends1 and this circuit may have upon it #e or #ej stations1 all of which may be required to listen upon the line at the same time4 ,it will be seen from this that the telephone dispatching circuit partakes somewhat of the nature of a long-distance commercial circuit in its length1 and it also resembles a rural line in that it has a large number of telephones upon it4 ,regarding three other characteristics1 namely1 that many of these stations may be required to be in on the circuit simultaneously1 that they must all be signaled selectively1 and that it must also be possible to talk and signal on the circuit simultaneously1 a telephone train-dispatching circuit resembles nothing in the commercial field4 ,these requirements are the ones which have necessitated the development of special equipment4 ,transmitting ,orders4 ,the method of giving orders is the same as that followed with the telegraph1 with one important exception4 ,when the dispatcher transmits a train order by telephone1 he writes out the order as he speaks it into his transmitter4 ,in this way the speed at which the order is given is regulated so that everyone receiving it can easily get it all down1 and a copy of the transmitted order is retained by the dispatcher4 ,all figures and proper names are spelled out4 ,then after an order has been given1 it is repeated to the dispatcher by each man receiving it1 and he underlines each word as it comes in4 ,this is now done so rapidly that a man can repeat an order more quickly than the dispatcher can underline4 ,the doubt as to the accuracy with which it is possible to transmit information by telephone has been dispelled by this method of procedure1 and the safety of telephone dispatching has been fully established4 ,apparatus4 ,the apparatus which is employed at waystations may be divided into two groups--the selector equipment and the telephone equipment4 ,the selector is an electro-mechanical device for ringing a bell at a waystation when the dispatcher operates a key corresponding to that station4 ,at first1 as in telegraphy1 the selector magnets were connected in series in the line1 but today all systems bridge the selectors across the telephone circuit in the same way and for the same reasons that it is done in bridging party-line work4 ,there are at the present time three types of selectors in general use1 and the mileage operated by means of these is probably considerably over #ie per cent of the total mileage so operated in the country4 ,illustration3 ,fig4 #dge4 ,western ,electric ,selector ,illustration3 ,fig4 #dgf4 ,western ,electric ,selector ,the ,western ,electric ,selector4 ,this selector is the latest and perhaps the simplest4 ,fig4 #dge shows it with its glass dust-proof cover on1 and ,fig4 #dgf shows it with the cover removed4 ,this selector is adapted for operating at high speed1 stations being called at the rate of ten per second4 ,the operating mechanism1 which is mounted on the front of the selector so as to be readily accessible1 works on the central-energy principle--the battery for its operation1 as well as for the operation of the bell used in connection with it1 both being located at the dispatcher's office4 ,the bell battery may1 however1 be placed at the waystation if this is desired4 ,the selector consists of two electromagnets which are bridged in series across the telephone circuit and are of very high impedance4 ,it is possible to place as many of these selectors as may be desired across a circuit without seriously affecting the telephonic transmission4 ,direct-current impulses sent out by the dispatcher operate these magnets1 one of which is slow and the other quick-acting4 ,the first impulse sent out is a long impulse and pulls up both armatures1 thereby causing the pawls above and below the small ratchet wheel1 shown in ,fig4 #dgf1 to engage with this wheel4 ,the remaining impulses operate the quick-acting magnet and step the wheel around the proper number of teeth1 but do not affect the slow-acting magnet which remains held up by them4 ,the pawl connected to the slow-acting magnet merely serves to prevent the ratchet wheel from turning back4 ,attached to the ratchet wheel is a contact whose position can be varied in relation to the stationary contact on the left of the selector with which this engages4 ,this contact is set so that when the wheel has been rotated the desired number of teeth1 the two contacts will make and the bell be rung4 ,any selector may thus be adjusted for any station1 and the selectors are thus interchangeable4 ,when the current is removed from the line at the dispatcher's office1 the armatures fall back and everything is restored to normal4 ,an answer-back signal is provided with this selector dependent upon the operation of the bell4 ,when the selector at a station operates1 the bell normally rings for a few seconds4 ,the dispatcher1 however1 can hold this ring for any length of time desired4 ,the keys employed at the dispatcher's office for operating selectors are shown in ,fig4 #dgg4 ,there is one key for each waystation on the line and the dispatcher calls any station by merely giving the corresponding key a quarter turn to the right4 ,fig4 #dgh shows the mechanism of one of these keys and the means employed for sending out current impulses over the circuit4 ,the key is adjustable and may be arranged for any station desired by means of the movable cams shown on the rear in ,fig4 #dgh1 these cams1 when occupying different positions1 serving to cover different numbers of the teeth of the impulse wheel which operate the impulse contacts4 ,illustration3 ,fig4 #dgg4 ,dispatcher's ,keys ,illustration3 ,fig4 #dgh4 ,dispatcher's ,key ,mechanism ,the ,gill ,selector4 ,the second type of selector in extensive use throughout the country today is known as the ,gill1 after its inventor4 ,it is manufactured for both local-battery and central-energy types1 the latter being the latest development of this selector4 ,with the local-battery type1 the waystation bell rings until stopped by the dispatcher4 ,with the central-energy type it rings a definite length of time and can be held for a longer period as is the case with the ,western ,electric selector4 ,the selector is operated by combinations of direct-current impulses which are sent out over the line by keys in the dispatcher's office4 ,illustration3 ,fig4 #dgi4 ,gill ,selector ,the dispatcher has a key cabinet1 and calls in the same way as already described1 but these keys instead of sending a series of quick impulses1 send a succession of impulses with intervals between corresponding to the particular arrangement of teeth in the corresponding waystation selector wheel4 ,each key1 therefore1 belongs definitely with a certain selector and can be used in connection with no other4 ,a concrete example may make this clearer4 ,the dispatcher may operate key ,no4 #adba4 ,this key starts a clockwork mechanism which impresses at regular intervals1 on the telephone line1 direct-current impulses1 with intervals between as follows3 #a-d-b-a4 ,there is on the line one selector corresponding to this combination and it alone1 of all the selectors on the circuit1 will step its wheel clear around so that contact is made and the bell is rung4 ,in all the others1 the pawls will have slipped out at some point of the revolution and the wheels will have returned to their normal positions4 ,the ,gill selector is shown in ,fig4 #dgi4 ,it contains a double-wound relay which is bridged across the telephone circuit and operates the selector4 ,this relay has a resistance of #d1ejj ohms and a high impedance1 and operates the selector mechanism which is a special modification of the ratchet and pawl principle4 ,the essential features of this selector are the step-up selector wheel and a time wheel1 normally held at the bottom of an inclined track4 ,the operation of the selector magnet pushes the time wheel up the track and allows it to roll down4 ,if the magnet is operated rapidly1 the wheel does not get clear down before being pushed back again4 ,a small pin on the side of the pawl1 engaging the selector wheel normally1 opposes the selector wheel teeth near their outer points4 ,when the time wheel rolls to the bottom of the track1 however1 the pawl is allowed to drop to the bottom of the tooth4 ,some of the teeth on the selector wheel are formed so that they will effectually engage with the pawl only when the latter is in normal position1 while others will engage only while the pawl is at the bottom position2 thus innumerable combinations can be made which will respond to certain combinations of rapid impulses with intervals between4 ,the correct combination of impulses and intervals steps the selector wheel clear around so that a contact is made4 ,the selector wheels at all other stations fail to reach their contact position because at some point or points in their revolution the pawls have slipped out1 allowing the selector wheels to return home4 ,the answer-back is provided in this selector by means of a few inductive turns of the bell circuit which are wound on the selector relay4 ,the operation of the bell through these turns induces an alternating current in the selector winding which flows out on the line and is heard as a distinctive buzzing noise by the dispatcher4 ,illustration3 ,fig4 #dhj4 ,cummings-,wray ,dispatcher's ,sender ,the ,cummings-,wray ,selector4 ,both of the selectors already described are of a type known as the individual-call selectors1 meaning that only one station at a time can be called4 ,if a plurality of calls is desired1 the dispatcher calls one station after another4 ,the third type of selector in use today is of a type known as the multiple-call1 in which the dispatcher can call simultaneously as many stations as he desires4 ,the ,cummings-,wray selector and that of the ,kellogg ,switchboard and ,supply ,company are of this type and operate on the principle of synchronous clocks4 ,when the dispatcher wishes to put through a call1 he throws the keys of all the stations that he desires and then operates a starting key4 ,the bells at all these stations are rung by one operation4 ,the dispatcher's sending equipment of the ,cummings-,wray system is shown in ,fig4 #dhj1 and the waystation selector in ,fig4 #dha4 ,it is necessary with this system for the clocks at all stations to be wound every eight days4 ,illustration3 ,fig4 #dha4 ,cummings-,wray ,selector ,in the dispatcher's master sender the clock-work mechanism operates a contact arm which shows on the face of the sender in ,fig4 #dhj4 ,there is one contact for every station on the line4 ,the clock at this office and the clocks at all the waystation offices start together1 and it is by this means that the stations are signaled1 as will be described later1 when the detailed operation of the circuits is taken up4 ,telephone ,equipment4 ,of no less importance than the selective devices is the telephone apparatus4 ,that which is here illustrated is the product of the ,western ,electric ,company1 to whom we are indebted for all the illustrations in this chapter4 ,dispatcher's ,transmitter4 ,the dispatcher1 in most cases1 uses the chest transmitter similar to that employed by switchboard operators in every-day service4 ,he is connected at all times to the telephone circuit1 and for this reason equipment easy for him to wear is essential4 ,in very noisy locations he is equipped with a double head receiver4 ,on account of the dispatcher being connected across the line permanently and of his being required to talk a large part of the time1 there is a severe drain on the transmitter battery4 ,for this reason storage batteries are generally used4 ,illustration3 ,fig4 #dhb4 ,waystation ,desk ,telephone ,waystation ,telephones4 ,at the waystations various types of telephone equipment may be used4 ,perhaps the most common is the familiar desk stand shown in ,fig4 #dhb1 which1 for railroad service1 is arranged with a special hook-switch lever for use with a head receiver4 ,often some of the familiar swinging-arm telephone supports are used1 in connection with head receivers1 but certain special types developed particularly for railway use are advantageous1 because in many cases the operator who handles train orders is located in a tower where he must also attend to the interlocking signals1 and for such service it is necessary for him to be able to get away from the telephone and back to it quickly4 ,the ,western ,electric telephone arm developed for this use is shown in ,fig4 #dhc4 ,in this the transmitter and the receiver are so disposed as to conform approximately to the shape of the operator's head4 ,when the arm is thrown back out of the way it opens the transmitter circuit by means of a commutator in its base4 ,illustration3 ,fig4 #dhc4 ,telephone ,arm ,siding ,telephones4 ,two types of sets are employed for siding purposes4 ,the first is an ordinary magneto wall instrument1 which embodies the special apparatus and circuit features employed in the standard waystation sets4 ,these are used only where it is possible to locate them indoors or in booths along the line4 ,these sets are permanently connected to the train wire1 and since the chances are small that more than one of them will be in use at a time1 they are rung by the dispatcher1 by means of a regular hand generator1 when it is necessary for him to signal a switching4 ,illustration3 ,fig4 #dhd4 ,weather-,proof ,telephone ,set ,in certain cases it is not feasible to locate these siding telephone sets indoors1 and to meet these conditions an iron weather-proof set is employed1 as shown in ,figs4 #dhd and #dhe4 ,the apparatus in this set is treated with a moisture-proofing compound1 and the casing itself is impervious to weather conditions4 ,illustration3 ,fig4 #dhe4 ,weather-,proof ,telephone ,set ,portable ,train ,sets4 ,portable telephone sets are being carried regularly on wrecking trains and their use is coming into more and more general acceptance on freight and passenger trains4 ,fig4 #dhf shows one of these sets equipped with a five-bar generator for calling the dispatcher4 ,fig4 #dhg shows a small set without generator for conductors' and inspectors' use on lines where the dispatcher is at all times connected in the circuit4 ,illustration3 ,fig4 #dhf4 ,portable ,telephone ,set ,illustration3 ,fig4 #dhg4 ,portable ,telephone ,set ,these sets are connected to the telephone circuit at any point on the line by means of a light portable pole arranged with terminals at its outer extremity for hooking over the line wires1 and with flexible conducting cords leading to the portable set4 ,the use of these sets among officials on their private cars1 among construction and bridge gangs working on the line1 and among telephone inspectors and repairmen for reporting trouble1 is becoming more and more general4 ,western ,electric ,circuits4 ,as already stated1 a telephone train-dispatching circuit may be from #be to #cjj miles in length1 and upon this may be as many stations as can be handled by one dispatcher4 ,the largest known number of stations upon an existing circuit of this character is #fe4 ,illustration3 ,fig4 #dhh4 ,dispatcher's ,station--,western ,electric ,system ,dispatcher's ,circuit ,arrangement4 ,the circuits of the dispatcher's station in the ,western ,electric system are shown in ,fig4 #dhh1 the operation of which is briefly as follows3 ,when the dispatcher wishes to call any particular station1 he gives the key corresponding to that station a quarter turn4 ,this sends out a series of rapid direct-current impulses on the telephone line through the contact of a special telegraph relay which is operated by the key in a local circuit4 ,the telegraph relay is equipped with spark-eliminating condensers around its contacts and is of heavy construction throughout in order to carry properly the sending current4 ,voltage4 ,the voltage of the sending battery is dependent on the length of the line and the number of stations upon it4 ,it ranges from #ajj to #cjj volts in most cases4 ,when higher voltages are required in order successfully to operate the circuit1 it is generally customary to install a telegraph repeater circuit at the center of the line1 in order to keep the voltage within safe limits4 ,one reason for limiting the voltage employed is that the condensers used in the circuit will not stand much higher potentials without danger of burning out4 ,it is also possible to halve the voltage by placing the dispatcher in the center of the line1 from which position he may signal in two directions instead of from one end4 ,simultaneous ,talking and ,signaling4 ,retardation coils and condensers will be noticed in series with the circuit through which the signaling current must pass before going out on the line4 ,these are for the purpose of absorbing the noise which is caused by high-voltage battery1 thus enabling the dispatcher to talk and signal simultaneously4 ,the #bej-ohm resistance connected across the circuit through one back contact of the telegraph relay absorbs the discharge of the #f-microfarad condenser4 ,illustration3 ,fig4 #dhi4 ,selector ,set--,western ,electric ,system ,waystation ,circuit4 ,the complete selector set for the waystations is shown in ,fig4 #dhi1 and the wiring diagram of its apparatus in ,fig4 #dij4 ,the first impulse sent out by the key in the dispatcher's office is a long direct-current impulse1 the first tooth being three or four times as wide as the other teeth4 ,this impulse operates both magnets of the selector and attracts their armatures1 which1 in turn1 cause two pawls to engage with the ratchet wheel1 while the remaining quick impulses operate the stepping-up pawl and rotate the wheel the requisite number of teeth4 ,retardation coils are placed in series with the selector in order to choke back any lightning discharges which might come in over the line4 ,the selector contact1 when operated1 closes a bell circuit1 and it will be noted that both the selector and the bell are operated from battery current coming over the main line through variable resistances4 ,there are1 of course1 a number of selectors bridged across the circuit1 and the variable resistance at each station is so adjusted as to give each approximately #aj milliamperes1 which allows a large factor of safety for line leakage in wet weather4 ,the drop across the coils at #aj milliamperes is #ch volts4 ,if these coils were not employed1 it is clear that the selectors nearer the dispatcher would get most of the current and those further away very little4 ,illustration3 ,fig4 #dij4 ,selector ,set--,western ,electric ,system ,a time-signal contact is also indicated on the selector-circuit diagram of ,fig4 #dij4 ,this is common to all offices and may be operated by a special key in the dispatcher's office1 thereby enabling him to send out time signals over the telephone circuit4 ,illustration3 ,fig4 #dia4 ,gill ,dispatcher's ,station ,gill ,circuits4 ,the circuit arrangement for the dispatcher's outfit of the ,gill system is shown in ,fig4 #dia4 ,this is similar to that of the ,western ,electric system just described4 ,the method of operation also is similar1 the mechanical means of accomplishing the selection being the main point of difference4 ,in ,fig4 #dib the wiring of the ,gill selector at a waystation for local-battery service is shown4 ,the selector contact closes the bell circuit in the station and a few windings of this circuit are located on the selector magnets1 as shown4 ,these provide the answer-back by inductive means4 ,illustration3 ,fig4 #dib4 ,gill ,selector--,local ,battery ,fig4 #dic shows the wiring of the waystation1 central-energy ,gill selector4 ,in this case1 the local battery for the operation of the bell is omitted and the bell is rung1 as is the case of the ,western ,electric selector1 by the main sending battery in the dispatcher's office4 ,illustration3 ,fig4 #dic4 ,gill ,selector--,central ,energy ,the sending keys of these two types of circuits differ1 in that with the local-battery selector the key contact is open after the selector has operated1 and the ringing of the bell must be stopped by the dispatcher pressing a button or calling another station4 ,either of these operations sends out a new current impulse which releases the selector and opens its circuit4 ,with the central-energy selector1 however1 the contacts of the sending key at the dispatcher's office remain closed after operation for a definite length of time4 ,this is obviously necessary in order that battery may be kept on the line for the operation of the bell4 ,in this case the contacts remain closed during a certain portion of the revolution of the key1 and the bell stops ringing when that portion of the revolution is completed4 ,if1 however1 the dispatcher desires to give any station a longer ring1 he may do so by keeping the key contacts closed through an auxiliary strap key as soon as he hears the answer-back signal from the called station4 ,cummings-,wray ,circuits4 ,the ,cummings-,wray system1 as previously stated1 is of the multiple-call type1 operating with synchronous clocks4 ,instead of operating one key after another in order to call a number of stations1 all the keys are operated at once and a starting key sets the mechanism in motion which calls all these stations with one operation4 ,fig4 #did shows the circuit arrangement of this system4 ,illustration3 ,fig4 #did4 ,cummings-,wray ,system ,in order to ring one or more stations1 the dispatcher presses the corresponding key or keys and then operates the starting key4 ,this starting key maintains its contact for an appreciable length of time to allow the clock mechanism to get under way and get clear of the releasing magnet clutch4 ,closing the starting key operates the clock-releasing magnet and also operates the two telegraph-line relays4 ,these send out an impulse of battery on the line operating the bridged #b1ejj-ohm line relays and1 in turn1 the selector releasing magnets2 thus1 all the waystation clocks start in unison with the master clock4 ,the second hand arbor of each clock carries an arm1 which at each waystation is set at a different angle with the normal position than that at any other station4 ,each of these arms makes contact precisely at the moment the master-clock arm is passing over the contact corresponding to that station4 ,if1 now1 a given station key is pressed in the master sender1 the telegraph-line relays will again operate when the master-clock arm reaches that point1 sending out another impulse of battery over the line4 ,the selector contact at the waystation is closed at this moment2 therefore1 the closing of the relay contact operates the ringing relay through a local circuit1 as shown4 ,the ringing relay is immediately locked through its own contact1 thus maintaining the bell circuit closed until it is opened by the key and the ringing is stopped4 ,as the master-clock arm passes the last point on the contact dial1 the current flows through the restoring relay operating the restoring magnet which releases all the keys4 ,a push button is provided by means of which the keys may be manually released1 if desired4 ,this is used in case the dispatcher presses a key by mistake4 ,retardation coils and variable resistances are provided at the waystation just as with the other selector systems which have been described and for the same reasons4 ,the circuits of the operator's telephone equipment shown in ,fig4 #die1 are also bridged across the line4 ,this apparatus is of high impedance and of a special design adapted to railroad service4 ,there may be any number of telephones listening in upon a railroad train wire at the same time1 and often a dispatcher calls in five or six at once to give orders4 ,these conditions have necessitated the special circuit arrangement shown in ,fig4 #die4 ,illustration3 ,fig4 #die4 ,telephone ,circuits ,the receivers used at the waystations are of high impedance and are normally connected1 through the hook switch1 directly across the line in series with a condenser4 ,when the operator1 at a waystation wishes to talk1 however1 he presses the key shown4 ,this puts the receiver across the line in series with the retardation coil and in parallel with the secondary of the induction coil4 ,it closes the transmitter battery circuit at the same time through the primary of the induction coil4 ,the retardation coil is for the purpose of preventing excessive side tone1 and it also increases the impedance of the receiver circuit1 which is a shunt on the induction coil4 ,this latter coil1 however1 is of a special design which permits just enough current to flow through the receiver to allow the dispatcher to interrupt a waystation operator when he is talking4 ,the key used to close the transmitter battery is operated by hand and is of a non-locking type4 ,in some cases1 where the operators are very busy1 a foot switch is used in place of this key4 ,the use of such a key or switch in practical operation has been found perfectly satisfactory1 and it takes the operators but a short time to become used to it4 ,the circuits of the dispatcher's office are similarly arranged1 ,fig4 #die1 being designed especially to facilitate their operation4 ,in other words1 as the dispatcher is doing most of the work on the circuit1 his receiver is of a low-impedance type1 which gives him slightly better transmission than the waystations obtain4 ,the key in his transmitter circuit is of the locking type1 so that he does not have to hold it in while talking4 ,this is for the reason that the dispatcher does most of the talking on this circuit4 ,foot switches are also employed in some cases by the dispatchers4 ,test ,boards4 ,it is becoming quite a general practice among the railroads to install more than one telephone circuit along their rights-of-way4 ,in many cases in addition to the train wire1 a message circuit is also equipped1 and quite frequently a block wire also operated by telephone1 parallels these two4 ,it is desirable on these circuits to be able to make simple tests and also to be able to patch one circuit with another in cases of emergency4 ,illustration3 ,fig4 #dif4 ,test ,board ,test boards have been designed for facilitating this work4 ,these consist of simple plug and jack boxes1 the general appearance of which is shown in ,fig4 #dif4 ,the circuit arrangement of one of these is shown in ,fig4 #dig4 ,each wire comes into an individual jack as will be noted on one side of the board1 and passes through the inside contact of this jack1 out through a similar jack on the opposite side4 ,the selector and telephone set at an office are taken off these inside contacts through a key1 as shown4 ,the outside contacts of this key are wired across two pairs of cords4 ,now1 assume the train wire comes in on jacks #a and #c1 and the message wire on jacks #i and #aa4 ,in case of an accident to the train wire between two stations1 it is desirable to patch this connection with a message wire in order to keep the all-important train wire working4 ,the dispatcher instructs the operator at the last station which he can obtain1 to insert plugs #a and #b in jacks #a and #aj1 and plugs #c and #d in jacks #c and #ab1 at the same time throwing the left-hand key4 ,then1 obtaining an operator beyond the break by any available means1 he instructs him likewise to insert plugs #a and #b in jacks #i and #b1 and plugs #c and #d in jacks #aa and #d1 similarly throwing the left-hand key4 ,by tracing this out1 it will be observed that the train wire is patched over the disabled section by means of the message circuit1 and that the selector and the telephone equipment are cut over on to the patched connections2 in other words1 bridged across the patching cords4 ,illustration3 ,fig4 #dig4 ,circuits of ,test ,board ,it will also be seen that with this board it is possible to open any circuit merely by plugging into a jack4 ,two wires can be short-circuited or a loop made by plugging two cords of corresponding colors into the two jacks4 ,a ground jack is provided for grounding any wire4 ,in this way1 a very flexible arrangement of circuits is obtained1 and it is possible to make any of the simple tests which are all that are usually required on this type of circuit4 ,blocking ,sets4 ,as was just mentioned1 quite frequently in addition to train wires and message circuits1 block wires are also operated by telephone4 ,in some cases separate telephone instruments are used for the blocking service1 but in others the same man handles all three circuits over the same telephone4 ,the block wire is generally a converted telegraph wire between stations1 usually of iron and usually grounded4 ,it seldom ranges in length over six miles4 ,illustration3 ,fig4 #dih4 ,blocking ,set ,where the block wires are operated as individual units with their own instruments1 it is unnecessary to have any auxiliary apparatus to be used in connection with them4 ,where1 however1 they are operated as part of a system and the same telephone is used on these that is used on the train wire and message wire1 additional apparatus1 called a blocking set1 is required4 ,this blocking set1 shown in ,figs4 #dih and #dii1 was developed especially for this service by the ,western ,electric ,company4 ,as will be noted1 a repeating coil at the top and a key on the front of the set are wired in connection with a pair of train wire cords4 ,this repeating coil is for use in connecting a grounded circuit to a metallic circuit1 as1 for instance1 connecting a block wire to the train wire1 and is1 of course1 for the purpose of eliminating noise4 ,below the key are three combined jacks and signals4 ,one block wire comes into each of these and a private line may be brought into the middle one4 ,when the next block rings up1 a visual signal is displayed which operates a bell in the office by means of a local circuit4 ,the operator answers by plugging the telephone cord extending from the bottom of the set into the proper jack4 ,this automatically restores the signal and stops the bell4 ,illustration3 ,fig4 #dii4 ,blocking ,set ,below these signals appear four jacks4 ,one is wired across the train wire2 one across the message wire2 and the other two are bridged across the two pairs of patching cords on each side of the set4 ,the operator answers a call on any circuit by plugging his telephone cord into the proper jack4 ,if a waystation is not kept open in the evening1 or the operator leaves it for any reason and locks up1 he can connect two blocks together by means of the block-wire cords4 ,these are arranged simply for connecting two grounded circuits together and serve to join two adjacent blocks1 thereby eliminating one station4 ,a jack is wired across these cords1 so that the waystation operator can listen in on the connection if he so desires4 ,in some cases not only are the telephone circuits brought into the test board1 but also two telegraph wires are looped through this board before going to the peg switchboard4 ,this is becoming quite a frequent practice and1 in times of great emergency1 enables patches to be made to the telegraph wires as well as to the telephone wires4 ,dispatching on ,electric ,railways4 ,as interurban electric railways are becoming more extended1 and as their traffic is becoming heavier1 they approximate more closely to steam methods of operation4 ,it is not unusual for an electric railway to dispatch its cars exactly as in the case of a steam road4 ,there is a tendency1 however1 in this class of work1 toward slightly different methods1 and these will be briefly outlined4 ,on those electric railways where the traffic is not especially heavy1 an ordinary magneto telephone line is frequently employed with standard magneto instruments4 ,in some cases the telephone sets are placed in waiting rooms or booths along the line of the road4 ,in other cases it is not feasible to locate the telephone indoors and then iron weather-proof sets1 such as are shown in ,figs4 #dhd and #dhe1 are mounted directly on the poles along the line of railway4 ,with a line of this character there is usually some central point from which orders are issued and the trainmen call this number when arriving at sidings or wherever they may need to do so4 ,another method of installing a telephone system upon electric railways is as follows3 ,instead of instruments being mounted in booths or on poles along the line1 portable telephone sets are carried on the cars and jacks are located at regular intervals along the right-of-way on the poles4 ,the crew of the car wishing to get in touch with the central office or the dispatcher1 plugs into one of these jacks and uses the portable telephone set4 ,at indoor stations1 in offices or buildings belonging to the railroad1 the regular magneto sets may be employed1 as in the first case outlined4 ,on electric railway systems where the traffic is heavy1 the train or car movements may be handled by a dispatcher just as on the steam railroad4 ,there is usually one difference1 however4 ,on a steam road1 the operators who give the train crews their orders and manipulate the semaphore signals are located at regular intervals in the different waystations4 ,no such operators are usually found on electric railways1 except1 perhaps1 at very important points1 and1 therefore1 it is necessary for the dispatcher to be able to signal cars at any point and to get into communication with the crews of these cars4 ,he does this by means of semaphores operated by telephone selectors over the telephone line4 ,the telephone circuit may be equipped with any number of selectors desired1 and the dispatcher can operate any particular one without operating any other one on the circuit4 ,each selector1 when operated1 closes a pair of contacts4 ,this completes a local circuit which throws the semaphore arm to the danger position1 at the same time giving the dispatcher a distinctive buzz in his ear1 which informs him that the arm has actually moved to this position4 ,he can get this signal only by the operation of the arm4 ,each semaphore is located adjacent to a telephone booth in which is also placed the restoring lever1 by means of which the semaphore is set in the clear position by the crew of the car which has been signaled4 ,the wall-type telephone set is usually employed for this class of service1 but if desired1 desk stands or any of the various transmitter arms may be used4 ,it is necessary for the crew of the car which first approaches a semaphore set at danger1 to get out1 communicate with the dispatcher1 and restore the signal to the clear position4 ,the dispatcher can not restore the signal4 ,the signal is set only in order that the train crew may get into telephonic communication with the dispatcher1 and in order to do this1 it is necessary for them to go into the booth in any case4 ,footnote ,a3 ,we wish particularly to acknowledge the courtesy of the ,western ,electric ,company in their generous assistance in the preparation of this chapter4 ,,review ,,questions ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #aa--fh 99 99 99 99 99 #a4 ,what are the advantages of a common-battery system8 #b4 ,when is the local battery to be preferred to the common-battery8 #c4 ,enumerate the different kinds of line signals4 #d4 ,make a diagram of the arrangement of a direct line lamp signal4 #e4 ,what is a direct line lamp with ballast8 ,give sketch4 #f4 ,describe a line lamp with relay4 #g4 ,what is a pilot lamp and what are its functions8 #h4 ,sketch three different kinds of batteries applied to cord circuits4 #i4 ,what is a supervisory signal8 #aj4 ,make diagram of a complete simple common-battery switchboard circuit4 #aa4 ,when will the supervisory signal become operative8 #ab4 ,what is the candle-power of incandescent lamps used for line and supervisory signals8 #ac4 ,at what voltages do they operate8 #ad4 ,what are visual signals8 #ae4 ,describe the mechanical signal of the ,western ,electric ,company4 #af4 ,give a short description of the general assembly of the parts of a simple common-battery switchboard4 #ag4 ,what is a transfer switchboard8 #ah4 ,outline the limitations of a simple switchboard4 #ai4 ,describe and sketch a plug-ended transfer line4 #bj4 ,why is the plug-seat switch not more widely adopted for use8 #ba4 ,make diagram of an order-wire arrangement4 #bb4 ,what are the limitations of the transfer system8 #bc4 ,what are the fundamental features of the multiple switchboard8 #bd4 ,what is a multiple jack8 #be4 ,what is an answering jack8 #bf4 ,make a diagram showing the principle of multiple switchboards4 #bg4 ,what is the busy signal8 #bh4 ,what determines the size of a multiple switchboard8 #bi4 ,what is the use of the intermediate distributing frame8 #cj4 ,make diagram of the series magneto multiple switchboard and describe its operation4 #ca4 ,what are the defects of this system8 #cb4 ,give a diagram of the branch terminal magneto multiple switchboard4 #cc4 ,give a diagram and a short description of the ,monarch magneto multiple switchboard4 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #fi--acd 99 99 99 99 99 #a4 ,sketch and describe the line circuit of the common-battery multiple switchboard of the ,bell companies4 #b4 ,make a diagram of the cord circuit of the ,western ,electric standard multiple common-battery switchboard4 #c4 ,describe the busy test in this system4 #d4 ,what is the function of the order-wire circuits8 #e4 ,what is jumper wire8 #f4 ,give a short description of the relay mounting in the standard ,no4 #a relay board of the ,western ,electric ,company4 #g4 ,what is the ultimate capacity of the ,no4 #a ,western ,electric switchboard8 #h4 ,what is the capacity of the ,no4 #aj ,western ,electric switchboard8 #i4 ,how does this switchboard ,no4 #aj differ from ,no4 #a8 #aj4 ,give a diagram of the two-wire line circuit of the ,kellogg ,company4 #aa4 ,what is the capacity of the condenser of the cord circuit in the foregoing system8 #ab4 ,give a complete diagram of the ,kellogg two-wire board4 #ac4 ,describe the busy test in this system4 #ad4 ,give diagram of the ,stromberg-,carlson multiple-board circuit4 #ae4 ,what is the most important piece of apparatus in a multiple switchboard8 #af4 ,what is the spacing of the multiple jacks in the ,no4 #a ,western ,electric switchboard8 #ag4 ,how do the relays of the ,western ,electric ,company differ from those of other companies8 #ah4 ,describe the relay construction of the ,monarch ,telephone ,company4 #ai4 ,what is meant by inter-office trunking8 #bj4 ,what is the present practice in ,america as to the capacity of multiple hoards8 #ba4 ,what is the tendency in ,europe regarding the capacity of multiple boards8 #bb4 ,discuss the preferences in ,american practice4 #bc4 ,state the different methods of trunking between exchanges4 #bd4 ,when are two-way trunks employed8 #be4 ,make diagram of the ,western ,electric inter-office connection system4 #bf4 ,describe the standard four-party line trunk ringing key of the ,western ,electric ,company4 #bg4 ,sketch and describe a keyless trunk4 #bh4 ,give diagram of the inter-office connection of the ,kellogg system4 #bi4 ,how does this system differ from the ,western ,electric in regard to the ringing8 #cj4 ,why are the ,a and ,b switchboards in large exchanges entirely separated8 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #ace--bbf 99 99 99 99 99 #a4 ,what is the general object of automatic telephone systems8 #b4 ,what are the common arguments against these systems and how are they met8 #c4 ,give the operations that the calling subscriber has to go through in any one of the successful systems4 #d4 ,during calling what is happening at the central office8 #e4 ,describe the action of the ,strowger or ,automatic ,electric ,company selecting switch4 #f4 ,what is the function of a line switch8 #g4 ,describe the ,strowger scheme of trunking and illustrate its action by diagram4 #h4 ,make a diagram of the sub-station apparatus and connections4 #i4 ,make a diagram of the line switch unit4 #aj4 ,describe the action of the various guarding features necessary to protect a busy line4 #aa4 ,make a simple diagram of the circuits of the first selector4 #ab4 ,give the functions and operations of the connector4 #ac4 ,give a diagram of connecting circuits4 #ad4 ,tell all you can regarding the battery supply to the connected subscriber4 #ae4 ,how are subscribers disconnected after they are through talking8 #af4 ,describe a multi-office system4 #ag4 ,give a diagram of circuits of the trunk repeater4 #ah4 ,make a complete diagram of the connections between a calling and a called subscriber in an automatic system4 #ai4 ,what is the rotary connector8 #bj4 ,describe the sub-station equipment of the ,lorimer automatic system4 #ba4 ,describe the ,lorimer central-office apparatus4 #bb4 ,give a description of the progress of a call from its institution to the final disconnection in the ,lorimer system4 #bc4 ,what is the automanual system8 #bd4 ,give general features of the operation in the automanual system4 #be4 ,describe the automanual system subscribers' apparatus4 #bf4 ,give a description of the automanual central-office equipment4 ,,review ,,questions ,,on ,,subject ,,of ,,telephony ,,pages #bbg--bgj 99 99 99 99 99 #a4 ,what kinds of currents are employed8 #b4 ,what types of power plants are used8 #c4 ,describe the sources of current supplied for the operator's transmitter current and ringing current4 #d4 ,make a diagram of the ,warner pole changer4 #e4 ,make a diagram of pole changers for harmonic ringing4 #f4 ,what is a multi-cyclic generator set8 #g4 ,make a diagram of governor for harmonic ringing generators4 #h4 ,describe the various primary sources of power4 #i4 ,make a diagram of the mercury-arc-rectifier circuits4 #aj4 ,what provision against breakdown is made8 #aa4 ,tell all you can about the storage battery--its construction and its operation4 #ab4 ,what is a pilot cell8 #ac4 ,describe the switches1 meters1 and protective devices used on the power switchboard4 #ad4 ,give a diagram showing a typical example of a common-battery manual switchboard equipment and circuits4 #ae4 ,give the main points concerning the construction of a central-office building4 #af4 ,what provision should be made for cable runways8 #ag4 ,make a sketch of a small central-office floor plan4 #ah4 ,describe the ,western ,electric main and intermediate frames4 ,give diagrams4 #ai4 ,give principal points regarding small office terminal apparatus4 #bj4 ,give types of line circuits4 #ba4 ,describe the typical equipment of a large manual office4 ,give floor plans4 #bb4 ,give floor plan of an automatic office4 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #bga--cbj 99 99 99 99 99 #a4 ,what is a private-branch exchange8 #b4 ,what does ,p4 ,b4 ,x4 mean8 #c4 ,what is the function of the private-branch exchange operator8 #d4 ,describe the key type of a small private-branch exchange switchboard4 #e4 ,describe the different methods of supervision of private-branch connections4 #f4 ,describe the automatic equipment of the common-battery type in private-branch exchanges4 #g4 ,how is secrecy of individual lines obtained in a private-exchange equipment8 #h4 ,what is an intercommunicating system8 #i4 ,sketch a magneto intercommunicating system4 #aj4 ,sketch and describe a plug type common-battery intercommunicating system4 #aa4 ,sketch and describe the action of the push button in the ,monarch system and in the ,western ,electric system4 #ab4 ,sketch and describe the ,monarch intercommunicating system4 #ac4 ,what is the office of the junction box in this system8 #ad4 ,what is a long-distance message8 #ae4 ,what is the function of the repeating coil in the long-distance line8 #af4 ,which is the simplest form of long-distance switch8 #ag4 ,what is a phantom circuit8 #ah4 ,under what control is the ringing of the subscriber in long-distance calls8 #ai4 ,what is meant by ticket passing8 #bj4 ,what particular advantage has a common-battery set on long-distance lines8 #ba4 ,give a typical load curve for telephone traffic4 #bb4 ,why is traffic a study of importance8 #bc4 ,state the function of the intermediate distributing frame4 #bd4 ,state the different methods of traffic study4 #be4 ,what is the trunking factor8 #bf4 ,define trunking efficiency4 #bg4 ,enumerate some of the elements upon which the quality of service in a manual system depends4 #bh4 ,what is team work8 #bi4 ,how does the cost of telephone service vary8 #cj4 ,what two general methods of charging for telephone service are in use8 #ca4 ,describe a calculagraph and how is it used8 #cb4 ,how are toll connections timed by the ,monarch ,telephone ,company8 #cc4 ,sketch and describe the ,western ,electric ,company line circuit and service meter4 ,,review ,,questions ,,on ,,the ,,subject ,,of ,,telephony ,,pages #cba--ceh 99 99 99 99 99 #a4 ,describe a phantom circuit with diagram4 #b4 ,explain how two phantoms may be joined by a physical circuit4 #c4 ,which are the better1 phantom or physical circuits1 and why8 #d4 ,explain how the simplex circuit differs from the phantom telephone circuit4 #e4 ,why are not telegraph wires as serviceable for telephone work as telephone wires are for telegraph work8 #f4 ,give the names of the different parts of a railway composite set and explain method of operating4 #g4 ,state the causes of the introduction of the telephone into the train dispatching field and explain the advantages it has over the telegraph for this work4 #h4 ,in transmitting orders for train dispatching1 how are mistakes avoided8 #i4 ,describe the ,western ,electric selector and explain its use4 #aj4 ,in what way does the ,gill selector differ from the ,western ,electric8 #aa4 ,what special feature does the multiple coil selector possess8 #ab4 ,what special arrangement is provided for the train dispatcher in noisy locations8 #ac4 ,how can a man on a wrecking train get connection with the train dispatcher8 #ad4 ,what is the usual limit in length of a telephone train dispatching circuit and what is the largest number of stations at present existing on such a circuit8 #ae4 ,what is the voltage of the sending battery for a train dispatcher's circuit and upon what is it dependent8 #af4 ,for what purpose is a repeater circuit used8 #ag4 ,how is the noise caused by a high voltage battery absorbed so that the dispatcher may talk and signal simultaneously8 #ah4 ,draw a diagram showing the circuit arrangement for the dispatcher's outfit of the ,gill system4 #ai4 ,explain fully the purpose of the retardation coil in connection with a waystation set4 #bj4 ,in case of accident to a train wire between two stations1 how can the connection be patched if the road is also equipped with a message circuit in addition to the train wire8 #ba4 ,why do some railroads have block wires in addition to train wires and message circuits8 #bb4 ,if a waystation on a block wire is to be cut out for any length of time1 by what method can the two adjacent blocks be connected1 eliminating the station between8 #bc4 ,what are some of the methods used for dispatching on electric railways where the traffic is not especially heavy8 #bd4 ,on an electric road in case a car approaches a semaphore set at danger1 what must the crew of the car do8 ,,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 ,automanual system #bah automatic distribution of calls #bbc automatic switching equipment #bbb building up a connection #bbd characteristics of #bah operation #bai operator's equipment #bbj setting up a connection #bbd speed in handling calls #bbd subscriber's apparatus #bai ,automatic desk stand #aeh ,automatic ,electric ,company's telephone system #adi automatic sub-offices #bja connector #ahe function of #ahe location of #ahf operation of #ahf first selector operation #agi function of line switch #aeb line switch #aec1 #afc bridge cut-off #agc circuit operations #afg guarding functions #agc line and trunk contacts #afd locking segment #agb master switch #aga relation of1 to connectors #agd structure of #aff summary of operation #agd trunk ratio #afe trunk selection #afe multi-office system #aif party lines #bjb release after conversation #aif rotary connector #bjb second selector operation #ahb selecting switches #aec1 #age release mechanism #agh side switch #age subdivision of subscribers' lines #aeb subscribers' station apparatus #aeh operation #afj bell and transmitter springs #afj ground springs #afj impulse springs #afa release springs #afc ringing springs #afc salient points #afc trunking #aed connector action #aeg first selector action #aef line switch action #aed second selector action #aef two-wire automatic systems #bjc two-wire and three-wire systems #aeg underlying feature of trunking system #aec ,automatic telephone systems #ace arguments against #ace attitude of public #ada complexity #acf expense #adj flexibility #adj subscriber's station equipment #adb automatic vs4 manual #adc comparative costs #adb definition #ace methods of operation #adc fundamental idea #adg grouping of subscribers #ade local and inter-office trunks #adh ,lorimer system #add magnet vs4 power-driven switches #add ,automatic telephone systems methods of operation multiple vs4 trunking #ade outline of action #adf ,strowger system #adc testing #adh trunking between groups #ade ,automatic wall set #aeh ,b ,blocking sets #cee ,busy test #dh busy-test faults #ej potential of test thimbles #di principle #di ,c ,circuits #cba applications #cbb composite #cbf phantom #cba transmission over #cbd transpositions #cbc railway composite #cbg ringing #cbg simplex #cbd ,common-battery multiple switchboard #fi assembly #ajf ,dean multiple board #ic cord circuit #id line circuit #ic listening key #id ringing keys #id test #id ,kellogg two-wire multiple board #hd battery feed #hh busy test #ij complete cord and line circuit #hh cord circuit #hf line circuit #he summary of operation #ia supervisory signals #hg wiring of line circuit #ib multiple switchboard apparatus #ig jacks #ii lamp jacks #ajj relays #aja ,stromberg-,carlson multiple board #if cord circuit #if supervisory signals #ig test #ig ,western ,electric ,no4 #a relay board #fi capacity range #hj cord circuit #ga functions of distributing frames #gg line circuit #fi modified relay windings #gi operation #gb operator's circuit detail #ge order-wire circuits #gh pilot signals #gi relay mounting #hj testing--called line busy #ge testing--called line idle #gd wiring of line circuit #gf ,western ,electric ,no4 #aj board #hj circuits #ha economy #hd operation #hc test #hc ,common-battery switchboard #aa advantages of operation #aa common battery vs4 magneto #ab cord circuit #bj battery supply #bj complete circuit #ba supervisory signals #ba cycle of operations #bc jacks #cj lamps #bd mounting #be line signals #ad direct-line lamp #ad direct-line lamp with ballast #ae line lamp with relay #ag pilot signals #ag mechanical signals #bg ,kellogg #bh ,monarch #bh ,western ,electric #bg relays #bh switchboard assembly #ca ,composite circuits #cbf ,connector #ahe ,cord circuit #bj ,cord circuit battery supply #bj complete circuit #ba supervisory signals #ba ,cord-rack connectors #ff ,cummings-,wray selector #cdb ,d ,dean multiple board #ic ,dispatchers' keys #cci ,dispatching on electric railways #cef ,g ,gill selector #cda ,h ,housing central-office equipment #bdi arrangement of apparatus in small manual offices #beb combined main and intermediate frames #bec floor plans for #beb types of line circuits #bee automatic offices #bfg typical automatic office #bgj central-office building #bdi fire hazard #bdi provision for cable runways #bea provision for employes #bea size of building #bej strength of building #bej large manual office #bef ,i ,intercommunicating systems #bhb common-battery systems #bhc ,kellogg plug type #bhd ,kellogg push-button type #bhe ,monarch system #bhg ,western ,electric system #bhe definition #bhb limitations #bhb for private-branch exchanges #bij simple magneto system #bhb ,j ,jacks #cj ,k ,kellogg mechanical signal #bh ,kellogg trunk circuits #abe ,kellogg two-wire multiple board #hd ,keyboard wiring #fg ,l ,lamp mounting #be ,lamps #bd ,line signals #ad direct-line lamp #ad direct-line lamp with ballast #ae line lamp with relay #ag pilot signals #ag ,line switch #afc ,long-distance switching #bic definitions #bic center-checking #big operators' orders #bid by call circuits #bid by telegraph #bid particular party calls #bie switching through local board #bic ticket passing #bif trunking #bie high-voltage toll trunks #bie through ringing #bie two-number calls #bid use of repeating coil #bic waystations #big ,lorimer automatic system #add1 #bje central-office apparatus #bjh connective division #baj sectional apparatus #bji switches #bac interconnector #bad interconnector selector #bad primary connector #bac rotary switch #bac secondary connector #bad signal transmitter controller #bad operation #bae subscriber's station equipment #bjf ,m ,magneto multiple switchboard #ec branch-terminal multiple board #eh arrangement of apparatus #fa magnet windings #fa operation #fj field of utility #ec modern magneto multiple board #fc assembly #ff cord circuit #fd test #fb ,magneto multiple switchboard series-multiple board #ed defects #eg operation #ef ,measured service #caj local service #caf meter method #caf prepayment method #cah ticket method #caf rates #caj toll service #caa long haul #caa short haul #caa timing toll connections #cab units of charging #caa ,mechanical signals #bg ,kellogg #bh ,monarch #bh ,western ,electric #bg ,mercury-arc rectifier circuits #bcg ,monarch visual signal #bh ,multi-office exchanges1 necessity for #aji ,multiple switchboard #dc busy test #dh cord circuits #df diagram showing principle of #dg double connections #df field of each operator #ea field of utility #dc influence of traffic #eb line signals #de multiple feature #dc ,p ,phantom circuit #cba ,pilot signals #ag ,plug-seat switch #ch ,pole changers for harmonic ringing #bca ,power plants #bbg auxiliary signaling currents #bcc currents employed #bbg alternating current #bbg direct current #bbg operator's transmitter supply #bbh power plant circuit #bdh power switchboard #bdf meters #bdf protective devices #bdh switches #bdf primary sources #bcd charging from direct-current mains #bcd charging dynamos #bce mercury-arc rectifiers #bcf rotary converters #bcd provision against breakdown #bcg capacity of power units #bch duplicate charging machines #bch duplicate primary sources #bch duplicate ringing machines #bch ringing-current supply #bbi magneto generators #bbi pole changers #bbi ringing dynamos #bcb storage battery #bci initial charge #bda installation #bdj low cells #bdd operation #bdb overcharge #bdc pilot cell #bdc regular charge #bdd replacing batteries #bde sediment #bde types #bbg common-battery systems #bbh magneto systems #bbh ,power switchboard #bdf ,private-branch exchanges #bga with automatic offices #bgh secrecy #bgi battery supply #bgi circuits1 key-type board #bgf definitions #bga desirable features #bha functions of the private-branch exchange operator #bgb marking of apparatus #bha private-branch switchboards #bgc common-battery type #bgc cord type #bge key type #bge magneto type #bgc ringing current #bhj supervision of private-branch connections #bgg ,r ,relays #bh ,rotary connector #bjb ,s ,selecting switches #age ,selector #age ,simplex circuits #cbd ,storage battery #bci ,storage cell #bdj ,stromberg-,carlson multiple board #if ,strowger automatic system #adc ,subscribers' board #bei-bfa ,switchboard assembly #ca ,t ,table automanual system time data #bbe automatic systems1 messages per trunk in #cje calling rates #cjb long-distance groups1 messages per trunk in #cje manual system1 messages per trunk in #cjd out-trunking1 effect of1 on operator's capacity #cjc subscribers' waiting time #bbf ,telephone traffic #bih importance of traffic study #cjj methods of traffic study #cja observation of service #cjh quality of service #cje accuracy and promptness #cjg answering time #cjf busy and don't answer calls #cjg courtesy and form #cjg disconnecting time #cjf enunciation #cjh team work #cjh rates of calling #cjj representative traffic data #cjb calling rates #cjb operators' loads #cjb toll traffic #cjd trunk efficiency #cjc trunking factor #cjc traffic variations #bih busy hour ratio #bii unit of traffic #bih ,telephone train dispatching #ccc advantages #cce apparatus #cch ,cummings-,wray selector #cdb dispatcher's transmitter #cdc ,gill selector #cda portable train sets #cde siding telephones #cde waystation telephones #cdd ,western ,electric selector #cch blocking sets #cee causes of its introduction #ccd ,cummings-,wray circuits #cej on electric railways #cef ,gill circuits #cdi railroad conditions #ccg rapid growth #ccc test boards #cec transmitting orders #ccg waystation circuits #cdh ,western ,electric circuits #cdg ,telephone train-dispatching circuit ,cummings-,wray #cej ,gill #cdi waystation #cdh ,western ,electric #cdg ,test boards #cec ,transfer switchboard #cd field of usefulness #da handling transfers #ch limitations #dj plug-seat switch #ch transfer lines #ce jack-ended trunk #ce plug-ended trunk #cg ,trunking in multi-office systems #aji classification #aab one-way trunks #ajc two-way trunks #aab ,kellogg trunk circuits #abe necessity for exchanges #aji ,western ,electric trunk circuits #aaf ,w ,warner pole changer #bcj ,waystation telephones #cdd ,western ,electric mechanical signal #bg selector #cch trunk circuits #aaf ,transcriber's ,notes4 ,spelling variants where it wasn't possible to determine the author's intent were left as is4 ,these include3 clockwork and clock-work2 doorkeeper and door-keeper2 interrelation and inter-relation2 multicyclic and multi-cyclic2 redesign and re-design1 along with derivatives4 ,added closing double quote in ,steinmetz entry in list of authorities3 ,theoretical ,elements of ,electrical ,engineering4 ,changed switch-hook to switch hook on page #hh3 the subscriber's switch hook4 ,page #ajg says there is room for #cjj banks of #ajj multiple jacks1 but then says this allows for #c1jjj multiple jacks in all1 rather than #cj1jjj4 ,based on the figure1 #cjj banks should be #cj banks1 which would correct the arithmetic4 ,however1 ,i did not change this4 ,changed bi-paths to by-paths on page #ahe3 circuits or by-paths4 ,changed appararus to apparatus on page #bji3 ,the sectional apparatus4 ,changed two number to two-number on page #cab3 the two-number calls are ticketed4 ,on page #ccc1 a paragraph begins with ,it has been only within the past three few4 ,perhaps the author meant ,it has been only within the past three years or ,it has been only within the past few years4 ,but since ,i didn't know1 ,i left is as is4 ,changed them ain to the main on page #ccc3 on the main line4 ,changed weatherproof to weather-proof on page #ceg3 iron weather-proof sets4 ,changed interoffice to inter-office three times on page #cfd1 to match the spelling in the body of the document3 meant by inter-office trunking2 inter-office connection system2 of the inter-office connection4 ,changed break-down to breakdown on page #cfg3 provision against breakdown4 ,changed way-station to waystation twice on page #cgb3 with a waystation set2 and a waystation on a block wire4 ,changed way stations to waystations on page #cge1 in the entry for ,long-distance switching4 ,each page of the ,index repeated this text3 ,note4--,for page numbers see foot of pages4 ,they were removed4