,this file was produced from page images at ,google ,books4 ,transcriber's ,note ,this book was transcribed from scans of the original found at ,google ,books4 ,i have rotated some images4 ,the more complex tables are treated as images4 ,illustration3 ,a ,,boy',s ,,wireless ,,outfit ,,made ,,up ,,of ,,the ,,apparatus ,,described ,,in ,,chapter ,,xiv4 ,,the ,,junior ,,dynamo ,,and ,a ,,coherer ,,outfit ,,can ,,be ,,seen ,,on ,,the ,,lower ,,part ,,of ,,the ,,table4 ,the ,boy ,electrician ,practical ,plans for ,electrical ,apparatus for work and play1 with an explanation ,of the principles of every-day electricity4 ,by ,,alfred ,p ,,morgan ,with illustrations by the author ,,boston ,,lothrop1 ,,lee ,,shepard ,,co4 ,copyright1 #aiac1 by ,lothrop1 ,lee ,shepard ,co4 ,entered at ,stationers' ,hall1 ,london ,published ,july1 #aiad ,all rights reserved ,,the ,,boy ,,electrician ,,norwood ,,press ,berwick ,smith ,co4 ,norwood1 ,mass4 ,u4,s4,a4 ,,to ,,the ,,self-,,reliant 99,,boys ,,of ,,america199 ,,our ,,future ,,engineers ,,and ,,scientists1 ,,than ,,whom ,,none ,,in ,,the ,,whole ,,world ,,are ,,better ,,able ,,to ,,work ,,out ,,and ,,solve ,,the ,,problems ,,that ,,ever ,,confront ,,young ,,manhood1 ,,this ,,book ,,is ,,cordially ,,dedicated4 ,,the ,,boy ,,electrician ,,introduction ,once upon a time1 and this is a true tale1 a boy had a whole railroad system for a toy4 ,the train ran automatically1 propelled by tiny electric motors1 the signals went up and down1 the station was reached1 a bell rang1 the train moved on again and was off on its journey around many feet of track to come back over the old route4 ,the boy viewed his gift with raptured eyes1 and then his face changed and he cried out in the bitterness of his disappointment3 ,but what do ,i do8 ,the toy was so elaborate that the boy was left entirely out of the play4 ,of course he did not like it4 ,his cry tells a long story4 ,the prime instinct of almost any boy at play is to make and to create4 ,he will make things of such materials as he has at hand1 and use the whole force of dream and fancy to create something out of nothing4 ,the five-year-old will lay half a dozen wooden blocks together with a spool on one end and tell you it is a steam train4 ,and it is4 ,he has both made and created an engine1 which he sees but which you don't1 for the blocks and spool are only a symbol of his creation4 ,give his older brother a telephone receiver1 some wire and bits of brass1 and he will make a wireless telegraph outfit and listen to a steamship hundreds of miles away spell out its message to the shore4 ,the wireless outfit is not a symbol1 but something that you can both hear and see in operation even though you may not understand the whispering of the dots and dashes4 ,and as soon as the mystery of this modern wonder more firmly grips your imagination1 you perhaps may come to realize that we are living more and more in the age of electricity and mechanism4 ,electricity propels our trains1 lights our houses and streets1 makes our clothes1 cures our ills1 warms us1 cooks for us and performs an innumerable number of other tasks at the turning of a little switch4 ,a mere list is impossible4 ,almost every boy experiments at one time or another with electricity and electrical apparatus4 ,it is my purpose in writing this book to open this wonderland of science and present it in a manner which can be readily understood1 and wherein a boy may do something4 ,of course there are other books with the same purport1 but they do not accomplish their end4 ,they are not practical4 ,i can say this because as a boy ,i have read and studied them and they have fallen far short of teaching me or my companions the things that we were seeking to learn4 ,if they have failed in this respect1 they have done so perhaps not through any inability of the author1 but from the fact that they have not been written from the boy's standpoint4 ,they tell what the author thought a boy ought to know but not what he really does want to know4 ,the apparatus described therein is for the most part imaginary4 ,the author thought it might be possible for a boy to build motors1 telegraph instruments1 etc41 out of old bolts and tin cans1 but he never tried to do so himself4 ,the apparatus and experiments that ,i have described have been constructed and carried out by boys4 ,their problems and their questions have been studied and remedied4 ,i have tried to present practical matter considered wholly from a boy's standpoint1 and to show the young experimenter just what he can do with the tools and materials in his possession or not hard to obtain4 ,to the boy interested in science1 a wide field is open4 ,there is no better education for any boy than to begin at the bottom of the ladder and climb the rungs of scientific knowledge1 step by step4 ,it equips him with information which may prove of inestimable worth in an opportune moment4 ,there is an apt illustration in the boy who watched his mother empty a jug of molasses into a bowl and replace the cork4 ,his mother told him not to disturb the jug1 as it was empty4 ,he persisted1 however1 and turned the jug upside down4 ,no more molasses came1 but out crawled a fly4 ,new developments in science will never cease4 ,invention will follow invention4 ,the unexpected is often a valuable clue4 ,the ,edisons and ,teslas of to-day have not discovered everything4 ,there is a fly in the molasses1 to be had by persistence4 ,inspiration is but a starting-point4 ,success means work1 days1 nights1 weeks1 and years4 ,there can be no boy who will follow exactly any directions given to him1 or do exactly as he is told1 of his own free will4 ,he will bolt at the first opportunity4 ,if forced or obliged to do as he is directed1 his action will be accompanied by many a why8 ,therefore in presenting the following chapters ,i have not only told how to make the various motors1 telegraphs1 telephones1 radio receivers1 etc4 but have also explained the principles of electricity upon which they depend for their operation1 and how the same thing is accomplished in the every-day world4 ,in giving directions or offering cautions1 ,i have usually stated the reason for so doing1 in the hope that this information may be a stimulant to the imagination of the young experimenter and a useful guide in enabling him to proceed along some of the strange roads on which he will surely go4 ,,alfred ,p4 ,,morgan ,,upper ,,montclair1 ,n4 ,j4 ,,the ,,boy ,,electrician 4444444444444444444444444444444444444444 4444444 ,,introduction 4444444444444444444444444444444444444444 444444444444 ,,chapter ,i ,,magnets ,,and ,,magnetism 444444444444444444444444444444444 ,,chapter ,,ii ,,static ,,electricity 44444444444444444444444444444444444 ,,chapter ,,iii ,,static ,,electric ,,machines 4444444444444444444444444444 ,,chapter ,,iv ,,cells ,,and ,,batteries 4444444444444444444444444444444444 ,,chapter ,v ,,electro-,,magnetism ,,and ,,magnetic ,,induction 44444444444444 ,,chapter ,,vi ,,electrical ,,units 4444444444444444444444444444444444444 ,,chapter ,,vii ,,electrical ,,appurtenances 4444444444444444444444444444 ,,chapter ,,viii ,,electrical ,,measuring ,,instruments 4444444444444444444 ,,chapter ,,ix ,,bells1 ,,alarms1 ,,and ,,annunciators 4444444444444444444444 ,,chapter ,x ,,electric ,,telegraphs 44444444444444444444444444444444444 ,,chapter ,,xi ,,microphones ,,and ,,telephones 444444444444444444444444444 ,,chapter ,,xii ,,induction ,,coils 4444444444444444444444444444444444444 ,,chapter ,,xiii ,,transformers 444444444444444444444444444444444444444 ,,chapter ,,viv ,,wireless ,,telegraphy 444444444444444444444444444444444 ,,chapter ,,xv ,a ,,wireless ,,telephone 444444444444444444444444444444444 ,,chapter ,,xvi ,,electric ,,motors 4444444444444444444444444444444444444 ,,chapter ,,xvii ,,dynamos 4444444444444444444444444444444444444444 4444 ,,chapter ,,xviii ,,an ,,electric ,,railway 4444444444444444444444444444444 ,,chapter ,,xix ,,miniature ,,lighting 4444444444444444444444444444444444 ,,chapter ,,xx ,,miscellaneous ,,electrical ,,apparatus 4444444444444444444 ,the ,full ,project ,gutenberg ,license 44444444444444444444444444444444 ,section #a4 ,general ,terms of ,use ,redistributing ,project ,gutenberg electronic works 4444444444444444444444444444444444444 ,section #b4 ,information about the ,mission of ,project ,gutenberg 44 ,section #c4 ,information about the ,project ,gutenberg ,literary ,archive ,foundation 4444444444444444444444444444444444444444 444444 ,section #d4 ,information about ,donations to the ,project ,gutenberg ,literary ,archive ,foundation 4444444444444444444444444444444444444 ,section #e4 ,general ,information ,about ,project ,gutenberg electronic works4 4444444444444444444444444444444444444444 444444444444444444 ,a ,,boy',s ,,wireless ,,outfit ,,made ,,up ,,of ,,the ,,apparatus ,,described ,,in ,,chapter ,,xiv4 ,,the ,,junior ,,dynamo ,,and ,a ,,coherer ,,outfit ,,can ,,be ,,seen ,,on ,,the ,,lower ,,part ,,of ,,the ,,table4 44444444444444444444444444444444444444 ,fig4 #a4--,the ,card of a ,mariner's ,compass1 ,showing the ,points4 4444 ,fig4 #b4--;,a ,bar ,magnet 4444444444444444444444444444444444444444 444444 ,fig4 #c4--;,a ,horseshoe ,magnet 4444444444444444444444444444444444444444 ,fig4 #d4--;,a ,magnetized ,needle and a ,bar ,magnet which have been dipped in ,iron ,filings4 4444444444444444444444444444444444444444 4444444444 ,fig4 #e4--,the ,lifting ,power of a ,bar ,magnet4 ,it must be brought closer to the nails than the tacks because they are heavier4 44444 ,fig4 #f4--;,a ,simple ,compass4 4444444444444444444444444444444444444444 4 ,fig4 #g4--,several ,different ,methods of ,making a ,simple ,compass4 44444 ,fig4 #h4--,the ,attraction of an ,iron ,nail through ,glass4 44444444444 ,fig4 #i4--;,a ,magnetic ,chain4 4444444444444444444444444444444444444444 4 ,fig4 #aj4--;,an ,experiment ,illustrating that ,like ,poles ,repel ,each ,other and ,unlike ,poles ,attract4 4444444444444444444444444444444444444444 4 ,fig4 #aa4--;,a ,magnetic ,boat4 4444444444444444444444444444444444444444 4 ,fig4 #ab4--,repulsion between ,similar ,poles1 ,shown by ,floating ,needles4 ,fig4 #ac4--;,a ,magnetic ,phantom1 ,showing the ,field of ,force about a ,magnet4 4444444444444444444444444444444444444444 4444444444444444444 ,fig4 #ad4--,magnetic ,phantom showing the ,lines of ,force about a ,horseshoe ,magnet4 4444444444444444444444444444444444444444 444444444 ,fig4 #ae4--,lines of ,force between ,like and ,unlike ,poles4 444444444444 ,fig4 #af4--;,a ,simple ,dipping ,needle4 444444444444444444444444444444444 ,fig4 #ag4--;,an ,electrified ,glass ,rod will ,attract ,small ,bits of ,paper4 ,from the author's ,wireless ,telegraphy and ,telephony by permission4 99,a ,double ,lightning ,discharge from a ,cloud to the ,earth499 4444444444444444444444444444444444444444 4444444444444444444 ,fig4 #ai4--;,a ,piece of ,dry ,writing-,paper may be ,electrified by ,rubbing4 ,fig4 #bj4--;,a ,surprise for the ,cat4 4444444444444444444444444444444444 ,fig4 #ba4--;,a ,paper ,electroscope4 444444444444444444444444444444444444 ,fig4 #bb4--;,a ,pith-,ball ,electroscope4 44444444444444444444444444444444 ,fig4 #bc4--;,a ,double ,pith-,ball ,electroscope4 4444444444444444444444444 ,fig4 #bd4--;,a ,gold-,leaf ,electroscope4 44444444444444444444444444444444 ,fig4 #be4--,method of ,suspending an ,electrified ,rod in a ,wire ,stirrup4 ,fig4 #bf4--,similarly ,electrified ,bodies ,repel ,each ,other4 ,dissimilarly ,electrified ,ones ,attract ,each ,other4 444444444444444444444444444444 ,fig4 #bg4--,the ,electrophorous 444444444444444444444444444444444444444 ,fig4 #bh4--;,an ,electric ,frog-,pond4 44444444444444444444444444444444444 ,fig4 #bi4--;,front ,view of a ,cylinder ,electric ,machine4 444444444444444 ,fig4 #cj4--,method of ,finding the ,center of a ,circle4 4444444444444444 ,fig4 #ca4--,the ,rubber4 4444444444444444444444444444444444444444 4444 ,fig4 #cb4--,the ,prime ,conductor or ,collector4 444444444444444444444444 ,fig4 #cc4--,the ,complete ,cylinder ,electric ,machine4 444444444444444444 ,fig4 #cd4--,paper ,pattern for laying out the ,plates4 44444444444444444 ,fig4 #ce4--,plate with ,sectors in ,position1 and a ,pattern for the ,sectors4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #cf4--;,a ,side ,view of one of the ,bosses1 showing the ,brass ,bushing used4 4444444444444444444444444444444444444444 444444444444444444444 ,fig4 #cg4--,the ,frame4 4444444444444444444444444444444444444444 4444444 ,fig4 #ch4--,the ,upright4 4444444444444444444444444444444444444444 44444 ,fig4 #ci4--,the ,driving-,wheels and ,axle4 44444444444444444444444444444 ,fig4 #dj--,the ,boss and ,axle4 ,for sake of clearness1 the ,plate is not shown4 4444444444444444444444444444444444444444 44444444444444444444 ,fig4 #da--,showing how the ,ball1 ,comb1 etc41 are mounted on the ,glass ,rod4 4444444444444444444444444444444444444444 4444444444444444444444 ,fig4 #db4--;,a ,comb or ,collector4 4444444444444444444444444444444444444 ,fig4 #dc4--,showing how the ,tinsel ,brushes are arranged on the ,neutralizer ,rods4 4444444444444444444444444444444444444444 4444444 ,fig4 #dd4--,the ,complete ,wimshurst ,electric ,machine4 ,b ,b ,b ,b1 ,brushes4 ,c ,c1 ,combs4 ,d ,b1 ,discharge ,ball4 ,i ,i1 ,glass ,rods4 ,h1 ,handle4 ,q ,q1 ,quadrant ,rods4 ,s ,s ,s ,s ,s1 ,sectors4 ,s ,g1 ,spark-,gap4 ,p ,p1 ,driving-,wheels4 ,for the sake of clearness1 several of the sectors are not shown4 44444444444444444444444444444 ,fig4 #de4--,the ,leyden ,jar4 4444444444444444444444444444444444444444 44 ,fig4 #df4--;,a ,wooden ,mortar for ,igniting ,gunpowder4 444444444444444444 ,fig4 #dg4--;,an ,electric ,umbrella4 444444444444444444444444444444444444 ,fig4 #dh4--;,a ,lightning ,board4 444444444444444444444444444444444444444 ,fig4 #di4--;,an ,electric ,dance4 444444444444444444444444444444444444444 ,fig4 #ej4--;,an ,electric ,whirl4 444444444444444444444444444444444444444 ,fig4 #ea4--,lichtenberg's ,figures4 44444444444444444444444444444444444 ,fig4 #eb4--,the ,voltaic ,cell4 4444444444444444444444444444444444444444 ,fig4 #ec4--,the ,elements of ,simple ,voltaic ,cell4 444444444444444444444 ,fig4 #ed4--;,a ,home-,made ,voltaic ,cell4 44444444444444444444444444444444 ,fig4 #ee4--;,carbon-,cylinder ,cell1 and ,cylinder4 4444444444444444444444 ,fig4 #ef4--;,a ,leclanche ,cell1 showing the ,porous ,cup4 4444444444444444 ,fig4 #eg4--;,a ,dry ,cell4 4444444444444444444444444444444444444444 444444 ,fig4 #eh4--,the ,different ,operations involved in ,making a ,dry ,cell4 44 ,fig4 #ei4--;,a ,zinc-,carbon ,element1 made from ,heavy plates4 44444444444 ,fig4 #fj4--;,a ,method of making a ,cell ,element from ,carbon ,rods4 444444 ,fig4 #fa4 ,an ,element made from two ,carbon ,plates and a ,zinc ,rod4 444 ,fig4 #fb4 ,a ,method of ,mounting four ,carbon ,plates4 44444444444444444 ,fig4 #fc4--;,a ,battery ,element arranged for three ,cells4 44444444444444 ,fig4 #fd4--;,a ,plunge ,battery1 with ,windlass4 4444444444444444444444444 ,fig4 #fe4--;,a ,plunge ,battery adapted to a ,set of ,elements1 as shown in ,figure #fc4 ,they may be lifted out and placed on the ,arms to drain4 ,fig4 #ff4--;,an ,edison-,lalande ,cell4 4444444444444444444444444444444444 ,fig4 #fg4--;,a ,tomato-,can ,cell2 ,sectional ,view4 44444444444444444444444 ,fig4 #fh4--,the ,tomato-,can ,cell ,complete4 4444444444444444444444444444 ,fig4 #fi4--,two ,methods of ,connecting ,cells so as to obtain ,different ,voltage and ,amperage ,values4 44444444444444444444444444444444444444 ,fig4 #gj4--,small ,storage ,cells4 4444444444444444444444444444444444444 ,fig4 #ga4--;,how to make the ,plates for a ,storage ,cell4 444444444444444 ,fig4 #gb4--,the ,wood ,separator4 44444444444444444444444444444444444444 ,fig4 #gc4--,the ,complete ,element for a ,storage ,cell4 44444444444444444 ,fig4 #gd4--;,a ,battery of ,home-,made ,storage ,cells4 44444444444444444444 ,fig4 #ge4--;,gravity ,cells4 ,these consist of zinc and copper elements1 immersed in a zinc-copper sulphate solution4 ,they cannot be easily made1 and are best purchased4 ,the illustration also shows the star-shaped copper and crowfoot zinc element used in a gravity cell4 4444444444444444444444444444444444444444 444444444444444444444 ,fig4 #gf4--;,a ,current of ,electricity flowing through a ,wire will deflect a ,compass ,needle4 4444444444444444444444444444444444444444 4 ,fig4 #gg4--;,if a ,loop of ,wire is formed about a ,compass ,needle1 the ,deflection will be greater4 444444444444444444444444444444444444444 ,fig4 #gh4--;,iron ,filings clustered on a ,wire carrying a ,current of ,electricity4 4444444444444444444444444444444444444444 44444444444444 ,fig4 #gi4--,magnetic ,phantom formed about a ,wire carrying a ,current of ,electricity4 4444444444444444444444444444444444444444 44444444444444 ,fig4 #hj4--,magnetic ,phantom formed about several ,turns of wire4 44444 ,fig4 #ha4--,paper ,tube wrapped with ,wire for ,experimental ,purposes4 44 ,fig4 #hb4--,showing how the ,lines of ,force ,leak at the sides of the coil1 from a ,coil of ,wire1 and how they are concentrated by an ,iron ,core4 4444444444444444444444444444444444444444 444444444444444444444 ,fig4 #hc4--,the ,principle of an ,electro-,magnet4 4444444444444444444444 ,fig4 #hd4--;if you wrap some insulated ,wire around an ,ordinary ,nail and connect it to a ,battery1 it will become an ,electro-,magnet4 44444444 ,fig4 #he4--;,if you wind the ,wire around a small ,paper ,tube into which a ,nail will slide easily1 the ,coil will draw the ,nail in when the ,current is turned on4 4444444444444444444444444444444444444444 44444 ,by permission1 from ,solenoids by ,c4 ,r4 ,underhill4 ,lifting-,magnets of the type known as ,plate1 ,billet1 and ,ingot ,magnets4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #hf4--,showing how a ,current of ,electricity may be induced by a ,bar ,magnet and a ,coil4 4444444444444444444444444444444444444444 4444 ,fig4 #hg4--;,a ,horseshoe ,magnet and a ,coil arranged to produce ,electric ,currents by ,induction4 4444444444444444444444444444444444444444 44 ,fig4 #hh4--;,graphic ,representation of a ,direct and an ,alternating ,current4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #hi4--,staples and ,wooden ,cleat used for running ,low ,voltage ,wires4 4444444444444444444444444444444444444444 44444444444444444444 ,fig4 #ij4--,porcelain ,insulators to support ,electric ,light ,wires4 4444 ,fig4 #ia4--;,glass ,insulator ,binding-,posts and ,pin used to support ,telegraph and ,telephone wires4 444444444444444444444444444444444444 ,fig4 #ib4--,types of ,binding-,posts4 4444444444444444444444444444444444 ,fig4 #ic4--;,home-made ,binding-,posts4 444444444444444444444444444444444 ,fig4 #id4--;,binding-,post removed from the ,carbon of a ,dry ,cell4 444444 ,fig4 #ie4--,simple ,switches4 ,a1 ,single-,point ,switch4 ,b1 ,two-,point ,switch4 ,c1 ,three-,point ,switch4 ,d1 ,five-,point ,switch4 ,e1 ,lever with ,end ,rolled up to form ,handle4 ,f1 ,lever with ,handle made from part of a ,spool4 4444444444444444444444444444444444444444 4444444444 ,fig4 #if4--,knife ,switches4 4444444444444444444444444444444444444444 44 ,fig4 #ig4--,metal ,parts for the ,knife ,switches4 4444444444444444444444 ,fig4 #ih4--,simple ,fuses4 ,a1 ,fuse-,block with plain ,wire ,fuse4 ,d1 ,fuse-,block with ,mica ,fuse in position4 4444444444444444444444444444 ,fig4 #ii4--,lightning-,arrester and ,ground-,wire ,switch4 444444444444444 ,fig4 #ajj4--;,home-made ,lightning-,arrester4 444444444444444444444444444 ,fig4 #aja4--,lightning-,arrester for ,telephone ,wires4 44444444444444444 ,fig4 #ajb4--;,a1 ,base1 showing ,slot4 ,b and ,c1 ,sides and ,top of the ,bobbin4 ,d1 ,base and ,bobbin in ,position4 4444444444444444444444444 ,fig4 #ajc4--;,arrangement of the ,needle and ,pointer4 444444444444444444 ,fig4 #ajd4--;,a1 ,bearings4 ,b1 ,how the ,needle is mounted4 4444444444 ,fig4 #aje4--,the ,completed ,meter4 444444444444444444444444444444444444 ,fig4 #ajf4--;,details of the ,bobbin4 4444444444444444444444444444444444 ,fig4 #ajg4--,the ,bobbin partly cut away so as to show the ,bearing4 ,details of the ,armature and ,shaft4 44444444444444444444444444444444 ,fig4 #ajh4--;,completed ,voltmeter4 444444444444444444444444444444444444 ,fig4 #aji4--;,circuits for ,calibrating the ,ammeter and ,voltmeter4 44444 ,fig4 #aaj4--,simple ,compass ,galvanoscope4 4444444444444444444444444444 ,fig4 #aaa4--;,galvanoscope4 4444444444444444444444444444444444444444 444 ,fig4 #aab4--;,astatic ,galvanoscope4 44444444444444444444444444444444444 ,fig4 #aac4--;,astatic ,needles4 4444444444444444444444444444444444444444 ,fig4 #aad4--;,bobbin for ,astatic ,galvanometer4 444444444444444444444444 ,fig4 #aae4--;,completed ,astatic ,galvanometer4 4444444444444444444444444 ,fig4 #aaf4--,wheatstone ,bridge4 44444444444444444444444444444444444444 ,fig4 #aag4--,knife-,contact4 4444444444444444444444444444444444444444 44 ,fig4 #aah4--,resistance-,coil4 ,a shows how the ,wire is doubled and wound on the ,spool4 ,b is the completed ,coil4 44444444444444444444 ,fig4 #aai4--;,details of the ,magnet ,spools1 and ,yoke for an ,electric ,bell4 4444444444444444444444444444444444444444 444444444444444444444 ,fig4 #abj4--;,details of the ,armature1 and ,contact ,screw4 4444444444444 ,fig4 #aba4--,the ,completed ,bell4 4444444444444444444444444444444444444 ,fig4 #abb4--;,diagram showing how to connect a ,bell1 ,battery1 and ,push-,button4 4444444444444444444444444444444444444444 44444444444444 ,fig4 #abc4--,two ,simple ,push-,buttons4 44444444444444444444444444444444 ,fig4 #abd4--;,diagram showing how to arrange a ,bell ,system of ,return ,signals4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #abe4--;,burglar-,alarm ,trap4 4444444444444444444444444444444444444 ,fig4 #abf4--;,an ,early-,riser's ,electric ,alarm ,attachment for a ,clock4 4 ,fig4 #abg4--;,details of the ,chain ,electrodes1 etc4 4444444444444444444 ,fig4 #abh4--;,an ,annunciator ,drop4 444444444444444444444444444444444444 ,fig #abi4--;,details of the ,drop-,frame and ,armature4 444444444444444444 ,fig4 #acj4--;,a ,typical ,telegraph ,key1 showing the ,various ,parts4 44444 ,fig4 #aca4--;,a ,typical ,telegraph ,sounder1 showing the ,various ,parts4 4 ,fig4 #acb4--;,a ,simple ,home-made ,telegraph ,key4 44444444444444444444444 ,fig4 #acc4--;,a ,simple ,home-made ,telegraph ,sounder4 4444444444444444444 ,fig4 #acd4--;,a ,diagram showing how to connect two ,simple ,telegraph ,stations4 4444444444444444444444444444444444444444 44444444444444444 ,fig4 #ace4--;,a ,complete ,telegraph ,set1 consisting of a ,keyboard and a ,sounder4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #acf4--;,details of the ,telegraph ,set shown in ,figure #ace4 4444444 ,fig4 #acg4--;,a ,diagram showing how to connect two ,complete ,telegraph ,sets1 using one ,line ,wire and a ,ground4 ,the ,two-,point ,switches throw the ,batteries out of ,circuit when the ,line is not in use4 444444444 ,fig4 #ach4--;,details of the ,relay ,parts4 44444444444444444444444444444 ,fig4 #aci4--,the ,completed ,relay4 444444444444444444444444444444444444 ,fig4 #adj4--;,a ,diagram showing how to connect a ,relay1 ,sounder1 and ,key4 ,closing the ,key will operate the ,relay4 ,the ,relay will then operate the ,sounder in turn4 44444444444444444444444444444444444444 ,fig4 #ada4--;,how to hold a ,telegraph ,key4 4444444444444444444444444444 ,fig4 #adb4--,the ,morse ,telegraphic ,code4 44444444444444444444444444444 ,fig4 #adc4--;,a ,microphone connected to a ,telephone ,receiver1 and a ,battery4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #add4--;,a ,very ,sensitive ,form of ,microphone1 with which the ,footsteps of a ,fly can be heard4 4444444444444444444444444444444444 ,fig4 #ade4--;,a ,telephone ,system1 consisting of a ,receiver1 ,transmitter1 and a ,battery connected in ,series4 ,words spoken into the ,transmitter are reproduced by the ,receiver4 44444444444444444444444444444444444 ,fig4 #adf4--;,a ,watch-,case ,telephone ,receiver4 444444444444444444444444 ,fig4 #adg4--;,a ,simple ,form of ,telephone ,receiver4 44444444444444444444 ,fig4 #adh4--;,a ,home-made ,telephone ,transmitter4 4444444444444444444444 ,fig4 #adi4--;,a ,complete ,telephone ,instrument4 ,two ,instruments such as this are necessary to form a simple ,telephone ,system4 4444444444444 ,fig4 #aej4--;,diagram of ,connection for the ,telephone ,instrument shown in ,fig4 #adi4 4444444444444444444444444444444444444444 44444444444444 ,fig4 #aea4--;,a ,desk-,stand ,type of ,telephone4 4444444444444444444444444 ,fig4 #aeb4--;,a ,telephone ,induction ,coil4 44444444444444444444444444444 ,fig4 #aec4--;,diagram of ,connection for a ,telephone ,system employing an ,induction ,coil at each ,station4 44444444444444444444444444444444444 ,fig4 #aed4--;,details of ,various ,parts of a ,medical ,coil4 4444444444444 ,fig4 #aee4--;,details of ,interrupter for ,medical ,coil4 4444444444444444 ,fig4 #aef4--;,completed ,medical ,coil4 444444444444444444444444444444444 ,fig4 #aeg4--;,diagram showing ,essential ,parts of ,induction ,coil4 444444 ,fig4 #aeh4--;,empty ,paper ,tube1 and ,tube filled with ,core ,wire preparatory to winding on the ,primary4 4444444444444444444444444444 ,fig4 #aei4--;,illustrating the ,various ,steps in winding on the ,primary and fastening the ,ends of the ,wire4 4444444444444444444444444444444 ,fig4 #afj4--;,complete ,primary ,winding and ,core4 4444444444444444444444 ,fig4 #afa4--,the ,primary covered with ,insulating ,layer of ,paper ready for the ,secondary4 4444444444444444444444444444444444444444 44444444 ,fig4 #afb4--,simple ,winding ,device for winding the ,secondary4 44444444 ,fig4 #afc4--;,completed ,secondary ,winding4 4444444444444444444444444444 ,fig4 #afd4--;,interrupter ,parts4 44444444444444444444444444444444444444 ,fig4 #afe4--;,condenser4 4444444444444444444444444444444444444444 444444 ,fig4 #aff4--;,completed ,coil4 4444444444444444444444444444444444444444 4 ,fig4 #afg4--;,diagram showing how to connect the ,apparatus for the ,electric ,hands ,experiment4 44444444444444444444444444444444444444 ,fig4 #afh4--;,geissler ,tubes4 4444444444444444444444444444444444444444 4 ,fig4 #afi4--,the ,bulb will emit a ,peculiar ,greenish ,light4 44444444444 ,fig4 #agj4--;,an ,electrified ,garbage-can4 44444444444444444444444444444 ,fig4 #aga4--;,jacob's ,ladder4 4444444444444444444444444444444444444444 4 ,fig4 #agb4--;,an ,x-,ray ,tube4 4444444444444444444444444444444444444444 44 ,fig4 #agc4--;,fluoroscope4 4444444444444444444444444444444444444444 4444 ,fig4 #agd4--;,how to connect an ,x-,ray ,tube to a ,spark-,coil4 44444444444 ,an ,x-,ray ,photograph of the hand taken with the ,outfit shown in ,figure #agd4 ,the arrows point to injuries to the bone of the third finger near the middle ,joint ,resulting in a ,stiff ,joint4 4444444444 ,fig4 #age4--;,comparison between ,electric ,current and ,flow of ,water4 44 ,fig4 #agf4--;,alternating ,current ,system for ,light and ,power4 444444444 ,fig4 #agg4--,motor ,generator ,set for changing ,alternating ,current to ,direct ,current4 4444444444444444444444444444444444444444 44444444444 ,fig4 #agh4--,step-,up ,transformer4 444444444444444444444444444444444444 ,fig4 #agi4--,step-,down ,transformer4 4444444444444444444444444444444444 ,fig4 #ahj4--;,core ,dimensions4 4444444444444444444444444444444444444444 ,fig4 #aha4--,the ,core1 ,assembled and ,taped4 44444444444444444444444444 ,fig4 #ahb4--,transformer ,leg4 4444444444444444444444444444444444444444 ,fig4 #ahc4--;,fiber ,head4 4444444444444444444444444444444444444444 44444 ,fig4 #ahd4--,leg with ,heads in ,position for ,winding4 44444444444444444 ,fig4 #ahe4--;,how to make a ,tap in the ,primary by soldering a ,copper ,strip to the ,wire4 4444444444444444444444444444444444444444 44444444 ,fig4 #ahf4--,the ,transformer completely ,wound and ready for ,assembling4 ,fig4 #ahg4--,wooden ,strips for mounting the ,transformer on the ,base4 4 ,fig4 #ahh4--;,details of the ,switch ,parts4 4444444444444444444444444444 ,fig4 #ahi4--,the ,complete ,switch4 444444444444444444444444444444444444 ,fig4 #aij4--;,diagram of ,connections4 444444444444444444444444444444444 ,fig4 #aia4--,top ,view of the ,transformer4 4444444444444444444444444444 ,fig4 #aib4--,side ,view of the ,transformer4 444444444444444444444444444 ,fig4 #aic4--,little ,waves spread out from the ,spot4 444444444444444444 ,fig4 #aid4--;,a ,simple ,transmitter4 44444444444444444444444444444444444 ,fig4 #aie4--;,a ,simple ,receptor4 44444444444444444444444444444444444444 ,fig4 #aif4--,molded ,aerial ,insulator 444444444444444444444444444444444 ,fig4 #aig4--;,a ,porcelain ,cleat will make a ,good ,insulator for ,small ,aerials4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #aih4--,method of ,arranging the ,wires and ,insulating them from the ,cross ,arm or ,spreader4 4444444444444444444444444444444444444444 4444 ,fig4 #aii4--,various ,types of ,aerials4 4444444444444444444444444444444 ,fig4 #bjj4--;,a ,ground ,clamp for ,pipes4 4444444444444444444444444444444 ,fig4 #bja4--;,details of the ,tuning ,coil4 44444444444444444444444444444 ,fig4 #bjb4--,side and ,end ,views of the ,tuning ,coil4 444444444444444444 ,fig4 #bjc4--;,complete ,double-,slider ,tuning ,coil4 444444444444444444444 ,fig4 #bjd4--;,a ,simple ,loose ,coupler4 444444444444444444444444444444444 ,fig4 #bje4--;,details of the ,wooden ,parts4 4444444444444444444444444444 ,fig4 #bjf4--,side ,view of the ,loose ,coupler4 4444444444444444444444444 ,fig4 #bjg4--,top ,view of the ,loose ,coupler4 44444444444444444444444444 ,fig4 #bjh4--;,end ,views of the ,loose ,coupler4 4444444444444444444444444 ,fig4 #bji4--;,complete ,loose ,coupler4 444444444444444444444444444444444 ,fig4 #baj4--;,a ,crystal ,detector4 4444444444444444444444444444444444444 ,fig4 #baa4--;,details of the ,crystal ,detector4 444444444444444444444444 ,a ,double ,slider ,tuning ,coil4 44444444444444444444444444444444444444 ,a ,junior ,loose ,coupler4 4444444444444444444444444444444444444444 444 ,crystal ,detectors4 4444444444444444444444444444444444444444 44444444 ,fig4 #bab ,details of the ,cat ,whisker ,detector4 4444444444444444444 ,fig4 #bac4--;,another ,form of the ,cat-,whisker ,detector4 4444444444444 ,fig4 #bad4--;,cat-,whisker ,detector4 444444444444444444444444444444444 ,fig4 #bae4--;,building up a ,fixed ,condenser4 44444444444444444444444444 ,fig4 #baf4--;,a ,fixed ,condenser enclosed in a ,brass ,case made from a ,piece of ,tubing fitted with ,wooden ,ends4 44444444444444444444444444 ,fig4 #bag4--;,a ,telephone ,head ,set4 44444444444444444444444444444444444 ,fig4 #bah4--;,a ,circuit showing how to connect a ,double-,slider ,tuning ,coil4 4444444444444444444444444444444444444444 444444444444444444444 ,fig4 #bai4--;,circuit showing how to connect a ,loose ,coupler4 444444444 ,fig4 #bbj4--;,a ,diagram showing how to connect some of the ,instruments described in this ,chapter4 4444444444444444444444444444444444444444 ,fig4 #bba4--;,a ,wireless ,spark ,coil4 4444444444444444444444444444444444 ,fig4 #bbb4--,small ,spark ,gaps4 444444444444444444444444444444444444444 ,fig4 #bbc4--;,diagram showing how to connect a ,simple ,transmitter4 4444 ,fig4 #bbd4--;,a ,test-,tube ,leyden ,jar4 444444444444444444444444444444444 ,fig4 #bbe4--;,eight ,test-,tube ,leyden ,jars mounted in a ,wooden ,rack4 444 ,fig4 #bbf4--;,a ,helix and ,clip4 444444444444444444444444444444444444444 ,fig4 #bbg4--;,an ,oscillation ,transformer4 44444444444444444444444444444 ,,an ,,oscillation ,,helix4 4444444444444444444444444444444444444444 44444 ,,an ,,oscillation ,,condenser4 4444444444444444444444444444444444444444 4 ,fig #bbh4--;,circuit showing how to connect a ,helix and a ,condenser4 44 ,fig #bbi4--;,circuit showing how to connect an ,oscillation ,transformer and a ,condenser4 4444444444444444444444444444444444444444 4444444444 ,fig #bcj4--;,an ,aerial ,switch4 4444444444444444444444444444444444444444 ,fig #bca4--;,a ,complete ,wiring ,diagram for both the ,transmitter and the ,receptor4 4444444444444444444444444444444444444444 44444444444444444 ,fig4 #bcb4--,the ,continental ,alphabet4 4444444444444444444444444444444 ,fig4 #bcc4--;,a ,coherer and a ,decoherer4 444444444444444444444444444444 ,fig4 #bcd4--;,details of the ,coherer4 444444444444444444444444444444444 ,fig4 #bce4--,the ,relay4 4444444444444444444444444444444444444444 444444 ,fig4 #bcf4--,the ,complete ,coherer ,outfit4 4444444444444444444444444444 ,fig4 #bcg4--;,a ,simple ,arrangement showing the ,inductive ,action between two ,coils4 4444444444444444444444444444444444444444 4444444444444444 ,fig4 #bch4--;,a ,simple ,wireless ,telephone4 ,speech directed into the ,transmitter can be heard in the ,receiver1 although there is no direct electrical connection between the two4 444444444444444444444 ,fig4 #bci4--;,a ,double-,contact ,strap-,key4 ,the ,dotted ,lines show how the ,binding-,posts are connected4 44444444444444444444444444444444444444 ,fig4 #bdj4--,the ,circuit of the ,wireless ,telephone4 ,when the ,key is up1 the ,receiver is ready for ,action4 ,when the ,key is pressed1 the ,transmitter and ,battery are thrown into the ,circuit4 44444444444444 ,fig4 #bda4--;,a ,complete ,wireless ,telephone and ,telegraph ,station for ,amateurs4 #a4 ,the ,telephone ,coil4 #b4 ,the ,telephone ,transmitter4 #c4 ,double-,contact ,strap-,key4 #d4 ,the ,battery4 #e4 ,spark ,coil4 #f4 ,key4 #g4 ,spark-,gap4 #h4 ,aerial ,switch4 #i4 ,loose ,coupler4 #aj4 ,detector1 #aa4 ,fixed ,condenser4 #ab4 ,code ,chart4 #ac4 ,amateur ,license4 #ad4 ,aerial4 #ae4 ,telephone ,receivers4 4444444444444444444444444444444444444444 44 ,fig4 #bdb4--;,a ,simple ,electric ,motor which may be made in ,fifteen ,minutes4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #bdc4--;,details of the ,armature of the ,simplex ,motor4 44444444444 ,fig4 #bdd4--,the ,armature4 4444444444444444444444444444444444444444 444 ,fig4 #bde4--,the ,field4 4444444444444444444444444444444444444444 444444 ,fig4 #bdf4--,the ,field and ,commutator4 4444444444444444444444444444444 ,fig4 #bdg4--,the ,bearings4 4444444444444444444444444444444444444444 444 ,fig4 #bdh4--,the ,complete ,motor4 4444444444444444444444444444444444444 ,fig4 #bdi4--;,details of the ,motor4 44444444444444444444444444444444444 ,fig4 #bej4--;,complete ,motor4 4444444444444444444444444444444444444444 4 ,fig4 #bea--;,a ,telephone ,magneto4 4444444444444444444444444444444444444 ,fig4 #beb4--,the ,principle of the ,alternator and the ,direct-,current ,dynamo4 4444444444444444444444444444444444444444 4444444444444444444 ,fig4 #bec4--;,details of the ,armature1 ,commutator1 and ,brushes4 4444444 ,fig4 #bed4--,the ,complete ,generator4 444444444444444444444444444444444 ,fig4 #bee4--;,details of the ,field ,casting4 444444444444444444444444444 ,fig4 #bef4--;,details of the ,armature ,casting4 444444444444444444444444 ,fig4 #beg4--;,details of the ,commutator4 444444444444444444444444444444 ,fig4 #beh4--;,diagram showing how to connect the ,armature ,winding to the ,commutator4 4444444444444444444444444444444444444444 444444444444444 ,fig4 #bei4--;,details of the ,wooden ,base4 44444444444444444444444444444 ,fig4 #bfj4--,the ,pulley and ,bearings4 44444444444444444444444444444444 ,fig4 #bfa4--,the ,brushes4 4444444444444444444444444444444444444444 4444 ,,the ,,junior ,,dynamo ,,mounted ,,on ,a ,,long ,,wooden ,,base ,,and ,,belted ,,to ,a ,,grooved ,,wheel ,,fitted ,,with ,a ,,crank ,,so ,,that ,,the ,,dynamo ,,can ,,be ,,run ,,at ,,high ,,speed ,,by ,,hand ,,power4 ,,the ,,illustration ,,also ,,shows ,a ,,small ,,incandescent ,,lamp ,,connected ,,to ,,the ,,dynamo ,,so ,,that ,,when ,,the ,,crank ,,is ,,turned ,,the ,,lamp ,,will ,,light4 444444444444444444444444444444444444444 ,fig4 #bfb4--;,complete ,dynamo4 4444444444444444444444444444444444444444 ,fig4 #bfc4--;,complete ,electric ,railway operated by ,dry ,cells4 ,note how the ,wires from the ,battery are connected to the ,rails by means of the ,wooden ,conductors illustrated in ,figure #bgg4 444444444444444444 ,fig4 #bfd4--;,details of the ,floor of the ,car4 444444444444444444444444 ,fig4 #bfe4--;,details of the ,bearing which supports the ,wheel and ,axle4 ,fig4 #bff4--,the ,wheels and ,axle4 444444444444444444444444444444444444 ,fig4 #bfg4--,the ,motor4 4444444444444444444444444444444444444444 444444 ,fig4 #bfh4--,the ,complete ,truck of the ,car without the ,body4 444444444 ,fig4 #bfi4--,pattern for the ,sides and ,ends of the ,car4 44444444444444 ,fig4 #bgj4--,the ,roof of the ,car4 444444444444444444444444444444444444 ,fig4 #bga4--,the ,completed ,car4 44444444444444444444444444444444444444 ,fig4 #bgb4-;,details of a ,wooden ,tie4 44444444444444444444444444444444 ,fig4 #bgc4-;,arrangement of ,track4 44444444444444444444444444444444444 ,fig4 #bgd4--,three ,different ,patterns for laying out the ,track4 444444 ,fig4 #bge4--;,details of the ,base of the ,cross-over4 444444444444444444 ,fig4 #bgf4--,the ,completed ,cross-over4 4444444444444444444444444444444 ,fig4 #bgg4--;,a ,connector for joining the ,ends of the ,rails4 4444444444 ,fig4 #bgh4--;,a ,bumper for preventing the ,car from leaving the ,rails4 4 ,fig4 #bgi4--;,a ,design for a ,railway ,bridge4 44444444444444444444444444 ,fig4 #bhj4--;,a ,design for a ,railway ,station4 4444444444444444444444444 ,fig4 #bha4--,miniature ,carbon ,battery ,lamp4 44444444444444444444444444 ,fig4 #bhb4--,miniature ,tungsten ,battery ,lamp4 444444444444444444444444 ,fig4 #bhc4--,lamps fitted respectively with ,miniature1 ,candelabra1 and ,ediswan ,bases4 4444444444444444444444444444444444444444 444444444444 ,fig4 #bhd4--,miniature ,flat-,base ,porcelain ,receptacle4 444444444444444 ,fig4 #bhe4--,weather-proof and ,pin-,sockets4 44444444444444444444444444 ,fig4 #bhf4--,types of ,battery ,switches suitable for ,miniature ,lighting4 ,fig4 #bhg4--;,how ,lamps are ,connected in ,multiple4 44444444444444444444 ,fig4 #bhh4--;,how ,lamps are ,connected in ,series4 4444444444444444444444 ,fig4 #bhi4--,three-way ,wiring ,diagram4 ,the ,light may be turned off or on from either ,switch4 4444444444444444444444444444444444444444 4444 ,fig4 #bij4--;,a ,lamp ,bracket for ,miniature ,lighting4 444444444444444444 ,fig4 #bia4--;,a ,home-made ,bracket4 444444444444444444444444444444444444 ,fig4 #bib4--;,a ,hanging ,lamp4 4444444444444444444444444444444444444444 4 ,fig4 #bic4--;,how the ,reflector is made4 444444444444444444444444444444 ,fig4 #bid4--;,a ,three-,cell ,dry ,battery for use in ,hand-,lanterns1 etc4 4 ,fig4 #bie4--;,an ,electric ,hand-,lantern4 4444444444444444444444444444444 ,fig4 #bif4--;,an ,electric ,ruby ,lantern4 4444444444444444444444444444444 ,fig4 #big4--,the ,electric ,ruby ,lamp with ,glass and ,shield ,removed4 444 ,fig4 #bih4--;,an ,electric ,night-,light for telling the ,time during the ,night4 4444444444444444444444444444444444444444 44444444444444444444 ,fig4 #bii4--;,a ,watch-,light4 4444444444444444444444444444444444444444 44 ,fig4 #cjj4--;,a ,pea ,lamp attached to a ,flexible ,wire and a ,plug4 4444 ,fig4 #cja4--;,four ,steps in ,carving a ,skull ,scarf-,pin4 #a4 ,the ,bone4 #b4 ,hole drilled in ,base4 #c4 ,roughed out4 #d4 ,finished4 4444444444444444 ,fig4 #cjb4--,the ,completed ,pin ready to be connected to a ,battery by removing the ,lamp from a ,flashlight and screwing the ,plug into its ,place4 4444444444444444444444444444444444444444 44444444444444444444 ,fig4 #cjc4--;,how the ,copper ,wires 7,c7 and the ,silver ,wires 7,i7 are twisted together in ,pairs4 4444444444444444444444444444444444444444 ,fig4 #cjd4--,wooden ,ring4 4444444444444444444444444444444444444444 4444 ,fig4 #cje4--;,complete ,thermopile4 ,an ,alcohol ,lamp should be lighted and placed so that the ,flame heats the ,inside ,ends of the ,wires in the ,center of the ,wooden ,ring4 4444444444444444444444444444444444444444 ,fig4 #cjf4--;,a ,reflectoscope4 4444444444444444444444444444444444444444 ,fig4 #cjg4--;,how the ,lens is ,arranged and ,mounted4 4444444444444444444 ,fig4 #cjh4--;,a ,view of the ,reflectoscope from the ,rear1 showing the ,door1 etc4 4444444444444444444444444444444444444444 4444444444444444 ,fig4 #cji4--;,a ,view of the ,reflectoscope with the ,cover removed1 showing the ,arrangement of the ,lamps1 etc4 444444444444444444444444 ,fig4 #caj4--;,a ,socket for holding the ,lamp4 44444444444444444444444444 ,fig4 #caa4--,the ,tin ,reflector4 44444444444444444444444444444444444444 ,fig #cab4--,top ,view of ,lamp ,bank1 showing how the ,circuit is arranged4 ,a and ,b are the ,posts to which should be connected any ,device it#8s desirable to operate4 4444444444444444444444444444444444444444 44444 ,fig4 #cac4--;,a ,glass ,jar arranged to serve as an ,electro-,plating ,tank4 ,fig4 #cad4--;,a ,rheostat4 4444444444444444444444444444444444444444 44444 ,fig4 #cae4--;,a ,pole-,changing ,switch or ,current ,reverser4 ,the ,connecting ,strip is pivoted so that the ,handle will operate both the ,levers1 ,a and ,b4 4444444444444444444444444444444444444444 44444444444444444444 ,,complete ,,receiving ,,set1 ,,consisting ,,of ,,double ,,slider ,,tuning ,,coil1 ,,detector ,,and ,,fixed ,,condenser4 4444444444444444444444444444444444444 ,,complete ,,receiving ,,set1 ,,consisting ,,of ,a ,,loose ,,coupler ,,in ,,place ,,of ,,the ,,tuning ,,coil1 ,,detector ,,and ,,fixed ,,condenser4 44444444444444444444 ,fig4 #caf4 ,a ,complete ,wireless ,receiving ,outfit4 4444444444444444444 ,fig4 #cag4--;,illustrating the ,principle of the ,tesla ,coil4 ,a ,leyden ,jar discharges through the ,primary ,coil and a ,high-,frequency ,spark is produced at the ,secondary4 4444444444444444444444444444444444444444 ,fig4 #cah4--;,details of the ,wooden ,rings used as the ,primary ,heads4 44 ,fig4 #cai4--;,details of the ,cross ,bars which support the ,primary ,winding4 4444444444444444444444444444444444444444 444444444444444444 ,fig4 #cbj4--,the ,secondary ,head4 4444444444444444444444444444444444444 ,a ,,complete ,,coherer ,,outfit ,,as ,,described ,,on ,,page #bgd4 444444444444444 ,,the ,,tesla ,,high ,,frequency ,,coil4 444444444444444444444444444444444444 ,fig4 #cba4--;,end ,view of the ,complete ,tesla ,coil4 44444444444444444444 ,fig4 #cbb4--,the ,complete ,tesla ,coil4 44444444444444444444444444444444 ,fig #cbc4--,showing how a ,glass-,plate ,condenser is built up of ,alternate ,sheets of ,tinfoil and ,glass4 4444444444444444444444444444 ,fig4 #cbd4--;,a ,diagram showing the ,proper ,method of ,connecting a ,tesla ,coil4 4444444444444444444444444444444444444444 444444444444444444444 ,,chapter ,i ,,magnets ,,and ,,magnetism ,over two thousand years ago1 in far-away ,asia ,minor1 a shepherd guarding his flocks on the slope of ,mount ,ida suddenly found the iron-shod end of his staff adhering to a stone4 ,upon looking further around about him he found many other pieces of this peculiar hard black mineral1 the smaller bits of which tended to cling to the nails and studs in the soles of his sandals4 ,this mineral1 which was an ore of iron1 consisting of iron and oxygen1 was found in a district known as ,magnesia1 and in this way soon became widely known as the ,magnesstone1 or magnet4 ,this is the story of the discovery of the magnet4 ,it exists in legends in various forms4 ,as more masses of this magnetic ore were discovered in various parts of the world1 the stories of its attractive power became greatly exaggerated1 especially during the ,middle ,ages4 ,in fact1 magnetic mountains which would pull the iron nails out of ships1 or1 later1 move the compass needle far astray1 did not lose their place among the terrors of the sea until nearly the eighteenth century4 ,for many hundreds of years the magnet-stone was of little use to mankind save as a curiosity which possessed the power of attracting small pieces of iron and steel and other magnets like itself4 ,then some one1 no one knows who1 discovered that if a magnet-stone were hung by a thread in a suitable manner it would always tend to point ,north and ,south2 and so the ,magnes-stone became also called the lodestone1 or leading-stone4 ,these simple bits of lodestone suspended by a thread were the forerunners of the modern compass and were of great value to the ancient navigators1 for they enabled them to steer ships in cloudy weather when the sun was obscured and on nights when the pole-star could not be seen4 ,the first real compasses were called gnomons1 and consisted of a steel needle which had been rubbed upon a lodestone until it acquired its magnetic properties4 ,then it was thrust through a reed or short piece of wood which supported it on the surface of a vessel of water4 ,if the needle was left in this receptacle1 naturally it would move against the side and not point a true position4 ,therefore it was given a circular movement in the water1 and as soon as it came to rest1 the point on the horizon which the north end designated was carefully noted and the ship's course laid accordingly4 ,the modern mariners' compass is quite a different arrangement4 ,it consists of three parts1 the bowl1 the card1 and the needle4 ,the bowl1 which contains the card and needle1 is usually a hemispherical brass receptacle1 suspended in a pair of brass rings1 called gimbals1 in such a manner that the bowl will remain horizontal no matter how violently the ship may pitch and roll4 ,the card1 which is circular1 is divided into #cb equal parts called the points of the compass4 ,the needles1 of which there are generally from two to four1 are fastened to the bottom of the card4 ,illustration3 ,fig4 #a4--,the ,card of a ,mariner's ,compass1 ,showing the ,points4 ,in the center of the card is a conical socket poised on an upright pin fixed in the bottom of the bowl1 so that the card hanging on the pin turns freely around its center4 ,on shipboard1 the compass is so placed that a black mark1 called the lubber's line1 is fixed in a position parallel to the keel4 ,the point on the compass-card which is directly against this line indicates the direction of the ship's head4 ,experiments with ,magnetism ,the phenomena of magnetism and its laws form a very important branch of the study of electricity1 for they play an important part in the construction of almost all electrical apparatus4 ,dynamos1 motors1 telegraphs1 telephones1 wireless apparatus1 voltmeters1 ammeters1 and so on through a practically endless list1 depend upon magnetism for their operation4 99,artificial ,magnets99 are those made from steel by the application of a lodestone or some other magnetizing force4 ,the principal forms are the ,bar and ,horseshoe1 so called from their shape4 ,the process of making such a magnet is called ,magnetization4 ,illustration3 ,fig4 #b4--;,a ,bar ,magnet ,small horseshoe and bar magnets can be purchased at toy-stores4 ,they can be used to perform very interesting and instructive experiments4 ,illustration3 ,fig4 #c4--;,a ,horseshoe ,magnet ,stroke a large darning-needle from end to end1 always in the same direction1 with one end of a bar magnet4 ,then dip the needle in some iron filings and it will be found that the filings will cling to the needle4 ,the needle has become a magnet4 ,dip the bar magnet in some iron filings and it will be noticed that the filings cling to the magnet in irregular tufts near the ends1 with few if any near the middle4 ,illustration3 ,fig4 #d4--;,a ,magnetized ,needle and a ,bar ,magnet which have been dipped in ,iron ,filings4 ,this experiment shows that the attractive power of a magnet exists in two opposite places4 ,these are called the poles4 ,there exists between magnets and bits of iron and steel a peculiar unseen force which can exert itself across space4 ,the power with which a magnet attracts or repels another magnet or attracts bits of iron and steel is called 99,magnetic ,force499 ,the force exerted by a magnet upon a bit of iron is not the same at all distances4 ,the force is stronger when the magnet is near the iron and weaker when it is farther away4 ,illustration3 ,fig4 #e4--,the ,lifting ,power of a ,bar ,magnet4 ,it must be brought closer to the nails than the tacks because they are heavier4 ,place some small carpet-tacks on a piece of paper and hold a magnet above them4 ,gradually lower the magnet until the tacks jump up to meet it4 ,then try some nails in place of the tacks4 ,the nails are heavier than the tacks1 and it will require a greater force to lift them4 ,the magnet will have to be brought much closer to the nails than to the tacks before they are lifted1 showing that the force exerted by the magnet is strongest nearest to it4 ,magnetize a needle and lay it on a piece of cork floating in a glass vessel of water4 ,it will then be seen that the needle always comes to rest lying nearly in a north and south line1 with the same end always toward the north4 ,illustration3 ,fig4 #f4--;,a ,simple ,compass4 ,the pole of the magnet which tends to turn towards the north is called the north-seeking pole and the opposite one is called the south-seeking pole4 ,the name is usually abbreviated to simply the north and south poles4 ,the north pole of a magnet is often indicated by a straight line or a letter ,n stamped into the metal4 ,a magnetized needle floating on a cork in a basin of water is a simple form of ,illustration3 ,fig4 #g4--,several ,different ,methods of ,making a ,simple ,compass4 99,compass499 ,figure #g shows several other different ways of making compasses4 ,the first method is to suspend a magnetized needle from a fine silk fiber or thread4 ,the second method illustrates a very sensitive compass made from paper4 ,two magnetized needles are stuck through the sides with their north poles both at the same end4 ,the paper support is mounted upon a third needle stuck through a cork4 ,a compass which more nearly approaches the familiar type known as a pocket compass may be made from a small piece of watch-spring or clock-spring4 ,the center of the needle is annealed or softened by holding it in the flame of an alcohol lamp and then allowing it to cool4 ,lay the needle on a piece of soft metal such as copper or brass1 and dent it in the center with a punch4 ,balance the needle on the end of a pin stuck through the bottom of a pill-box4 99,magnetic ,substances99 are those which are attracted by a magnet4 ,experiment with a number of different materials1 such as paper1 wood1 brass1 iron1 copper1 zinc1 rubber1 steel1 chalk1 etc4 ,it will be found that only iron and steel are capable of being attracted by your magnet4 ,ordinary magnets attract but very few substances4 ,iron1 steel1 cobalt1 and nickel are about the only ones worthy of mention4 99,attraction through ,bodies499 ,a magnet will attract a nail or a tack through a piece of paper1 just as if nothing intervened4 ,illustration3 ,fig4 #h4--,the ,attraction of an ,iron ,nail through ,glass4 ,it will also attract through glass1 wood1 brass1 and all other substances4 ,through an iron plate1 however1 the attraction is reduced or entirely checked because the iron takes up the magnetic effect itself and prevents the force from passing through and reaching the nail4 ,a number of carpet-tacks may be supported from a magnet in the form of a chain4 ,each individual tack in the series becomes a temporary magnet by induction4 ,if the tack in contact with the magnet be taken in the hand and the magnet suddenly withdrawn1 the tacks will at once lose their magnetism and fall apart4 ,illustration3 ,fig4 #i4--;,a ,magnetic ,chain4 ,it will furthermore be found that a certain magnet will support a certain number of tacks in the form of a chain1 but that if a second magnet is placed beneath the chain1 so that its south pole is under the north pole of the original magnet1 the chain may be lengthened by the addition of several other tacks4 ,the reason for this is that the magnetism in the tacks is increased by induction4 99,magnets will ,attract or ,repel99 each other1 depending upon which poles are nearest4 ,magnetize a sewing-needle and hang it from a thread4 ,bring the north pole of a bar magnet near the lower end of the needle4 ,if the lower end of the needle happens to be a south pole it will be attracted by the north pole of the bar magnet4 ,if1 on the other hand1 it is a north pole1 it will be repelled and you cannot touch it with the north pole of the bar magnet unless you catch it and hold it4 ,this fact gives rise to the general law of magnetism3 ,like poles repel each other and unlike poles attract each other4 ,illustration3 ,fig4 #aj4--;,an ,experiment ,illustrating that ,like ,poles ,repel ,each ,other and ,unlike ,poles ,attract4 ,another interesting way of illustrating this same law is by making a small boat from cigar-box wood and laying a bar magnet on it4 ,place the north pole of the bar magnet in the bow of the boat4 ,float the boat in a basin of water4 ,bring the south pole of a second magnet near the stern of the boat and it will sail away to the opposite side of the basin4 ,present the north pole of the magnet and it will sail back again4 ,illustration3 ,fig4 #aa4--;,a ,magnetic ,boat4 ,if the south pole of the magnet is presented to the bow of the boat the little ship will follow the magnet all around the basin4 ,the repulsion of similar poles may be also illustrated by a number of magnetized sewing-needles fixed in small corks so that they will float in a basin of water with their points down4 ,illustration3 ,fig4 #ab4--,repulsion between ,similar ,poles1 ,shown by ,floating ,needles4 ,the needles will then arrange themselves in different symmetrical groups1 according to their number4 ,a bar magnet thrust among them will attract or repel them depending upon its polarity4 ,the upper ends of the needles should all have the same polarity1 that is1 all be either north or south poles4 ,magnetism flows along certain lines called 99,lines of ,magnetic ,force499 ,these lines always form closed paths or circuits4 ,the region in the neighborhood of a magnet through which these lines are passing is called the field of force1 and the path through which they flow is called the 99,magnetic ,circuit499 ,the paths of the lines of force can be easily demonstrated by placing a piece of paper over a bar magnet and then sprinkling iron filings over the paper1 which should be jarred slightly in order that the filings may be drawn into the magnetic paths4 ,illustration3 ,fig4 #ac4--;,a ,magnetic ,phantom1 ,showing the ,field of ,force about a ,magnet4 ,the filings will arrange themselves in curved lines1 diverging from one pole of the magnet and meeting again at the opposite pole4 ,the lines of force are considered as extending outward from the north pole of the magnet1 curving around through the air to the south pole and completing the circuit back through the magnet4 ,illustration3 ,fig4 #ad4--,magnetic ,phantom showing the ,lines of ,force about a ,horseshoe ,magnet4 ,figure #ad shows the lines of force about a horseshoe magnet4 ,it will be noticed that the lines cross directly between the north and south poles4 ,the difference between the magnetic fields produced by like and unlike poles is shown in ,figure #ae4 ,illustration3 ,fig4 #ae4--,lines of ,force between ,like and ,unlike ,poles4 ,a study of this illustration will greatly assist the mind in conceiving how attraction and repulsion of magnetic poles take place4 ,it will be noticed the lines of force between two north poles resist each other and meet abruptly at the center4 ,the lines between a north and a south pole pass in regular curves4 99,the ,earth is a ,great ,magnet499 ,the direction assumed by a compass needle is called the magnetic meridian4 ,the action of the earth on a compass needle is exactly the same as that of a permanent magnet4 ,the fact that a magnetized needle places itself in the magnetic meridian is because the earth is a great magnet with lines of force passing in a north and south direction4 ,the compass needle does not generally point exactly toward the true ,north4 ,this is because the magnetic pole of the earth toward which the needle points is not situated at the same place as the geographical pole4 99,magnetic ,dip499 ,if a sewing-needle is balanced so as to be perfectly horizontal when suspended from a silk thread and is then magnetized1 it will be found that it has lost its balance and that the north end points slightly downward4 ,illustration3 ,fig4 #af4--;,a ,simple ,dipping ,needle4 ,this is due to the fact that the earth is round and that the magnetic pole which is situated in the far ,north is therefore not on a horizontal line with the compass1 but below such a line4 ,a magnetic needle mounted so as to move freely in a vertical plane1 and provided with a scale for measuring the inclination1 is called a 99,dipping ,needle499 ,a dipping needle may be easily made by thrusting a knitting-needle through a cork before it has been magnetized4 ,a second needle is thrust through at right angles to the first and the arrangement carefully balanced1 so that it will remain horizontal when resting on the edge of two glasses4 ,then magnetize the first needle by stroking it with a bar magnet4 ,when it is again rested on the glasses it will be found that the needle no longer balances1 but dips downward4 99,permanent ,magnets99 have a number of useful applications in the construction of scientific instruments1 voltmeters1 ammeters1 telephone receivers1 magnetos and a number of other devices4 ,in order to secure a very powerful magnet for some purposes a number of steel bars are magnetized separately1 and then riveted together4 ,a magnet made in this way is called a compound magnet1 and may have either a bar or a horse-shoe shape4 ,magnets are usually provided with a soft piece of iron called an armature or keeper4 ,the keeper is laid across the poles of the magnet when the latter is not in use and preserves its magnetism4 ,a blow or a fall will disturb the magnetic arrangement of the molecules of a magnet and greatly weaken it4 ,the most powerful magnet becomes absolutely demagnetized at a red heat1 and remains so after cooling4 ,therefore if you wish to preserve the strength of a magnetic appliance or the efficiency of any electrical instrument provided with a magnet1 do not allow it to receive rough usage4 ,,chapter ,,ii ,,static ,,electricity ,if you take a glass rod and rub it with a piece of flannel or silk1 it will be found to have acquired a property which it did not formerly possess3 namely1 the power of attracting to itself such light bodies as dust or bits of thread and paper4 ,hold such a rod over some small bits of paper and watch them jump up to meet it1 just as if the glass rod were a magnet attracting small pieces of iron instead of paper4 ,the agency at work to produce this mysterious power is called electricity1 from the ,greek word ,elektron1 which means amber4 ,amber was the first substance found to possess this property4 ,illustration3 ,fig4 #ag4--;,an ,electrified ,glass ,rod will ,attract ,small ,bits of ,paper4 ,the use of amber begins with the dawn of civilization4 ,amber beads have been found in the royal tombs at ,mycenae and at various places throughout ,sardinia1 dating from at least two thousand years before our era4 ,amber was used by the ancient world as a jewel and for decoration4 ,the ancient ,syrian woman used distaffs made of amber for spinning4 ,as the spindle whirled around it often rubbed against the spinner's garments and thus became electrified1 as amber always does when it is rubbed4 ,then on nearing the ground it drew to itself the dust or bits of chaff or leaves lying there1 or sometimes perhaps attracted the fringe of the clothing4 ,the spinner easily saw this1 because the bits of chaff which were thus attracted would become entangled in her thread unless she were careful4 ,the amber spindle was1 therefore1 called the harpaga or clutcher1 for it seemed to seize such light bodies as if it had invisible talons1 which not only grasped but held on4 ,this was probably the first intelligent observation of an electrical effect4 ,in the eighteenth century1 when ,benjamin ,franklin performed his famous kite experiment1 electricity was believed to be a sort of fiery atmospheric discharge which could be captured in small quantities and stored in receptacles such as ,leyden jars4 ,franklin was the first to prove that the lightning discharges taking place in the heavens are electrical4 ,the story of his experiment is very interesting4 ,he secured two light strips of cedar wood1 placed cross-wise and covered with a silk handkerchief for a kite4 ,to the top of the upright stick of the kite was fastened a sharp wire about a foot long4 ,the twine was of the usual kind1 but was provided with a short piece of silk ribbon and a key4 ,the purpose of the ribbon was possible protection against the lightning running through his body1 silk being a non-conductor1 as will be explained a little farther on4 ,the key was secured to the junction of the silk ribbon and the twine1 to serve as a convenient conductor from which to draw the sparks--if they came4 ,he did not have to wait long for a thunderstorm1 and as he saw it gathering he went out with his son1 then a young man twenty-two years of age4 ,the great clouds rolled up from the horizon1 and the gusts of wind grew fitful and strong4 ,the kite felt a swishing blast and began to rise steadily1 swooping this way and that as the breeze caught it4 ,the thunder muttered nearer and nearer and the rain began to patter on the grass as the kite flew higher4 ,the rain soon began to fall heavily1 compelling ,franklin and his son to take refuge under a near-by shed4 ,the heavy kite1 wet with water1 was sailing sluggishly when suddenly a huge low-lying black cloud traveling overhead shot forth a forked flame and the flash of thunder shook the very earth4 ,the kite moved upward1 soaring straight into the black mass1 from which the flashes began to come rapidly4 ,franklin watched the silk ribbon and the key4 ,there was not a sign4 ,had he failed8 ,suddenly the loose fibers of the twine erected themselves4 ,the moment had come4 ,without a tremor he advanced his knuckle to the key4 ,and between his knuckle and the key passed a spark6 then another and another4 ,they were the same kind of little sparks that he had made hundreds of times with a glass tube4 ,and then as the storm abated and the clouds swept off towards the mountains and the kite flew lazily in the blue1 the face of ,franklin gleamed in the glad sunshine4 ,the great discovery was complete1 his name immortal4 ,the cause of lightning is the accumulation of the electric charges in the clouds1 the electricity residing on the surface of the particles of water in the cloud4 ,these charges grow stronger as the particles of water join together and become larger4 ,as the countless multitude of drops grows larger and larger the potential is increased1 and the cloud soon becomes heavily charged4 ,through the effects of a phenomenon called induction1 and which we have already stumbled against in the experiment with the tacks and the magnetic chain1 the force exerted by the charge grows stronger because of a charge of the opposite kind on a neighboring cloud or some object on the earth beneath4 ,these charges continually strive to burst across the intervening air4 ,as soon as the charge grows strong enough a vivid flash of lightning1 which may be from one to ten miles long1 takes place4 ,the heated air in the path of the lightning expands with great force2 but immediately other air rushes in to fill the partial vacuum1 thus producing the terrifying sounds called thunder4 ,in the eighteenth century1 electricity was believed to be a sort of fiery atmospheric discharge1 as has been said4 ,later it was discovered that it seemed to flow like water through certain mediums1 and so was thought to be a fluid4 ,modern scientists believe it to be simply a vibratory motion1 either between adjacent particles or in the ether surrounding those particles4 ,it was early discovered that electricity would travel through some mediums but not through others4 ,these were termed respectively conductors and non-conductors or insulators4 ,metals such as silver1 copper1 gold1 and other substances like charcoal1 water1 etc41 are good conductors4 ,glass1 silk1 wool1 oils1 wax1 etc41 are non-conductors or insulators1 while many other substances1 like wood1 marble1 paper1 cotton1 etc41 are partial conductors4 ,there seems to be two kinds of electricity1 one called static and the other current electricity4 ,the former is usually produced by friction while the latter is generated by batteries or dynamos4 ,a very simple and well-known method of generating static electricity is by shuffling or sliding the feet over the carpet4 ,the body will then become charged1 and if the knuckles are presented to some metallic object1 such as a gas-jet or radiator1 a stinging little spark will jump out to meet it4 ,illustration3 ,from the author's ,wireless ,telegraphy and ,telephony by permission4 99,a ,double ,lightning ,discharge from a ,cloud to the ,earth499 ,the electricity is produced by the friction of the feet sliding over the carpet and causes the body to become electrified4 ,warm a piece of writing-paper1 then lay it on a wooden table and rub it briskly with the hand4 ,it soon will become stuck to the table and will not slide along as it did at first4 ,if one corner is raised slightly it will tend to jump right back4 ,if the paper is lifted off the table it will tend to cling to the hands and the clothing4 ,if held near the face it will produce a tickling sensation4 ,all these things happen because the paper is electrified4 ,it is drawn to the other objects because they are neutral1 that is1 do not possess an electrical charge4 ,illustration3 ,fig4 #ai4--;,a ,piece of ,dry ,writing-,paper may be ,electrified by ,rubbing4 ,all experiments with static electricity perform better in the winter time1 when it is cool and clear1 than in the summer4 ,the reason is that the air in winter is drier than in summer4 ,summer air contains considerable moisture and water vapor4 ,water vapor is a partial conductor of electricity1 and the surrounding air will therefore conduct the static electricity away from your apparatus almost as fast as it can be produced in the summer time4 ,illustration3 ,fig4 #bj4--;,a ,surprise for the ,cat4 ,some day during the winter time1 when it is cool and clear1 and the cat is near a fire or a stove1 stroke the cat rapidly with the hand4 ,the fur will stand up towards the hand and a faint crackling noise will be heard4 ,the crackling is caused by small sparks passing between the cat and the hand4 ,if the experiment is performed in a dark room1 the sparks may be plainly seen4 ,if you present your knuckle to the cat's nose a spark will jump to your knuckle and somewhat surprise the cat4 ,if the day is brisk and cool1 so that everything outside is frozen and dry1 try combing the hair with a rubber comb4 ,your hair will stand up all over your head instead of lying down flat1 and the faint crackling noise1 showing that sparking is taking place as the comb passes through the hair1 will be plainly heard4 ,the electricity is produced by the friction between the hair and the comb4 ,electricity may be produced by friction between a number of substances4 ,a hard rubber rod1 a glass rod1 a rubber comb or a stick of sealing-wax may be very easily electrified by rubbing them briskly with a piece of dry1 warm flannel4 99,electroscopes99 are devices for detecting the presence of static electricity4 ,illustration3 ,fig4 #ba4--;,a ,paper ,electroscope4 ,a very simple form of electroscope may be made in much the same manner as the paper compass described in the last chapter4 ,it may be cut out of writing-paper and mounted on a pin stuck through a cork4 ,if an electrified rod is held near the electroscope it may be made to whirl around in the same manner as a compass needle when a bar magnet is brought to it4 99,the ,pith-,ball ,electroscope99 is a very simple device1 in which a ball of cork or elder pith is hung by a fine silk thread from an insulated support4 ,a suitable electroscope may be made from a glass bottle having a piece of wire thrust into the cork to support the pith ball4 ,when the electrified rod is presented to the pith ball1 it will fly out towards the rod4 ,illustration3 ,fig4 #bb4--;,a ,pith-,ball ,electroscope4 ,if the pith ball is permitted to touch the glass rod1 the latter will transfer some of its electricity and charge the ball4 ,almost immediately the pith ball will fly away from the glass rod1 and no matter how near the rod is brought1 it will refuse to be touched again4 ,this action is much the same as that of the magnetized needle suspended from a thread when the similar pole of the magnet is presented to it4 ,when the rod is first presented to the pith ball1 the latter is neutral and does not possess an electrical charge4 ,when the rod has touched the ball1 however1 some of the electricity from the rod passes to the ball1 and after this they will repel each other4 ,the reason is that the rod and the ball are similarly charged and similarly charged bodies will repel each other4 ,illustration3 ,fig4 #bc4--;,a ,double ,pith-,ball ,electroscope4 ,if you are a good observer you might have noticed when experimenting with an electrified rod and the small bits of paper1 that some of the little papers were first attracted and flew upwards to the rod1 but having once touched it1 were quickly repelled4 ,the repulsion between two similarly electrified bodies may be shown by a double electroscope4 ,a double electroscope is made by hanging two pith balls on two silk threads from the same support4 ,electrify a glass rod and touch it to the pith balls4 ,they will immediately fly apart because they are electrified with the same kind of electricity4 99,the ,gold-leaf ,electroscope99 is one of the most sensitive means which can be employed to detect small amounts of static electricity4 ,illustration3 ,fig4 #bd4--;,a ,gold-,leaf ,electroscope4 ,it is a very simple instrument and is easily made in a short time4 ,a couple of narrow strips of the thinnest tissue paper1 or1 better still1 two strips of gold leaf1 are hung from a support in a wide-mouthed glass bottle which serves at once to insulate and protect the strips from draughts of air4 ,the mouth of the jar is closed by a plug of paraffin wax1 through the center of which passes a small glass tube4 ,a stiff copper wire passes through the tube4 ,the lower end of the wire is bent at right angles to furnish support for the strips of gold leaf4 ,a round sheet metal disk about the size of a quarter is soldered to the upper end of the rod4 ,if an electrified stick of sealing-wax or a glass rod is presented to the disk of the electroscope1 the strips will repel each other very strongly4 ,if the instrument is sensitive1 the strips should begin to diverge some time before the rod reaches the disk4 ,it is possible to make an electroscope so sensitive that chips formed by sharpening a pencil will cause the strips to diverge4 99,there are two kinds of static electricity499 ,rub a glass rod with a piece of silk and then suspend it in a wire stirrup as shown in ,figure #be4 ,excite a second rod also with a piece of silk and bring it near one end of the suspended one4 ,the suspended rod is repelled and will swing away from the one held in the hand4 ,illustration3 ,fig4 #be4--,method of ,suspending an ,electrified ,rod in a ,wire ,stirrup4 ,now rub a stick of sealing-wax with a piece of flannel until the sealing-wax is electrified4 ,then bring the stick of sealing-wax near the end of the suspended rod4 ,the rod will be attracted to the sealing-wax4 ,if you experiment further you will find that two sticks of sealing-wax will repel each other4 ,illustration3 ,fig4 #bf4--,similarly ,electrified ,bodies ,repel ,each ,other4 ,dissimilarly ,electrified ,ones ,attract ,each ,other4 ,this experiment indicates that there are two kinds of electrification3 one developed by rubbing glass with silk and the other developed by rubbing sealing-wax with flannel4 ,in the first instance1 the glass rod is said to be positively electrified1 and in the latter case the sealing-wax is negatively electrified4 ,the same law that applies to magnetism also holds true in the case of static electricity1 and similarly electrified bodies will repel each other and dissimilar ones attract4 99,the ,electrophorus99 is an instrument devised by ,volta in #agge for the purpose of obtaining static electricity4 ,illustration3 ,fig4 #bg4--,the ,electrophorous ,it is easily constructed and will furnish a source of electricity for quite a number of interesting experiments4 ,an electrophorus consists of two parts1 a round cake of resinous material cast in a metal dish or pan1 and a round metal disk which is provided with an insulating handle4 ,to make an electrophorus1 first procure an old cake or pie tin1 and fill it with bits of resin or sealing-wax4 ,place the pan in a warm spot upon the stove where the resin will melt1 taking care not to overheat or it will spatter and possibly take fire4 ,as the resin melts1 add more until the pan is nearly full4 ,when all is melted1 remove from the fire and set it away where it may cool and harden in the pan without being disturbed4 ,cut a circular disk out of sheet tin1 zinc1 or copper1 making the diameter about two inches less than that of the pie pan4 ,solder a small cylinder of tin or sheet brass to the center of the disk to aid in supporting the handle4 ,the latter is a piece of glass tubing about three-quarters of an inch in diameter and four or five inches long1 placed in the center of the cylinder and secured with molten sealing-wax4 ,in order to use the electrophorus the resinous cake must first be beaten or briskly rubbed with a piece of warm woolen cloth or flannel4 ,then place the disk on the cake holding the insulating handle with the right hand4 ,touch the cover or the disk momentarily with the forefinger of the left hand4 ,after the finger is removed1 raise the disk from the cake by picking it up with the glass insulating handle4 ,the disk will now be found heavily charged with positive electricity1 and if the knuckles are presented to the edge1 a spark will jump out to meet them4 ,illustration3 ,fig4 #bh4--;,an ,electric ,frog-,pond4 ,the cover may then be replaced1 touched1 and once more removed4 ,it will yield any number of sparks1 the resinous cake only needing to be recharged by rubbing once in a long while4 99,an ,electric ,frog-,pond99 may be experimented with by cutting out some small tissue-paper frogs4 ,moisten them a little and lay them on the cover of the electrophorus4 ,touch the electrophorus with the finger and then raise it with the insulating handle4 ,if the frogs are not too wet they will jump from the cover upon the table as soon as the cover is raised4 ,,chapter ,,iii ,,static ,,electric ,,machines ,a ,cylinder ,electric ,machine ,the electrophorus described in the last chapter is capable of furnishing sufficient electricity for many interesting experiments1 but for the purpose of procuring larger supplies of electricity1 a static electric machine is necessary4 ,an electric machine is composed of two parts1 one for producing the electricity by the friction of two surfaces rubbing against each other1 and the other an arrangement for collecting the electricity thus formed4 ,the earliest form of electric machine consisted of a ball of sulphur fixed upon a spindle which could be rotated by means of a crank4 ,when the dry hands were pressed against the sulphur by a person standing on a cake of resin1 which insulated him1 sparks could be drawn from his body4 ,later a leather cushion was substituted for the hands1 and a glass cylinder for the ball of sulphur1 so that the frictional electric machine now consists of a cylinder or a disk of glass mounted upon a horizontal axis capable of being turned by a handle4 ,a leather cushion1 stuffed with horsehair and covered with a powdered amalgam of zinc or tin1 presses against one side of the cylinder4 ,a prime conductor in the shape of an elongated cylinder presents a row of fine metal spikes1 like the teeth of a rake1 to the opposite side4 ,a flap of silk attached to the leather cushion passes over the cylinder and covers the upper half4 ,illustration3 ,fig4 #bi4--;,front ,view of a ,cylinder ,electric ,machine4 ,when the handle of the machine is turned1 the friction produced between the leather cushion and the glass generates a supply of positive electricity on the glass1 which is collected1 as the cylinder revolves1 by the row of sharp points1 and transferred to the prime conductor4 ,the first thing required in the construction of an electric machine is a large glass bottle having a capacity of from two to four quarts4 ,the insulating power of glass varies considerably4 ,common green glass 7not white glass colored green by copper1 but glass such as the telegraph insulators are made from7 generally insulates the best4 ,some sorts of white glass1 the ,bohemian especially1 are good insulators1 but this quality will not usually be found in ordinary bottles4 ,illustration3 ,fig4 #cj4--,method of ,finding the ,center of a ,circle4 ,select a smooth bottle which has no lettering embossed upon it1 and stand it upon a piece of white paper4 ,trace on the paper a line around the circumference of the bottle so that the circle thus formed is of the same size as the bottom of the bottle4 ,lay a carpenter's square on the circle so that the point ,c just touches the circumference4 ,draw a line from ,a to ,b where the sides of the square cut the circumference4 ,the point in the middle of this line is the center of the circle4 ,place the paper on the bottom of the bottle so that the circle coincides with the circumference1 and mark the center of the bottle4 ,the bottle must now be drilled4 ,this is accomplished with a small three-cornered file1 the end of which has been broken off so as to form a ragged cutting edge4 ,the file is set in a brace and used like an ordinary drill4 ,during the boring process the drill must be frequently lubricated with a mixture of gum camphor and turpentine4 ,the drilling1 which will require almost an hour before the glass is pierced1 if the bottle is a thick one1 should be performed slowly and carefully1 so as to avoid all danger of cracking the glass4 ,the hole1 when finished1 should be from one-quarter to three-eighths of an inch in diameter4 ,after the hole has been bored1 fit a wooden plug into the neck of the bottle and cement it there with a mixture composed of one-half a pound of resin1 five ounces of beeswax1 one-quarter of an ounce of plaster of ,paris1 and three-quarters of an ounce of red ocher1 melted together over a moderately warm stove4 ,dip the plug in the molten cement and force it into the neck of the bottle4 ,when the cement dries it will be impossible to remove it4 ,the sizes of bottles vary1 so that it is quite impossible to give dimensions which must be closely followed in constructing the machine4 ,those in the text are approximate4 ,the drawings have been made to scale so as to show the proportions the parts bear to each other4 ,a heavy wooden base will be required to mount the machine on4 ,two uprights are mounted on the base to support the axis of the bottle4 ,through one of these bore a hole of the same diameter as the wooden plug fitted in the neck of the bottle4 ,the end of the wooden plug projecting through the upright is notched and fitted with a crank so that the bottle may be revolved4 ,the handle of the crank is an ordinary spool having one flange cut off and mounted with a screw and a washer4 ,illustration3 ,fig4 #ca4--,the ,rubber4 ,the machine is now ready for the rubber and prime conductor4 ,the rubber is a piece of wood one inch square and from six to eight inches long4 ,a piece of undressed leather is tacked on as shown in the illustration and stuffed with horsehair4 ,the wood is shellacked and covered with tin-foil previous to tacking on the leather4 ,a strip of wood1 two inches wide and one-half an inch thick1 is fastened to the back of the rubber4 ,the strip should be just long enough so that when the lower end rests on the base the rubber is level with the axis of the bottle4 ,the lower end may be fastened to the base by means of a small brass hinge4 ,two rubber bands stretch from hooks between the rubber and the base so as to pull the former tightly against the bottle4 ,the illustration shows a method of mounting the rubber on a foot-piece held to the base with a thumb-nut so that it may be slid back and forth and the pressure varied at will4 ,the prime conductor is formed from a piece of curtain-pole two inches in diameter and eight inches long4 ,the ends are rounded with a rasp and then smoothed with sandpaper4 ,the whole surface is then shellacked and covered with a layer of tinfoil4 ,the heads of a number of dressmaker's pins are cut off1 and the pins forced into the side of the prime conductor with a pair of pincers4 ,they should form a row like the teeth of a rake about three-eighths of an inch apart4 ,a hole is bored in the center of the under side of the prime conductor to receive a glass rod one-half inch in diameter4 ,a second hole of the same size is bored in the base in such a position that when the glass rod is in place1 the teeth on the prime conductor are on a level with the axis of the bottle1 and their points about #c-cb of an inch away from the glass4 ,the glass rod must be used in order to insulate the prime conductor and prevent the escape of the electricity4 ,it is secured with some of the cement described on page #cc4 ,a piece of water-gauge glass may be used in place of a glass rod4 ,illustration3 ,fig4 #cb4--,the ,prime ,conductor or ,collector4 ,a strip of oiled silk1 or in its place a strip of silk which has been shellacked1 eight or nine inches wide1 and long enough to reach half-way around the bottle1 is tacked to the rubber so that the silk covers the upper half of the cylinder and comes over to within one-quarter of an inch of the steel points4 ,the machine is now complete1 and when the handle is turned rapidly1 you will be able to draw sparks from the prime conductor4 ,the sparks will probably be very short1 about one-half of an inch long4 ,these can be increased1 however1 to three inches1 if the glass is of the right quality1 by treating the rubber with amalgam4 ,the amalgam is formed by melting one ounce of tin and adding to it one ounce of zinc in small bits4 ,as soon as the zinc has also melted add to the mixture two ounces of mercury which has been previously warmed4 ,be careful not to inhale any of the vapor during this operation4 ,pour the mixture into a vessel of cold water1 which will reduce the metal to small grains4 ,pour off the water and grind the amalgam to a powder by pounding the grains with a hammer4 ,the leather rubber should be thinly smeared with lard and the powdered amalgam rubbed on it4 ,in order to obtain the greatest effect from an electric machine1 it must be carefully freed from dust and particles of amalgam adhering to the glass1 and the insulating column rubbed with a warm woolen cloth4 ,the best results are obtained by placing the machine near a stove or radiator where it is warm4 ,illustration3 ,fig4 #cc4--,the ,complete ,cylinder ,electric ,machine4 ,a ,wimshurst ,machine ,the ,wimshurst ,machine consists of two varnished glass plates revolving in opposite directions4 ,on the outside of each of these plates are cemented a number of tinfoil sectors1 arranged radially4 ,two conductors at right angles to each other extend obliquely across the plates1 one at the back and the other at the front4 ,these conductors each terminate in brushes of tinsel which electrically excite the sectors as the plates revolve4 ,the electricity is collected by a set of collectors arranged in a somewhat similar manner to the collector on the cylinder electric machine4 99,the ,glass ,plates99 are each eighteen inches in diameter4 ,purchase two panes of clear glass twenty inches square from a glass dealer4 ,the white glass is far preferable to the green glass and will make the best electric machine4 ,the plates should be of the thickness known as single light and should be perfectly free from wavy places1 bubbles1 or other imperfections4 ,illustration3 ,fig4 #cd4--,paper ,pattern for laying out the ,plates4 ,the work is first laid out on a piece of stiff paper twenty inches square as a pattern4 ,describe a circle four inches in diameter4 ,using the same center1 draw other circles1 making them respectively eight1 sixteen1 and eighteen inches in diameter4 ,then mark sixteen radial lines1 from the center1 making them equal distances apart1 as shown in ,figure #cd4 ,illustration3 ,fig4 #ce4--,plate with ,sectors in ,position1 and a ,pattern for the ,sectors4 ,lay one of the glass panes over the pattern and cut out a glass circle eighteen inches in diameter1 or perhaps you may be able to have a glazier do the cutting for you and so save considerable trouble and possible breakage4 ,two such plates should be made4 ,the ,sectors are cut from heavy flat tinfoil according to the pattern shown in ,figure #ce4 ,they should be made one inch and one-half wide at the wide end and three-quarters of an inch at the other end4 ,they are each four inches long4 ,thirty-two such sectors are required4 ,the easiest way to make them is to cut out a pattern from heavy cardboard to serve as a guide4 ,clean and dry both of the glass plates very carefully and then give them each two thin coats of white shellac4 ,after they have been dried1 lay one of the plates on the paper pattern so that the outside of the plate will coincide with the largest circle on the paper4 ,then place a weight in the center of the plate so that it will not move1 and stick sixteen of the tinfoil sectors on the plate with thick shellac4 ,the sectors are arranged symmetrically on the plate1 using the eight-inch and sixteen-inch circles and the radial lines as guides4 ,both plates should be treated in this manner4 ,each sector should be carefully pressed down on the glass1 so that it will stick smoothly without air-bubbles or creases4 ,when all the sectors are in place the plates will appear like that shown in ,figure #ce4 99,the ,bosses99 will have to be turned out at a wood-working mill or at some place where they have a turning-lathe4 ,the bosses are four inches in diameter at the large end and one inch and one-half at the other4 ,a groove is turned near the small end of each to accommodate a round leather belt4 ,a hole should be made in each boss about half-way through from the small end4 ,these holes should be bushed with a piece of brass tubing having an inside diameter of one-half inch4 ,the tubing should go into the hole very snugly and be a driven fit4 ,illustration3 ,fig4 #cf4--;,a ,side ,view of one of the ,bosses1 showing the ,brass ,bushing used4 ,the bosses should both be given a coat of shellac1 and after this is dry1 fastened to the glass plates on the same side to which the tinfoil sectors are attached4 ,the best plan is to lay the disks on the paper pattern and adjust them until the outer edge coincides with the largest circle4 ,then apply some bichromate glue to the flat surface of one of the bosses and place the latter in the center of the plate in line with the smallest circle4 ,place a weight on the boss to hold it down firmly against the plate and leave it over night1 or for ten or twelve hours1 until thoroughly dry4 ,the glue is prepared by placing some high-grade glue in a tin cup and covering it with cold water4 ,allow it to stand until the glue absorbs all the water it will and becomes soft4 ,then pour the water off and add enough glacial acetic acid to cover the glue4 ,heat the mixture until it is reduced to a liquid1 stirring it until it is perfectly smooth4 ,add a teaspoonful of powdered bichromate of potash to the glue4 ,the glue must now be kept in the dark1 for sunlight will set the glue so that it becomes insoluble4 ,the ,frame of the machine is composed of two strips twenty-five inches long1 three inches wide1 and an inch and one-half in thickness1 and two cross-pieces of the same thickness and width fifteen inches long4 ,illustration3 ,fig4 #cg4--,the ,frame4 ,notches are cut at both sides of the base to admit the feet of the uprights4 99,the ,uprights99 are seventeen inches long1 three inches wide1 and one and one-half inches thick4 ,illustration3 ,fig4 #ch4--,the ,upright4 ,the notch at the foot is cut the same width as the thickness of the long members of the frame and is arranged so that when fitted in place1 the foot of the upright will rest on the table in line with the bottom of the cross-pieces4 99,the ,driving-,wheels99 are turned out of wood on a lathe4 ,they are seven inches in diameter and seven-eighths of an inch thick4 ,a groove should be turned in the edge to carry a small round leather belt4 ,the wheels are mounted on a wooden axle made from a round curtain-pole4 ,they are glued to the axle and arranged so that the grooves will fall directly underneath the pulleys turned in the bosses4 ,illustration3 ,fig4 #ci4--,the ,driving-,wheels and ,axle4 ,the ends of the axle pass through the uprights1 five inches above the bottom4 ,the front end of the axle is fitted with a crank and a handle4 ,illustration3 ,fig4 #dj--,the ,boss and ,axle4 ,for sake of clearness1 the ,plate is not shown4 ,the plates are mounted on short iron axles passing through the top of the upright into the brass bushings4 ,one end of each of the axles is filed flat where it passes through the wood upright so that it may be firmly held by a set-screw and prevented from revolving4 ,fasten a small fiber washer to the center of one glass disk so that it will separate the plates and prevent them from touching when revolving4 ,the collectors1 quadrant rods1 etc41 are mounted on glass rods one inch in diameter4 ,the bottoms of the rods fit in holes 7,h ,h7 bored in the cross-pieces of the base1 ,figure #cg4 ,the upper ends are each fitted with a brass ball two inches in diameter4 ,the balls are mounted on the rods by soldering a piece of brass tubing to the ball and slipping it over the rod4 ,the rods should be of the proper length to bring the center of the balls on a line with the center of the plates4 ,illustration3 ,fig4 #da--,showing how the ,ball1 ,comb1 etc41 are mounted on the ,glass ,rod4 ,make two forks as shown in ,figure #db out of brass rod1 three-sixteenths of an inch in diameter and solder brass balls at the ends4 ,the forks are eleven inches long4 ,a number of small holes must be bored in the prongs and pins made by cutting ordinary dressmakers' pins in half and soldering them in place4 ,these pins1 mounted on the forks1 form the combs or collectors4 ,bore a horizontal hole through each of the brass rods on the top of the glass rods and pass the shanks of the forks through and solder them in place4 ,one of the shanks may be provided with a discharge ball at the end as shown by ,d ,b in ,figure #dd4 ,the other is provided with a hard rubber handle made from a piece of rod4 ,bore a three-eighths hole directly in the top of each brass ball to receive the quadrant rods forming the spark-gap4 ,illustration3 ,fig4 #db4--;,a ,comb or ,collector4 ,the quadrant rods extend over the top of the plates and are three-eighths of an inch in diameter4 ,they are loose in the tops of the balls so that they may be moved about or removed entirely4 ,a small brass ball three-quarters of an inch in diameter should be soldered to the top of one of the quadrant rods and a similar ball two inches in diameter to the other4 ,illustration3 ,fig4 #dc4--,showing how the ,tinsel ,brushes are arranged on the ,neutralizer ,rods4 ,two large brass balls1 two inches in diameter1 are fitted over the ends of the axles1 which project through the uprights4 ,bore a one-quarter-inch hole through each ball at right angles to the axle and slip a one-quarter-inch brass rod through and solder it fast4 ,illustration3 ,fig4 #dd4--,the ,complete ,wimshurst ,electric ,machine4 ,b ,b ,b ,b1 ,brushes4 ,c ,c1 ,combs4 ,d ,b1 ,discharge ,ball4 ,i ,i1 ,glass ,rods4 ,h1 ,handle4 ,q ,q1 ,quadrant ,rods4 ,s ,s ,s ,s ,s1 ,sectors4 ,s ,g1 ,spark-,gap4 ,p ,p1 ,driving-,wheels4 ,for the sake of clearness1 several of the sectors are not shown4 ,the ends of the rods should be tipped with a bunch of tinsel or fine copper wires and be curved so that the brushes so formed will just touch the sectors on the disks when the latter are revolved4 ,these are the neutralizers and are arranged in the approximate positions shown in ,figure #dd4 ,the driving-wheels are connected to the bosses by means of small round leather belts4 ,the belt at the rear of the machine is crossed in order to make the plates revolve in opposite directions4 ,if the machine has been properly built it is now ready for operation4 ,it may be necessary to charge the machine the first time that it is used by touching several of the sectors with the charged cover of an electrophorus4 ,then if the handle is turned the accumulated electricity should discharge across the spark-gap at the top of the machine in the form of bright blue sparks4 ,experiments with an ,electric ,machine ,many interesting experiments can be performed with an electric machine4 ,the number is almost unlimited4 ,a few of the most instructive ones are described below4 ,others can be found in almost any text book on physics4 99,the ,leyden jar99 consists of a glass jar coated with tinfoil part way up on both the outside and inside4 ,through the wooden stopper passes a brass rod or a heavy copper wire which connects with the inner coating of tinfoil by means of a small brass chain4 ,the upper and outside end of the rod usually terminates in a brass ball or knob4 ,it is a very simple matter to make a good ,leyden jar4 ,illustration3 ,fig4 #de4--,the ,leyden ,jar4 ,the jar must be thoroughly cleaned and dried before coating4 ,the inside is then given a thorough brushing over with shellac or varnish4 ,before it is dry1 carefully insert the tin-foil and press it smoothly against the glass4 ,the outside of the jar is treated and coated in the same manner4 ,the inside and outside of the bottom are also coated by cutting the tinfoil in circular pieces and shellacking them on4 ,in order to charge the ,leyden jar1 grasp it in the hand near the bottom and hold the knob against the prime conductor while turning the handle of the machine4 ,illustration3 ,fig4 #df4--;,a ,wooden ,mortar for ,igniting ,gunpowder4 99,igniting gunpowder499 ,bore a hole one-half inch in diameter and one inch deep in a block of hardwood4 ,pass two small brass wires through holes in the sides1 letting the ends of the wires be about one-eighth of an inch apart4 ,pour a little gunpowder in loosely over the wires4 ,tie a piece of thoroughly moistened cotton twine1 three inches long1 to one of the wires and attach it to the outside coating of a charged ,leyden jar4 ,connect the knob of the jar to the other wire4 ,the gunpowder will immediately explode4 ,keep the face and hands away from the gunpowder when performing this experiment4 ,illustration3 ,fig4 #dg4--;,an ,electric ,umbrella4 99,electric ,umbrella499 ,the repulsion of similarly electrified bodies which was illustrated by the action of the pith ball electroscope may be better illustrated by pasting some narrow streamers of tissue paper about one-eighth of an inch wide and four inches long to a small cork covered with tinfoil4 ,the cork is mounted on the upper end of a stiff copper wire supported in a bottle4 ,when the wire is connected to the prime conductor and the machine set in motion1 the strips will spread out like an umbrella4 99,lightning ,board499 ,a pane of glass is thoroughly cleaned and then given a coat of shellac or varnish4 ,before the varnish is dry1 press on a piece of tinfoil large enough to cover one side of the glass and rub it down smoothly4 ,illustration3 ,fig4 #dh4--;,a ,lightning ,board4 ,after the shellac or varnish is dry1 cut the tinfoil up into innumerable little squares with a sharp knife and ruler1 leaving two solid strips of tinfoil at the ends of the glass pane4 ,the pane is mounted by cementing it in a slot in the cork of a bottle4 ,connect one of the tinfoil strips to the prime conductor and the other to the earth or the body4 ,when the machine is turned1 innumerable little sparks will pass between the tinfoil squares and give an appearance very similar to that of lightning4 ,illustration3 ,fig4 #di4--;,an ,electric ,dance4 99,the ,electrical ,dance499 ,a number of little balls of cork or pith are enclosed in a cylinder of glass about two and one-half or three inches high formed by cutting off the top of a lamp chimney4 ,the top and bottom of the cylinder are closed by two circular pieces of sheet brass or copper4 ,the top disk is connected to the prime conductor while the bottom one is connected to the rubber4 ,when the machine is set in motion1 the little balls will dance up and down4 ,bits of feather or paper cut to represent figures of men and women may be used as well as pith or cork balls4 99,the ,electric ,whirl499 ,the whirl consists of an ,s shaped piece of brass wire1 pointed at both ends and supported on a needle by a little conical depression made in the center with a punch4 ,illustration3 ,fig4 #ej4--;,an ,electric ,whirl4 ,the needle is stuck in a cork in the top of a bottle and connected with the prime conductor of the electric machine4 ,when the latter is set in motion1 the whirl will commence to revolve at a high rate of speed4 99,lichtenberg's ,figures99 can be produced by charging a ,leyden jar by connecting the knob or inside coating with the prime conductor and holding the outside coating in the hand4 ,then trace a small circle on the electrophorus bed with the knob4 ,charge a second ,leyden jar by connecting the outside coating with the prime conductor4 ,the inside coating should be connected to the rubber by means of a wire fastened to the knob4 ,the same result may be obtained by connecting the outside coating with the prime conductor and touching the knob with the hand4 ,then trace a cross on the electrophorus bed with the knob1 making the cross inside of the circle4 ,illustration3 ,fig4 #ea4--,lichtenberg's ,figures4 ,shake a mixture of red lead and sulphur through a muslin bag from a height of several inches over the electrophorus4 ,the red lead will accumulate around the cross and the sulphur around the circle4 ,,chapter ,,iv ,,cells ,,and ,,batteries ,in order that the young experimenter may obtain electricity for driving his various electrical devices it is necessary to resort to batteries1 a small dynamo1 or the house-lighting current4 ,all houses are not supplied with electric current4 ,furthermore1 many boys have no source of power from which to drive a small dynamo4 ,batteries must therefore be resorted to in the majority of cases4 ,a number of different cells and batteries are described in this chapter4 ,all of them are practical1 but after buying zinc1 chemicals1 etc41 for any length of time1 figure out what your batteries cost you to make4 ,the real value is not their cost in dollars and cents but in what you have learned in making them4 ,if you have a continuous use for electrical current for running small electrical devices it is cheaper to buy dry cells1 or what is better1 a storage battery1 and have it recharged when necessary4 ,build your own batteries first4 ,then after you have learned how they are made and something about their proper care buy them from some reliable electrical house4 ,batteries are always interesting to the average experimenter1 and when properly made are one of the most useful pieces of apparatus around the home1 laboratory1 or shop that it is possible to construct4 ,many hundreds of thousands of experiments have been carried out by capable men in an effort to discover or devise a perfect battery1 and the list of such cells is very great4 ,only the most common forms1 which are simple and inexpensive to construct but will at the same time render fair service1 have been chosen for description4 ,cells are usually considered one element or jar of a battery4 ,a cell means only one1 while a battery is a group of cells4 ,it is not a proper use of the word to say battery when only one cell is implied4 ,this is a very common error4 99,the ,voltaic cell99 is called after its inventor1 ,volta1 a professor in the ,university of ,pavia1 and dates back to about the year #aghf4 ,illustration3 ,fig4 #eb4--,the ,voltaic ,cell4 ,a simple voltaic cell is easily made by placing some water mixed with a little sulphuric acid in a glass tumbler and immersing therein two clean strips1 one of zinc and the other of copper4 ,the strips must be kept separate from each other4 ,the sulphuric acid must be diluted by mixing it with about ten times its volume of water4 ,in mixing acid with water always remember never to pour water into acid but to perform the operation the other way and pour the acid into the water4 ,a copper wire is fastened with a screw or by soldering to the top of each of the strips1 and care must be exercised to keep the wires apart4 ,as has been said1 the zinc and copper must never be allowed to touch each other in the solution1 but must be kept at opposite sides of the jar4 ,the sulphuric acid solution attacks the zinc1 causing it slowly to waste away and disappear4 ,this action is called oxidation1 and in reality is a very slow process of burning4 ,the consumption of the zinc furnishes the electric energy1 which in the case of this cell will be found to be sufficient to ring a bell or buzzer1 or run a very small toy motor4 ,as soon as the plates are immersed in the acid solution1 bubbles will begin to rise from the zinc4 ,these bubbles contain a gas called hydrogen and they indicate that a chemical action is taking place4 ,the zinc is being dissolved and the hydrogen gas is being set free from the acid4 ,it will be noticed that no bubbles arise from the copper plate and that there is little if any chemical action there4 ,in other words1 it seems that the chemical action at one plate is stronger than that at the other4 ,a cell might be likened to a furnace in which the zinc is the fuel which is burned to furnish the energy4 ,we know that if the zinc is burned or oxidized in the open air it will give out energy in the form of heat4 ,when it is burned or oxidized slowly in acid in the presence of another metal it gives out its energy in the form of electricity4 ,the acid might be likened to the fire1 and the copper to a hand which dips into the cell to pick up the current and takes no part chemically4 ,if a wire is connected to each of the plates and the free ends of the wires touched to the tip of the tongue it will produce a peculiar salty taste in the mouth indicating the presence of a current of electricity4 ,if the wires are connected to an electric bell1 the bell will ring1 or1 instead1 the current may be used to run a small motor4 ,if the cell is made of two zinc plates or two copper plates1 the bell will not ring1 because no electricity will be produced4 ,in order to produce a current1 the electrodes must be made of two different materials upon which the acid acts differently4 ,current may be obtained from a cell made with a zinc and carbon plate or from one with zinc and iron4 ,therefore1 in order to make a battery it is necessary to have a metal which may be consumed1 a chemical to consume or oxidize it1 and an inactive element which is merely present to collect the electricity4 ,when the wires connected to the two plates are joined together1 a current of electricity will flow from the copper plate through the wire to the zinc4 ,the copper is known as the positive pole and the zinc as the negative4 ,a simple voltaic cell may be easily made by cutting out a strip of zinc and a strip of copper1 each #c #a_/b inches long1 and one inch wide4 ,a small hole bored through the upper end of the strips will permit them to be mounted on a wooden strip with a screw as shown in ,figure #ec4 ,the connecting wires are placed under the heads of the screws4 ,care should be exercised to arrange the screws used for mounting the electrodes to the wooden strip so that they do not come exactly opposite1 and there is no danger of the points touching and forming a short circuit4 ,illustration3 ,fig4 #ec4--,the ,elements of ,simple ,voltaic ,cell4 ,illustration3 ,fig4 #ed4--;,a ,home-,made ,voltaic ,cell4 ,an ordinary tumbler or jelly glass will make a good battery jar4 ,the exciting liquid should be composed of ,one part of sulphuric acid ,ten parts of water ,one of the disadvantages of the voltaic cell is that it becomes polarized1 that is1 small bubbles of hydrogen which are liberated by the chemical action collect on the copper plate and cause the strength of the battery to fall off rapidly4 ,there are a great number of elements1 as the zinc and copper are called1 and an even greater number of different solutions or excitants which can be employed in place of sulphuric acid to make a cell1 forming an almost endless number of possible combinations4 99,leclanche ,cell499 ,one of the most common forms of cell employed for bell-ringing1 telephones1 etc41 is called the ,leclanche cell1 after its inventor1 and consists of two elements1 one of zinc and the other of carbon1 immersed in a solution of sal ammoniac or ammonium chloride4 ,this cell has an ,e4 ,m4 ,f4 of #a4d volts1 which is about half as much again as the voltaic cell4 ,illustration3 ,fig4 #ee4--;,carbon-,cylinder ,cell1 and ,cylinder4 ,the most common form of ,leclanche cell is illustrated in ,figure #ee4 ,this type is usually known as a carbon cylinder cell because the positive element is a hollow carbon cylinder4 ,the zinc is in the form of a rod passing through a porcelain bushing set in the center of the carbon cylinder4 ,a battery of such cells can only be used successfully for open circuit work4 ,the open circuit is used for bells1 burglar alarms1 telephone circuits1 etc41 or wherever the circuit is such that it is open most of the time and current is only drawn occasionally and then only for short periods4 ,if the current is drawn for any appreciable length of time hydrogen gas will collect on the carbon cylinder and the cell will become polarized4 ,when polarized it will not deliver much current4 ,many methods have been devised for overcoming this difficulty1 but even the best of them are only partially successful4 ,the usual method is to employ a chemical depolarizing agent4 ,figure #ef shows a ,leclanche cell provided with a depolarizer4 ,the carbon is in the form of a plate placed in a porous cup made of earthenware and filled with manganese dioxide4 ,chemists class manganese dioxide as an oxidizing agent1 which means that it will furnish oxygen with comparative ease4 ,oxygen and hydrogen have a strong chemical affinity or attraction for each other4 ,illustration3 ,fig4 #ef4--;,a ,leclanche ,cell1 showing the ,porous ,cup4 ,if the carbon plate is packed in manganese dioxide any hydrogen which tends to collect on the carbon and polarize the cell is immediately seized by the oxygen of the manganese dioxide and united with it to form water4 ,this form of ,leclanche cell is called the disk type4 ,it is capable of delivering a stronger current for a longer period of time than the carbon cylinder battery4 ,the zinc is usually made in the form of a cylinder1 and fits around the outside of the porous cup4 99,dry ,cells99 are used extensively nowadays for all open circuit work on account of their convenience and high efficiency4 ,the dry cell is not1 as its name implies1 dry1 but the exciting agent or electrolyte1 instead of being a liquid1 is a wet paste which cannot spill or run over4 ,the top of the cell is poured full of molten pitch1 thus effectively sealing it and making it possible to place the cell in any position4 ,dry cells can be purchased from almost any electrical house or garage for twenty-five cents each4 ,it will therefore hardly pay the young experimenter to make his own dry cells4 ,for the sake of those who may care to do so1 however1 directions for building a simple but efficient dry cell of the type used for door-bells and ignition work1 will be found below4 ,illustration3 ,fig4 #eg4--;,a ,dry ,cell4 ,the principle of a dry cell is the same as that of a ,leclanche cell of the disk type4 ,the exciting solution is ammonium chloride1 the electrodes or elements are zinc and carbon1 and the carbon is surrounded by manganese dioxide as a depolarizing agent4 ,obtain some sheet zinc from a plumbing shop or a hardware store and cut out as many rectangles1 #h x #f inches1 as it is desired to make cells4 ,also cut out an equal number of circles #b #c_/h inches in diameter4 ,roll the sheets up into cylinders #b #c_/h inches in diameter inside and #f inches long4 ,the edges are lapped and soldered4 ,fit one of the round circles in one end of each of the cylinders and solder them securely into place1 taking care to close up all seams or joints which might permit the electrolyte to escape or evaporate4 ,secure some old carbon rods or plates by breaking open some old dry cells4 ,the carbons will be in the form of a flat plate1 a round rod1 or a star-shaped corrugated rod1 depending upon the manufacture of the cell4 ,any of these types of carbons will serve the purpose well1 provided that they are fitted with a thumb-screw or a small bolt and nut at the top so as to make wire connections with the carbon4 ,make a wooden plunger of the same shape as the carbon which you may select1 but make it slightly larger4 ,smooth it with sandpaper and give it a coat of shellac to prevent it from absorbing moisture4 ,this wooden plunger is temporarily inserted in the center of one of the zinc cups and supported so that it will be about one-half inch above the bottom4 ,the electrolyte is prepared by mixing together the following ingredients in the proportions shown3 ,sal ,ammoniac4 #a part ,zinc ,chloride4 #a part ,plaster of ,paris4 #c parts ,flour4 #c_/d part ,water4 #b parts ,illustration3 ,fig4 #eh4--,the ,different ,operations involved in ,making a ,dry ,cell4 ,the above paste is then firmly packed into the zinc shell around the wooden plunger1 leaving a space of about #c_/d of an inch at the top4 ,the paste can be poured in very readily when first mixed but sets and hardens after standing a short while4 ,after it has set1 withdraw the wooden plunger1 thus leaving a space inside of the dry cell a little larger than the carbon4 ,the carbon is now inserted in this hole and the surrounding space is filled with a mixture composed of3 ,sal ,ammoniac4 #a part ,zinc ,chloride4 #a part ,manganese ,dioxide4 #a part ,granulated ,carbon4 #a part ,flour4 #a part ,plaster of ,paris4 #c parts ,water4 #b parts ,the granular carbon may be had by crushing up some old battery carbons4 ,the parts given in both of the above formulas are proportioned so that they may be measured by bulk and not by weight4 ,an old teaspoon or a small cup will make a good measure4 ,each one of the zinc shells should be filled in this manner4 ,after they have all been filled1 clean off the top edge of the zinc and pour the remaining space in the cell full of molten tar or pitch so as to seal it over4 ,solder a small binding-post to the top edge of the zinc to facilitate connection4 ,then wrap the cells in several thicknesses of heavy paper to prevent them from short circuiting1 and they are ready for use4 ,a small hole bored through the sealing material after it is dry will provide a vent for the escape of gases4 99,recharging dry cells99 is a subject that interests most experimenters4 ,dry cells very often become useless before the zinc shell is used up or the chemicals are exhausted1 due to the fact that the water inside of the cell dries up and the resistance therefore becomes so great that it is practically impossible for the current to pass4 ,the life of such cells may be partially renewed by drilling several holes in the cell and permitting it to soak in a strong solution of sal ammoniac until some of the liquid is absorbed4 ,the holes should then be plugged up with some sealing wax in order to prevent evaporation4 ,an old dry cell may be easily turned into a wet cell by drilling the zinc full of holes and then setting it in a jar containing a sal ammoniac solution4 ,the battery should be allowed to remain in the solution4 99,wet batteries99 are very much easier to make than dry batteries and are capable of delivering more current4 ,they have the disadvantage1 however1 of wasting away more rapidly1 when not in service1 than dry cells4 ,the ,leclanche cell is the type generally first attempted by most experimenters4 99,carbon plates99 for making such a battery are most easily and cheaply obtained from old dry cells4 ,about the only way that a dry cell can be broken open is with a cold-chisel and a hammer4 ,care must be taken1 however1 in order not to break the carbon4 ,ordinary jelly-glasses make good jars for small cells4 ,fruit-jars may be used for larger batteries by cutting the tops off so that the opening is larger4 ,the carbon plate contained in a dry cell is usually too long for a jar of this sort and must be broken off before it can be used4 ,the lower end is the one which should be broken because the top carries a binding-post1 with which connections can be made4 ,a small hole is bored in the carbon rod at a distance from the bottom equal to the height of the jar which is to be used4 ,illustration3 ,fig4 #ei4--;,a ,zinc-,carbon ,element1 made from ,heavy plates4 ,considerable care must be used in boring carbon because it is very brittle and easily cracks4 ,only very light pressure should be used on the drill4 ,the carbon is fastened to a strip of wood1 about an inch and one-quarter wide1 one-half an inch thick1 and a little longer than the top of the glass jar is wide4 ,illustration3 ,fig4 #fj4--;,a ,method of making a ,cell ,element from ,carbon ,rods4 ,a piece of heavy sheet zinc is fastened on the other side opposite the carbon1 with a screw4 ,it is a good idea to paint the screws and the surrounding portions of both the zinc and the carbon with hot paraffin wax so that the solution will not creep and attack the screws4 ,it is also a good plan first to soak the wooden strip in some hot paraffin until it is thoroughly impregnated4 ,ammonium chloride1 or1 as it is more commonly called1 sal ammoniac1 should be added to a jar of water until it will dissolve no more4 ,the zinc and carbon elements may then be placed in the solution4 ,one of the great disadvantages of the voltaic cell is that the zinc is attacked by the acid when the battery is not in use and cannot be allowed to remain in the solution without quickly wasting away4 ,this is true in the case of the ,leclanche cell only to a very small extent4 ,the voltaic cell is more powerful than the ,leclanche cell1 but the elements must be carefully lifted out and rinsed with water every time that you are through using the cell4 ,by using several carbon plates instead of one1 the cell may be made more powerful4 ,the illustrations show several ways of accomplishing this4 ,the simplest method is to place a carbon plate on each side of the wooden strip and use a zinc in the form of a rod which passes through a hole between the two4 ,care must always be used to keep any screws which are used to hold the carbons or zincs in position in the cells from touching each other4 ,illustration3 ,fig4 #fa4 ,an ,element made from two ,carbon ,plates and a ,zinc ,rod4 ,in ,figure #fb an arrangement of using four carbons is shown4 ,the drawing is self-explanatory4 ,in any of the cells using more than one carbon element1 the carbons should all be connected4 ,in discussing the voltaic cell we mentioned the fact that it becomes polarized1 and explained this phenomenon as being caused by hydrogen bubbles collecting on the copper or positive pole4 ,the same thing happens in the case of carbon or any other material which is used as a positive4 99,polarization99 is the bugbear of batteries4 ,it can be eliminated to a certain extent1 however1 by the use of a depolarizer placed in the solution4 ,there are several such substances1 the most common being sodium bichromate and potassium bichromate4 ,these are used in battery preparations on the market called ,electric ,sand1 ,electropoian ,fluid1 etc4 ,illustration3 ,fig4 #fb4 ,a ,method of ,mounting four ,carbon ,plates4 ,when one of these is added to a sulphuric acid solution1 using zinc and carbon as the battery elements1 it forms a very powerful cell1 having ,e4 ,m4 ,f4 of two volts4 ,a battery solution of this kind may be prepared by adding four ounces of bichromate of potash to a solution composed of four ounces of sulphuric acid mixed with sixteen ounces of water4 ,the battery will give a more powerful current for a longer time when this solution is used instead of the plain sulphuric acid and water or sal ammoniac4 ,illustration3 ,fig4 #fc4--;,a ,battery ,element arranged for three ,cells4 ,it might be well at this time to caution the experimenter against the careless handling of sulphuric acid4 ,it is not dangerous if handled properly1 but if spilled or spattered around carelessly it is capable of doing considerable damage to most things with which it comes in contact4 ,do not attempt to use it in any place but a shop or cellar4 ,the smallest drop coming in contact with any organic matter such as woodwork1 clothing1 carpets1 etc41 will not only discolor any of the latter1 but eat a hole in it4 ,the best thing to use to counteract the effects of the acid which has been spilled or spattered is water in sufficient quantity to drench things and dilute the acid enough to render it harmless4 ,a little strong ammonia will neutralize the acid and sometimes restore the color to clothing which has been burned by acid4 ,illustration3 ,fig4 #fd4--;,a ,plunge ,battery1 with ,windlass4 ,all acid batteries of this sort have the one objection that it is impossible to leave the elements in the solution without wasting the zinc4 ,the usual way to arrange the battery cells so that the elements may be removed from the solution most easily is to fasten the elements to a chain or cord passing over a windlass fitted with a crank so that when the crank is turned the elements may be raised or lowered as desired4 ,a plunge battery of this sort is illustrated in ,figure #fd4 ,the construction is so plainly shown by the drawing that it is hardly necessary to enter into the details4 ,the crank is arranged with a dowel-pin which passes through into a hole in the frame1 so that when the elements are lifted out of the solution the pin may be inserted in the hole and the windlass prevented from unwinding4 ,illustration3 ,fig4 #fe4--;,a ,plunge ,battery adapted to a ,set of ,elements1 as shown in ,figure #fc4 ,they may be lifted out and placed on the ,arms to drain4 ,a somewhat easier method of accomplishing the same result is that shown by ,figure #fe4 ,in this1 the elements are simply raised up out of the jars and laid across the two arms to drain4 99,the ,edison-,lalande99 cell employs a block of pressed copper oxide as the positive element1 while two zinc plates form the negative4 ,the exciting liquid is a strong solution of caustic soda4 ,illustration3 ,fig4 #ff4--;,an ,edison-,lalande ,cell4 ,the copper oxide acts both as the positive element and as a depolarizer1 for the oxygen of the oxide immediately combines with any hydrogen tending to form on the plate4 ,this type of cell has some advantages but also many disadvantages1 chief among which is the fact that the ,e4 ,m4 ,f4 is very low4 ,it is used principally for railway signal work1 slot-machines1 etc4 99,a ,tomato-,can ,battery99 using caustic soda as the exciting liquid is a simple form of home-made battery whose only disadvantage is the low voltage that it delivers4 ,illustration3 ,fig4 #fg4--;,a ,tomato-,can ,cell2 ,sectional ,view4 ,illustration3 ,fig4 #fh4--,the ,tomato-,can ,cell ,complete4 ,the cell is liable to polarization1 but the large surface of its positive elements protects it to some extent4 ,the positive element and the outer vessel is a tomato can4 ,within it is a porous cup made out of blotting paper or unglazed earthenware such as a flower pot4 ,the space between the can and the porous cup is filled with fine scrap-iron such as borings and turnings4 ,a zinc plate is placed in the porous cup4 ,the cell is filled with a ten-per-cent solution of caustic soda4 ,the following table gives the names1 elements1 fluids1 voltage1 etc41 of the most useful batteries1 all of which may be easily constructed by the experimenter4 ,secondary or ,storage ,batteries ,the storage battery is a very convenient means of taking energy at one time or place and using it at some other time or place4 ,small storage batteries are used in automobiles to supply current for the headlights and spark-coils4 ,many automobiles are now equipped with electric starters1 consisting of a dynamo-motor and a storage battery4 ,throwing a switch will cause the current from the storage battery to drive the motor and crank the engine4 ,after the engine is started1 the motor acts as a dynamo and generates a current for recharging the storage battery4 ,storage batteries are also used to drive electric vehicles and cars4 ,many central lighting and power stations employ storage batteries to supply the extra current demanded during rush hours4 ,in the middle of the day1 when the load is light1 the surplus current of the dynamos is used to recharge the storage batteries4 ,what is really effected in the storage battery is the electrical storage of energy1 not the storage of electricity4 ,properly speaking1 the energy is put into the form of chemical energy1 and there is really no more electricity in the cell when it is charged than after it is discharged4 ,illustration3 ,fig4 #fi4--,two ,methods of ,connecting ,cells so as to obtain ,different ,voltage and ,amperage ,values4 ,storage batteries are made up of plates of lead 7the electrodes7 or an alloy of lead cast into a grid or framework4 ,the framework may be one of a large number of patterns1 but usually consists of a set of bars crossing one another at right angles1 leaving a space between4 ,the spaces are filled with a paste of lead oxide4 ,they are then formed by placing in a tank of acid solution and connected to a source of electric current4 ,illustration3 ,fig4 #gj4--,small ,storage ,cells4 ,the plate connected to the positive wire gradually turns dark-brown in color1 due to the changes in the paste1 which gradually turns into lead peroxide4 ,the paste in the negative plate becomes gray in color and changes into a form of metallic lead called spongy lead4 ,the positive and negative plates are placed in a bundle after the forming process has been completed4 ,they are kept apart by strips of wood or rubber called separators4 ,the negative plates of one cell are all connected in parallel at one end of the cell4 ,the positive plates are connected at the other end4 ,the liquid surrounding the plates is diluted sulphuric acid4 ,when the battery has been exhausted1 it is charged by connecting a dynamo with the terminals of the battery and sending a current through it4 ,this current reverses the chemical action1 which goes on during the discharge of the battery4 99,a ,storage ,battery99 furnishes the most convenient source of current for performing all sorts of electrical experiments4 ,it is capable of giving more current for a longer period than dry cells and is not expensive1 for it merely requires recharging and does not have to be thrown away each time the current is used up4 ,the storage cell described below is made in a very simple manner and will well repay any time or expense spent in its construction4 ,illustration3 ,fig4 #ga4--;,how to make the ,plates for a ,storage ,cell4 ,the plates are cut out of a large sheet of lead1 one-quarter of an inch thick4 ,they may be made any convenient size to fit the jars which the experimenter may have at hand4 ,we will assume that they are to be made two and seven-eighths inches wide and three and one-half inches long4 ,they will then fit the rectangular glass storage cell which is already on the market and can be procured from dealers in electrical supplies4 ,a long terminal or lug is left projecting from the plate as shown in ,figure #ga4 ,any number of plates may be placed in a single cell1 depending of course upon the size of the glass jar4 ,we will suppose that three will just fit the jar nicely4 ,an odd number of plates should always be used1 so that a positive plate may come between two negatives4 ,each cell will give two volts regardless of the number of plates4 ,increasing the number of plates1 however1 will give the cell a greater amperage capacity and make the charge last longer4 ,three cells 7six volts7 will form a convenient set for running small fan-motors1 miniature lights1 etc4 ,cut out nine plates and pile them up in sets of three with a piece of thin wood 7cigar-box wood7 between each pair of plates4 ,clamp them together in a vise and bore full of one-quarter-inch holes4 ,the plates are now ready for pasting4 ,they are placed on a smooth slab of stone or glass and pasted with a stiff mixture of red lead and sulphuric acid 7two parts water to one part acid74 ,the paste must be pressed carefully into the recesses of the plates with a flat stick4 ,they are then laid aside to dry and harden4 ,illustration3 ,fig4 #gb4--,the ,wood ,separator4 ,after they have thoroughly dried they should be assembled as in ,figure #gc with one positive plate between two negative ones4 ,the wooden separators are easily cut out of wood with a saw and penknife4 ,the thin wood used in the construction of peach baskets is the best for the purpose4 ,the separators should be made the same size as the lead battery plates4 ,each group of plates is then placed in a jar containing a mixture of sulphuric acid and water 7#d parts water to one part acid74 ,in mixing the acid be very careful to pour the acid into the water1 stirring the mixture slowly at the same time1 and not the water into the acid4 ,illustration3 ,fig4 #gc4--,the ,complete ,element for a ,storage ,cell4 ,the plates are now ready for forming4 ,the binding-posts on the lugs of the plates may be secured from the carbons of some old dry cells4 ,the simplest method of forming the plates is to use four gravity cells and form one storage cell at a time4 ,illustration3 ,fig4 #gd4--;,a ,battery of ,home-,made ,storage ,cells4 ,connect the positive pole 7copper7 of the gravity battery to the positive pole 7center-plate7 of the storage cell and the negative 7zinc7 of the gravity battery to the negative 7outside plates7 of the storage cell4 ,allow the current to flow through the storage battery for several days or until the positive plate turns to a dark chocolate-brown color and the negatives to a gray-slate4 ,illustration3 ,fig4 #ge4--;,gravity ,cells4 ,these consist of zinc and copper elements1 immersed in a zinc-copper sulphate solution4 ,they cannot be easily made1 and are best purchased4 ,the illustration also shows the star-shaped copper and crowfoot zinc element used in a gravity cell4 ,after the cells have once been formed all that they require is occasional recharging from gravity cells or from a dynamo1 by connecting the positive pole of the charging current to the positive plates of the storage cells and the negative pole to the negative plates4 ,when the cells are fully charged1 bubbles of gas will rise freely from the plates4 ,if a dynamo is used it must be shunt wound and not a series machine4 ,recharging will only require about one-quarter of the time consumed in forming4 ,it is a very good plan to connect twelve gravity cells in series and use them to recharge the storage battery4 ,the gravity cells can always be kept connected to the storage cells when the latter are not in use and thus remain fully charged and ready to supply their maximum current4 ,after the cells have been in use for some time1 it is a good plan to lift out the plates and remove all sediment which has settled to the bottom of the jars4 ,a set of three such storage cells will have an ,e4 ,m4 ,f4 of over six volts4 ,any number may be connected up in series in order to obtain a higher voltage4 ,storage batteries are usually rated in ampere hours4 ,an ampere hour is the amount of current represented by one ampere flowing for one hour4 ,a ten-ampere-hour storage battery will deliver3 ,one ampere for ten hours ,two amperes for five hours ,five amperes for two hours ,ten amperes for one hour ,in other words1 the result obtained by multiplying the number of amperes by the time in hours is the ampere hour capacity4 ,a dynamo must have an ,e4 ,m4 ,f4 of about ten volts in order to charge a three-cell storage battery4 ,,chapter ,v ,,electro-,,magnetism ,,and ,,magnetic ,,induction ,connect two copper wires to a voltaic cell and stretch a portion of the wire over a compass needle1 holding it parallel to it and as near as possible without touching4 ,then bring the free ends of the wires together and observe that the needle is deflected and after a few movements back and forth comes to rest at an angle with the wire4 ,illustration3 ,fig4 #gf4--;,a ,current of ,electricity flowing through a ,wire will deflect a ,compass ,needle4 ,next form a rectangular loop of wire and place the needle within it as in ,figure #gg4 ,a greater deflection will now be obtained4 ,if a loop of several turns is formed1 the deflection will be still greater4 ,these experiments were first performed by ,oersted1 in #ahai1 and show that the region around a wire carrying a current of electricity has magnetic properties4 ,illustration3 ,fig4 #gg4--;,if a ,loop of ,wire is formed about a ,compass ,needle1 the ,deflection will be greater4 ,another interesting experiment showing the magnetic effect of a current of electricity when passing through a wire may be performed by connecting a heavy copper wire to two or three bichromate-of-potash cells4 ,dip the wire into a pile of fine iron filings and a thick cluster of them will adhere to the wire as in ,figure #gh4 ,as soon as the circuit is broken so that the current of electricity ceases flowing1 the filings will fall off1 showing that the magnetic effect ceases with the current4 ,illustration3 ,fig4 #gh4--;,iron ,filings clustered on a ,wire carrying a ,current of ,electricity4 ,these three simple experiments have shown that if a current of electricity is passed through a copper wire1 the wire will deflect a compass needle1 attract to itself iron filings1 etc41 as long as the current continues to flow4 ,as soon as the current is shut off1 the magnetic effect is destroyed4 ,the region in the neighborhood of a wire carrying a current is a field of force through which lines of magnetism are flowing in exactly the same way that they do in the neighborhood of a bar or horseshoe magnet4 ,illustration3 ,fig4 #gi4--,magnetic ,phantom formed about a ,wire carrying a ,current of ,electricity4 ,this is readily shown by punching a small hole in a piece of cardboard1 and passing a wire carrying a strong current of electricity through the hole4 ,if a few iron filings are sifted on the cardboard and the latter jarred slightly with a pencil as they fall1 they will arrange themselves in circles with the wire at the center1 forming a magnetic phantom and showing the paths of the lines of magnetic force4 ,illustration3 ,fig4 #hj4--,magnetic ,phantom formed about several ,turns of wire4 ,by forming the wire into a coil as in ,figure #hj the magnetic field generated is much stronger and more plainly seen1 for then the combined effect of the wires is secured4 ,illustration3 ,fig4 #ha4--,paper ,tube wrapped with ,wire for ,experimental ,purposes4 ,roll up a small paper tube about #a_/b inch in diameter and four inches long4 ,wind neatly on the tube three layers of ,no4 #ah insulated copper wire4 ,pass an electric current through it from two or three cells of a battery1 and test its magnetic properties by bringing it near a compass needle4 ,it will be found that the coil possesses very marked magnetic properties1 and will readily cause the needle to swing about1 even though it is held quite a distance away4 ,if an iron bar is placed inside of the paper tube1 the magnetic effect will be greatly increased4 ,illustration3 ,fig4 #hb4--,showing how the ,lines of ,force ,leak at the sides of the coil1 from a ,coil of ,wire1 and how they are concentrated by an ,iron ,core4 ,the presence of the iron bar inside of the coil of wire greatly increases the number of lines of force running through the coil4 ,illustration3 ,fig4 #hc4--,the ,principle of an ,electro-,magnet4 ,when a bar is not used1 many of the lines of force leak out at the sides of the coil1 and but few extend from end to end4 ,the effect of the iron core is not only to diminish the leakage of the lines of force1 but also to add many more to those previously existing4 ,hence the magnetic strength of a coil is greatly increased by the iron core4 ,a coil of wire wrapped around an iron core forms an electro-magnet4 ,illustration3 ,fig4 #hd4--;if you wrap some insulated ,wire around an ,ordinary ,nail and connect it to a ,battery1 it will become an ,electro-,magnet4 ,if you wrap some insulated wire around an ordinary nail and connect it to one or two cells of a battery it will become an electro-magnet and pick up bits of iron and steel4 ,if you wind the wire around a small paper tube into which a nail will slide easily1 the coil will draw the nail in when the current is turned ,on4 ,a hollow coil of this sort is called a solenoid4 ,electro-magnets and solenoids play a part in the construction of almost all electrical machinery4 ,they form the essential parts of dynamos1 motors1 telephone receivers1 telegraph relays and sounders1 and a host of other devices4 ,the form usually given to an electro-magnet depends upon the use to which it is to be put4 ,the horseshoe is the most common4 ,this consists of two electro-magnets mounted on a yoke and connected so that the two free poles are ,north and ,south4 ,illustration3 ,fig4 #he4--;,if you wind the ,wire around a small ,paper ,tube into which a ,nail will slide easily1 the ,coil will draw the ,nail in when the ,current is turned on4 ,electro-magnets are made on a huge scale for lifting large castings and heavy pieces of iron4 ,such magnets are used in the great steel mills and in factories where nails1 bolts1 etc41 are manufactured4 ,monster electro-magnets can be seen in wonderful perfection at the great steel mill at ,gary1 ,indiana4 ,ships bring the ore down the lakes to ,gary1 where great steel jaws lift it out of the hold of the boat and carry it to the furnaces4 ,after being melted1 great machines pour it out4 ,it is divided into huge ingots1 and these1 while hot1 are carried to the first part of the rolling mill4 ,the ingot is squeezed by a machine1 made longer and narrower1 then squeezed again and made still longer and narrower4 ,it is started on its journey along the rollers of the mill1 squeezed and pressed here and there1 as it travels hundreds of yards--no hand ever touching it4 ,it finally arrives1 a red-hot steel rail1 the right shape and the right length4 ,during this time the steel has made a long journey and changed from a shapeless ingot to a finished rail1 handled entirely by machinery guided and controlled by one or two operators1 pressing levers and switches4 ,when almost finished1 the rail slides down an incline before a man who grasps the rail with huge pinchers1 and standing at one end1 runs his eye along it4 ,as he looks along the rail he sees the defects1 moves the left or the right hand1 and another man at the levers of the straightening machine1 straightens the rail as directed4 ,and soon there are ten rails1 perfectly straight1 side by side1 with more coming down the incline to meet the glance of the man's eye4 ,they are still too hot for any man's touch and so a man sitting in a tower touches an electric switch1 and along the overhead rails there comes gliding a monster electro-magnet4 ,the magnet moves along1 drops down upon the ten rails1 lying side by side and weighing thousands of pounds4 ,the man in the tower presses another switch1 thus turning on the current1 and electricity glues the rails to the magnet4 ,illustration3 ,by permission1 from ,solenoids by ,c4 ,r4 ,underhill4 ,lifting-,magnets of the type known as ,plate1 ,billet1 and ,ingot ,magnets4 ,the ten rails are lifted at once1 as easily as you would lift a needle with your horseshoe magnet2 they are carried to a flat-car1 and when lowered in position1 the current is turned off1 releasing the rails1 and the magnet travels back for another load4 ,induction ,in #ahca1 ,michael ,faraday1 a famous ,english chemist and physicist1 discovered that if a magnet be suddenly plunged into a hollow coil of wire1 a momentary current of electricity is generated in the coil4 ,as long as the magnet remains motionless1 it induces no current in the coil1 but when it is moved back and forth1 it sets up the currents4 ,the source of electrical energy is the mechanical work done in moving the magnet4 ,illustration3 ,fig4 #hf4--,showing how a ,current of ,electricity may be induced by a ,bar ,magnet and a ,coil4 ,the medium which changes the mechanical energy into electricity is the magnetic field which we have already seen exists in the neighborhood of a magnet4 ,a current of electricity produced in a coil in such a manner is said to be an induced current and the phenomenon is that known as magnetic induction4 ,magnetic induction is met in the dynamo1 induction coil1 telephone1 transformer1 some forms of motors1 and a number of other electrical devices4 ,illustration3 ,fig4 #hg4--;,a ,horseshoe ,magnet and a ,coil arranged to produce ,electric ,currents by ,induction4 ,a simple experiment in which electricity is produced by magnetic induction may be performed by winding a number of turns of fine insulated wire around the armature or keeper of a horseshoe magnet1 leaving the ends of the iron free to come in contact with the poles of the permanent magnet4 ,connect the ends of the coil to a sensitive galvanometer1 the ends of the armature being in contact with the poles of the horseshoe magnet as shown in ,figure #hg4 ,keeping the magnet fixed1 suddenly pull off the armature4 ,the galvanometer will show a momentary current4 ,suddenly bring the armature up to the poles of the magnet2 another momentary current in the reverse direction will flow through the circuit4 ,the fact that it is a reverse current is shown by the actions of the galvanometer for it will be noticed that the needle swings in the opposite direction this time4 ,it will also be noticed that no current is produced when the coil and magnet are stationary4 ,current is only generated when the coil and magnet are approaching one another or moving apart suddenly4 ,this is because it is only then that the magnetic field is changing4 ,the field is strongest nearest the magnet1 and therefore if either the magnet or the coil of wire is moved1 the strength of that part of the field which intersects the coil is changed4 ,induced currents can only be generated by a changing magnetic field4 ,see chapter on ,measuring ,instruments4 ,,chapter ,,vi ,,electrical ,,units ,the ,ampere ,there are certain terms used in the electrical field to distinguish various properties and qualities of the electrical current with which it is well for the young experimenter to acquaint himself4 ,one of the first units usually required1 in order to make intelligent comparisons1 is a unit of measure4 ,the quart is the unit of measure commonly applied to liquids and is based upon the amount of space occupied by a certain volume4 ,the pound is a unit of weight which determines a certain amount of any substance by comparing the force which gravity exerts in pulling it to the earth with the same effect of gravity on another standard weight4 ,electric current is invisible and weightless1 and for these and other reasons cannot be measured by the quart or weighed by the pound4 ,the only way that it can be measured is by means of some of the effects which it produces4 ,either the chemical1 electro-magnetic1 or the heating effects may be made the basis of a system of measurement4 ,the first method used to measure electric current was the chemical one4 ,if a current is passed through a solution of a chemical called copper sulphate 7blue vitriol7 by means of two copper plates1 copper will be deposited on one plate and dissolved from the other4 ,if the current is furnished by a battery the copper will be deposited on the plate connected with the zinc of the battery4 ,if the current is allowed to flow for a short time and the two copper plates are then taken out and weighed it will be found that one plate is considerably heavier than the other4 ,the copper has been taken from one plate and deposited on the other by the electric currents4 ,the amount of electric current which will deposit #a4agg grammes of copper in an hour is called an ampere4 ,the ampere is the unit of electrical current measurement1 and implies quantity or amount4 ,the chemical method of measuring current was at one time put to practical service in the distribution of electric current for lighting and power4 ,many years ago the house meters1 used to measure the current1 consisted of a jar containing two copper plates4 ,the current used in the house would cause copper to deposit on one plate1 and by weighing the plate the power company could determine the amount of current used1 and thereby the amount of the bill4 ,the meters nowadays make use of the magnetic effects of the current instead of the chemical1 as described later on4 ,the ,volt ,for purposes of explanation the electric current may be likened to a stream of water flowing through a pipe4 ,if you hold your thumb over the end of a water-pipe through which water is flowing it will push your thumb away because of the pressure which the water exerts4 ,electric currents also exert a pressure1 only it is not called pressure in electrical parlance1 but1 spoken of as electromotive force or potential4 ,the pressure of the water enables it to pass through small openings and to overcome the resistance offered by the pipe4 ,wires and other electrical conductors do not offer a perfectly free path to an electric current1 but also possess a resistance4 ,it is the potential of the electro-motive force which overcomes the resistance and pushes the current through the wire4 ,advantage has been taken of the fact to fix a unit of electrical pressure called the volt4 ,the pressure of the water in a water-pipe is measured in pounds1 but the pressure of an electric current in a wire is measured by volts4 ,the volt is the unit of electrical force which will cause a current of one ampere to flow through a resistance of one ohm4 ,the ,ohm ,the ohm is the unit of electrical resistance4 ,the standard ohm is the resistance offered by a column of pure mercury having a section of one square millimeter and a length of #ajf4bh centimeters at a temperature of #j centigrade4 ,the pressure which will force sufficient current through such a column of mercury to deposit #a4agg grammes of copper in one hour is a volt1 and in doing so has passed a current of one ampere through a resistance of one ohm4 ,the units ohm1 ampere1 and volt1 were named in honor of the three great electricians3 ,ohm1 ,ampre1 and ,volta4 ,these three units bear a very close relation to each other which is explained by ,ohm's ,law4 ,ohm's ,law is a simple statement of facts which it is well for the young electrician thoroughly to understand1 for it might almost be said to be the basis of design of almost all electrical instruments4 ,it is simply this3 ,the strength of a current equals the voltage divided by the resistance4 ,it may be expressed in symbols by3 ,c ,e_/,r4 ,where ,c is the current in amperes1 ,e is the potential in volts1 and ,r the resistance in ohms4 ,by way of a simple example1 we will suppose that a small telegraph sounder is connected to a battery and that the voltage of the battery is ten volts4 ,we will further suppose that the resistance of the sounder connecting wires and the battery itself is five ohms4 ,knowing these two facts1 it is very easy to find out how many amperes are flowing through the sounder by substituting these values in the equation as follows3 ,c ,e_/,r ,e #aj volts and ,r #e ohms therefore ,c #aj_/e or #b amperes ,in order to indicate fractions or very large values of the ampere1 volt1 and ohm1 it is customary to use the following terms3 ,milli-volt #a_/ajjj of a volt ,mill-ampere #a_/ajjj of an ampere ,kilo-volt #ajjj volts ,meg-ohm #a1jjj1jjj ohms ,the ,watt ,it is no doubt perfectly plain that the water in a certain size of pipe at a pressure of #ajj lbs4 is more powerful than a stream of water in the same size of pipe at #be lbs4 pressure4 ,likewise a current of electricity represents more power at #ajj volts potential than the same current would at #be volts4 ,the unit of electrical power is called the watt4 ,a watt is represented by a current of one ampere flowing through a wire at a potential of one volt4 ,the number of watts is found by multiplying the voltage by the amperage4 ,in the case of the sounder and battery used as an example to explain ,ohm's ,law1 and where the voltage was #aj and the amperage found to be #b1 the number of watts is #aj x #b1 or #bj watts4 ,seven hundred and forty-six watts represent one electrical horse-power4 ,one thousand watts are called a kilo-watt4 ,the ,coulomb ,so far1 none of the units have taken into consideration the element of time4 ,if water should be permitted to run out of a pipe into a tank until ten gallons had passed it would not be possible to tell at what rate the water was flowing by knowing that ten gallons had passed unless it were also known how long the water had been flowing4 ,ten gallons per minute or ten gallons per hour would indicate the rate of flow4 ,one ampere flowing for one second is the electrical unit of flow4 ,this unit is called the coulomb4 ,one ampere flowing for one hour is called an ampere hour4 ,the number of ampere hours is found by multiplying the current in amperes by the time in hours4 ,a battery may be said to have a capacity of #aj ampere hours4 ,this means that it will deliver one ampere for #aj hours 7#a ampere x #aj hours #aj ampere hours7 or #b amperes for #e hours 7#b amperes x #e hours #aj ampere hours74 ,the same element of time enters into consideration in connection with the watt4 ,one watt flowing for one hour is a watt hour and one kilowatt flowing for one hour is a kilo-watt hour4 ,the ,difference between ,alternating and ,direct ,currents ,there are two distinct kinds of electric current supplied for lighting and power1 one known as direct current and the other as alternating4 ,a direct current is one which passes in one direction only4 ,it may be represented by a straight line1 as ,a in ,figure #hh4 ,an alternating current is one which reverses its direction and passes first one way and then the other4 ,it may be represented by a curved line1 shown in ,figure #hh4 ,it starts at zero1 and gradually grows stronger and stronger4 ,then it commences to die away until no current is flowing4 ,at this point it reverses and commences to flow in the opposite direction1 rising gradually and then dying away again4 ,this is repeated a definite number of times per second2 when the current rises from zero1 reverses and returns to zero1 it is said to pass through a cycle4 ,illustration3 ,fig4 #hh4--;,graphic ,representation of a ,direct and an ,alternating ,current4 ,the part of the curved line from a to b in ,figure #hh represents the first part of the current1 when it is rising4 ,from b to c represents its fall4 ,the point at which the curved line crosses the straight line is zero4 ,at c the current crosses the line and commences to flow in the opposite direction until it reaches d1 at which point it dies away and again crosses the line to flow in its original direction and repeat the cycle4 ,in electrical parlance1 that part of the current from a to c or from c to e is known as an alternation4 ,from a to e is called a cycle4 ,the reason why alternating current is often used in place of direct current is that it can be sent over the wires for long distances more economically than direct current4 ,this is more fully explained farther on in the chapter dealing with a step-down transformer4 ,the number of cycles taking place in one second is known as the frequency of the current4 ,the usual frequency of commercial alternating currents is #fj cycles per second or #gbjj alternations per minute4 ,,chapter ,,vii ,,electrical ,,appurtenances ,wires ,electric currents are usually led from place to place1 at will1 by means of conductors called wires4 ,there are a great many kinds of wires1 each adapted to some special purpose4 ,wires are usually covered with a material called an insulator1 in order to prevent the loss of electric current due to the wires coming into contact with other bodies or circuits4 ,insulators are substances which do not conduct electricity4 ,wires which are insulated by heavy braids of cotton fiber and then impregnated with some compound1 such as creosote1 are called weather-proof wires1 and are best adapted to outside service1 where they must be exposed to the action of the elements4 ,the wires used for interior wiring in buildings1 etc41 are usually insulated with rubber1 over which is placed a cotton braid to protect the rubber4 ,rubber cannot well be used as an insulator for all wires1 although its insulating value is very great1 owing to the fact that it deteriorates under many conditions4 ,rubber-covered and weather-proof wires are made in a variety of insulations4 ,some may have only one insulating layer1 while others have a great many4 ,different substances are used as insulators to adapt the wire to some special purpose4 ,copper is usually the only metal used to form the wire or conductor itself4 ,the reason for this is that copper is a better conductor than any other metal except those known as precious metals1 such as gold and silver1 the cost of which prohibits their use for such purposes4 ,the wire may be solid1 or made up of a number of small conductors so that it is flexible4 ,the various combinations of insulating layers1 together with either a solid or a stranded conductor1 have made possible a variety of current-carriers1 known as3 ,theater or ,stage ,cable ,elevator ,cable ,fixture ,wire ,telephone ,wire ,mining ,cable ,feeder ,cable ,brewery ,cord ,heater ,cord1 etc4 depending upon the special use for which they were designed4 ,the wires which the young experimenter is likely to use in his work the most are known as magnet wires1 and are used for making electro-magnets1 coils1 and various windings4 ,magnet wires may be insulated with either silk1 cotton1 or enamel4 ,silk-covered and cotton-covered wires may be obtained with either a single or double covering4 ,wires with a single covering of silk or enamel are used when it is desirable to save space1 for the covering of these two classes of magnet wires is thinner than either the cotton or double-silk-covered wire1 and consequently they require less room for winding4 ,the size of the wire is indicated by its diameter1 and in the ,united ,states is measured by the ,brown and ,sharpe gauge1 often indicated by the term1 ,b4 ,s4 ,the preceding table shows the various sizes of wire of the ,brown and ,sharpe gauge1 and also several of their characteristics1 such as weight1 resistance1 etc4 ,insulators ,the covering placed over wires is not the only precaution taken to insulate them1 but in the case of permanent wiring they are usually mounted on glass or porcelain supports4 ,illustration3 ,fig4 #hi4--,staples and ,wooden ,cleat used for running ,low ,voltage ,wires4 ,wires used for batteries1 bells1 telephones1 etc41 operated by batteries and where the voltage is not over #bj volts1 may be run under insulated staples or wooden cleats inside of a building4 ,if outside and exposed to the weather1 they should be mounted on suitable glass or porcelain knobs4 ,illustration3 ,fig4 #ij4--,porcelain ,insulators to support ,electric ,light ,wires4 ,electric-light wires for inside use are commonly supported by insulators made of porcelain and known as cleats1 knobs1 and tubes according to the shape4 ,telegraph1 telephone1 and power lines are usually supported by glass knobs or large porcelain insulators which screw on to wooden pins4 ,illustration3 ,fig4 #ia4--;,glass ,insulator ,binding-,posts and ,pin used to support ,telegraph and ,telephone wires4 ,binding-,posts ,binding-posts are the most convenient device to make quick connections between wires and other parts of electrical apparatus4 ,binding-posts may be either made or purchased4 ,those which are purchased are of course the best1 and will add greatly to the appearance of any instrument upon which they are mounted4 ,several of the best-known types of manufactured posts are shown in ,figure #ib4 ,illustration3 ,fig4 #ib4--,types of ,binding-,posts4 ,figure #ic shows different ways of making simple binding-posts and connectors from screws1 washers1 screw-eyes1 and strips of metal4 ,the drawings are self-explanatory and should need no comment4 ,illustration3 ,fig4 #ic4--;,home-made ,binding-,posts4 ,the screws and nuts obtainable from old dry cells are very convenient to use for binding-posts and other similar purposes4 ,switches and ,cut-,outs ,switches and cut-outs are used in electrical work for turning the current on and off4 ,if the experimenter chooses to make them himself1 care should be taken1 to construct them in a strong and durable fashion1 for they usually are subjected to considerable use1 with consequent wear and tear4 ,illustration3 ,fig4 #id4--;,binding-,post removed from the ,carbon of a ,dry ,cell4 ,several very simple home-made switches are illustrated in ,figure #ie4 ,illustration3 ,fig4 #ie4--,simple ,switches4 ,a1 ,single-,point ,switch4 ,b1 ,two-,point ,switch4 ,c1 ,three-,point ,switch4 ,d1 ,five-,point ,switch4 ,e1 ,lever with ,end ,rolled up to form ,handle4 ,f1 ,lever with ,handle made from part of a ,spool4 ,the first one shown 7,a7 has one contact1 formed by driving a brass-headed tack through a small strip of copper or brass4 ,the movable arm is a strip of copper or brass1 rolled up to form a handle at one end4 ,the other end is pivoted with a brass screw4 ,the brass screw passes through a small strip of copper or brass having a binding-post mounted on the end4 ,a small copper washer should be placed between the movable arm and the copper strip to make the switch work more easily4 ,a somewhat similar switch is shown by ,b in the same illustration1 only in this case a handle made from half of a spool is used1 instead of rolling up the end of the arm4 ,the other illustrations show how the same method of construction may be applied to make switches having more than one point or contact4 ,no dimensions have been given for constructing these switches1 because it is doubtless easier for the young experimenter to use materials which he may have at hand1 and construct a switch of his own proportions4 ,only one suggestion is necessary1 and that is to bevel the under edges of the arm with a file1 so that it will slip over the head of the brass tack more easily4 ,the switches shown in ,figure #if are capable of carrying heavier currents than those just described1 and more nearly approach the type used on lighting and power switchboards4 ,the base may be made of wood1 but preferably should be made of some insulating substance such as fiber or slate4 ,illustration3 ,fig4 #if4--,knife ,switches4 ,the patterns for the metal parts are shown in ,figure #ig4 ,these are cut from heavy sheet-brass or sheet-copper1 and then bent into shape with a pair of flat-nosed pliers4 ,the handle of the single-pole switch is driven on over the metal tongue4 ,the double-pole switch is almost a duplicate of the single-pole type1 but has two sets of levers and contacts1 actuated by the handle1 in place of one4 ,illustration3 ,fig4 #ig4--,metal ,parts for the ,knife ,switches4 ,the ends of the blades to which the handle is connected are turned over at right angles and a hard-wood cross-bar fastened between the ends4 ,the handle is fastened to the center of the cross-bar4 ,after the switch is assembled1 bend the various parts until they line up that is1 are in proper position in respect to each other1 so that the blades will drop into the contacts without bringing pressure to bear on either one side or the other of the handle in order to force the blades into line4 ,fuses ,fuses are used to prevent electrical instruments and wires from damage due to too much current flowing through4 ,when an electric current passes through a resistance it produces heat4 ,a fuse is usually a short piece of lead or some alloy which melts at a low temperature1 and which is inserted in the circuit so that the current must flow through it4 ,if too much current flows through the fuse it will become hot and melt1 because of its low melting-point1 thus interrupting the circuit and shutting the current off until the cause which occasioned the surplus current to flow can be ascertained4 ,fuses are rated according to the amount of current which is required to blow them out1 and therefore are called #a1 #c1 #e1 or #aj ampere fuses1 as the case may be4 ,illustration3 ,fig4 #ih4--,simple ,fuses4 ,a1 ,fuse-,block with plain ,wire ,fuse4 ,d1 ,fuse-,block with ,mica ,fuse in position4 ,when a fuse burns out in a trolley car or in a light or power circuit1 it is because a greater amount of current is trying to pass than the circuit can safely carry4 ,if a fuse were not placed in such a circuit so as to shut the current off before the danger point is reached1 any electrical device might become burned out1 or in extreme cases1 the wires become so hot as to cause a serious fire4 ,figure #ih shows several simple forms of fuses which the experimenter may easily make to protect the batteries1 etc41 from short circuits4 ,the simplest possible fuse consists merely of a small piece of lead wire or a strip of thick tinfoil held between two binding-posts mounted upon a wooden block4 ,the same form of fuse may be made from a strip of mica about two and one-half inches long and one-half an inch wide4 ,a strip of thin sheet-copper is bent around the ends of the mica strip4 ,a piece of fuse wire is stretched between the two copper contacts and fastened to each with a drop of solder4 ,fuse wire of any desired ampere-carrying capacity can be obtained from most electrical supply houses4 ,such a fuse is held in a mounting as shown by ,d4 ,the contacts are made from sheet-copper or brass4 ,they should spring together very tightly1 so as to make perfect contact with the copper ends on the mica strip4 ,lightning-,arresters ,lightning-arresters are used to protect all wires which run into a building from outdoors1 especially telegraph or telephone wires1 so that static electricity due to lightning will not damage the instruments4 ,lightning-arresters may be constructed in many ways and of different materials1 but there are only two types for which the average experimenter will have any use4 ,illustration3 ,fig4 #ii4--,lightning-,arrester and ,ground-,wire ,switch4 ,the arrester shown in ,figure #ii is the type known as lightning-arrester and ground-wire switch4 ,it is used principally on telegraph lines4 ,it consists of three pieces of sheet-brass about one-sixteenth of an inch thick1 and shaped as shown by ,a1 ,b1 and ,c in ,figure #ajj4 ,the metal pieces are mounted on a wooden block with a narrow space of about one-thirty-second of an inch separating them4 ,illustration3 ,fig4 #ajj4--;,home-made ,lightning-,arrester4 ,the two outside pieces are each fitted with two binding-posts1 and the center triangular-shaped piece is fitted with one post4 ,a hole about one-eighth of an inch in diameter is bored between each of the metal pieces4 ,make a tapered metal pin which can be placed tightly in the holes1 and will make contact between the metal pieces4 ,the two outside line wires of the telegraph circuit are connected to the outside metal pieces ,c and ,b4 ,a is connected to the earth or ground4 ,in case of a lightning storm1 if the wires become charged1 the small space between the metal plates will permit the charge to jump across and pass harmlessly into the ground4 ,if complete protection is desired1 it is merely necessary to insert the plug in one of the holes1 and thus ground either wire or short-circuit both of them4 ,illustration3 ,fig4 #aja4--,lightning-,arrester for ,telephone ,wires4 ,the lightning-arrester shown in ,figure #aja is designed for service on telephone wires4 ,it is an ordinary fuse provided with an arrester in the shape of two carbon blocks about one inch square4 ,the blocks rest on a copper strip1 and are held in place by a spring-strip connected to ,b4 ,the carbon blocks are separated by a piece of thin sheet-mica1 of the same size as the blocks4 ,the post1 ,b1 is connected to one of the telephone-line wires near the point where it enters the building from outdoors4 ,the post1 ,a1 is connected to the instrument2 ,c is connected to the ground4 ,an arrester of this kind should be connected to each one of the telephone wires4 ,if the line wires should happen to come into contact with a power wire1 there is danger of damage to the instruments1 but if an arrester is connected in the circuit such an occurrence would be prevented by the blowing out of the fuse4 ,if the lines become charged by lightning1 the charge can easily pass over the edge of the mica between the two blocks and into the ground4 ,,chapter ,,viii ,,electrical ,,measuring ,,instruments ,an instrument designed to measure electromotive force 7electrical pressure7 is called a voltmeter4 ,an instrument designed to measure volume of current is called an ammeter4 ,there are many forms of reliable meters for measuring current and voltage1 but all are more or less expensive and out of the reach of an ordinary boy4 ,some meters are more carefully made than a watch1 and are provided with fine hair-springs and jeweled bearings1 but all depend upon the same principle for their action1 namely1 the mutual effects produced between a magnetic needle and a coil of insulated wire carrying a current of electricity4 ,the little meters described in this chapter are simple and inexpensive but quite sensitive4 ,unlike a meter making use of a hair-spring1 they will stand considerable rough handling1 but of course should not be subjected to such treatment unnecessarily4 ,two types of meters are described4 ,both operate on exactly the same principle1 but one is more elaborate than the other4 ,a ,simple ,voltmeter and ,ammeter ,a base-board five inches long1 two and one-half inches wide and one-half inch thick is cut out of hard wood4 ,in its center1 cut a slot three-eighths of an inch wide and one and one-half inches long1 with the slot running lengthwise the board4 ,along each side of the slot glue two small wooden blocks one and one-half inches long1 one-quarter of an inch thick1 and one-half of an inch high4 ,illustration3 ,fig4 #ajb4--;,a1 ,base1 showing ,slot4 ,b and ,c1 ,sides and ,top of the ,bobbin4 ,d1 ,base and ,bobbin in ,position4 ,when they are firmly in position1 glue a strip of wood1 two and one-half inches long1 three-quarters of an inch wide and one-eighth inch thick to the top as shown by ,d in ,figure #ajb4 ,using these as a support1 wind a horizontal coil composed of #bjj feet of ,no4 #cf ,b4 ,s4 gauge silk-covered wire4 ,a needle is next made from a piece of watch-spring4 ,it should be about one and one-quarter inches long1 and one-eighth of an inch wide4 ,straighten it out by bending1 and then heat the center in a small alcohol flame until the center is red-hot1 taking care to keep the ends as cool as possible4 ,the spring is mounted on a small steel shaft made by breaking up an ordinary sewing-needle4 ,make the piece one-half of an inch long4 ,it must have very sharp points at both ends4 ,the ends may be pointed by grinding4 ,illustration3 ,fig4 #ajc4--;,arrangement of the ,needle and ,pointer4 ,bore a small hole just large enough to receive the needle through the center of the spring4 ,insert the needle in the hole and fasten it in the center by two small circular pieces of wood which fit tightly on the needle4 ,a little glue or sealing-wax will serve to help make everything firm4 ,the pointer is a piece of broom-straw1 about three inches long4 ,bore a small hole in the top of one of the wooden clamps and insert the pointer in the hole1 fastening it with a little glue4 ,the pointer should be perfectly straight1 and in a position at right angles to the spring4 ,bore a small hole in the bottom of one of the wooden clamps and glue a small wire nail in the hole4 ,the purpose of the nail is to serve as a counterweight and keep the pointer in a vertical position4 ,the spring should be magnetized by winding ten or twelve turns of magnet wire around one end and connecting it with a battery for a moment4 ,illustration3 ,fig4 #ajd4--;,a1 ,bearings4 ,b1 ,how the ,needle is mounted4 ,the needle is mounted in two small pieces of thin sheet-brass1 one inch long and one-half inch wide4 ,bend each strip at right angles in the middle1 and at one-quarter of an inch from one end make a small dent by means of a pointed nail and a hammer4 ,the strips are now slipped down in the center of the slot in the coil with the dents inside of the coil and exactly opposite one another4 ,after the exact position is found1 they may be fastened into position by two very small screws4 ,the sharp-pointed sewing-needle1 together with the magnetized spring1 pointer1 and counterweight1 should slip down into the dents made in the strips and swing freely there4 ,it may require a little filing and bending1 but the work should be done patiently1 because the proper working of the meter will depend upon having the needle swing freely and easily in its place4 ,fasten an upright board1 four inches wide and one-quarter of an inch thick1 to the base-board1 back of the bobbin4 ,attach a piece of thick cardboard to the upright by means of small blocks1 in such a position that the pointer swings very close to it but does not touch it4 ,the meter is now complete1 except for marking or calibrating the scale4 ,the method of accomplishing this will be described farther on4 ,illustration3 ,fig4 #aje4--,the ,completed ,meter4 ,if the meter is wound with ,no4 #cf ,b4 ,s4 gauge wire it is a voltmeter for measuring voltage4 ,if it is wound with ,no4 #af ,b4 ,s4 gauge wire it will constitute an ammeter for measuring amperes4 ,a ,portable ,voltmeter and ,ammeter ,the bobbin upon which the wire is wound is illustrated in ,figure #ajf4 ,the wood is the ,spanish cedar1 of which cigar boxes are made4 ,it should be one-eighth of an inch thick1 and can be easily worked with a pocket-knife4 ,in laying out the work1 scratch the lines on the wood with the point of a darning-needle4 ,pencil lines are too thick to permit of accuracy in small work4 ,the bobbin when finished must be perfectly true and square4 ,the dimensions are best understood from the illustrations4 ,in putting the bobbin together1 do not use any nails4 ,use strong glue only4 ,two bobbins are required1 one for the ammeter and one for the voltmeter4 ,after completing the bobbins1 sandpaper them and coat them with shellac4 ,illustration3 ,fig4 #ajf4--;,details of the ,bobbin4 ,the bobbin for the ammeter is wound with ,no4 #ad ,b4 ,s4 double-cotton-covered magnet wire4 ,the voltmeter requires ,no4 #dj ,b4 ,s4 silk-covered wire4 ,in both cases the wire should be wound carefully in smooth1 even layers4 ,a small hole is bored in the flange through which to pass the end of the wire when starting the first layer4 ,after finishing the winding1 about six inches of wire should be left at both ends to make connection with the terminals4 ,the whole winding is then given a coat of shellac4 ,a strip of passe-partout tape1 one-half of an inch wide wound over the wire around the bobbin will not only protect the wire from injury1 but also give the bobbin a very neat appearance4 ,the armature is a piece of soft steel one inch long1 one-eighth of an inch thick and three-eighths wide4 ,a one-eighth-inch hole is bored one-sixteenth of an inch above the center for the reception of the shaft4 ,the center of gravity is thus thrown below the center of the mass of the armature1 and the pointer will always return to zero if the instrument is level4 ,the shaft is a piece of one-eighth-inch ,bessemer steel rod1 seven-sixteenths of an inch long4 ,the ends are filed to a sharp knife-edge on the under side1 as indicated in the figure4 ,illustration3 ,fig4 #ajg4--,the ,bobbin partly cut away so as to show the ,bearing4 ,details of the ,armature and ,shaft4 ,a one-sixteenth-inch hole is bored in the top of the armature to receive the lower end of the pointer1 which is a piece of ,no4 #af aluminum wire1 four and one-half inches long4 ,after the holes have been bored1 the armature is tempered so that it will retain its magnetism4 ,it is heated to a bright red heat and dropped into a basin of strong salt water4 ,the armature is then magnetized by rubbing one end against the pole of a strong magnet4 ,the bearings are formed by two strips of thin sheet-brass1 three-sixteenths of an inch wide1 and one and one-quarter inches long1 bent and glued to the sides of the bobbin4 ,in the illustration1 part of the bobbin is represented as cut away4 ,the center of the bearing is bent out so that the end of the shaft will not come in contact with the sides of the bobbin4 ,the top of the center is notched with a file to form a socket for the knife-edges of the shaft4 ,illustration3 ,fig4 #ajh4--;,completed ,voltmeter4 ,the bobbin is glued to the center of a wooden base1 seven inches long1 four inches wide and three-quarters of an inch thick4 ,the terminals of the coil lead down through two small holes in the base and thence to two large binding-posts4 ,the wires are inlaid on the under side of the base1 i4e41 they pass from the holes to the binding-posts through two grooves4 ,this precaution avoids the possibility of their becoming short-circuited or broken4 ,the case is formed of two sides1 a back and top of one-half-inch wood4 ,it is six inches high1 four inches wide1 and two inches deep4 ,a glass front slides in two shallow grooves cut in the wooden sides1 one-eighth of an inch from the front4 ,the case is held down to the base by four round-headed brass screws1 which pass through the base into the sides4 ,it is then easily removable in case it ever becomes necessary to repair or adjust the instrument4 ,the meter and case1 as illustrated in ,figure #ajh1 are intended for portable use and are so constructed that they will stand up4 ,a small brass screw1 long enough to pass all the way through the base1 serves to level the instrument4 ,if a little brass strip is placed in the slot in the screw-head and soldered so as to form what is known as a winged screw1 the adjustment may be made with the fingers and without the aid of a screw-driver4 ,where the instrument is intended for mounting upon a switch-board1 it can be given a much better appearance by fitting with a smaller base1 similar in size and shape to the top4 ,the binding-posts are then mounted in the center of the sides4 ,to calibrate the meters properly1 they are compared with some standard4 ,the scale is formed by a piece of white cardboard glued by two small blocks on the inside of the case4 ,the various values are marked with a pen and ink4 ,the glass front1 therefore1 cannot be put in place until they are located4 ,the zero value on the meters will normally be in the center of the scale4 ,when a current is passed through the bobbin1 the armature tends to swing around at right angles to the turns of wire4 ,but since the armature is pivoted above the center of the mass1 when it swings1 the center of gravity is displaced and exerts a pull in opposition to that of the bobbin1 and the amount of swing indicated by the pointer will be greater as the current is stronger4 ,the pointer will swing either to the right or the left1 depending upon the direction in which the current passes through the bobbin4 ,the pointer of the instrument illustrated in ,figure #ajh is at zero when at the extreme left of the scale4 ,the pointer is bent to the left1 so that the current will be registered when passing through the meter only in one direction1 but the scale will have a greater range of values4 ,it will also be necessary to cut a small groove in the base of the instrument in this case so that the armature will have plenty of room in which to swing4 ,illustration3 ,fig4 #aji4--;,circuits for ,calibrating the ,ammeter and ,voltmeter4 ,when calibrating the ammeter1 it is placed in series with the standard meter1 a set of strong batteries1 and a rheostat4 ,the rheostat is adjusted so that various current readings are obtained4 ,the corresponding positions of the pointer on the meter being calibrated are then located for each value4 ,the voltmeters must be placed in parallel1 or shunt with each other1 and in series with several battery cells4 ,a switch is arranged so that the voltage of a varying number of cells may be passed through the meters4 ,to secure fractional values of a volt1 the rheostat is placed in shunt with the first cell of the battery4 ,then1 by adjusting both the switch and the rheostat1 any voltage within the maximum range of the battery may be secured4 ,this means of regulating voltage is a common one1 and of much use in wireless telegraph circuits1 as will be explained later4 ,when using the meters1 it is always necessary that the ammeter shall be in series and the voltmeter in parallel or in shunt with the circuit4 ,galvanoscopes and ,galvanometers ,in the first part of ,chapter ,v it was explained that several turns of wire surrounding a compass-needle would cause the needle to move and show a deflection if a current of electricity were sent through the coil4 ,such an instrument is called a galvanoscope and may be used for detecting very feeble currents4 ,a galvanoscope becomes a galvanometer by providing it with a scale so that the deflection may be measured4 ,a galvanometer is really1 in principle1 an ammeter the scale of which has not been calibrated to read in amperes4 ,illustration3 ,fig4 #aaj4--,simple ,compass ,galvanoscope4 ,a very simple galvanoscope may be made by winding fifty turns of ,no4 #cf ,b4 ,s4 gauge single-silk-covered wire around an ordinary pocket compass4 ,the compass may be set in a block of wood1 and the wood provided with binding-posts so that connections are easily made4 ,another variety of the same instrument is shown in ,figure #aaa4 ,illustration3 ,fig4 #aaa4--;,galvanoscope4 ,wind about twenty-five turns of ,no4 #cj ,b4 ,s4 gauge cotton-covered wire around the lower end of a glass tumbler4 ,leave about six inches of each end free for terminals1 and then1 after slipping the coil from the glass1 tie the wire with thread in several places so that it will not unwind4 ,press two sides of the coil together so as to flatten it1 and then attach it to a block of wood with some hot sealing-wax4 ,make a little wooden bridge as shown in ,figure #aaa1 and mount a compass-needle on it in the center4 ,the compass-needle may be made out of a piece of spring-steel in the manner already described in ,chapter ,i4 ,mount two binding-posts to the corners of the block1 and connect the ends of the wire coil to them4 ,turn the block so that the needle points ,north and ,south and parallel to the coil of wire4 ,if a battery is connected to the binding-posts1 the needle will fly around to a position at right angles to that which it first occupied4 ,an astatic galvanoscope is one having two needles with their poles in opposite directions4 ,the word astatic means having no directive magnetic tendency4 ,if the needles of an astatic pair are separated and pivoted separately1 they will each point to ,north and ,south in the ordinary manner4 ,but when connected together with the poles arranged in opposite directions they neutralize each other4 ,an astatic needle requires but very little current in order to turn it either one way or the other1 and for this reason an astatic galvanoscope is usually very sensitive4 ,a simple instrument of this sort may be made by winding about fifty turns of ,no4 #cj-cf ,b4 ,s4 gauge single-silk or cotton-insulated wire into a coil around a glass tumbler4 ,after removing the coil from the glass1 shape it into the form of an ellipse and fasten it to a small base-board4 ,separate the strands of wire at the top of the coil so that they are divided into two groups4 ,illustration3 ,fig4 #aab4--;,astatic ,galvanoscope4 ,make a bridge or standard in the shape of an inverted ,u out of thin wooden strips and fasten it to the block4 ,the needles are ordinary sewing-needles which have been magnetized and shoved through a small carrier-bar1 made from a strip of cardboard1 with their poles opposite one another1 as shown in the illustration4 ,illustration3 ,fig4 #aac4--;,astatic ,needles4 ,they may be held in place in the cardboard strip by a small drop of sealing-wax4 ,a small hole is punched in the top of the carrier1 through which to pass the end of a thread4 ,the upper end of the thread passes through a hole in the bridge and is tied to a small screw-eye in the center of the upper side of the bridge4 ,the carrier-bar is passed through the space where the coil is split at the top4 ,the lower needle should hang in the center of the coil4 ,the upper needle should be above and outside the coil4 ,the terminals of the coil are connected to two binding-posts mounted on the base-block4 ,owing to the fact that this galvanoscope is fitted with an astatic needle1 the instrument does not have to be turned so that the coil may face ,north and ,south4 ,the slightest current of electricity passing into the coil will instantly affect the needles4 ,an astatic galvanometer for the detection of exceedingly weak currents and for use in connection with a ,wheatstone bridge for measuring resistance1 as described farther on1 will form a valuable addition to the laboratory of the boy electrician4 ,make two small bobbins similar to those already described in connection with the volt and ammeter1 but twice as long1 as shown in ,figure #aad4 ,wind each of the bobbins in the same direction with ,no4 #cf silk-covered or cotton-covered wire1 leaving about six inches free at the ends for connection to the binding-posts4 ,fasten each of the bobbins to the base-board with glue4 ,do not nail or screw them in position1 because the presence of nails or screws may impair the sensitiveness of the instrument4 ,in mounting the bobbins1 leave about one-sixteenth of an inch of space between the inside flanges1 through which the needle may pass4 ,connect the coils wound on the bobbins so that the end of the outside layer of the first coil is connected to the inside layer of the other coil4 ,this arrangement is so that the current will travel through the windings in the same continuous direction1 exactly the same as though the bobbin were one continuous spool4 ,illustration3 ,fig4 #aad4--;,bobbin for ,astatic ,galvanometer4 ,magnetize two small sewing-needles and mount them in a paper stirrup made from good1 strong paper1 as shown in ,figure #aad4 ,take care that the poles are reversed so that the north pole of one magnet will be on the same side of the stirrup as the south pole of the other4 ,they may be fastened securely by a drop of shellac or melted sealing-wax4 ,cut out a cardboard disk and divide it into degrees as in ,figure #aae4 ,glue the disk to the top of the bobbins4 ,a small slot should be cut in the disk so that it will pass the lower needle4 ,a wooden post should be glued to the back of the base4 ,to the top of this post is fastened an arm from which are suspended the magnetic needles4 ,a fine fiber for suspending the needle may be secured by unraveling a piece of embroidery silk4 ,illustration3 ,fig4 #aae4--;,completed ,astatic ,galvanometer4 ,the upper end of the fiber is tied to a small hook in the end of the arm4 ,the wire hook may be twisted so that the needles may be brought to zero on the scale4 ,zero should lie on a line parallel to the two coils4 ,the fiber used for suspending the needles should be as fine as possible4 ,the finer the fiber is1 the more sensitive will the instrument be4 ,the lower needle should swing inside of the two coils1 and the upper needle above the disk4 ,how to ,make a ,wheatstone ,bridge ,the amateur experimenter will find many occasions when it is desirable to know the resistance of some of his electrical apparatus4 ,telephone receivers1 telegraph relays1 etc41 are all graded according to their resistance in ohms4 ,the measurement of resistance in any electrical instrument or circuit is usually accomplished by comparing its resistance with that of some known circuit1 such as a coil of wire which has been previously tested4 ,the simplest method of measuring resistance is by means of a device known as the ,wheatstone bridge4 ,this instrument is very simple but at the same time is remarkably sensitive if properly made4 ,a ,wheatstone bridge is shown in ,figure #aaf4 ,the base is a piece of well-seasoned hard wood1 thirty inches long1 six inches wide1 and three-quarters of an inch thick4 ,secure a long strip of ,no4 #ah ,b4 ,s4 gauge sheet-copper1 one inch wide1 and cut it into three pieces1 making two of the pieces three inches long1 and the other piece twenty-three and one-half inches long4 ,mount the copper strips on the base1 as shown1 being very careful to make the distance between the inside edges of the end-pieces just twenty-five inches4 ,the strips should be fastened to the base with small round-headed brass screws4 ,mount two binding-posts on each of the short strips in the positions shown in the illustration1 and three on the long strip4 ,these binding-posts should pass through the base and make firm contact with the strips4 ,illustration3 ,fig4 #aaf4--,wheatstone ,bridge4 ,then make a paper scale twenty-five inches long1 and divide it into one hundred equal divisions one-quarter of an inch long4 ,mark every fifth division with a slightly longer line1 and every tenth division with a double-length line4 ,start at one end and number every ten divisions1 then start at the other end and number them back1 so that the scale reads #j1 #aj1 #bj1 #cj1 #dj1 #ej1 #fj1 #gj1 #hj1 #ij1 #ajj1 from right to left at the top and #j1 #aj1 #bj1 #cj1 #dj1 #ej1 #fj1 #gj1 #hj1 #ij1 #ajj1 from left to right at the bottom4 ,solder a piece of ,no4 #cj ,b4 ,s4 gauge ,german-silver wire to one of the short copper strips opposite the end of the scale1 and then stretch it tightly across the scale and solder it to the strip at the other end4 ,make a knife-contact by flattening a piece of heavy copper wire as shown in ,figure #aag4 ,solder a piece of flexible wire1 such as lamp cord1 at the other end4 ,it is well to fit the contact with a small wooden handle1 made by boring out a piece of dowel4 ,the instrument is now practically complete4 ,illustration3 ,fig4 #aag4--,knife-,contact4 ,in order to use the ,wheatstone bridge1 it is necessary to have a set of resistances of known value4 ,the resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils4 ,it is just like going to a store and buying a pound of sugar4 ,the grocer weighs out the sugar by balancing it on the scales with an iron weight of known value1 and taking it for granted that the weight is correct1 we would say that we have one1 five1 or ten pounds of sugar1 as the case may be4 ,the ,wheatstone bridge might be called a pair of electrical scales for weighing resistance by comparing an unknown coil with one which we know has a certain value4 ,the next step is to make up some standard resistance coils4 ,secure some ,no4 #cb ,b4 ,s4 gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths1 laying it straight on the floor but using care not to pull or stretch it4 #a_/b ohm coil--#c feet #a_/b inch #a ohm coil--#f feet #a #a_/d inches #b ohm coil--#ab feet #b #a_/b inches #e ohm coil--#cj feet #f #a_/d inches #aj ohm coil--#fa feet #bj ohm coil--#abb feet #cj ohm coil--#ahc feet #ej ohm coil--#cje feet ,these lengths of wire are then wrapped on the spools in the following manner4 ,illustration3 ,fig4 #aah4--,resistance-,coil4 ,a shows how the ,wire is doubled and wound on the ,spool4 ,b is the completed ,coil4 ,this method of winding is known as the non-inductive method1 because the windings do not generate a magnetic field1 which might affect the galvanometer needle used in connection with the ,wheatstone bridge as described later on4 ,each length of wire should be doubled exactly in the middle1 then wrapped on the spools like a single wire1 the two ends being left free for soldering to the terminals as shown in ,figure #aah1 ,b4 ,the spools may be the ordinary reels upon which cotton and sewing-silk are wrapped4 ,the terminals of the spools are pieces of stout copper wire1 ,no4 #ab or ,no4 #ad ,b4 ,s4 gauge4 ,two pieces of wire about three inches long are driven into holes bored in the ends of each spool4 ,a small drop of solder is used permanently to secure the ends of the coil to each of the heavy wire terminals4 ,the spools are then dipped into a pan of molten paraffin and boiled until the air bubbles cease to rise4 ,the spools should be marked #a1 #b1 #aj1 #bj1 #cj1 and #ej1 according to the amount of wire each one contains as indicated in the table above4 ,how to ,use a ,wheatstone ,bridge for ,measuring ,resistance ,the instrument is connected as in ,figure #aaf4 ,the unknown resistance or device to be measured is connected across the gap at ,b4 ,one of the standard known coils is connected across the gap at ,a4 ,a sensitive galvanometer or a telephone receiver and two cells of battery are also connected as shown4 ,if a telephone receiver is used1 place it to the ear4 ,if a galvanometer is used instead1 watch the needle carefully4 ,then move the sharp edge of the knife-contact over the scale along the ,german-silver slide wire until a point is reached when there is no deflection of the needle or no sound in the telephone receiver4 ,if this point lies very far on one side or the other of the center division on the scale1 substitute the next higher or lower known resistance spool until the point falls as near as possible to the center of the scale4 ,when this point is found1 note the reading on the scale carefully4 ,now comes the hardest part4 ,almost all my readers have no doubt progressed far enough in arithmetic to be able to carry on the following simple calculation in proportion which must be made in order to find out the resistance of the unknown coil4 ,the unknown resistance1 connected to ,b1 bears the same ratio to the known coil1 at ,a1 that the number of divisions between the knife-contact and the right-hand end of the scale 7lower row of figures7 bears to the number of divisions between the knife-edge and the left-hand end of the scale 7upper row of figures74 ,we will suppose that a #e-ohm coil was used at ,a in a test1 and the needle of the galvanometer stopped swinging when the knife-contact rested on the #fjth division from the left-hand end1 or on the #djth from the right4 ,then1 in order to find the value of the unknown resistance at ,b1 it is simply necessary to multiply the standard resistance at ,a by the number of left-hand divisions and divide the product by the number of right-hand divisions4 ,the answer will be the resistance of ,b in ohms4 ,the calculation in this case would be as follows3 #e ,x #dj #bjj #bjj_/fj #c4cc ohms #c4cc ohms is the resistance of ,b4 ,this explanation may seem very long and complex1 but if you will study it carefully you will find it to be very simple4 ,when once you master it1 you will be enabled to make many measurements of resistance which will add greatly to the interest and value of your experiments4 ,,chapter ,,ix ,,bells1 ,,alarms1 ,,and ,,annunciators ,an electric bell may be bought almost anywhere for twenty-five cents1 and from the standpoint of economy it does not pay to build one4 ,a bell is not a hard thing to construct1 and the time and money spent will be amply repaid by the more intimate knowledge of this useful piece of apparatus which will be gained by constructing it4 ,the base is four inches wide and five and one-half inches long4 ,the magnets consist of two machine bolts1 wound with ,no4 #bb cotton-covered magnet wire4 ,fiber ends are fitted on the bolts to hold the wire in place4 ,the wire is wound on each of the magnets separately4 ,cover the cores with two or three layers of paper before winding on the wire4 ,the ends of the wire are led through holes in the core ends4 ,the ends of the bolts are passed through the yoke1 and the nuts applied to hold them in place4 ,the magnets are clamped down to the bell-base by means of a hard-wood strip having a screw passed through it between the magnets into the base4 ,illustration3 ,fig4 #aai4--;,details of the ,magnet ,spools1 and ,yoke for an ,electric ,bell4 ,the armature of the bell is shown in ,figure #abj4 ,it is made of a piece of iron having a steel spring riveted to it1 as illustrated4 ,the armature is fastened to a small block mounted on the lower left-hand corner of the base4 ,illustration3 ,fig4 #abj4--;,details of the ,armature1 and ,contact ,screw4 ,a second block with a contact-point made from an ordinary brass screw by filing the end into the shape shown in the illustration1 is mounted on the base so that it is opposite the end of the contact-spring fastened to the armature4 ,the gong may be secured from an old bell or alarm clock4 ,it is mounted on the upper part of the base in such a position that the hammer will strike it on its lower edge4 ,the instrument is connected as shown in ,figure #aba4 ,one terminal of the magnets is connected to the contact-screw4 ,the other end is connected to the binding-post4 ,a second binding-post is connected to the armature4 ,illustration3 ,fig4 #aba4--,the ,completed ,bell4 ,the armature spring should be bent so that the armature is pushed over against the contact4 ,if a battery is connected to the bell1 the electromagnets will pull the armature and cause the hammer to strike the gong4 ,as soon as the armature has moved a short distance1 the spring will move away from the contact and break the circuit4 ,the magnets cease pulling as soon as the current is cut off and the armature spring then causes the armature to move back and touch the contact4 ,as soon as the contact is made1 the armature is drawn in again and the process is repeated4 ,illustration3 ,fig4 #abb4--;,diagram showing how to connect a ,bell1 ,battery1 and ,push-,button4 ,a little experimenting with the bell will soon enable one to find its best adjustment4 ,figure #abb shows how to connect a bell to a battery and a push-button4 ,a push-button is simply a small switch which closes the circuit when pressed4 ,do not make the armature spring too weak1 or the hammer will move very slowly and with very little life4 ,each time that the armature moves toward the magnets1 it should barely touch the iron cores before the ball strikes the bell4 ,after you get the bell in good working order1 it is well to make a small box to serve as a cover for the working parts of the instrument1 leaving only the gong exposed4 ,illustration3 ,fig4 #abc4--,two ,simple ,push-,buttons4 ,illustration3 ,fig4 #abd4--;,diagram showing how to arrange a ,bell ,system of ,return ,signals4 ,it is sometimes desirable to arrange two bells and two push-buttons1 so that a return signal can be sent4 ,in that case the circuit shown in ,figure #abd may be employed4 ,it is then possible for the person answering the bell to indicate that he has heard the call by pushing the second button4 ,for instance1 one push-button and bell might be located on the top floor of a house and the other bell and button in the basement4 ,a person in the basement wishing to call another on the top floor would push the button4 ,the person answering could return the signal by pushing the button on the top floor and cause the bell in the basement to ring4 ,a ,burglar ,alarm ,a simple method of making an efficient burglar alarm is shown in ,figure #abe4 ,the base is a piece of wood about five by six inches1 and half an inch thick4 ,a small brass strip1 ,a1 is fastened to the base by means of two round-headed wood screws and the ends turned up at right angles4 ,the lever1 ,b1 is also a strip of brass4 ,one end is bent out1 so as to clear the strip and the screws that are under it4 ,the lever is pivoted in the middle with a screw and a washer4 ,a small hole1 ,d1 is bored in the lower end through which a spring and a string are passed4 ,the other end of the spring is fastened under a screw and washer1 ,c4 ,illustration3 ,fig4 #abe4--;,burglar-,alarm ,trap4 ,in order to set the alarm1 first fasten the base in any convenient place4 ,carry the string across the room and fasten it4 ,adjust the string so that the lever is half-way between the two ends of the strip1 ,a4 ,if the string is disturbed1 it will pull the lever over against the strip1 ,a4 ,if the string is cut1 the spring will pull the lever over to the opposite side4 ,in either case1 if the alarm is properly connected to a bell and battery1 the circuit will be closed if the string is disturbed1 and the bell will ring4 ,one wire leading from the bell and the battery should be connected to ,a1 and the other to the screw and washer1 ,c4 ,the alarm may be arranged across a window or doorway and a black thread substituted for the string4 ,any one entering in the dark and unaware of the existence of the alarm is liable to break the thread and ring the bell4 ,an ,electric ,alarm ,it is often desirable to arrange an electrical alarm clock so that a bell will ring continuously until shut off4 ,illustration3 ,fig4 #abf4--;,an ,early-,riser's ,electric ,alarm ,attachment for a ,clock4 ,figure #abf shows an electrical alarm attachment4 ,it consists of a wooden box1 large enough to receive an ordinary dry cell4 ,a bell is fastened on the outside of the box4 ,connect one terminal of the battery to one terminal of the bell4 ,connect the other bell and battery terminals1 each to a short piece of brass chain1 about four inches long4 ,the ends of the chain are then fastened to a small piece of sheet fiber or hard rubber1 so that they are insulated from each other4 ,the opposite end of the fiber is fastened to a piece of wire spring having a garter or suspender clip soldered to the end4 ,illustration3 ,fig4 #abg4--;,details of the ,chain ,electrodes1 etc4 ,the operation of this electrical attachment is very simple4 ,wind up the alarm key of an ordinary alarm clock and place the clip on the key4 ,place the clock in such a position that the two chains do not touch each other4 ,set the clock4 ,when the mechanical alarm goes off1 the key will revolve and twist the two chains1 thus closing the electric circuit and causing the bell to ring4 ,the bell will ring until the clamp is removed4 ,the outfit can be attached to any ordinary alarm clock4 ,an ,annunciator ,annunciators are often placed in bell and burglar alarm-circuits to indicate where the button ringing the bell was pushed1 in case there are several4 ,the separate indicators used on an annunciator are called drops4 ,illustration3 ,fig4 #abh4--;,an ,annunciator ,drop4 ,a drop may be made from an electromagnet and some brass strips1 etc4 ,the frame is cut from heavy sheet-brass and shaped as shown in ,figures #abh and #abi4 ,the drop bar is a strip of metal which is pivoted on the frame at its lower end and has the upper end turned up to receive a numeral or letter4 ,the armature is made from a strip of sheet-iron4 ,it is pivoted on the frame at its upper end4 ,the strip is bent at right angles so as to fall in front of the magnet4 ,the lower part of the armature is bent into a hook4 ,the hook fits into a slot cut in the drop bar4 ,a fine wire spring is placed between the frame and the upper end of the armature so as to pull the armature away from the core when the current is not passing through the magnet4 ,the electromagnet should be wound with ,no4 #be ,b4 ,s4 cotton-covered magnet wire4 ,when a current is sent through the magnet1 it will draw the armature in4 ,this action releases the hook from the edge of the slot in the drop bar and permits the bar to drop and bring the number or letter down into view4 ,illustration3 ,fig #abi4--;,details of the ,drop-,frame and ,armature4 ,a number of drops may be arranged on a board and placed in different circuits so as to indicate which circuit is closed at any time4 ,it is a good plan to arrange a bar to act as a stop1 so that the numeral will not drop down too far4 ,each time that any one of the drops falls1 it must be reset by pushing the bar back into position4 ,,chapter ,x ,,electric ,,telegraphs ,experiments in telegraphy were carried out as far back as the year #agec1 when it was proposed to transmit messages by representing the letters of the alphabet by combinations of sparks produced by a static machine2 but these were of little practical value and nothing of any importance was accomplished until after the discovery of galvanic current4 ,many of these old experiments were very crude and appear somewhat ridiculous when compared with the methods of nowadays4 ,the earliest proposal for an electric telegraph appeared in the ,scots' ,magazine for ,february1 #agec1 and shows several kinds of proposed telegraphs acting by the attractive power of electricity1 conveyed by a series of parallel wires1 one wire corresponding to each letter of the alphabet and supported by glass rods at every twenty yards4 ,words were to be spelled by the action of the electricity in attracting paper letters1 or by striking bells corresponding to letters4 ,the modern telegraph consists essentially of four things1 namely3 ,a battery which produces an electric current4 ,a wire which conducts the electric current from one point to another4 ,a transmitter for shutting the current off and on4 ,an electro-magnetic receiving apparatus1 which gives out in sounds1 the signals made by the pulsations of the current from a distant point4 ,the battery may be almost any form of battery4 ,gravity cells are preferred1 however1 for telegraph work4 ,heavy galvanized iron wire is usually employed as the line4 ,it is necessary to use non-conductors wherever the wire is fastened4 ,glass insulators placed on a wooden pin or bracket1 which is fastened to the pole or building on which the wire is to be supported1 are used for outside work4 ,inside of buildings1 rubber tubes are used where the wires pass through walls1 etc4 ,the operation of a telegraph is not1 as many people suppose1 a complicated or difficult matter to understand1 but is quite simple4 ,the key is a contrivance for controlling the passage of the electric current in much the same manner as an ordinary switch4 ,it consists of a steel lever1 swung on trunnion-screws mounted in a frame1 and provided with a rubber knob which the operator grasps lightly with the thumb and forefinger4 ,on pressing the lever downward1 a platinum point fastened on the under side of the lever is brought into contact with another point set into a rubber bushing in the base of the key1 so that there is no electrical connection between the two points unless the key is pressed down or closed1 as it is often termed4 ,the key is usually fastened to the operating bench by two rods called legs4 ,the lever is provided with screws which permit the stroke of the key to be very closely adjusted4 ,illustration3 ,fig4 #acj4--;,a ,typical ,telegraph ,key1 showing the ,various ,parts4 ,the line wire and battery are connected to the key1 so that no current can flow until the key is pressed and the contacts brought together4 ,a sounder consists of two electromagnets mounted on a base under a movable flat piece of iron which is attracted by the magnetism of the electromagnets when a current flows through them and is withdrawn by a spring when no magnetism excites the windings4 ,this piece of iron1 which is called the armature1 is mounted upon a strip of brass or aluminum called the lever4 ,the lever strikes against a brass anvil and produces the clicks1 which form the dots and dashes of the telegraph alphabet4 ,illustration3 ,fig4 #aca4--;,a ,typical ,telegraph ,sounder1 showing the ,various ,parts4 ,every time that the key is pressed1 an electric current is sent out into the line4 ,the current flows through the magnets of the sounder and causes the armature to be drawn downward4 ,the lever strikes the anvil and produces a click4 ,when the key lever is released1 the current is shut up and the lever flies up and clicks against the top of the anvil4 ,the period of time between the first click and the second click may be varied at will according to the length of time that the key is held down4 ,a short period is called a dot and a long period a dash4 ,combinations of dots1 dashes1 and spaces arranged according to the ,morse ,alphabet1 make intelligible signals4 ,how ,to ,make a ,simple ,key and ,sounder ,the little telegraph instruments shown in ,figures #acb and #acc are not practical for long lines but may be used for ticking messages from one room to another1 and can be made the source of much instruction and pleasure4 ,illustration3 ,fig4 #acb4--;,a ,simple ,home-made ,telegraph ,key4 ,the key is a strip of brass fastened to a wooden base in the manner shown in ,figure #acb4 ,it is fitted with a knob of some sort on the front end1 so that it is conveniently gripped with the fingers4 ,the little bridge is made from heavy sheet-brass and prevents the lever from moving too far away from the contact on the upward stroke4 ,connections are made to the key lever at the back end and the contact in front by the binding-posts1 ,a and ,b4 ,the post1 ,c1 connects with the bridge4 ,the sounder consists of two small electromagnets mounted in a vertical position on a wooden base4 ,the magnets are connected at the bottom by a strip of heavy sheet-iron which acts as a yoke4 ,illustration3 ,fig4 #acc4--;,a ,simple ,home-made ,telegraph ,sounder4 ,the armature is made out of sheet-iron1 rolled up in the manner shown in the illustration4 ,one end of the armature is fastened to a wooden block in such a position that the armature comes directly over the magnets and about one-eighth of an inch above them4 ,the opposite end of the armature moves up and down for about an eighth of an inch between two screws1 each fastened in a wooden block mounted on an upright board in the back of the magnets4 ,the purpose of the screws is to make the click of the sounder louder and clearer than it would be if the armature only struck the wood4 ,a rubber band or a small wire spring passing over a screw and connected at the other end to the armature will draw the latter away from the magnets when the current is not passing4 ,the terminals of the magnets are connected to binding-posts mounted on the base4 ,illustration3 ,fig4 #acd4--;,a ,diagram showing how to connect two ,simple ,telegraph ,stations4 ,the key and sounder should be placed in series with one or two cells of a battery4 ,pressing the key will then cause the armature of the sounder to be drawn down and make a click4 ,when the key is released1 the armature will be drawn up by the spring or rubber band and make a second click4 ,hardly a boy interested in mechanics and electricity has not at some time or other wished for a telegraph instrument with which to put up a line with his chum4 ,a practical working set of such instruments can be very easily constructed1 and with little expense1 by following the sketches and instructions given here4 ,the magnets for the sounder may either be constructed by the intending telegraph operator or secured from some old electrical instrument such as a magneto-bell4 ,in the latter case1 the hardest part of the work will be avoided4 ,if they are to be home-made1 the following suggestions may prove of value in carrying out their construction4 ,illustration3 ,fig4 #ace4--;,a ,complete ,telegraph ,set1 consisting of a ,keyboard and a ,sounder4 ,the cores are made from one-quarter-inch stove-bolts with the heads cut off4 ,the magnet heads are cut out of hard-wood fiber1 one-eighth of an inch thick and one inch in diameter4 ,they should fit tightly and be held in place with glue4 ,they are separated so as to form a winding space between of seven-eighths of an inch4 ,the magnets should be wound full of ,no4 #be ,b4 ,s4 gauge cotton-covered wire4 ,illustration3 ,fig4 #acf4--;,details of the ,telegraph ,set shown in ,figure #ace4 ,the yoke is made of enough strips of sheet-iron1 one-half inch wide and two inches long1 to form a pile one-quarter of an inch thick4 ,two one-quarter-inch holes are bored in the opposite ends of the yoke1 one and one-half inches apart4 ,the lower ends of the magnet cores are passed through these holes4 ,the ends should project one-half of an inch beyond the yoke4 ,they are passed through two holes in a base-board three-quarters of an inch thick4 ,the holes are countersunk from the lower side1 so that a nut can be screwed on the lower end of each and the magnets held tightly in an upright position4 ,the remaining parts of the instrument are very easily made1 and are so clearly shown by the drawing that it is hardly necessary to say more than a few words in explanation4 ,the lever or tongue1 the anvil1 the standard1 and the lever of the key are all cut out of hard-wood according to the pattern shown in the illustration4 ,the armature is a piece of soft iron fastened to the lever with a small brass screw4 ,tacks are placed under the heads of the adjusting screws on the sounder so that it will click more loudly4 ,the rubber band acts as a spring to counteract the weight of the armature and lever and draw it up as soon as the current is cut off4 ,the movement of the lever should be so adjusted that it is only sufficient to make an audible click4 ,use care to avoid friction between the lever and the standard1 so that the former will move with perfect freedom4 ,all the screws used in the work should be round-headed brass wood screws with the points filed flat4 ,bore a small hole before screwing them into place so as to avoid splitting the wood4 ,the construction of the key is even more simple than that of the sounder4 ,it should move up and down without any side motion4 ,the circuit-closer should be kept closed when the instruments are not in use1 and when you are receiving a message4 ,as soon as you are through receiving and wish to transmit1 you should open your circuit-closer and your friend close his4 ,the tension of the spring under the lever of the key must be adjusted to suit the needs of each individual operator4 ,illustration3 ,fig4 #acg4--;,a ,diagram showing how to connect two ,complete ,telegraph ,sets1 using one ,line ,wire and a ,ground4 ,the ,two-,point ,switches throw the ,batteries out of ,circuit when the ,line is not in use4 ,the diagram for connecting the instruments is self-explanatory4 ,in cities or towns where a ground is available by connecting to the gas or water pipes1 one line wire may be easily dispensed with4 ,or1 if desirable1 a ground may be formed by burying a large plate of zinc 7three or four feet square7 in a moist spot and leading the wire to it4 ,how ,to ,build a ,telegraph ,relay ,in working a telegraph over a long line or where there are a large number of instruments on one circuit1 the currents are often not strong enough to work the sounder directly4 ,in such a case a relay is used4 ,the relay is built on the same principle as a sounder1 but the parts are made much lighter1 so that the instrument is more sensitive4 ,the armature of a relay is so small and its movement so little that its clicking is scarcely audible4 ,it is therefore fitted with a second set of contacts and connected to a battery and a sounder1 which is to set in operation every time the contacts close4 ,the principle of a relay is that a weak current of insufficient strength to do the work itself may set a strong local current to do its work for it4 ,there are many forms of relays1 and while that which is described below is not of the type commonly used on telegraph lines1 it has the advantage of being far more sensitive than any instrument of the regular line relay type that the average experimenter could build4 ,illustration3 ,fig4 #ach4--;,details of the ,relay ,parts4 ,make the magnets from one-quarter-inch stove-bolts1 and cut them off so that they will form a core about two and one-quarter inches long4 ,fit each of the cores with two fiber heads to hold the wire in place4 ,insulate the legs with paper and wind each with about fifty layers of ,no4 #cj ,b4 ,s4 gauge single-cotton-covered magnet wire4 ,the winding space between the magnet's heads should be one and one-eighth inches4 ,the upper ends of the magnet cores should be allowed to project about one-quarter of an inch beyond the fiber head4 ,the end of the core is filed flat1 as shown in the illustration4 ,the magnets are mounted upon an iron yoke1 three-sixteenths of an inch thick4 ,the holes in the yoke should be spaced so that there is a distance of one and one-half inches between the centers of the magnet cores4 ,the armature of the relay is mounted on a small steel shaft with sharp points at each end4 ,the exact shape of the armature may be best understood from the illustrations4 ,the lower end of the shaft rests in a small cone-shaped depression made by driving a center punch into the yoke half-way between the two magnets4 ,the top bearing is a strip of brass projecting from a wooden support4 ,the end of the shaft rests in a depression similar to that in the yoke4 ,the contact lever is made of brass and forced on the shaft below the armature4 ,it swings between a small brass clip fastened to one side of the support and a little screw held in a similar clip on the opposite side4 ,the contact clip is made of spring brass about ,no4 #bb gauge in thickness4 ,it may be adjusted by a screw passing through the support4 ,the armature may be controlled in its movement so that the latter will be very slight by adjusting the screws4 ,there should not be any friction in the bearings and the armature should move with perfect freedom4 ,the armature should approach the ends of the magnet cores until a space about the thickness of heavy paper separates them and should not touch them4 ,illustration3 ,fig4 #aci4--,the ,completed ,relay4 ,the spring is made of fine brass wire4 ,it is fastened to the armature shaft1 and the screw mounted on the wooden support with a piece of silk thread4 ,the thread is passed around the shaft once or twice so that the tension of the spring will cause the armature to move away from the pole pieces just as soon as the current flowing through the magnets ceases4 ,illustration3 ,fig4 #adj4--;,a ,diagram showing how to connect a ,relay1 ,sounder1 and ,key4 ,closing the ,key will operate the ,relay4 ,the ,relay will then operate the ,sounder in turn4 ,the tension of the spring may be adjusted by turning the screw with a screw-driver4 ,if the armature tends to stick to the magnet poles fasten a small piece of paper to the poles with some shellac4 ,the terminals of the magnets are connected to two binding-posts marked ,a and ,b4 ,the binding-posts marked ,c and ,d are connected respectively to the contact clip and the brass bearing on the top of the wooden support4 ,the diagram in ,figure #adj shows how the relay is connected to a telegraph line4 ,how ,to ,learn ,to ,telegraph ,the instruments so far described have been practical working telegraph instruments1 but they lack the fine points of commercial apparatus and it is not possible to become as efficient an operator with their aid as with a real key and sounder4 ,if the young experimenter desires to become a proficient telegraph operator1 the first thing to do is to purchase a ,learner's telegraph key and sounder4 ,connect a dry cell to the binding-posts on the back of the instrument4 ,screw the set down on a table about eighteen inches from the front edge1 so that there is plenty of room for the arm to rest4 ,see that none of the various adjustment screws about the instrument are loose and that the armature of the sounder moves freely up and down through a distance of about one-sixteenth of an inch4 ,the spring which draws the lever upwards away from the magnets should be set only at sufficient tension to raise the lever when no current is passing4 ,if too tight1 the spring will not allow the armature to respond to the current flowing through the magnets4 ,the key is provided with several adjustment-screws to regulate the tension and the play of the lever to suit the hand of the operator4 ,a little practice will enable the student to judge best for himself just how the key should be set4 ,the next step is to memorize the alphabet1 so that each character can instantly be called to mind at will4 ,the punctuation marks are not used very frequently1 and the period is the only one which the student need learn at first4 ,the ,morse alphabet consists of dots1 dashes1 and spaces4 ,combinations of these signals spell letters and words4 ,many of the characters are the reverse of others4 ,for example1 ,a is the reverse of ,n4 ,b and ,f1 ,d and ,u1 ,c and ,r1 ,q and ,x1 ,z and 1 are the other reverse letters1 so if the formation of one of each of these letters is memorized the reverse is easily mastered4 ,it is important that the beginner should learn how properly to grasp the key1 for habits are easily formed and a poor position will limit the sending speed of the operator4 ,place the first or index finger on the top of the key-handle1 with the thumb under the edge2 and the second finger on the opposite side4 ,the fingers should be curved so as to form a quarter-section of a circle4 ,bring the third and fourth fingers down so that they are almost closed on the palm of the hand4 ,rest the arm on the table in front of the key and allow the wrist to be perfectly limber4 ,illustration3 ,fig4 #ada4--;,how to hold a ,telegraph ,key4 ,the grasp on the key should be firm but not rigid4 ,avoid using too much strength or a light hesitating touch4 ,endeavor to acquire a positive1 firm up and down motion of the key4 ,avoid all side pressure1 and do not allow the fingers to leave the key when making the signals4 ,the movement is made principally with the wrist1 with the fingers and hand perfectly elastic4 ,a dot is made by a single instantaneous1 downward stroke of the key4 ,a dash is made by holding the key down for the same period of time that it takes to make three dots4 ,a long dash is made by holding the key down for the same time that it takes to make five dots4 ,a space in the letters1 such as1 for instance1 the space between the first and last two dots in the letter ,r should occupy the time of one dot4 ,the space between each letter should occupy the time required for two dots1 and the space between words should occupy the time required for three dots4 ,commence the use of the key by making dots in succession1 first at the rate of two every second1 and increasing the speed until ten can be made4 ,practice should be continued until three hundred and sixty dots a minute can be made with perfect regularity4 ,then begin making dashes at the rate of two every three seconds1 and continue until one hundred and twenty a minute can be made with perfect regularity4 ,practise the long dashes at the rate of one a second1 and increase until ninety can be made in a minute4 ,illustration3 ,fig4 #adb4--,the ,morse ,telegraphic ,code4 ,when this has been accomplished1 practise the following letters until they can be perfectly made4 ,each row of letters is an exercise which should be practised separately until mastered4 ,dot ,letters ,e ,i ,s ,h ,p #f ,dot and ,space ,letters ,o ,c ,r ,y ,z ,dash ,letters ,t ,l ,m #e ,o ,dots and ,dashes ,a ,u ,v #d ,dashes and ,dots ,n ,d ,b #h ,mixed ,dots and ,dashes ,f ,g ,j ,k ,q ,w ,x #a #b #c #g #i ,period ,after you can write these different letters1 practise making words4 ,select a list of commonly used words4 ,when words seem easy to write1 practise sending pages from a book4 ,systematic and continual practice will enable the student to make surprising progress in mastering the art of sending4 ,reading and receiving messages must be practised with a companion student4 ,place two instruments in separate rooms or in separate houses so that the operators will be entirely dependent upon the instruments for their communication with each other4 ,start by transmitting and receiving simple messages4 ,then use pages from a book1 and increase the speed until it is possible to send and receive at least #ae words a minute without watching the sounder but merely depending upon the clicks to determine the duration of the dots and dashes4 ,figure #adj shows how to arrange a regular telegraph line for two stations4 ,gravity batteries should be used for regular telegraph work4 ,it is necessary that the key should be kept closed by having its circuit-closer shut when messages are not being sent4 ,if one of the keys is left open the circuit is broken1 and it is not possible for a person at the other end of the line to send a message4 ,every telegraph office has a name or call usually consisting of two letters2 thus for ,new ,york the call might be ,n4 ,y4 and for ,chicago1 ,c4 ,h4 ,if ,new ,york should desire to call ,chicago1 he would repeat the call letters1 ,c ,h41 until answered4 ,chicago would answer by sending ,i1 several times and signing1 ,c ,h4 ,when so answered1 ,new ,york would proceed with the message4 ,,chapter ,,xi ,,microphones ,,and ,,telephones ,in #ahgh1 ,david ,edward ,hughes discovered that the imperfect contact formed between two pieces of some such substance as carbon or charcoal is very sensitive to the slightest changes in pressure1 and when included in an electric circuit with a battery and a telephone receiver1 will transmit sounds4 ,such an instrument is called a microphone4 ,it has various forms but in most of them one piece of carbon or charcoal is held loosely between two other pieces in such a manner as to be easily affected by the slightest vibrations conveyed to it through the air or any other medium4 ,illustration3 ,fig4 #adc4--;,a ,microphone connected to a ,telephone ,receiver1 and a ,battery4 ,figure #adc illustrates a simple form of instrument embodying this principle4 ,a small pencil of carbon is supported loosely between two blocks of the same substance glued to a thin wooden sounding-board of pine4 ,the sounding-board is mounted in an upright position on a wooden base4 ,the carbon pencil rests loosely in two small indentations in the carbon blocks4 ,the blocks are connected1 by means of a very fine wire or a strip of tinfoil1 with one or two cells of battery and a telephone receiver4 ,any vibration or sounds in range of the microphone will cause the sounding-board to vibrate4 ,this will affect the pressure of the contact between the carbon pencil and the two blocks4 ,when the pressure between the two is increased the resistance in the path of the electric current is decreased and more current immediately flows through the circuit4 ,on the other hand1 when the pressure is decreased1 the resistance is increased and less current flows through the telephone receiver4 ,the amount of current flowing in the circuit thus keeps step with the changes in the resistance1 and accordingly produces sounds in the telephone receiver4 ,the vibrations emitted from the receiver are usually much greater than those of the original sounds1 and so the microphone may be used to magnify weak sounds such as the ticking of clock-wheels or the footfalls of insects4 ,if a watch is laid on the base of the microphone1 the ticking of the escapement wheel can be heard with startling loudness4 ,the sounds caused by a fly walking on a microphone may be made to sound as loud as the tramp of a horse4 ,illustration3 ,fig4 #add4--;,a ,very ,sensitive ,form of ,microphone1 with which the ,footsteps of a ,fly can be heard4 ,the electrical stethoscopes used by physicians to listen to the action of the heart are in principle only a microphone and telephone receiver connected to a battery4 ,the drawing in ,figure #add illustrates a very sensitive microphone that is quite easy to make4 ,with this instrument it is possible to hear the tramping of a fly's feet or the noise of its wings4 ,the base upon which the apparatus is mounted serves as the sounding-board and is made in the form of a hollow wooden box4 ,it can be made from an ordinary cigar-box by removing the paper and taking the box apart4 ,the piece forming the top of the box must be planed down until it is only three thirty-seconds of an inch thick4 ,the box should measure about five inches square and three-quarters of an inch thick when finished4 ,do not use any nails or small brads whatsoever in its construction1 but fasten it together with glue4 ,if you use any nails you will decrease the sensitiveness of the instrument quite appreciably4 ,the bottom of the box should be left open4 ,the result is a sounding-board of the same principles as that of the banjo head4 ,small feet1 one-quarter of an inch square1 are glued to the four under corners so as to raise the bottom clear of the table1 or whatever the microphone may be placed upon4 ,the bottom of each one of the small feet is cushioned with a layer of felt so that no jars will be transmitted to the instrument by any object upon which it is resting4 ,the carbon pencil used on this type of instrument is pivoted in the center and rests at one end upon a carbon block4 ,the carbon block is made about one inch long1 one-quarter of an inch thick1 and one-half of an inch wide4 ,a small hole is drilled near each end to receive a screw which fastens the block to the sounding-board4 ,a fine wire is led from one of these screws to a binding-post mounted at the side of the box4 ,another wire leads from a second binding-post to a standard which is also fastened to the sounding-board with a small screw4 ,the standard is made from a sheet of thin brass and is bent into the shape shown in the illustration4 ,the pencil is a piece of one-quarter-inch carbon rod1 two and three-quarter inches long4 ,a small hole is drilled one and five-eighths of an inch from one end with a sewing-needle1 and a piece of fine brass wire1 pointed at both ends1 pushed in4 ,the wire should be a tight fit in the hole4 ,it should be about one-half of an inch long1 and may be made from an ordinary pin4 ,the slide-bar is used to regulate the pressure of the pencil upon the carbon block and is simply a piece of soft copper wire about one-eighth of an inch in diameter4 ,it is bent into the shape shown in the illustration so that it will slide over the carbon pencil4 ,the sides of the standard should press just tightly enough against the ends of the pivot which passes through the carbon pencil to hold it in position without slipping1 and at the same time allow it to swing freely up and down4 ,the two binding-posts should be connected in series with two dry cells and a pair of good telephone receivers4 ,place the receivers against the ears4 ,move the slide-bar gently back and forth until the voice of any one talking in another part of the room can be heard distinctly in the telephone receivers4 ,in order to hear faint whispers1 move the slide-bar away from the carbon block4 ,in order to hear a fly walk it is necessary to have the carbons very dry and clean4 ,the instrument must be very carefully adjusted4 ,cover the microphone with a large glass globe and place a fly inside of the globe4 ,whenever the fly walks on any part of the microphone you will be able to hear each footstep in the telephone receivers4 ,when he flies about inside of the globe1 his wings will cause a loud roaring and buzzing noise to be heard in the receivers4 ,telephones ,not many years ago1 when the telephone made its first appearance1 it was the wonder of the times just as wireless telegraphy is to-day4 ,starting as an exceedingly simple and inexpensive apparatus1 it has gradually developed into a wonderful and complex system1 so that at the present time1 instead of experiencing difficulty in telephoning over distances of fifty or one hundred miles1 as at first1 it is possible to carry on a conversation over a line two thousand miles long as easily as it is face to face4 ,like the telegraph1 the principle of the telephone is that of a current of electricity flowing over a line wire into a pair of electro-magnets1 but with many important differences4 ,when compared with telegraph apparatus1 the telephone is found to be exceedingly sensitive4 ,a telegraph relay requires perhaps about one-hundredth of an ampere to work it properly4 ,a telegraph sounder will require about one-tenth of an ampere1 but a telephone receiver will render speech audible with less than a millionth of an ampere1 and therefore may almost be said to be a hundred thousand times more sensitive than a sounder4 ,another difference between the telephone and the telegraph lies in the fact that the currents flowing over a telegraph line do not usually vary at a rate greater than twenty or thirty times a second1 whereas telephone currents change their intensity hundreds of times a second4 ,the telephone is an instrument for the transmission of speech to a distance by means of electricity1 wherein the speaker talks to an elastic plate of thin sheet-iron which vibrates and sends out a pulsating current of electricity4 ,the transmission of the vibrations depends upon well-known principles of electricity1 and does not consist of the actual transmission of sounds1 but of electrical impulses which keep perfect accord or step with the sound waves produced by the voice in the transmitter4 ,these electrical currents pass through a pair of small electro-magnets acting upon a plate or diaphragm1 which in turn agitates the air in a manner similar to the original voice speaking into the transmitter and thus emits sounds4 ,that part of the apparatus which takes up the sounds and changes them into electric currents composes the transmitter4 ,when words are spoken into the mouthpiece they strike a diaphragm1 on the back of which is fastened a small cup-shaped piece of carbon4 ,a second cup is mounted in a rigid position directly back of the first4 ,the space between them is filled with small polished granules of carbon4 ,when these granules are in a perfectly loose state and are undisturbed1 their resistance to an electric current is very great and they allow almost none to flow4 ,when slightly compressed their resistance is greatly lowered and they permit the current to pass4 ,the vibrations of the diaphragm cause the carbon cup mounted on its back to move and exert a varying pressure upon the granules with a corresponding variation in their resistance and the amount of current which will pass through4 ,illustration3 ,fig4 #ade4--;,a ,telephone ,system1 consisting of a ,receiver1 ,transmitter1 and a ,battery connected in ,series4 ,words spoken into the ,transmitter are reproduced by the ,receiver4 ,the receiver1 or that part of the apparatus which transforms the pulsating current back into sound waves consists of a thin iron disk1 placed very near but not quite touching the end of a small steel bar1 permanently magnetized1 and about which is wound a coil of fine insulated wire4 ,the transmitter and the receiver are connected together in series with a battery as in ,figure #ade4 ,when words are spoken into the transmitter the little carbon granules are immediately thrown into motion1 and being alternately compressed and released cause corresponding changes in the current flowing through the receiver from the battery4 ,the magnetism of the receiver changes with each change in the electric current1 and thus by alternately attracting and repelling the diaphragm causes it to vibrate and emit sounds4 ,such is the principle of the telephone4 ,the telephones in actual service to-day are complicated with bells1 magnetos1 induction coils1 condensers1 relays1 and various other apparatus1 which fact renders them more efficient4 ,the bells and magnetos are for the purpose of calling the central operator or the person at the other end of the line and drawing attention to the fact that some one wishes to get into communication with him4 ,the older styles of telephones used what is known as a polarized bell and a hand magneto for this purpose4 ,a polarized bell is a very sensitive piece of apparatus which will operate with very little current4 ,a magneto is a small hand dynamo which when turned with a crank will generate a current causing the bell at the other end of the line to ring4 ,when the telephone receiver is raised off its hook in order to place it to the ear the bell and magneto are automatically disconnected from the line and the receiver and the transmitter are connected in their place4 ,the current necessary to supply the telephone and receiver is supplied by two or three dry cells placed inside of each telephone4 ,the latest types of instruments employ what is known as the central energy system1 wherein the current is supplied by a large storage battery located at the central office and serving as a current supply to all the telephones connected to that system4 ,it would be impossible to enter into the history of the telephone far enough to explain the details of some of the various systems in every-day use in such a book as this because of the immense amount of material it would be necessary to present4 ,such a work would occupy a volume of its own4 ,additional information may be readily found in any reference library4 ,however1 the boy electrician who wishes to make a telephone for communicating between the house and barn1 or with his chum down the street1 will find the necessary information in the following pages4 ,if this work is carried out carefully and a home-made telephone system built and installed it will not only prove a very interesting undertaking but will also serve to dispel all mystery which may surround this device in the mind of the young experimenter4 ,how to ,build a ,telephone ,telephone receivers are useful for many purposes in electrical work other than to receive speech4 ,they are used in connection with wireless instruments1 in place of a galvanometer in measuring electrical circuits1 and for testing in various ways4 ,telephone receivers are of two types4 ,one of them is long and cumbersome1 and is very similar to the original ,bell telephone receiver4 ,the other is small and flat1 and is called a watch-case receiver4 ,a watch-case receiver is shown in ,figure #adf4 ,it consists of a ,u-shaped permanent magnet so mounted as to exert a polarizing influence upon a pair of little electro-magnets1 before the poles of which is placed an iron diaphragm4 ,for convenience1 these parts are assembled in a small cylindrical casing1 usually of hard rubber4 ,the permanent magnet exerts a continual pull upon the diaphragm1 tending to draw it in4 ,when the telephone currents pass through the little magnets1 they will either strengthen the permanent magnet and assist it in attracting the diaphragm1 or detract from its strength and allow the diaphragm to recede1 depending upon which direction the current flows4 ,illustration3 ,fig4 #adf4--;,a ,watch-,case ,telephone ,receiver4 ,watch-case receivers are usually employed for wireless telegraph work because they are very light in weight and can easily be attached to a head-band in order to hold them to the ears and leave the hands free4 ,watch-case receivers can be purchased for forty-five to seventy-five cents at almost any electrical supply house4 ,they are very useful to the amateur experimenter in many ways4 ,a telephone receiver capable of giving fair results on a short telephone line can be very easily made1 but of course will not prove as efficient as one which is purchased ready-made from a reliable electrical manufacturer4 ,the first practical telephone receiver was invented by ,alexander ,graham ,bell and was made somewhat along the same lines as that shown in ,figure #adg4 ,such a receiver may be made from a piece of curtain-pole1 three and three-quarter inches long and about one and one-eighth inches in diameter4 ,a hole1 three-eighths of an inch in diameter1 is bored along the axis throughout its entire length1 to receive the permanent magnet4 ,the shell of the receiver is a cup-shaped piece of hard wood1 two and one-half inches in diameter and one inch deep4 ,it will have to be turned on a lathe4 ,its exact shape and dimensions are best understood from the dimensions shown in the cross section in ,figure #adg4 ,the shell is firmly attached to one end of the piece of curtain-pole by gluing4 ,the permanent magnet is a piece of hard steel1 three-eighths of an inch in diameter and four and five-eighths of an inch in length4 ,the steel will have to be tempered or hardened before it will make a suitable magnet1 and the best way to accomplish this is to have a blacksmith do it for you by heating the rod and then plunging it into water when just at the right temperature4 ,illustration3 ,fig4 #adg4--;,a ,simple ,form of ,telephone ,receiver4 ,one end of the bar is fitted with two thick fiber washers about seven-eighths of an inch in diameter and spaced one-quarter of an inch apart4 ,the bobbin so formed is wound full of ,no4 #cf ,b4 ,s4 gauge single-silk-covered magnet wire4 ,the ends of the wire are passed through two small holes in the fiber washers and then connected to a pair of heavier wires4 ,the wires are run through two holes in the curtain-pole1 passing lengthwise from end to end1 parallel to the hole bored to receive the bar magnet4 ,this bar magnet is then pushed through the hole until the end of the rod on which the spool is fixed is just below the level of the edges of the shell4 ,the two wires are connected to binding-posts1 ,a and ,b1 mounted on the end of the receiver4 ,a hook is also provided so that the receiver may be hung up4 ,the diaphragm is a circular piece of thin sheet-iron1 two and one-half inches in diameter4 ,it is placed over the shell1 and the bar magnet adjusted until the end almost touches the diaphragm4 ,the magnet should fit into the hole very tightly1 so that it will have to be driven in order to be moved back and forth4 ,the diaphragm is held in place by a hard-wood cap1 two and three-quarter inches in diameter and having a hole three-quarters of an inch in diameter in the center4 ,the cap is held to the shell by means of four small brass screws4 ,the receiver is now completed and should give a loud click each time that a battery is connected or disconnected from the two posts1 ,a and ,b4 ,the original ,bell telephone apparatus was made up simply of two receivers without any battery or transmitter4 ,in such a case the current is generated by induction4 ,the receiver is used to speak through as well as to hear through4 ,this method of telephoning is unsatisfactory over any appreciable distances4 ,the time utilized in making a transmitter will be well spent4 ,a simple form of transmitter is shown in ,figure #adh4 ,the wooden back1 ,b1 is three and one-half inches square and three-quarters of an inch thick4 ,the front face of the block is hollowed out in the center as shown in the cross-section view4 ,the face-plate1 ,a1 is two and one-half inches square and one-half an inch thick4 ,a hole1 seven-eighths of an inch in diameter1 is bored through the center4 ,one side is then hollowed out to a diameter of one and three-quarter inches1 so as to give space for the diaphragm to vibrate as shown in the cross-sectional drawing4 ,the carbon buttons are one inch in diameter and three-sixteenths of an inch thick4 ,a small hole is bored in the center of each to receive a brass machine screw4 ,the hole is countersunk1 so as to bring the head of the screw down as close to the surface of the carbon as is possible4 ,then1 using a sharp knife or a three-cornered file1 score the surface of the carbon until it is covered with criss-cross lines4 ,the diaphragm is a piece of thin sheet-iron cut in the form of a circle two and one-half inches in diameter4 ,a small hole is bored through the center of this4 ,one of the carbon buttons is fastened to the center of the diaphragm with a small screw and a nut4 ,cut out a strip of flannel or thin felt1 nine-sixteenths of an inch wide and three and one-half inches long4 ,around the edge of the carbon button mounted on the diaphragm1 bind this strip with silk thread in such a manner that the strip forms a cylinder closed at one end with the button4 ,fill the cylinder with polished carbon telephone transmitter granules to a depth of about one-eighth of an inch4 ,these granules will have to be purchased from an electrical supply house4 ,they are finely polished small carbon balls1 much like birdshot in appearance4 ,slip a long machine-screw through the hole in the second carbon button and clamp it in place with a nut4 ,then place the carbon button in the cylinder so that it closes up the end4 ,the space between the two buttons should be about three-sixteenths of an inch4 ,bind the flannel or felt around the button with a piece of silk thread so that it cannot slip out of place4 ,the arrangement of the parts should now be the same as that shown by the cross-sectional drawing in the upper right-hand corner of ,figure #adh4 ,the complete transmitter is assembled as shown in the lower part of ,figure #adh4 ,a small tin funnel is fitted into the hole in the face-plate1 ,a1 to act as a mouthpiece4 ,a screw passes through the back1 ,b1 and connects to the diaphragm4 ,the screw is marked ,e in the illustration4 ,a binding-post is threaded on the screw so that a wire may be easily connected4 ,the screw passing through the back carbon button also passes through a hole in the wooden back1 and is clamped firmly in position with a brass nut so that the button is held very rigidly and cannot move4 ,the front button1 being attached to the diaphragm1 is free to move back and forth with each vibration of the latter4 ,illustration3 ,fig4 #adh4--;,a ,home-made ,telephone ,transmitter4 ,the carbon granules should fill the space between the buttons three-quarters full4 ,they should lie loosely together1 and not be packed in4 ,when connected to a battery and a telephone receiver the current passes from the post1 ,d1 to the back button1 through the mass of carbon granules into the front button and out at the post1 ,e4 ,when the voice is directed into the mouthpiece1 the sound waves strike the diaphragm and cause it to vibrate4 ,the front button attached to it then also vibrates and constantly changes the pressure on the carbon granules4 ,each change in pressure is accompanied by an immediate change in resistance and consequently the amount of current flowing4 ,figure #adi shows a complete telephone ready for mounting on the wall4 ,it consists of a receiver1 telephone transmitter1 bell1 hook1 and push-button4 ,the bell is mounted on a flat base-board4 ,the transmitter is similar to that just described1 but is built into the front of a box-like cabinet4 ,the box is fitted with a push-button at the lower right-hand corner4 ,a simple method of making a suitable push-button is shown in the upper left-hand part of the illustration4 ,it consists of two small brass strips arranged so that pushing a small wooden plug projecting through the side of the cabinet will bring the two strips together and make an electrical connection4 ,the hook consists of a strip of brass1 pivoted at one end with a round-headed brass wood screw and provided with a small spring1 so that when the receiver is taken off of the hook it will fly up and make contact with a screw1 marked ,c in the illustration4 ,when the receiver is on the hook1 its weight will draw the latter down against the screw1 ,d4 ,the hook is mounted on the base-board of the telephone1 and projects through a slot cut in the side of the cabinet4 ,four binding-posts are mounted on the lower part of the base-board4 ,the two marked ,b and ,b are for the battery4 ,illustration3 ,fig4 #adi4--;,a ,complete ,telephone ,instrument4 ,two ,instruments such as this are necessary to form a simple ,telephone ,system4 ,that marked ,l is for the line1 and ,g is for the ground connection or the return wire4 ,illustration3 ,fig4 #aej4--;,diagram of ,connection for the ,telephone ,instrument shown in ,fig4 #adi4 ,the diagram of the connections is shown in ,figure #aej4 ,the line-wire coming from the telephone at the other station enters through the binding-post marked ,l1 and then connects to the hook4 ,the lower contact on the hook is connected to one terminal of the bell4 ,the other terminal of the bell leads to the binding-post marked ,g1 which is connected to the ground1 or to the second line-wire1 where two are used4 ,the post1 ,g1 and one post1 ,b1 are connected together4 ,the other post marked ,b connects to one terminal of the transmitter4 ,the other terminal of the transmitter is connected to the telephone receiver4 ,the other post of the telephone receiver leads to the upper contact on the hook marked ,c4 ,the push-button is connected directly across the terminals of the transmitter and the receiver so that when the button is pushed it short-circuits the transmitter and the receiver4 ,when the receiver is on the hook and the latter is down so that it makes contact with ,d any current coming over the line-wire will pass through the bell and down through the ground or the return-wire to the other station1 thus completing the circuit4 ,if the current is strong enough it will ring the bed4 ,when the receiver is lifted off the hook1 the spring will cause the hook to rise and make contact with the screw marked ,c4 ,this will connect the receiver1 transmitter1 and the battery to the line so that it is possible to talk4 ,if1 however1 it is desired to ring the bell on the instrument at the other end of the line1 all that it is necessary to do is to press the push-button4 ,this will short-circuit the receiver and the transmitter and ring the bell4 ,the battery current is flowing over the line all the time when the receiver is up1 but the transmitter and the receiver offer so much resistance to its flow that not enough current can pass to ring the bell until the resistance is cut out by short-circuiting them with the push-button4 ,the instruments at both ends of the line should be similar4 ,in connecting them together care should be taken to see that the batteries at each end of the line are arranged so that they are in series and do not oppose each other4 ,one side of the line may be a wire1 but the return may be the ground1 as already explained in the chapter on telegraph apparatus4 ,a transmitter of the desk-stand type may be made according to the scheme shown in ,figure #aea4 ,it consists simply of a transmitter mounted upon an upright1 and provided with a base so that it may stand on a desk or a table4 ,illustration3 ,fig4 #aea4--;,a ,desk-,stand ,type of ,telephone4 ,it is also fitted with a hook and a push-button1 so that it is a complete telephone instrument with the exception of the bell and the battery4 ,the battery and the bell may be located in another place and connected to the desk-stand by means of a flexible wire or electrical cord4 ,figure #aeb shows what is known as a telephone induction coil4 ,induction coils are used in telephone systems whenever it is necessary to work over a long distance4 ,such a system is more complicated1 and requires considerable care in making the connections1 but is far superior to the system just described4 ,illustration3 ,fig4 #aeb4--;,a ,telephone ,induction ,coil4 ,an induction coil consists of two fiber or hard-wood heads1 about one inch square and one-quarter of an inch thick1 mounted on the ends of an iron core composed of a bundle of small iron wires about two and one-half inches long4 ,the core should be about five-sixteenths of an inch in diameter4 ,the core is covered with a layer of paper and then wound with three layers of ,no4 #bb ,b4 ,s4 single-cotton-covered wire4 ,these three layers of wire form the primary4 ,the primary is covered with a layer of paper and then the secondary is wound on4 ,the secondary consists of twelve layers of ,no4 #cf ,b4 ,s4 single-silk-covered magnet wire4 ,it is advisable to place a layer of paper between layers of the secondary winding1 and to give each one a coating of shellac4 ,the two secondary terminals of the coil are led out through holes in the fiber head and kept separate from the primary terminals4 ,illustration3 ,fig4 #aec4--;,diagram of ,connection for a ,telephone ,system employing an ,induction ,coil at each ,station4 ,the wiring diagram of a telephone system using an induction coil at each station is shown in ,figure #aec4 ,the speech sent over a line using an induction coil system is much clearer and more easily understood than that on a line not using such a device4 ,in building telephone instruments or connecting them up1 care and accuracy will go a long way towards success4 ,telephony involves some very delicate and sensitive vibratory mechanical and electrical actions1 and such instruments must be very carefully made4 ,a transmitter is really a microphone built especially to receive the sounds of the human voice1 and operates on the same principle4 ,,chapter ,,xii ,,induction ,,coils ,a ,medical ,coil or shocking coil1 as it is properly termed1 is nothing more or less than a small induction coil1 and consists of a core1 a primary winding1 a secondary winding1 and an interrupter4 ,the principle of an induction coil and that of magnetic induction have already been explained in ,chapter ,v4 ,it might be well for the readers to turn back to pages #hi-ia and reread them4 ,the human body possesses considerable resistance1 and the voltage of one or two ordinary cells of battery is not sufficient to overcome that resistance and pass enough current through the body to be felt1 unless under exceptional conditions4 ,the simplest means employable for raising the voltage of a battery high enough to produce a shock is the medical coil4 ,the first step in making such a coil is to roll up a paper tube1 five-sixteenths of an inch in diameter inside1 and two and one-half inches long4 ,the outer end of the paper is carefully glued1 so that it will not unroll4 ,the tube is filled with pieces of iron wire two and one-half inches long which have been straightened by rolling between two boards4 ,the size of the iron wire may vary from ,no4 #bj to ,no4 #bd ,b4 ,s4 gauge4 ,enough should be slipped into the tube to pack it tightly and admit no more4 ,a square block1 #a x #a x #e-af inches1 is cut out of fiber or a close-grained hard wood and a hole three-eighths of an inch in diameter bored through the center4 ,one end of the tube containing the core is smeared with glue and slipped into the block4 ,the end of the tube is allowed to project through about one-sixteenth of an inch4 ,a second block1 in the form of a circle three-quarters of an inch in diameter1 one-quarter of an inch thick1 and having a three-eighths of an inch hole through the center1 is glued on the opposite end4 ,illustration3 ,fig4 #aed4--;,details of ,various ,parts of a ,medical ,coil4 ,after the glue has dried1 four small holes are drilled in the square head in the approximate positions shown by ,figure #aed4 ,four layers of ,no4 #bb ,b4 ,s4 gauge magnet wire 7it may be either silk or cotton1 double or single covered7 is wound smoothly and carefully over the core4 ,the terminals are led out of the holes a and b4 ,the primary is covered with two or three layers of paper1 and then enough secondary wound on to bring the total diameter of the coil to about eleven-sixteenths of an inch4 ,the secondary wire must be much finer than the primary4 ,it is possible to use any size from ,no4 #cb to ,no4 #cf ,b4 ,s4 gauge and obtain good results4 ,the insulation may be either single silk or single cotton4 ,illustration3 ,fig4 #aee4--;,details of ,interrupter for ,medical ,coil4 ,the secondary terminals are led out through the holes c and d4 ,it is perhaps a wise plan to re-enforce these leads with a heavier piece of wire1 because otherwise they are easily broken4 ,the interrupter is a simple arrangement capable of being made in several different ways4 ,the drawing shows an arrangement which can be improved upon by any experimenters who are familiar with a medical coil4 ,i have shown the simplest arrangement1 so that all my readers will be able to build it1 and those who want to improve it can do so4 ,if a small piece of silver is soldered to the spring and to the contact-point it will give better results4 ,the silver is easily secured by cutting up a ten-cent piece4 ,one terminal of the primary is connected to the interrupter spring and the other to a binding-post4 ,the contact-post is also connected to a binding-post4 ,if a battery is connected to the two binding-posts1 the current will flow from one post through the coil to the interrupter spring1 through the spring to the contact post1 and thence back to the battery1 making a complete circuit4 ,as soon as the current flows1 however1 it produces magnetism which draws the spring away from the contact and breaks the circuit1 cutting off the magnetic pull4 ,the spring flies back to the contact but is drawn forward again immediately and repeats the operation continuously at a high rate of speed4 ,illustration3 ,fig4 #aef4--;,completed ,medical ,coil4 ,the secondary terminals are led out to two binding-posts to which are connected two electrodes or handles by means of flexible wires4 ,the electrode may be made of two ordinary flat strips of sheet-metal or a piece of tubing4 ,in the latter case1 the wires may be connected by wedging them in with a cork4 ,if the handles are grasped while the battery is connected to the primary posts and the interrupter is in operation a powerful shock will be felt4 ,the shock may be regulated from a weak current that can hardly be felt to a very powerful one by providing the coil with a piece of iron tubing of about seven-eighths of an inch inner diameter and two inches long which will slip on and oh the coil4 ,when the tube is all the way on1 the shock is very mild1 and when all the way off1 the shock is very strong4 ,of course any intermediate strength may be secured at stages between the two extremes4 ,the current from medical coils is often prescribed by physicians for rheumatism and nervous disorders1 but must be properly applied4 ,the coil just described is harmless4 ,it will give a strong shock1 but the only result is to make the person receiving it drop the handles and not be anxious to try it again4 ,spark-,coils ,a spark-coil is one of the most interesting pieces of apparatus an experimenter can possess4 ,the experiments that may be performed with its aid are varied and many4 ,the purpose of a spark-coil is to generate enormously high voltages which are able to send sparks across an air space that ordinary currents of low voltage could not possibly pierce4 ,the spark-coil is the same in principle as the small induction coils used as medical or shocking coils1 but is made on a larger scale and is provided with a condenser connected across the terminals of the interrupter4 ,illustration3 ,fig4 #aeg4--;,diagram showing ,essential ,parts of ,induction ,coil4 ,it consists of a central iron core surrounded by a coil of heavy wire called the primary1 and by a second outside winding of wire known as the secondary4 ,the primary is connected to a few cells of battery in series with an interrupter4 ,the interrupter makes and breaks the circuit1 i4 e41 shuts the current on and off repeatedly4 ,every time that the current is made or broken1 a high voltage is induced in the secondary4 ,by means of the condenser connected across the interrupter terminals1 the current at make is caused to take a considerable fraction of time to grow1 while at break1 the cessation is instantaneous4 ,the currents induced in the secondary at break are so powerful that they leap across the space in a brilliant torrent of sparks4 ,building a ,spark-,coil ,perhaps more attempts are made by experimenters to construct a spark-coil than any other piece of apparatus1 and the results are usually poor4 ,a spark-coil is not hard to construct1 but it requires careful work and patience4 ,it is not a job to be finished in a day1 but time must be liberally expended in its construction4 ,satisfactory results are easily obtained by any one of ordinary mechanical ability if patience and care are used4 ,parts for spark-coils are for sale by many electrical houses1 and it is possible to purchase a set of such machine-made parts for less than the separate materials usually cost4 ,for the benefit of those who might wish to build a larger coil than the one described in the following text1 a table showing the dimensions of two other sizes will be found4 ,illustration3 ,fig4 #aeh4--;,empty ,paper ,tube1 and ,tube filled with ,core ,wire preparatory to winding on the ,primary4 99,the core99 is made of very soft iron wire about ,no4 #bj or #bb ,b4 ,s4 gauge1 cut to exact length4 ,each piece should be six inches long4 ,iron wire may be purchased from electrical supply houses already cut to various lengths for twenty cents a pound4 ,in view of the amount of labor required carefully to cut each piece to length and then straighten it out so that it will form a neat bundle1 it is cheaper to purchase the wire already cut4 ,such wire has been annealed1 i4e41 softened by bringing to a red heat and then cooling slowly4 ,in case the wire is purchased at a plumbing shop or a hardware store it must be annealed before it can be used4 ,this is accomplished by tying the wire in a compact bundle and placing it in a wood fire where it will grow red-hot4 ,when this stage is reached1 cover the wire with ashes and allow the fire to die away4 ,illustration3 ,fig4 #aei4--;,illustrating the ,various ,steps in winding on the ,primary and fastening the ,ends of the ,wire4 ,cut a piece of tough wrapping paper into strips six inches long and about five inches wide4 ,wrap it around a stick or metal rod one-half of an inch in diameter1 so as to form a tube six inches long and having a diameter of one-half of an inch4 ,glue the inside and outside edges of the paper so that the tube cannot unroll and then slip it off the stick4 ,illustration3 ,fig4 #afj4--;,complete ,primary ,winding and ,core4 ,fill the tube with the six-inch wires until it is packed tightly and no more can be slipped in4 99,the primary99 consists of two layers of ,no4 #ah ,b4 ,s4 gauge cotton-covered wire wound over the core for a distance of five inches4 ,one-half pound of wire is more than enough for one primary4 ,the wire must be wound on very smoothly and carefully4 ,in order to fasten the inside end so that it will not become loose1 place a short piece of tape lengthwise of the core and wind on two or three turns over it4 ,then double the end back and complete the winding4 ,after the first layer is finished1 give it a coat of shellac and wind on the second layer4 ,the end of the wire is wound with a piece of tape and fastened by slipping through a loop of tape embedded under the last few turns4 ,the illustrations will explain more clearly just how this is accomplished4 ,the second layer is then given a coat of shellac and allowed to dry4 ,after it is dry1 wrap about fifteen layers of paper which have been soaked in paraffin around the primary4 ,this operation should be performed in a warm place1 over a fire or lighted lamp where the paraffin may be kept soft1 so that the paper will go on tightly4 ,illustration3 ,fig4 #afa4--,the ,primary covered with ,insulating ,layer of ,paper ready for the ,secondary4 ,the coil is now ready to receive the secondary winding4 ,the core and primary which have been described are suit-able for a secondary giving sparks from one-half to three-fourths of an inch long4 99,the secondary99 winding consists of several thousand turns of very fine wire wound on in smooth even layers with paper between each two layers4 ,the following table shows the size and amount of wire required4 ,in addition1 about two pounds of paraffin and a pad of linen paper or typewriter paper will be required4 ,the wire may be either enamel1 cotton1 or silk insulated4 ,single silk-covered wire is preferred4 ,,size ,,of ,,coil ,,size ,,of ,,wire ,,amount #a_/b inch #cf ,b4 ,s4 #aj ounces #a inch #cd ,b4 ,s4 #a lb4 #a_/b inch #cd ,b4 ,s4 #b lbs4 ,the means for supporting and turning the coil in order to wind on the secondary may be left somewhat to the ingenuity of the young experimenter4 ,the following suggestion1 however1 is one which experience has proved to be well worth following out1 and may be applied to other things than the construction of an induction coil4 ,it seems to be the nature of most boys1 for some reason or other1 to be unwilling to spend time and labor on anything which will aid them in their work4 ,they are always in such a hurry and so anxious to see something completed that they direct all their energy to that end rather than spend part of their time in constructing some little device which would really lighten the other work and go a long way towards insuring its successful completion4 ,i have frequently given instructions for building an induction coil and placed particular stress upon winding the secondary1 only to have such suggestions ignored in the anxious endeavor of boys to finish the coil as soon as possible4 ,in every such instance the coil has been a failure4 ,illustration3 ,fig4 #afb4--,simple ,winding ,device for winding the ,secondary4 ,the illustration shows a simple form of winder1 with which the operation of winding the secondary is a very slow one1 but1 on the other hand1 it is possible to do very accurate1 careful winding with the aid of such a device4 ,the parts may all be made from wood4 ,the chucks fit tightly over the ends of the core so that when the handle is turned1 the coil will revolve also4 ,the spring serves to keep the chucks snugly against the coil ends1 so that they will not slip4 ,from one-half to five-eighths of a pound of wire will be required to wind the coil4 ,a large number of strips of thin paraffined or waxed paper must be cut five inches wide4 ,the inside terminal1 or beginning end of the wire is tied around the insulating tube near the left-hand end4 ,the spool of wire must be placed in a position where it will revolve freely without strain on the wire4 ,no4 #cf is very fine and easily broken1 so use the utmost care to guard against this mishap4 ,illustration3 ,fig4 #afc4--;,completed ,secondary ,winding4 ,wind on a smooth1 even layer of wire1 permitting each turn to touch the other1 but none to lap over4 ,carry the winding to within one-half inch of the ends of the insulating tube and then wind on two layers of the waxed paper4 ,the paper must be put on smoothly and evenly1 so as to afford a firm foundation for the next layer4 ,the wire is wrapped around with the paper1 so that the next layer starts one-half inch from the edge4 ,a second layer is then wound on very carefully1 stopping when it comes one-half inch from the edge4 ,two more layers of paper are put on1 and the process repeated1 alternately winding on paper and wire until the stated quantity of the latter has been used up4 ,the layers of wire may occasionally be given a coating of shellac4 ,this is a good insulator1 and will serve to hold them together and prevent the wire from becoming loose4 ,in winding the coil1 remember that if at any point you allow the winding to become irregular or uneven1 the irregularity will be much exaggerated on the succeeding layers4 ,for this reason1 do not allow any to occur4 ,if the wire tends to go on unevenly1 wrap an extra layer of thick paper around underneath so as to offer a smooth foundation1 and you will find the difficulty remedied4 ,illustration3 ,fig4 #afd4--;,interrupter ,parts4 ,an efficient vibrator for a coil cannot be easily made1 and it is best to buy one which is already fitted with platinum points4 ,the interrupter will play a very important part in the successful working of the coil1 and its arrangement and construction are important4 ,interrupters like that shown in the illustration and used for automobile will be found the best4 ,the condenser may be home-made4 ,it consists of alternate sheets of tinfoil and paraffined paper1 arranged in a pile as shown in the illustration4 ,the following table gives the proper sizes for condensers for three different coils4 ,,tinfoil ,,size ,,of ,,spark-,,coil ,,no4 ,,sheets ,,size ,,of ,,sheets #a_/b inch #ej #b x #b #a inch #ajj #g x #e #a #a_/b inch #ajj #h x #f ,the paper must be about one-half inch larger all the way around1 so as to leave a good margin4 ,the alternate sheets of tinfoil1 that is1 all on one side and all on the other1 are connected4 ,the condenser is connected directly across the terminals of the interrupter4 ,illustration3 ,fig4 #afe4--;,condenser4 ,there are various methods of mounting a coil1 the most common being to place it in a box with the interrupter at one end4 ,perhaps1 however1 one of the neatest and also the simplest methods is to mount it in the manner shown in the illustration4 ,the end-pieces are cut out of wood4 ,no specific dimensions can be given1 because the diameter of the coils will vary somewhat according to who winds them and how tightly they are made4 ,the coil is enclosed in a tube made by rolling up a strip of cardboard and then giving it a coat of shellac4 ,the tube may be covered by a strip of black cloth1 so as to improve its appearance4 ,illustration3 ,fig4 #aff4--;,completed ,coil4 ,the vibrator is mounted on the end4 ,the core projects through a hole in the wood near the end of the vibrator spring so that the latter will be drawn in by the magnetism of the core when the current flows4 ,the condenser may be placed in the hollow box which forms the base of the coil4 ,the secondary terminals of the coil are mounted on a small strip of wood bridging the two coil ends4 ,one terminal of the primary is connected to a binding-post mounted on the base1 and the other led to the vibrator spring4 ,the vibrator yoke is connected to a second binding-post on the base4 ,one terminal of the condenser is connected to the spring1 and the other to the yoke4 ,four cells of dry battery should be sufficient to run the coil and cause it to give a good one-half-inch spark if built according to the directions here given4 ,the vibrator or interrupter will require adjusting and a position of the adjusting screw will soon be found where the coil works best4 ,experiments with a ,spark-,coil 99,electrical ,hands499 ,many extraordinary and interesting experiments may be performed with the aid of a spark-coil4 ,the following experiment never fails to amuse a party of friends1 and is mystifying and weird to the ordinary person1 unacquainted with the secret of its operation4 ,figure #afg shows the arrangement of the apparatus4 ,the primary of an ordinary one-inch spark induction coil is connected in series with a twelve-volt battery and telephone transmitter4 ,a small switch is included in the circuit to break the current and prevent needless waste of the battery when the apparatus is not in immediate use4 ,the secondary terminals of the induction coil are led by means of an insulated wire to the adjoining room where they terminate in a pair of scissors1 or some other small metallic object which may be clasped in the hand4 ,each of two persons1 wearing dry shoes or rubber-soled slippers1 grasps the terminal of one wire in one hand4 ,the other hand is placed flat against the ear of a third person1 with a piece of dry linen paper intervening between the hands and the head4 ,if a fourth person1 in the room where the induction coil is located1 then closes the small switch and speaks into the telephone transmitter1 the person against whose ears the hands are being held will hear the speech very distinctly4 ,the ticking of a watch held against the mouthpiece of the transmitter will be heard with startling clearness4 ,illustration3 ,fig4 #afg4--;,diagram showing how to connect the ,apparatus for the ,electric ,hands ,experiment4 ,the principle governing the operation of the apparatus is very simple4 ,almost every experimenter is familiar with the ordinary electrical condenser1 which consists of alternate sheets of paraffined paper and tinfoil4 ,when this is connected to a source of electricity of high potential1 but not enough so as to puncture the paper dielectric1 the alternate sheets of tinfoil will become oppositely charged and attract each other4 ,if the circuit is then broken the sheets will lose their charge and also their attraction for one another4 ,if the tinfoil sheets and paper are not pressed tightly together1 there will be a slight movement of the tinfoil and paper which will correspond in frequency to any fluctuations of the charging current which may take place4 ,the head of the third person and the hands held against his head act like three tinfoil sheets of a condenser1 separated by two sheets of paper4 ,the words spoken in the transmitter cause the current to fluctuate and the induction coil raises the potential of the current sufficiently to charge the condenser and cause a slight vibration of the paper dielectric4 ,the vibrations correspond in strength and speed to those of the voice1 and so the words spoken in the transmitter are audible to the person over whose ears the paper is pressed4 ,everything about the apparatus must be as dry as possible1 to insure its successful operation4 ,the people holding the wires in their hands should stand on a carpeted floor4 ,always be very careful to tighten the adjusting screw and block the interrupter on the coil1 so that by no means can it possibly commence to operate1 or the person listening1 instead of hearing things will become the victim of a rather painful1 practical joke4 ,geissler ,tubes ,the most beautiful and surprising effects may be obtained by lighting ,geissler tubes with a coil4 ,the tubes are made in intricate and varied patterns of special glass1 containing fluorescent minerals and salts1 and are filled with different rarefied gases4 ,when the tubes are connected to the secondary of a spark-coil by means of a wire fastened to the little rings at the end1 and the coil is set in operation1 they light up in the most wonderful way imaginable4 ,the rarefied gases and minerals in the glass throw out beautiful iridescent colors1 lighting up a dark room with a weird flickering light4 ,every tube is usually of a different pattern and has a combination of different colors4 ,the most beautiful tubes are those provided with a double wall containing a fluorescent liquid1 which heightens the color effects when the tube is lighted4 ,illustration3 ,fig4 #afh4--;,geissler ,tubes4 ,eight to ten tubes may be lighted at once on an ordinary coil by connecting them in series4 ,ghost ,light ,if you grasp the bulb of an old incandescent electric lamp in one hand and touch the base to one side of the secondary when the coil is in operation the bulb will emit a peculiar greenish light in the dark4 ,puncturing ,paper ,if you place a piece of heavy paper or cardboard between two sharp wires connected to the secondary of a spark-coil and start the coil working1 the paper will be pierced4 ,a ,practical ,joke ,this action of the coil may be made the basis of an amusing joke4 ,offer a friend who may smoke cigarettes some cigarette paper which has been prepared in the following way4 ,illustration3 ,fig4 #afi4--,the ,bulb will emit a ,peculiar ,greenish ,light4 ,place several sheets of the paper on a piece of sheet-metal which is connected to one side of the secondary4 ,by means of an insulated handle so that you will not get a shock1 move the other wire all over the surface of the cigarette paper4 ,the paper will be pierced with numerous fine holes which are so fine that they can hardly be seen4 ,if your friend uses any of the paper in making a cigarette and tries to light it he will waste a box of matches without being able to get one good puff1 because the little invisible holes in the paper will spoil the draft4 ,perhaps he may quit smoking altogether4 ,an ,electric ,garbage-can ,illustration3 ,fig4 #agj4--;,an ,electrified ,garbage-can4 ,if there are any dogs in your neighborhood that have a habit of extracting things from your ash-barrel or garbage-can1 place the latter on a piece of dry wood4 ,lead a well insulated wire from one secondary terminal of your coil to the can4 ,ground the other secondary terminal4 ,if you see a dog with his nose in the can press your key and start the coil working4 ,it will not hurt the dog1 but he will get the surprise of his life4 ,he will go for home as fast as he can travel and will not touch that particular can again1 even if it should contain some of the choicest canine delicacies4 ,photographing an ,electric ,discharge ,the following experiment must be conducted in a dark room with the aid of a ruby photographic lamp1 as otherwise the plates used would become lightstruck and spoiled4 ,illustration3 ,fig4 #aga4--;,jacob's ,ladder4 ,placed an ordinary photographic plate on a piece of sheet-metal with the coated side of the plate upwards4 ,connect one of the secondary terminals of the spark-coil to the piece of sheet-metal4 ,then sift a thin film of dry starch powder1 sulphur1 or talcum through a piece of fine gauze on the plate4 ,lead a sharp-pointed wire from the other secondary terminal of the coil to the center of the plate and then push the key just long enough to make one spark4 ,wipe the powder off the plate and develop it in the usual manner of films and plates4 ,if you cannot do developing yourself1 place the plate back in its box and send it to some friend1 or to a photographer4 ,the result will be a negative showing a peculiar electric discharge1 somewhat like sea-moss in appearance4 ,no two such photographs will be alike and the greatest variety of new designs1 etc41 imaginable may be produced in this manner4 ,jacob's ,ladder ,take two pieces of bare copper wire about eight inches long and bend them at right angles4 ,place them in the secondary terminals of a spark-coil as in ,figure #aga4 ,bend them so that the vertical portions are about one-half of an inch apart at the bottom and one inch apart at the top4 ,start the coil working1 and the sparks will run up the wires from the bottom to the top and appear very much like the rungs in a ladder4 ,x-,rays ,most young experimenters are unaware what a wonderful and interesting field is open to the possessor of a small ,x-ray tube4 ,small ,x-ray tubes which will operate satisfactorily on an inch and one-half spark-coil may be obtained from several electrical supply houses4 ,they usually cost about four dollars and a half4 ,with such a tube and a fluoroscope it is possible to see the bones in the human hand1 the contents of a closed purse1 etc4 ,the tube is made of glass and contains a very high vacuum4 ,the long end of the tube contains a platinum electrode called the cathode4 ,the short end contains two electrodes called anodes1 one perpendicular to the tube and the other diagonal4 ,the tube is usually clamped in a wooden holder called an ,x-ray tube stand4 ,the tube should be so adjusted that the ,x-rays which are reflected from the diagonal anode will pass off in the direction shown by the dotted lines in ,figure #agd4 ,the fluoroscope is a cone-shaped wooden box fitted with a screen composed of a sheet of paper covered with crystals of a chemical called platinum-barium-cyanide4 ,illustration3 ,fig4 #agb4--;,an ,x-,ray ,tube4 ,the opposite end of the box is fitted with a covering of felt or velvet which shuts off the light around the eyes and nose when you look into the fluoroscope and hold it tightly against the face4 ,a fluoroscope may be purchased complete1 or the platinum-barium-cyanide screen purchased separately and mounted on a box as shown in ,figure #agc4 ,the two anodes of the tube should be connected1 and led to one terminal of a spark-coil capable of giving a spark at least one and one-half inches long4 ,another wire should be led from the cathode of the tube to the other terminal of the coil4 ,illustration3 ,fig4 #agc4--;,fluoroscope4 ,when it is desired to inspect any object1 such as the hand1 it must be held close to the screen of the fluoroscope and placed between the latter and the tube in the path of the ,x-rays4 ,the ,x-rays are thrown forth from the tube at an angle of #de degrees from the diagonal anode4 ,look into the fluoroscope and it should appear to be filled with a green light4 ,if not1 the battery terminals connected to the primary of the coil should be reversed1 so as to send the current through in the opposite direction4 ,the ,x-rays will cause the chemicals on the screen to light up and give forth a peculiar green light4 ,if the hand is held against the screen1 between the screen and the tube1 the ,x-rays will pass through the hand and cast a shadow on the screen4 ,they do not pass through the bones as easily as they do through the flesh and so will cast a shadow of the bones in the hand on the screen1 and if you look closely you will be able to see the various joints1 etc4 ,the interrupter on the coil should be carefully adjusted so that the light does not flicker too much4 ,illustration3 ,fig4 #agd4--;,how to connect an ,x-,ray ,tube to a ,spark-,coil4 ,if it is desired to take ,x-ray pictures1 a fluoroscope is unnecessary4 ,turn the tube around so that the ,x-rays point downward4 ,shut the battery current off so that the tube is not in operation until everything else is ready4 ,place an ordinary photographic plate1 contained in an ordinary plate-holder1 directly under the tube with the gelatin side of the plate upwards4 ,place the hand flat on the plate and lower the tube until it is only about three inches above the hand4 ,then start the coil working so that the tube lights up and permit it to run for about fifteen minutes without removing the hand4 ,then turn the current off and develop the plate in a dark room4 ,it is possible to obtain a very good ,x-ray photograph of the hand in this manner4 ,photographs showing the skeleton of a mouse1 nails in a board1 coins in a purse1 a bullet in a piece of wood1 etc41 are a few of the other objects which make interesting pictures4 ,illustration3 ,an ,x-,ray ,photograph of the hand taken with the ,outfit shown in ,figure #agd4 ,the arrows point to injuries to the bone of the third finger near the middle ,joint ,resulting in a ,stiff ,joint4 ,,chapter ,,xiii ,,transformers ,in most towns and cities where electricity for light and power is carried over long distances1 it will be noticed that small iron boxes are fastened to the poles at frequent intervals1 usually wherever there is a group of houses or buildings supplied with the current4 ,many boys know that the boxes contain transformers1 but do not quite understand exactly what their purpose is1 and how they are constructed4 ,when it is desired to convey electrical energy to a distance1 for the purpose of producing either light or power1 one of the chief problems to be faced is1 how to reduce to a minimum any possible waste or loss of energy during its transmission4 ,furthermore1 since wires and cables of large size are very costly1 it is desirable that they be as small as possible and yet still be able to carry the current without undue losses4 ,it has already been explained that wires offer resistance to an electrical current1 and that some of the energy is lost in passing through a wire because of this resistance4 ,small wires possess more resistance than large ones1 and if small wires are to be used1 in order to save on the cost of the transmission line1 the loss of energy will be greater1 necessitating some method of partially reducing or overcoming this fault4 ,in order to explain clearly how the problem is solved1 the electric current may for the moment be compared to a stream of water flowing through a pipe4 ,illustration3 ,fig4 #age4--;,comparison between ,electric ,current and ,flow of ,water4 ,the illustration shows two pipes1 a small one and a large one1 each supposed to be connected to the same tank1 so that the pressure in each is equal1 and it is clearly apparent that more water will flow out of the large one than out of the small one4 ,if ten gallons of water flow out of the large pipe in one minute1 it may be possible that the comparative sizes of the pipes are such that only one gallon of water will flow out of the small one in the same length of time4 ,but in case it should be necessary or desirable to get ten gallons of water a minute out of a small pipe such as ,b1 what could be done to accomplish it8 ,the pressure could be increased4 ,the water would then be able better to overcome the resistance of the small pipe4 ,this is exactly what is done in the distribution of electric currents for power and lighting4 ,the pressure or potential is increased to a value where it can overcome the resistance of the small wires4 ,but unfortunately it rarely happens that electrical power can be utilized at high pressure for ordinary purposes4 ,for instance1 #aaj volts is usually the maximum pressure required by incandescent lamps1 whereas the pressure on the line wires issuing from the power-house is generally #b1bjj volts or more4 ,such a high voltage is hard to insulate1 and would kill most people coming into contact with the lines1 and is otherwise dangerous4 ,before the current enters a house1 therefore1 some apparatus is necessary1 which is capable of reducing this high pressure to a value where it may be safely employed4 ,this is the duty performed by the transformer enclosed in the black iron box fastened on the top of the electric light poles about the streets4 ,if a transformer were to be defined it might be said to be a device for changing the voltage and current of an ,alternating circuit in pressure and amount4 ,the word1 alternating1 has been placed in italics because it is only upon alternating currents that a transformer may be successfully employed4 ,therein1 also1 lies the reason why alternating current is supplied in some cases instead of direct current4 ,it makes possible the use of transformers for lowering the voltage at the point of service4 ,many boys possessing electrical toys and apparatus operating upon direct current only1 have bemoaned the fact that the lighting system in their town furnished alternating current4 ,very often in the case of small cities or towns one power-house furnishes the current for several communities and the energy has to be carried a considerable distance4 ,alternating current is then usually employed4 ,illustration3 ,fig4 #agf4--;,alternating ,current ,system for ,light and ,power4 ,the illustration shows the general method of arranging such a system4 ,a large dynamo located at the power-house generates alternating current4 ,the alternating current passes into a step-up transformer which raises the potential to #b1bjj volts 7approximately74 ,it is then possible to use much smaller line wires1 and to transmit the energy with smaller loss than if the current were sent out at the ordinary dynamo voltage4 ,the current passes over the wires at this high voltage1 but wherever connection is established with a house or other building1 the service wires which supply the house are not connected directly to the line wires1 but to a a step-down transformer which lowers the potential of the current flowing into the house to about #aaj volts4 ,in larger cities where the demand for current in a given area is much greater than that in a small town1 a somewhat different method of distributing the energy is employed4 ,illustration3 ,fig4 #agg4--,motor ,generator ,set for changing ,alternating ,current to ,direct ,current4 ,the alternating current generated by the huge dynamos at the central station is passed into a set of transformers which in some cases raise the potential as high as five or six thousand volts4 ,the current is then sent out over cables or feeders to various sub stations1 or converter stations1 located in various parts of the city4 ,here the current is first sent through a set of step-down transformers which reduce the potential to the approximate value originally generated by the dynamos4 ,it then passes into the rotary converters which change the alternating current into direct current after which it is sent by underground cables direct to the consumers in the neighborhood4 ,a transformer in its simplest form consists of two independent coils of wire wound upon an iron ring4 ,when an alternating current is passed through one of the coils1 known as the primary1 it produces a magnetic field which induces a current of electricity in the other1 or secondary1 coil4 ,the potential or voltage of the current in the secondary is in nearly the same ratio to the potential of the current passed into the primary as the number of turns in the secondary is to the number of turns in the primary4 ,illustration3 ,fig4 #agh4--,step-,up ,transformer4 ,knowing this1 it is very easy to arrange a transformer to step the potential up or down as desired4 ,the transformer in ,figure #agh represents a step-up transformer having ten turns of wire on the primary and twenty turns on the secondary4 ,if an alternating current of #aj volts and #b amperes is passed into the primary1 the secondary winding will double the potential1 since it has twice as many turns as the primary and the current delivered by the secondary will be approximately #bj volts and #a ampere4 ,the action may be very easily reversed and a step-down transformer arranged by placing twenty turns of wire on the primary and ten turns on the secondary4 ,if a current of #bj volts and #a ampere is passed into the primary1 the secondary will deliver a current of only #aj volts and #b amperes1 since it contains only half as many turns4 ,a circular ring of iron wire wound with two coils would in many respects be somewhat difficult to construct1 and so the iron core is usually built in the form of a hollow rectangle and formed of sheets of iron4 ,illustration3 ,fig4 #agi4--,step-,down ,transformer4 ,it is often desirable to have at hand an alternating current of low voltage for experimental purposes4 ,such a current may be used for operating induction coils1 motors1 lamps1 toy railways1 etc41 and is quite as satisfactory as direct current for many purposes1 with the possible exception of electro-plating and storage-battery charging1 for which it cannot be used4 ,when the supply is drawn from the #aaj-volt lighting circuit and passed through a small step-down transformer1 the alternating current is not only cheaper but more convenient4 ,a transformer of about #ajj watts capacity1 capable of delivering a current of #aj volts and #aj amperes from the secondary will not draw more than approximately one ampere from the #aaj-volt circuit4 ,this current is only equal to that consumed by two ordinary #af-;candle-power lamps or one of #cb candle-power1 making it possible to operate the transformer to its full capacity for about one cent an hour4 ,a further advantage is the fact that a step-down transformer enables the small boy to use the lighting current for operating electrical toys without danger of receiving a shock4 ,illustration3 ,fig4 #ahj4--;,core ,dimensions4 ,the transformer described in the following pages can be easily built by any boy at all familiar with tools1 and should make a valuable addition to his electrical equipment1 provided that the directions are carefully followed and pains are taken to make the insulation perfect4 ,the capacity of the transformer is approximately #ajj watts4 ,the dimensions and details of construction described and illustrated are those of a transformer intended for use upon a lighting current of #aaj volts and #fj-;cycles frequency4 ,the frequency of most alternating current systems is #be1 #fj1 or #abj cycles4 ,the most common frequency is #fj4 ,dimensions and particulars of transformers for #be and #abj cycles will be found in the form of a table farther on4 ,the frequency of your light circuit may be ascertained by inquiring of the company supplying the power4 ,illustration3 ,fig4 #aha4--,the ,core1 ,assembled and ,taped4 ,the first part to be considered in the construction of a transformer is the core4 ,the core is made up of thin sheet-iron strips of the dimensions shown in ,figure #ahj4 ,the iron may be secured from almost any hardware store or plumbing shop by ordering stove-pipe iron4 ,have the iron cut into strips #a #a_/d inches wide and #bd inches long4 ,then1 using a pair of tinner's shears1 cut the long strips into pieces #c inches and #d #c_/d inches long until you have enough to make a pile of each #b #a_/b inches high when they are stacked up neatly and compressed4 ,the long strips are used to form the legs of the core1 and the short ones the yokes4 ,illustration3 ,fig4 #ahb4--,transformer ,leg4 ,the strips are assembled according to the diagram shown in ,figure #ahj4 ,the alternate ends overlap and form a hollow rectangle #d #a_/d x #f inches4 ,the core should be pressed tightly together and the legs bound with three or four layers of insulating tape preparatory to winding on the primary4 ,after the legs are bound1 the yoke pieces may be pulled out1 leaving the legs intact4 ,four fiber heads1 #b #a_/b inches square and #a_/h of an inch thick1 are made as shown in ,figure #ahc4 ,a square hole #a #a_/d x #a #a_/d inches is cut in the center4 ,two of these are placed on each of the assembled legs as shown in ,figure #ahd4 ,illustration3 ,fig4 #ahc4--;,fiber ,head4 ,the primary winding consists of one thousand turns of ,no4 #bj ,b4 ,s4 gauge single-cotton-covered magnet wire4 ,five hundred turns are wound on each leg of the transformer4 ,the wire should be wound on very smoothly and evenly with a layer of shellacked paper between each layer of wire4 ,the two legs should be connected in series4 ,the terminals are protected and insulated by covering with some insulating tape rolled up in the form of a tube4 ,the secondary winding consists of one hundred turns of ,no4 #aj ,b4 ,s4 gauge double-cotton-covered wire4 ,fifty turns are wound on each leg1 over the primary1 several layers of paper being placed between the two4 ,illustration3 ,fig4 #ahd4--,leg with ,heads in ,position for ,winding4 ,a tap is brought out at every ten turns4 ,the taps are made by soldering a narrow strip of sheet-copper to the wire at proper intervals4 ,care must be taken to insulate each joint and tap with a small strip of insulating tape so that there is no danger of a short circuit being formed between adjacent turns4 ,after the winding is completed the transformer is ready for assembling4 ,the yoke pieces of the core should be slipped into position and the whole carefully lined up4 ,the transformer itself is now ready for mounting4 ,illustration3 ,fig4 #ahe4--;,how to make a ,tap in the ,primary by soldering a ,copper ,strip to the ,wire4 ,the base-board measures #aa x #g #c_/d x #g_/h inches4 ,it is shown in ,figure #aib4 ,the transformer rests upon two wooden strips1 ,a and ,b1 #d #a_/d inches long1 #a #a_/d inches wide1 and #c_/d of an inch high4 ,the strips are nailed to the base so that they will come under the ends of the core outside of the fiber heads4 ,the transformer is held to the base by two tie-rods passing through a strip1 ,c1 #f inches long1 one-half of an inch thick and three-quarters of an inch wide4 ,the strip rests on the ends of the core4 ,the tie-rods are fastened on the under side of the base by means of a nut and washer on the ends4 ,when the nuts are screwed up tightly1 the cross-piece will pull the transformer firmly down to the base4 ,illustration3 ,fig4 #ahf4--,the ,transformer completely ,wound and ready for ,assembling4 ,the regulating switches1 two in number1 are mounted on the lower part of the base4 ,the contact points and the arm are cut out of sheet-brass1 one-eighth of an inch thick4 ,it is unnecessary to go into the details of their construction1 because the dimensions are plainly shown in ,figure #ahh4 ,the contacts are drilled out and countersunk so that they may be fastened to the base with small flat-headed wood screws4 ,each switch-arm is fitted with a small rubber knob to serve as a handle4 ,the arm works on a small piece of brass of exactly the same thickness as the switch-points4 ,care must be taken that the points and this washer are all exactly in line1 so that the arm will make good contact with each point4 ,there are five points to each switch1 as shown in ,figure #aij4 ,illustration3 ,fig4 #ahg4--,wooden ,strips for mounting the ,transformer on the ,base4 ,the switch1 ,d1 is arranged so that each step cuts in or out twenty turns of the secondary1 the first point being connected with the end of the winding4 ,the second point connects with the first tap1 the third contact with the second tap1 the fourth contact with the third tap1 and the fifth contact with the fourth tap4 ,illustration3 ,fig4 #ahh4--;,details of the ,switch ,parts4 ,the switch1 ,e1 is arranged so that each step cuts in or out five turns4 ,the contacts on this switch are numbered in the reverse direction4 ,the fifth contact of switch ,d1 and the fifth contact of switch ,e1 are connected together4 ,the fourth contact is connected to the fifth tap1 the third contact to the sixth tap1 the second contact to the seventh1 and the first contact to the end of the winding4 ,this arrangement makes it possible to secure any voltage from one-half to ten in one-half-volt steps from the secondary of the machine4 ,each step on the switch1 ,d1 will give two volts1 while those on ,e will each give one-half of a volt4 ,illustration3 ,fig4 #ahi4--,the ,complete ,switch4 ,two binding-posts 7marked ,p and ,p in the drawing7 mounted in the upper corners of the base are connected to the terminals of the primary winding4 ,the two posts in the lower corners 7marked ,s and ,s in the drawing7 are connected to the switch levers1 and are the posts from which the secondary or low voltage is obtained4 ,illustration3 ,fig4 #aij4--;,diagram of ,connections4 ,the transformer may be connected to the #aaj-,v4 alternating current circuit by means of an attachment plug and cord4 ,one end of the cord is placed in each of the primary binding-posts4 ,the other end of the cord is connected to the attachment plug so that the latter may be screwed into any convenient electric-light socket4 ,illustration3 ,fig4 #aia4--,top ,view of the ,transformer4 ,the transformer must not be connected directly to the line4 ,an instrument such as this is not designed for continuous service and is intended to be disconnected as soon as you are through using it4 ,illustration3 ,fig4 #aib4--,side ,view of the ,transformer4 ,it will be found a great convenience in operating many of the electrical devices described1 wherever direct current is not essential4 ,,chapter ,,viv ,,wireless ,,telegraphy ,probably no branch of electrical science ever appealed more to the imagination of the experimenter than that coming under the heading of wireless telegraphy4 ,wherever you go1 you are likely to see the ear-marks of amateur wireless telegraph stations in the aerials and masts set up in trees and on house-tops4 ,it is estimated that there are nearly a quarter of a million such stations in the ,united ,states4 ,there is really no great mystery about this wonderful art which made possible the instantaneous transmission of messages over immense distances without any apparent physical connection save that of the earth1 air1 or water4 ,did you ever throw a stone in a pool of water8 ,as soon as the stone struck1 little waves spread out from the spot in gradually enlarging circles until they reached the shore or died away4 ,by throwing several stones in succession1 with varying intervals of time between them1 it would be possible so to arrange a set of signals1 that they would convey a meaning to a second person standing on the opposite shore of the pool4 ,wireless telegraphy is based upon the principle of creating and detecting waves in a great pool of ether4 ,modern scientists suppose that all space is filled with an imaginary substance called ether4 ,the ether is invisible1 odorless1 and practically weightless4 ,this ether1 however1 bears no relation to the anaesthetic of that name which is used in surgical operations4 ,it surrounds and penetrates all substances and all space4 ,illustration3 ,fig4 #aic4--,little ,waves spread out from the ,spot4 ,it exists in a vacuum and in solid rocks4 ,since the ether does not make itself apparent to any of our physical senses1 some of these statements may seem contradictory4 ,its definite existence cannot be proved except by reasoning1 but by accepting and imagining its reality1 it is possible to understand and explain many scientific puzzles4 ,a good instance is offered by the sun4 ,light and heat can be shown to consist of extremely rapid vibrations4 ,that fact can be proved4 ,the sun is over #ij1jjj1jjj miles away from our earth and yet light and heat come streaming down to us through a space that is devoid even of air4 ,something must exist as a medium to transmit these vibrations2 it is the ether4 ,let us consider again the pool of water4 ,the waves or ripples caused by throwing in the stone are vibrations of the water4 ,the distance between two adjacent ripples is called the wave length4 ,the distances between two vibrations of light can also be measured4 ,they are so small1 however1 that they may be spoken of only in thousandths of an inch4 ,the waves created in the ether by wireless telegraph apparatus are the same as those of light except that their length usually varies from #ge to #i1jjj feet instead of a fraction of a thousandth of an inch4 ,illustration3 ,fig4 #aid4--;,a ,simple ,transmitter4 99,a ,simple ,transmitter99 is illustrated in ,figure #aid4 ,a telegraph key is connected in series with a set of cells and the primary of an induction coil1 which1 it will be remembered1 is simply a coil consisting of a few turns of wire4 ,this induces a high voltage in a second coil consisting of a larger number of turns and called the secondary4 ,the terminals of the secondary are led to a spark-gap--an arrangement composed of two polished brass balls1 separated by a small air-gap4 ,one of the balls1 in turn1 is connected to a metal plate buried in the earth1 and the other to a network of wires suspended high in the air and insulated from all surrounding objects4 ,when the key at the transmitter is pressed1 the battery current flows through the primary of the induction coil and generates in the secondary a current of very high voltage1 #bj1jjj volts or more1 which is able to jump an air-gap in the shape of a spark at the secondary terminals4 ,the latter are connected to the earth and aerial1 as explained above4 ,the high potential currents are therefore enabled to charge the aerial4 ,the charge in the aerial exerts a great tendency to pass into the ground1 but is prevented from doing so by the small air-gap between the spark-balls until the charge becomes so great that the air-gap is punctured and the charge passes across and flows down into the ground4 ,the passage of the charge is made evident by the spark between the two spark-balls4 ,the electrical charges flowing up and down the aerial disturb the ether1 strike it a blow1 as it were4 ,the effect of the blow is to cause the ether to vibrate and to send out waves in all directions4 ,it may be likened to the pond of water which is suddenly struck a blow by throwing a stone into it1 so that ripples are immediately sent out in widening circles4 99,these ,waves in the ,ether99 are called electro-magnetic or ,hertzian waves1 after their discoverer1 ,hertz4 ,the distance over which they pass is dependent upon the power of the transmitting station4 ,the waves can be made to correspond to the dots and dashes of the telegraphic code by so pressing the key4 ,if some means of detecting the waves is employed we may readily see how it is possible to send wireless messages4 99,the ,action of the ,receiving ,station99 is just the opposite of that of the transmitter4 ,when the waves pass out through the ether1 some of them strike the aerial of the receiving station and generate a charge of electricity in it which tends to pass down into the earth4 ,if the transmitting and receiving stations are very close together and the former is very powerful1 it is possible to make a very small gap in the receiving aerial across which the charge will jump in the shape of sparks4 ,thus the action of the receptor simply takes place in a reversed order from that of the transmitter4 ,if the stations are any considerable distance apart1 it is impossible for the currents induced in the receiving aerial to produce sparks1 and so some more sensitive means of detecting the waves from the transmitter is necessary1 preferably one which makes itself evident to the sense of hearing4 ,the telephone receiver is an extremely sensitive instrument1 and it only requires a very weak current to operate it and produce a sound4 ,the currents or oscillations generated in the aerial1 however1 are alternating currents 7see pages #ig-ii7 of high frequency1 that is1 they flow in one direction and then reverse and flow in the other several thousand times a second4 ,such a current cannot be made to pass through a telephone receiver1 and in order to do so the nature of the current must be changed by converting it into direct current flowing in one direction only4 ,certain minerals and crystals possess the remarkable ability to do this1 silicon1 galena1 and iron pyrites are among the best4 ,illustration3 ,fig4 #aie4--;,a ,simple ,receptor4 ,the diagram in ,figure #aie shows the arrangement of a simple receiving outfit4 ,the detector consists of a sensitive mineral placed between two contacts and connected so that the aerial currents must pass through it on their way to the ground4 ,a telephone receiver is connected to the detector so that the rectified currents 7currents which have been changed into direct current7 pass into it and produce a sound4 ,by varying the periods during which the key is pressed at the transmitting station1 according to a prearranged code1 the sounds in the receiver may be made to assume an intelligible meaning4 ,,how ,,to ,,build ,,wireless ,,instruments 99,the ,aerial99 ,every wireless station is provided with a system of wires elevated high in the air1 above all surrounding objects1 the purpose of which is to radiate or intercept the electromagnetic waves1 accordingly as the station is transmitting or receiving4 ,this system of wires is1 as already has been stated1 called the aerial or antenna4 ,the arrangement of the aerial will greatly determine the efficiency and range of the apparatus4 ,the aerial should be as long as it is reasonably possible to make it1 that is from #ej to #aej feet4 ,it will be necessary for most amateurs to put up their aerial in some one certain place1 regardless of what else may be in the vicinity1 but whenever possible the site selected should preferably be such that the aerial will not be in the immediate neighborhood of any tall objects1 such as trees1 smoke-stacks1 telephone wires1 etc41 because such objects will interfere with the aerial and noticeably decrease the range of the station1 both when transmitting and receiving4 ,bare copper wire makes the best aerials4 ,aluminum wire is very often used and on account of its light weight causes very little strain on the poles or cross arms4 ,iron wire should never be used for an aerial1 even if galvanized or tinned1 because it tends to choke the currents which must flow up and down the aerial when the station is in operation4 ,illustration3 ,fig4 #aif4--,molded ,aerial ,insulator ,the aerial must be very carefully insulated from its supports and all surrounding objects4 ,the insulation must be strong enough to hold the weight of the aerial and able to withstand any strain caused by storms4 ,special aerial insulators made of molded insulating material and having an iron ring imbedded in each end are the best4 ,illustration3 ,fig4 #aig4--;,a ,porcelain ,cleat will make a ,good ,insulator for ,small ,aerials4 ,ordinary porcelain cleats may be used on small aerials where the strain is light4 ,one insulator should be placed at each end of each wire close to the spreader or spar4 ,most aerials are made up of four wires4 ,the wires should be placed as far apart as possible4 ,there are several different forms of aerials1 the principal ones of which are shown in ,figure #aii4 ,they are known as the grid1 8,v1 inverted 8,l10 and 8,t0 types4 ,most amateurs support their aerials from a pole placed on the top of the house1 in a tree1 or erected in the yard4 ,many use two supports1 since such an aerial has many advantages4 ,the facilities to be had for supporting the aerial will largely determine which form to use4 ,illustration3 ,fig4 #aih4--,method of ,arranging the ,wires and ,insulating them from the ,cross ,arm or ,spreader4 ,the grid aerial has no particular advantages or disadvantages4 ,the 8,v0 aerial receives waves much better when they come from a direction opposite to that in which the free end points4 ,the free end of the aerial is the one not leading into the station4 ,the inverted 8,l0 aerial possesses the same characteristics as the 8,v0 type4 ,the 8,t0 aerial is the best 8all around and is to be recommended whenever it is possible to put up an aerial of this sort4 ,much of the detail of actually putting up an aerial or antenna must be omitted1 because each experimenter will usually meet different conditions4 ,it should be remembered1 however1 that the success of the whole undertaking will rest largely upon the construction of a proper aerial4 ,the most excellent instruments will not give very good results if connected to a poor aerial1 while1 on the other hand1 inferior instruments will often give fair results when connected to a good aerial4 ,illustration3 ,fig4 #aii4--,various ,types of ,aerials4 ,the aerial should be at least thirty feet high4 ,the wire should not be smaller than ,no4 #ad ,b4 ,s4 ,the masts which support the aerial should be of wood and provided with pulleys so that the wires may be lowered any time it may be necessary4 ,the mast should be thoroughly braced with stays or guys so as to counteract the strain of the aerial4 ,the aerial should not be hoisted up perfectly tight1 but should be allowed to hang somewhat loose1 as it will then put less strain on the ropes and poles that support it4 ,when an aerial is to be fastened in a tree1 it is best to attach it to a pole placed in the top of the tree1 so that it will come well above any possible interference from the branches4 ,the wires leading from the aerial to the instruments should be very carefully insulated throughout their length4 ,this part of the aerial is called the rat-tail or lead-in4 ,the illustrations in ,figure #aii show the proper place to attach the 8lead-in form of aerial4 ,the wires should gradually converge4 ,illustration3 ,fig4 #bjj4--;,a ,ground ,clamp for ,pipes4 ,it is very important that a good ground connection be secured for wireless instruments4 ,a good ground is absolutely necessary for the proper working of the apparatus4 ,amateur experimenters usually use the water or gas-pipes for a ground1 and fasten the wires by means of a ground clamp such as shown in ,figure #bjj4 ,in the country1 where such pipes are not available1 it is necessary to bury a sheet of copper1 three or four feet square1 in a moist spot in the earth and connect a wire to it4 99,the ,receiving ,apparatus99 ,the receiving instruments form the most interesting part of a wireless station and usually receive first attention from the amateurs4 ,they are the ears of the wireless station and are wondrously sensitive1 yet are very simple and easy of construction4 ,the instruments necessary for receiving are3 ,a ,detector1 ,a ,tuning ,coil or a ,loose ,coupler1 ,a ,fixed ,condenser1 ,a ,telephone ,receiver4 ,other devices1 such as a test buzzer1 variable condenser1 etc41 may be added and will improve the outfit4 ,after the aerial has been properly erected1 the first instrument necessary to construct will be either a tuning coil or a loose coupler4 ,it is a good plan to make a tuning coil first1 and a loose coupler after you have had a little experience with your apparatus4 99,a ,tuning ,coil99 is a very simple arrangement making it possible to receive messages from greater distances1 and also somewhat to eliminate any messages not desirable and to listen without confusion to the one wanted4 ,a tuning coil consists of a single layer of wire wound upon a cylinder and arranged so that connection may be had with any part of it by means of sliding contacts4 ,the cylinder upon which the wire is wound is a cardboard tube six and three-quarters inches long and two and seven-eighths inches in diameter outside4 ,it should be given two or three coats of shellac both inside and out so that it is thoroughly impregnated1 and then laid away until dry4 ,this treatment will prevent the wire from becoming loose after the tube is wound1 due to shrinkage of the cardboard4 ,illustration3 ,fig4 #bja4--;,details of the ,tuning ,coil4 ,after having become dry1 the tube is wound with a single layer of ,no4 #be ,b4 ,s4 gauge green silk or cotton-covered magnet wire4 ,the wire must be wound on very smoothly and tightly1 stopping and starting one-quarter of an inch back from each end4 ,the ends of the wire are fastened by weaving back and forth through two small holes punched in the cardboard tube with a pin4 ,the winding should be given a single coat of clear varnish or white shellac and allowed to dry4 ,the coil heads or end pieces are cut from one-half-inch wood according to the plan and dimensions shown in the accompanying illustration4 ,the top corners are beveled and notched to receive the slider-rods4 ,a circular piece of wood two and five-eighths inches in diameter and three-eighths of an inch thick is nailed to the inside of each of the coil heads to support the ends of the cylinder4 ,the wooden parts should be stained mahogany or some other dark color and finished with a coat of shellac or varnish4 ,the slider-rods are square brass #c-af x #c-af inches and seven and three-quarters inches long4 ,a small hole is bored near the ends of each1 one-quarter of an inch from the edge1 to receive a round-headed brass wood screw which holds the rod to the tuner end4 ,the sliders may be made according to the plan shown in ,figure #bja4 ,the slider is made from a small piece of brass tubing1 three-sixteenths of an inch square4 ,an #h-cb flat-headed brass screw is soldered to one face1 in the center4 ,a small strip of phosphor bronze sheet or spring copper soldered to the bottom of the slider forms a contact for making connection to the wire on the cylinder4 ,a small electrose knob screwed to the slider makes a neat and efficient handle4 ,two sliders are required1 one for each rod4 ,the tuning coil is assembled as shown in ,figure #bjc4 ,the cardboard tube is held in place by several small brass nails driven through it into the circular pieces on the coil heads4 ,a slider is placed on each of the slider-rods and the rods fastened in the slots in the coil ends by a small round-headed brass screw1 passing through the holes bored near the ends for that purpose4 ,illustration3 ,fig4 #bjb4--,side and ,end ,views of the ,tuning ,coil4 ,two binding-posts are mounted on one of the coil ends4 ,one should be connected to each of the slider-rods4 ,a third binding-post is placed below in the center of the head and connected to one end of the wire wound around the cylinder4 ,a small1 narrow path along the coil1 directly underneath each slider and to which the copper strip can make contact1 must be formed by scraping the insulation off the wire with a sharp knife4 ,the sliders should make contact with each one of the wires as they pass over1 and should slide smoothly without damaging or disarranging any of the wires4 ,illustration3 ,fig4 #bjc4--;,complete ,double-,slider ,tuning ,coil4 ,when scraping the insulation1 be very careful not to loosen the wires or remove the insulation from between them1 so that they are liable to short-circuit between adjacent turns4 99,a ,loose ,coupler99 is a much more efficient tuning device than a double-slider tuner1 and sooner or later most amateur wireless operators install one in their station4 ,the loose coupler shown in the figure given on the next page is a very simple one and is both easy and inexpensive to build4 ,its simplicity is a disadvantage in one respect1 however4 ,owing to its construction1 it is impossible to move the slider on the secondary when the latter is inside the primary4 ,the reason that ,i have chosen this sort of loose coupler to describe is to acquaint my young readers with the methods of making a loose coupler4 ,the ,junior loose coupler described farther on is a more elaborate instrument of greater efficiency1 but much harder to build4 ,illustration3 ,fig4 #bjd4--;,a ,simple ,loose ,coupler4 ,the base of the loose coupler is of wood and measures twelve by four inches4 ,the head supporting the primary is of the same size as those used on the ,junior double-slide tuning coil just described4 ,it may be made in the same manner1 and fitted with a circular block to support the tube4 ,the primary tube is of the same diameter as that on the tuning coil but is only four inches long4 ,it is fastened to the primary head with glue and then secured with a number of small tacks4 ,one or two coats of shellac liberally applied will render it non-shrinkable1 so that the wire will not be apt to loosen after the loose coupler has been in use a while4 ,the secondary is of the same length as the primary1 but of smaller diameter1 so that it will easily slip inside4 ,it also is treated with shellac4 ,the primary should be wound with a single layer of ,no4 #bb single-silk-covered magnet wire4 ,the secondary is wound with ,no4 #bi single-silk4 ,the head supporting the secondary is smaller than that used for the same purpose on the primary4 ,the round boss to which the tube is fastened1 however1 is much thicker4 ,the secondary slides on a guide-rod supported at one end by passing through the primary head and at the other by a brass upright4 ,the upright may also be made of wood4 ,if the secondary is offset1 that is1 placed out of center slightly to one side1 it will leave room so that the secondary slider will possibly pass inside of the primary without striking4 ,both the primary and the secondary must be fitted with sliders to make contact with the various turns of wire4 ,the method of constructing a slider has already been described4 ,the ends of the slider-rods are bent at right angles and fastened to the coil heads by two small screws passing through holes bored near the ends4 ,a small narrow path must be scraped in the insulation under each so that the slider will make contact with each turn4 ,the secondary head may be provided with a small electrose handle to facilitate sliding it back and forth4 ,two binding-posts are mounted on each of the coil heads4 ,one post on each is connected to the end of the coil farthest from the head1 and the other posts are each connected to the slider-rods4 ,figure #bbj shows how to connect the loose coupler in the receiving set4 ,in order to tune with a loose coupler1 first adjust the slider on the primary until the signals are the clearest4 ,then set the secondary slider in the best place and move the secondary in and out of the primary until the signals are clearest4 99,how to ,build the ,junior ,loose ,coupler99 ,a loose coupler of the sort just described is simple and quite easily constructed1 but will not be found to work as well as one in which the secondary may be varied by means of a switch while it is inside of the primary4 ,the base of the instrument measures twelve by three and five-eighths inches4 ,the primary is composed of a single layer of ,no4 #bd ,b4 ,s4 gauge single-silk-covered wire wound on a cardboard tube two and three-quarter inches in diameter and three and three-quarter inches long4 ,the winding is laid on in a single layer and should comprise about #aej turns4 ,after winding on tightly1 it should be given a coat of clean white shellac and allowed to dry4 ,the shellac is for the purpose of fastening the wire down tightly to the tube so that it will not loosen when the slider is moved back and forth4 ,the primary is mounted between two heads1 the details of which are shown in ,figure #bje4 ,one of the heads1 ,b1 has a flanged hole two and three-quarter inches in diameter cut through the center so as to receive the end of the tube and permit the secondary to pass inside4 ,illustration3 ,fig4 #bje4--;,details of the ,wooden ,parts4 ,the secondary winding is composed of a single layer of ,no4 #bh ,b4 ,s4 gauge silk-covered wire and divided into six equal sections4 ,the secondary is supported by two circular wooden pieces1 ,c and ,f1 and slides back and forth on two guide-rods4 ,the guide-rods should be brass4 ,iron or steel rods running through the center of a loose coupler will seriously weaken the signals1 and such practice must by all means be avoided4 ,illustration3 ,fig4 #bjf4--,side ,view of the ,loose ,coupler4 ,illustration3 ,fig4 #bjg4--,top ,view of the ,loose ,coupler4 ,the secondary sections are connected to six contacts and a switch-arm mounted on the end of the secondary so that by turning the switch either one1 two1 three1 four1 five1 or six sections of the winding may be connected4 ,illustration3 ,fig4 #bjh4--;,end ,views of the ,loose ,coupler4 ,illustration3 ,fig4 #bji4--;,complete ,loose ,coupler4 ,the two binding-posts near the secondary end of the coupler are connected to the terminals of the secondary winding by means of two flexible wires4 ,they have not been shown in several of the illustrations because they would be liable to confuse the drawing4 ,the primary is provided with a slider moving back and forth over a narrow path scraped through the insulation so that it may make contact with each wire independently4 99,detectors99 ,detectors are very simple devices and consist merely of an arrangement for holding a small piece of certain minerals and making a contact against the surface4 ,the crystal detector shown in ,figure #baj is a very efficient form that may be easily and quickly made4 ,when finished1 it will make a valuable addition to almost any amateur experimenter's wireless equipment4 ,illustration3 ,fig4 #baj4--;,a ,crystal ,detector4 ,the bracket is bent out of a piece of strip brass about one-eighth of an inch thick and five-eighths of an inch wide1 according to the shape shown in the illustration4 ,the bracket is mounted on a circular wooden base about three inches in diameter4 ,the circular wooden blocks used by electricians in putting up chandeliers1 called 8fixture blocks10 will make a satisfactory base4 ,an electrose knob of the typewriter type may be purchased from any good dealer in wireless supplies4 ,it should be fitted with a threaded shank which will screw into a hole in the upper part of the bracket4 ,the mineral is contained in a small brass cup mounted on the base below the end of the knob4 ,contact with the mineral in the cup is made by means of a fine wire spring soldered to the end of the adjusting screw4 ,moving the screw up or down will vary the pressure of the spring on the mineral and permit the most sensitive adjustment to be secured4 ,the bracket is connected to one of the binding-posts and the cup to the other4 ,illustration3 ,fig4 #baa4--;,details of the ,crystal ,detector4 ,the detector shown in ,figure #bab is of the type often termed a cat-whisker1 because of the long1 fine wire resting on the mineral4 ,it consists of a small clip1 formed by bending a strip of sheet-brass1 which grips a piece of galena4 ,illustration3 ,a ,double ,slider ,tuning ,coil4 ,illustration3 ,a ,junior ,loose ,coupler4 ,illustration3 ,crystal ,detectors4 ,galena may be obtained from any dealer in radio supplies4 ,a piece of ,no4 #cj phosphor bronze wire is soldered to the end of a short length of brass rod supported by a binding post4 ,the other end of the rod is fitted with an electrose knob4 ,this part of the detector is called the feeler4 ,illustration3 ,fig4 #bab ,details of the ,cat ,whisker ,detector4 ,the detector is fitted with binding posts and may be mounted upon any suitable small base4 ,the mineral clip is connected to one post and the binding-post supporting the feeler to the other4 ,the tension or pressure of the end of the fine wire upon the mineral may be regulated by twisting the electrose knob so as to twist the rod4 ,the different portions of the crystal may be searched for the most sensitive spot by sliding the rod back and forth4 ,a somewhat similar form of cat-whisker detector is shown in ,figure #bac4 ,it is provided with a cup to hold the mineral in place of a clip4 ,the detector shown in ,figure #bad is more elaborate than any of the others described so far4 ,illustration3 ,fig4 #bac4--;,another ,form of the ,cat-,whisker ,detector4 ,illustration3 ,fig4 #bad4--;,cat-,whisker ,detector4 ,the base is a wooden block1 three and one-half by one and three-quarters inches by one-half inch4 ,the binding-posts are of the type commonly used on electrical instruments4 ,one of the posts is pivoted so that it will swing from side to side4 ,a short piece of brass rod fitted with a rubber or fiber knob passes through the wire hole in the post4 ,a piece of ,no4 #cj ,b4 ,s4 gauge bronze wire is soldered to the end of the rod4 ,a small brass cup contains the mineral1 which may be either galena1 or silicon4 ,by twisting the post and sliding the rod back and forth1 any portions of the mineral surface may be selected4 99,fixed ,condenser499 ,the construction of the condenser is illustrated in ,figure #bje4 ,take twenty-four sheets of thin typewriter paper1 three by four inches1 and twenty-three sheets of tinfoil1 two by four inches4 ,pile them up1 using first a sheet of paper then a sheet of tinfoil1 then paper1 and so on1 so that every two sheets of tinfoil are separated by a sheet of paper4 ,each sheet of tinfoil must1 however1 project out beyond the edge of the paper4 ,connect all the tinfoil projections on one end of the condenser together and and attach a small wire4 ,connect all those on the opposite side in a similar manner4 ,then fasten a couple of rubber bands around the condenser to hold it together4 ,illustration3 ,fig4 #bae4--;,building up a ,fixed ,condenser4 ,illustration3 ,fig4 #baf4--;,a ,fixed ,condenser enclosed in a ,brass ,case made from a ,piece of ,tubing fitted with ,wooden ,ends4 ,if it is desired to give the condenser a finished appearance1 it may be placed in a brass tube fitted with two wooden or fiber ends4 ,the ends are provided with binding-posts to which the terminals of the condenser are connected4 99,telephone ,receivers99 for use with wireless instruments must be purchased4 ,their construction is such that they cannot be made by the experimenter4 ,illustration3 ,fig4 #bag4--;,a ,telephone ,head ,set4 ,a seventy-five ohm1 double-pole telephone receiver will do for stations not wishing to receive farther than fifty miles4 ,in order to secure the best results from wireless instruments1 it is necessary to have receivers especially made for wireless4 ,each receiver should have #ajjj ohms resistance4 ,some boys may find it necessary to purchase one receiver at a time4 ,two receivers1 a double headband1 and a double cord1 forming a complete head set as shown in ,figure #bag1 should be secured as soon as possible4 ,illustration3 ,fig4 #bah4--;,a ,circuit showing how to connect a ,double-,slider ,tuning ,coil4 99,connecting the ,receiving ,apparatus99 ,figure #bah shows how to connect a double-slide tuner1 a detector1 a fixed condenser and a pair of telephones to the aerial and ground4 ,the same instruments with a loose coupler in place of the double-slide tuner are shown in ,figure #bai4 ,the diagrams in ,figure #bbj are the same circuits as those shown in ,figures #bah and #bai1 but show different instruments4 ,illustration3 ,fig4 #bai4--;,circuit showing how to connect a ,loose ,coupler4 ,illustration3 ,fig4 #bbj4--;,a ,diagram showing how to connect some of the ,instruments described in this ,chapter4 ,after the instruments are connected1 place a piece of galena or silicon in the cup of the detector and bring the wire down on it4 ,then move the sliders on the tuning coil or loose coupler and adjust the detector until you can hear a message buzzing in the telephones4 ,it may require a little patience and practice1 but if you persist you will soon learn how to adjust the apparatus so as to receive the signals loudly and clearly with very little trouble4 99,the ,transmitting ,apparatus99 ,spark coils have already been described in ,chapter ,,xii4 ,they may be used to transmit wireless messages simply by connecting to a spark-gap and a key4 ,spark coils which are especially made for wireless telegraphy will usually send farther than an ordinary spark coil used for experimental purposes4 ,illustration3 ,fig4 #bba4--;,a ,wireless ,spark ,coil4 ,a good one-inch coil costs from #d4ej to #e4jj and will send from three to five miles if used with a fair aerial4 ,a spark coil requires considerable current for its successful operation and will give the best results if operated on storage cells1 dry cells1 or bichromate cells4 ,if dry cells are used1 it is a good plan to connect them in series multiple as shown in ,figure #fi4 ,spark-gaps may be made by mounting two double binding-posts on a wooden base as shown in ,figure #bbb4 ,zinc possesses some peculiar property which makes it very efficient for a spark-gap1 and for this reason the electrodes of a spark-gap are usually zinc4 ,illustration3 ,fig4 #bbb4--,small ,spark ,gaps4 ,the figure shows two different forms of electrodes4 ,in one1 they are made of zinc rods and provided with 8electrose0 handles4 ,in the other gap1 the zinc electrodes are in the shape of tips fitted on the ends of two short brass rods4 ,a one-inch spark coil will give very good results by connecting the spark-gap directly across the secondary of the coils4 ,the aerial is connected to one side of the gap and the ground to the other4 ,the transmitter may be tuned and the range sometimes increased by using a condenser and a helix4 ,a condenser is most easily made by coating the inside and outside of a test-tube with tinfoil so as to form a miniature ,leyden jar4 ,the end of the tube is closed with a cork through which passes a brass rod connecting to the inner coating of tinfoil4 ,illustration3 ,fig4 #bbc4--;,diagram showing how to connect a ,simple ,transmitter4 ,if such a condenser is connected directly across the spark-gap1 the spark will become very white and crackling4 ,several tubes may be arranged in a rack as shown in ,figure #bbe4 ,a helix consists of a spiral of brass ribbon set in a wooden frame4 ,the two strips composing the frame are each nine inches long4 ,the spiral consists of eight turns of brass ribbon1 three-eighths of an inch wide1 set in saw-cuts made in the frame4 ,a binding-post is connected to the outside end of the ribbon4 ,figure #bbh shows how to connect a helix and a condenser to a coil and a spark-gap4 ,the two clips are made by bending a strip of sheet brass and connecting a piece of flexible wire to one end4 ,illustration3 ,fig4 #bbd4--;,a ,test-,tube ,leyden ,jar4 ,in large stations1 the best position for the clips is found by placing a hot-wire ammeter in the aerial circuit and then moving the clips until the meter shows the highest reading4 ,the young experimenter will have to tune his set by moving the helix clips about until the best results are obtained in sending4 ,illustration3 ,fig4 #bbe4--;,eight ,test-,tube ,leyden ,jars mounted in a ,wooden ,rack4 ,if the spark coil is a good one and capable of giving a good hot spark1 it may be possible to tell when the set is in proper tune by placing a small miniature tungsten lamp in series with the aerial and changing the clips1 the condenser1 and the length of the spark-gap until the lamp lights the brightest4 ,an oscillation transformer is sometimes used to replace an ordinary helix when it is desirable to tune a station very closely so that its messages shall not be liable to be confused with those of another station when both are working at the same time4 ,illustration3 ,fig4 #bbf4--;,a ,helix and ,clip4 ,an oscillation transformer consists of two helixes arranged so that one acts as a primary and the other as a secondary4 ,an oscillation helix may be made by making two sets of helix frames similar to that in ,secondary ,figure #bbf4 ,illustration3 ,fig4 #bbg4--;,an ,oscillation ,transformer4 ,the primary should be provided with eight turns of brass ribbon and the secondary with twelve4 ,the primary supports a stiff brass rod upon which the secondary is mounted4 ,the secondary should slide up and down on the rod but move very stiffly so that it will stay where it is put4 ,illustration3 ,,an ,,oscillation ,,helix4 ,illustration3 ,,an ,,oscillation ,,condenser4 ,an ordinary double-throw1 double-pole knife switch having a porcelain base will make a very good aerial switch in a small station4 ,it is used to connect the aerial and ground to either the transmitting or receiving apparatus at will4 ,such a switch is shown in ,figure #bcj4 ,illustration3 ,fig #bbh4--;,circuit showing how to connect a ,helix and a ,condenser4 ,the aerial should be connected to the post ,a and the ground to ,b4 ,the posts ,e and ,f lead to the transmitter1 and ,c and ,d to the receptor1 or vice-versa according to which is the more convenient from the location of the apparatus on the table or operating bench4 ,a suitable table should be arranged to place the wireless instruments upon so that they may be permanently connected together4 ,illustration3 ,fig #bbi4--;,circuit showing how to connect an ,oscillation ,transformer and a ,condenser4 ,the ,continental ,code is the one usually employed in wireless telegraphy4 ,it differs slightly from ,morse as it contains no space letters4 ,it will be found easy to learn and somewhat easier to handle than ,morse4 ,illustration3 ,fig #bcj4--;,an ,aerial ,switch4 ,two or three months' steady practice with a chum should enable the young experimenter to become a very fair wireless telegraph operator4 ,then by listening for some of the high power wireless stations which send out the press news to ships at sea during the evening it should be possible to become very proficient4 ,the press news is sent more slowly than ordinary commercial wireless messages1 and is therefore easy to read and a good starting point for the beginner learning to read4 ,illustration3 ,fig #bca4--;,a ,complete ,wiring ,diagram for both the ,transmitter and the ,receptor4 ,illustration3 ,fig4 #bcb4--,the ,continental ,alphabet4 99,a ,coherer ,outfit99 99,a ,coherer ,outfit99 is usually capable of only receiving messages coming from a distance of under one mile4 ,in spite of this fact1 however1 it is an exceedingly interesting apparatus to construct and experiment with1 and for this reason is found fully described below4 ,a coherer set will ring a bell or work a sounder for short distances and therefore is the best sort of an arrangement for demonstrating the workings of your wireless apparatus to your friends4 ,the first thing that you need for a coherer is a pair of double binding-posts4 ,mount these about an inch and three-quarters apart on a wooden base1 six inches long and four inches wide as shown in ,figure #bcc4 ,illustration3 ,fig4 #bcc4--;,a ,coherer and a ,decoherer4 ,get a piece of glass tubing about an inch and one-half long and about one-eighth of an inch inside diameter4 ,you will also need some brass rod which will just slide into the tube tightly4 ,cut off two pieces of the brass rod each one and three-quarters inches long and slip these through the upper holes in the binding-posts and into the glass tube as shown in ,figure #bcd4 ,before putting the second rod in place1 however1 you must put some nickel and silver filings in the tube1 so that when the rods are pushed almost together1 with only a distance of about one-sixteenth of an inch between them1 the filings will about half fill the space4 ,the filings must be very carefully prepared1 and in order to make them1 first use a coarse-grained file on the edge of a five-cent piece4 ,do not use the fine dust and powder1 but only the fairly coarse filings4 ,mix a few silver filings from a ten-cent piece with the nickel in such proportion that the mixture is #ij nickel and #aj silver4 ,illustration3 ,fig4 #bcd4--;,details of the ,coherer4 ,you will have to experiment considerably to find out just the right amount of filings to place in the tube1 and how far apart to place the brass rods or plugs4 ,remove the gong from an old electric bell and mount the bell on the base as shown in ,figure #bcc4 ,it should be in such a position that the bell hammer will touch the coherer very lightly when the bell is ringing4 ,the two binding-posts1 tube rods1 and filings constitute the coherer4 ,the bell is the decoherer4 ,the next thing required in order to complete the apparatus is a relay4 ,you may use the relay described in ,chapter ,x or build one according to the plan shown in ,figure #bce4 ,this relay consists of a single electro-magnet mounted on a wooden base1 two inches wide and four inches long4 ,the armature is a piece of soft iron rod one-quarter of an inch in diameter and one-eighth of an inch long1 riveted to the end of a thin piece of spring brass1 about ,no4 #cd ,b4 ,s4 gauge in thickness4 ,illustration3 ,fig4 #bce4--,the ,relay4 ,the other end of the spring is fitted to a bracket and provided with a thumbscrew to adjust the tension of the spring4 ,the under side of the armature and the upper side of the magnet core are each fitted with a small silver contact4 ,the contacts should meet squarely when the armature is drawn down on to the core by a current of electricity passing through the electro-magnet4 ,by turning the adjusting screw1 the armature can be raised or lowered4 ,it should be adjusted so that it almost touches the core and is only just far enough away to slip a piece of thick paper under4 ,the terminals of the magnet are connected to the two binding-posts on the base marked ,s and ,s4 ,one of the binding-posts1 ,p1 is connected to the brass upright1 and the other is connected to the core of the magnet4 ,figure #bcf shows how to connect up the outfit4 ,it will require some very nice adjusting before you will be able to get it to working properly4 ,illustration3 ,fig4 #bcf4--,the ,complete ,coherer ,outfit4 ,if you wish to use the outfit for demonstration purposes or for sending messages for very short distances1 as for instance across a room1 you do not need an aerial but merely a pair of catch-wires4 ,the catch-wires are two pieces of stiff copper wire1 about two feet long1 placed in the lower holes in the double binding-posts forming part of the coherer4 ,in order to set the apparatus for operation1 raise the adjusting screw of the relay until the armature is quite far away from the core4 ,then push the armature down against the contact on the core4 ,the decoherer should then immediately operate and begin to tap the coherer4 ,then turn the thumbscrew until the armature is brought down to the core in such a position that it is as close as it is possible to get it without ringing the bell4 ,the transmitter should consist of a spark coil1 battery1 key1 and a spark-gap4 ,the gap should be connected to the secondary of the coil and adjusted so that the electrodes are only about one-eighth of an inch apart4 ,the key is placed in series with the primary of the coil and the battery1 so that pressing the key will send a stream of sparks across the gap4 ,fit the spark-gap with two catch-wires similar to those on the coherer and place the transmitter about four or five feet away from the coherer outfit4 ,you are now likely to find that if you press the key of the transmitter1 the decoherer will ring4 ,it is possible that it will continue to ring after you have stopped pressing the key4 ,if such is the case1 it will be necessary to turn the adjusting screw on the relay so as to move the armature upward a short distance away from the core4 ,if the decoherer will not operate each time when you press the key1 the brass plugs in the coherer need adjusting4 ,you must not be discouraged if you have some difficulty in making the apparatus work at first4 ,after you learn how to adjust it properly1 you will find that you can move the transmitter quite a distance away from the coherer and it will still operate very nicely4 ,after you manage that1 you can place the apparatus in separate rooms and find it possible to work it just the same1 because ordinary walls will not make any difference to wireless waves4 ,bear in mind that the nearer the coherer plugs are to each other1 the more sensitive the coherer will be1 but that if too close1 the decoherer will not be able to shake the filings properly and will not stop when you stop pressing the key4 ,the operation of the apparatus depends upon the fact that when properly adjusted the resistance of the filings between the two brass plugs is too great to allow sufficient battery current to flow to attract the armature of the relay4 ,as soon as any wireless waves from the transmitter strike the catch-wires of the coherer1 they cause the filings to cling together or cohere4 ,when in this state1 they have a low resistance and permit the current to flow in the relay circuit and draw down the armature4 ,the armature closes the second circuit and sets the decoherer into operation4 ,the decoherer shakes the filings and causes them to decohere or fall apart and so makes them ready again for the next signal4 ,a coherer set of this sort may be used on an aerial and ground by substituting the coherer for the detector1 but otherwise following any of the receiving circuits which have already been shown4 ,,chapter ,,xv ,a ,,wireless ,,telephone ,probably many readers of the ,boy ,electrician are amateur wireless operators and have constructed their own apparatus with which they are able to pick up commercial messages or communicate with other experimenters in the neighborhood1 but not many have ever built a wireless telephone4 ,the device described in the following pages is easy to make and arrange1 and will serve for some very interesting experiments4 ,it is of no practical value as a commercial wireless telephone1 because the distance over which it will transmit speech is limited to from #bej to #cjj feet4 ,if you have a chum who lives across the street and within the distance named above1 it is possible for you to construct a simple wireless telephone which will enable you to remain in your own rooms and talk to each other without any connecting wires4 ,the instruments operate by magnetic induction4 ,it has already been explained how it is possible for the current in the primary of an induction coil to induce a current in the secondary coil1 even though the two are not electrically connected4 ,this type of wireless telephone really consists of an induction coil in which the two windings are widely separated4 ,suppose that two coils of wire are connected as in ,figure #bcg4 ,the illustration shows that one coil1 ,a1 is connected in series with a set of batteries and a telegraph key4 ,the terminals of the other coil1 ,b1 are connected to a telephone receiver4 ,the coils are placed parallel to each other and a few inches apart4 ,if the key is pressed so that the battery current may flow through the coil1 ,a1 it will create a magnetic field1 and lines of force will be set up in the immediate vicinity4 ,the lines of force will pass through the coil1 ,b1 and induce in it a current of electricity which will cause a sound like a click to be heard in the telephone receiver4 ,illustration3 ,fig4 #bcg4--;,a ,simple ,arrangement showing the ,inductive ,action between two ,coils4 ,if a telephone transmitter is substituted for the key and words are spoken into it1 the current passing through the coil from the battery will vary with each vibration of the voice and the words will be distinctly repeated by the receiver connected to ,b4 ,this experiment may be tried by any boy with the equipment he probably has already around his shop4 ,twenty-five to thirty turns of wire wound around a cardboard tube five or six inches in diameter will serve as a coil4 ,two such coils1 an ordinary telephone transmitter1 a telephone receiver and a couple of dry cells are all that is required4 ,illustration3 ,fig4 #bch4--;,a ,simple ,wireless ,telephone4 ,speech directed into the ,transmitter can be heard in the ,receiver1 although there is no direct electrical connection between the two4 ,the diagram in the accompanying illustration shows how the apparatus is arranged4 ,the coils may be used several inches apart and the voice will be clearly heard in the receiver4 ,such an outfit is1 however1 only experimental1 and if it is desired to make a practical set1 the coils1 etc41 must be much larger in diameter and contain a greater number of turns4 ,larger coils are made by first drawing a circle four feet in diameter on the floor of the shop or attic4 ,then drive a number of small nails around the circumference1 about four inches apart4 ,procure two and one-half pounds of ,no4 #bj ,b4 ,s4 gauge cotton-covered magnet wire and wind it around the circumference of the circle4 ,the wire should form at least sixty complete turns4 ,about one foot should be left at each end to establish connections with4 ,after winding1 the coil should be tied about every six inches with a small piece of string so that it will retain its shape and not come apart4 ,the nails are then pulled out so that the coil may be removed4 ,the coil may be used just as it is for experimental purposes1 but if it is intended for any amount of handling it is wise to procure a large hoop such as girls use for rolling along the sidewalk1 and make the coil the same diameter as the hoop so that upon completion they may be firmly bound together with some insulating tape4 ,two binding-posts may then be fastened to the hoop and the terminals of the coil connected to them4 ,two such coils are required for a complete wireless telephone system1 one to be located at each station4 ,it is also necessary to make a double-contact strap-key4 ,such a key is easily built out of a few screws and some sheet-brass4 ,the illustration shows the various parts and construction so clearly that no detailed explanation is necessary4 ,illustration3 ,fig4 #bci4--;,a ,double-,contact ,strap-,key4 ,the ,dotted ,lines show how the ,binding-,posts are connected4 ,the telephone transmitter and the telephone receiver required for this experiment must be very sensitive1 and it is hardly possible for the young experimenter to build one which will be satisfactory4 ,they can be secured from a second-hand telephone or purchased at an electrical supply house4 ,the transmitter should be of the long distance type4 ,an #hj-ohm receiver will serve the purpose1 but if you also have a wireless station1 use the same #ajjj-ohm receivers belonging to your wireless set and you will secure very good results4 ,a battery capable of delivering about #aj volts and a good constant current is required4 ,the apparatus is connected as shown in ,figure #bdj4 ,when the key is pressed1 the coil is connected to the battery and the telephone transmitter4 ,if words are then spoken into the transmitter they will vary the amount of current flowing and the magnetic field which is set up in the neighborhood of the coil will induce currents in the coil at the other station1 provided that it is not too far away1 and cause the words to be reproduced in the telephone receiver4 ,when the key is released it will connect with the upper contact and place the telephone receiver in the circuit for receiving1 so that your chum at the other station can answer your message by pressing his key and talking into his transmitter4 ,illustration3 ,fig4 #bdj4--,the ,circuit of the ,wireless ,telephone4 ,when the ,key is up1 the ,receiver is ready for ,action4 ,when the ,key is pressed1 the ,transmitter and ,battery are thrown into the ,circuit4 ,the best plan is to mount each of the coils upon a tripod and experiment by placing them close together at first and gradually moving them apart until the maximum distance that the apparatus will work is discovered4 ,be very careful to keep the two coils exactly parallel4 ,much depends upon the battery4 ,be certain that it is capable of delivering a good strong current4 ,do not hold the key down any longer than is absolutely necessary1 or the telephone transmitter will become hot4 ,by making the coils six feet in diameter and placing from #bjj to #djj turns of wire in each coil you can make a set which is capable of transmitting speech #cjj feet or more4 ,illustration3 ,fig4 #bda4--;,a ,complete ,wireless ,telephone and ,telegraph ,station for ,amateurs4 #a4 ,the ,telephone ,coil4 #b4 ,the ,telephone ,transmitter4 #c4 ,double-,contact ,strap-,key4 #d4 ,the ,battery4 #e4 ,spark ,coil4 #f4 ,key4 #g4 ,spark-,gap4 #h4 ,aerial ,switch4 #i4 ,loose ,coupler4 #aj4 ,detector1 #aa4 ,fixed ,condenser4 #ab4 ,code ,chart4 #ac4 ,amateur ,license4 #ad4 ,aerial4 #ae4 ,telephone ,receivers4 ,the coil may be mounted on the wall of your shop in a position where it will be parallel to one located in your chum's house4 ,the success of a wireless telephone system of this sort lies in making the coils of large diameter and many turns1 in keeping the coils parallel1 using a sensitive transmitter and receiver1 and in employing a good strong battery4 ,storage cells are the best for the purpose4 ,,chapter ,,xvi ,,electric ,,motors ,the first ,american patentee and builder of an electric motor was ,thomas ,davenport4 ,the father of ,davenport died when his son was only ten years old4 ,this resulted in the young inventor being apprenticed to the blacksmith's trade at the age of fourteen4 ,some years later1 after having thoroughly learned his trade1 he married a beautiful girl of seventeen1 named ,emily ,goss1 and settled in the town of ,brandon1 ,vermont1 as an independent working blacksmith4 ,about this time ,joseph ,henry invented the electro-magnet4 ,davenport heard of this wonderful galvanic magnet which it was rumored would lift a blacksmith's anvil4 ,this was his undoing1 for never again was he to know peace of mind but was destined to always be a seeker after some elusive scientific will-o'-the-wisp4 ,although many times he needed iron for his shop1 the greater part of his money was spent in making electro-magnets and batteries4 ,in those days insulated wire could not be purchased1 and any one wishing insulated wire had to buy bare wire and insulate it himself4 ,it was then supposed by scientists that silk was the only suitable material for insulating wire and so ,davenport's brave young wife cut her silk wedding gown into narrow strips and with them wound the coils of the first electric motor4 ,continuing his experiments in spite of almost insurmountable difficulties and making many sacrifices which were equally shared by his family1 he was enabled to make a trip to ,washington in #ahce for the purpose of taking out a patent4 ,his errand was fruitless1 however1 and he was obliged to return home penniless4 ,nothing daunted1 he made the second and third trip and finally secured his memorable patent1 the first of the long line of electric-motor patents that have made possible both the electric locomotive that hauls its long train so swiftly and silently1 and the whirring little fan which stirs up a breeze during the hot and sultry days4 ,these are a few of the reasons why a modest country blacksmith1 in turn an inventor and an editor1 through perseverance in struggling against adversity and poverty succeeded in placing his name on the list which will be deservedly immortal among the scientists and engineers of the world4 99,a ,simple ,electric ,motor99 can be made in fifteen minutes by following the plan shown in ,figure #bdb4 ,the armature is made by sticking a pin in each end of a long cork4 ,the pins should be as nearly central as it is possible to make them1 so that when the cork is revolved upon them it will not wabble4 ,the pins form the shaft or spindle of the motor4 ,then take about ten feet of fine magnet wire 7,nos4 #bh-cb ,b4 ,s4 gauge7 and wind it on as shown in the illustration1 winding an equal number of turns on each side of the two pins4 ,illustration3 ,fig4 #bdb4--;,a ,simple ,electric ,motor which may be made in ,fifteen ,minutes4 ,when this is finished1 fasten the wire securely to the cork by binding it with thread4 ,bend the two free ends 7the starting and the finishing end7 down at right angles and parallel to the shaft so as to form two commutator sections as shown in the upper left hand corner of ,figure #bdb4 ,cut them off so that they only project about three-eighths of an inch4 ,bare the ends of the wire and clean them with a piece of fine emery paper or sandpaper4 ,the bearings are made by driving two pins into a couple of corks so that the pins cross each other as shown in the upper right-hand corner of ,figure #bdb4 ,they must not be at too sharp an angle1 or when the armature is placed in position1 the friction of the shaft will be so great that it may not revolve4 ,the motor is assembled by placing the armature in the bearings and then mounting two bar magnets on either side of the armature4 ,the magnets may be laid on small blocks of wood and should be so close to the armature that the latter just clears when it is spun around by hand4 ,the north pole of one magnet should be next to the armature and the south pole of the other1 opposite4 ,connect two wires about one foot long and ,no4 #bf ,b4 ,s4 gauge in diameter to a dry cell4 ,bare the ends of the wires for about an inch and one half4 ,take the ends of the two wires between the forefinger and thumb and bend them out1 so that when the armature is revolved they can be made just to touch the ends of the wire on the armature1 or the commutator sections1 as they are marked in the drawing4 ,give the armature a twist so as to start it spinning1 and hold the long wires in the hand so that they make contact with the commutator as it revolves4 ,very light pressure should be used4 ,if you press too hard1 you will prevent the armature from revolving1 while1 on the other hand1 if you do not press hard enough1 the wires will not make good contact4 ,the armature will run in only one direction1 and so try both ways4 ,if you start it in the right direction and hold the wires properly1 it will continue to revolve at a high rate of speed4 ,if carefully made1 this little motor will reward its maker by running very nicely4 ,although it is of the utmost simplicity it demonstrates the same fundamental principles which are employed in real electric motors4 99,the ,simplex ,motor99 is an interesting little toy which can be made in a couple of hours1 and when finished it will make an instructive model4 ,illustration3 ,fig4 #bdc4--;,details of the ,armature of the ,simplex ,motor4 ,as a motor itself1 it is not very efficient1 for the amount of iron used in its construction is necessarily small4 ,the advantage of this particular type of motor and the method of making it is that it demonstrates the actual principle and the method of application that is used in larger machines4 ,the field of the motor is of the type known as the simplex while the armature is the ,siemens ,h or two-pole type4 ,the field and the armature are cut from ordinary tin-plated iron such as is used in the manufacture of tin cans and cracker-boxes4 ,the simplest method of securing good flat material is to get some old scrap from a plumbing shop4 ,an old cocoa tin or baking-powder can may1 however1 be cut up and flattened and will then serve the purpose almost as well4 ,illustration3 ,fig4 #bdd4--,the ,armature4 99,the ,armature499 ,two strips of tin1 three-eighths of an inch by one and one-half inches1 are cut to form the armature4 ,they are slightly longer than will actually be necessary1 but are cut to length after the finish of the bending operations4 ,mark a line carefully across the center of each strip4 ,then1 taking care to keep the shape symmetrical so that both pieces are exactly alike1 bend them into the shape shown in ,figure #bdc4 ,the small bend in the center is most easily made by bending the strip over a knitting-needle and then bending it back to the required extent4 ,illustration3 ,fig4 #bde4--,the ,field4 ,a piece of knitting-needle one and one-half inches long is required for the shaft4 ,bind the two halves of the armature together in the position shown in ,figure #bdi4 ,bind them with a piece of iron wire and solder them together4 ,the wire should be removed after they are soldered4 ,illustration3 ,fig4 #bdf4--,the ,field and ,commutator4 99,the ,field ,magnet99 is made by first cutting out a strip of tin one-half by four and then bending it into the shape shown in ,figure #bde4 ,the easiest way of doing this with accuracy is to cut out a piece of wood as a form1 and bend the tin over the form4 ,the dimensions shown in ,figure #bde should be used as a guide for the form4 ,illustration3 ,fig4 #bdg4--,the ,bearings4 ,two small holes should be bored in the feet of the field magnet to receive ,no4 #c wood screws1 which fasten the field to the base4 99,the ,bearings99 are shown in detail in ,figure #bdg4 ,they are easily made by cutting from sheet-tin4 ,two small washers1 serving as collars1 should be soldered to the shaft as shown in ,figure #bdc4 99,the ,commutator ,core99 is formed by cutting a strip of paper five-sixteenths of an inch wide and about five inches long4 ,it should be given a coat of shellac on one side and allowed to get sticky4 ,the strip is then wrapped around the shaft until its diameter is three-sixteenths of an inch4 99,the ,base99 is cut from any ordinary piece of wood and is in the form of a block about two by one and one-half by one-half inch4 ,illustration3 ,fig4 #bdh4--,the ,complete ,motor4 99,assembling the ,motor499 ,the parts must be carefully prepared for winding by covering with paper4 ,cut a strip of paper one-half inch wide and one and one-eighth of an inch long and give it a coat of shellac on one side4 ,as soon as it becomes sticky1 wrap it around the top bar of the field magnet4 ,the armature is insulated in exactly the same way1 taking care that the paper covers the entire flat portion4 ,the field and armature are now ready for winding4 ,it is necessary to take proper precautions to prevent the first turn from slipping out of place4 ,this is accomplished by looping a small piece of tape or cord over it4 ,the next two turns are then taken over the ends of the loop so as to embed them4 ,wind on three layers of wire and when in the middle of the fourth layer embed the ends of another loop1 which may be used at the end of the fourth layer to fasten the end so that it will not unwind4 ,after the winding is finished1 give it a coat of shellac4 ,the winding of the armature is somewhat more difficult4 ,the wire used for winding both the armature and the field should be ,no4 #be or ,no4 #bf ,b4 ,s4 gauge double-cotton-covered4 ,in order to wind the armature1 cut off about five feet of wire and double it back to find the center4 ,then place the wire diagonally across the center of the armature so that there is an equal length on both sides4 ,place a piece of paper under the wire at the crossing point to insulate it4 ,then1 using one end of the wire1 wind four layers on half of the armature4 ,tie the end down with a piece of thread and wind on the other half4 ,the ends of the wire are cut and scraped to form the commutator segments4 ,figure #bdf shows how this is done4 ,bend the wires as shown so that they will fit closely to the paper core4 ,bind them tightly into position with some silk thread4 ,use care so that the two wires do not touch each other4 ,cut the free ends of the wires off close to the core4 ,when finished1 the relative positions of the armature and the commutator should be as shown in ,figure #bdh4 ,the brushes are made by flattening a piece of wire by a few light hammer blows4 ,the brushes are fastened under a small clamp formed by a strip of tin held down at each end with a wood screw4 ,they can be adjusted to the best advantage only under actual working conditions when the current is passing through the motor4 ,one or two dry cells should be sufficient to operate the motor4 ,illustration3 ,fig4 #bdi4--;,details of the ,motor4 ,one end of the field winding is connected to one of the brushes4 ,the other brush and the other end of the field form the terminals to which the battery is connected4 ,the motor1 being of the two-pole armature type1 must be started when the current is turned on by giving it a twist with the fingers4 99,a ,larger ,motor99 may be built in somewhat the same manner as the one just described by cutting armature and field out of sheet tin4 ,it will be more substantial if it is built up out of laminations and not bent into shape1 as in the case of the other4 ,lay out an armature disk and a field lamination on a sheet of tin in accordance with the dimensions and pattern shown in ,figure #bdi4 ,these pieces are used as patterns for laying out the rest of the laminations4 ,illustration3 ,fig4 #bej4--;,complete ,motor4 ,place them on some thin sheet-iron and trace the outline with a sharp-pointed needle4 ,then cut a sufficient number of pieces of each pattern to form a pile three-quarters of an inch thick4 ,four laminations for the field should be cut with extensions shown by the dotted lines4 ,they are bent out at right angles for mounting the motor and holding it upright4 ,assemble the armature and field by piling the pieces on top of each other and truing them up4 ,enough laminations should be used to form a pile three-quarters of an inch thick when piled up and clamped tightly4 ,file off any burrs and rough edges and then bind the laminations together with some string to hold them until wound4 ,wrap a couple of layers of paper around those portions of the armature and field which are liable to come into contact with the iron4 ,five or six layers of ,no4 #ah ,b4 ,s4 gauge double-cotton-covered magnet wire are sufficient to form the field coil4 ,the armature is wound with three or four layers of wire of the same size4 ,the commutator is made out of a circular piece of hard wood or fiber1 fitted with segments cut out of thin sheet-copper4 ,the segments may be fastened to the core with thick shellac or some melted sealing-wax4 ,the ends may be bound down tightly by wrapping with silk thread4 ,the brushes are cut out of thin sheet-copper similar to that used for the commutator segments4 ,the bearings are strips of heavy sheet-brass bent into the shape shown4 ,they are mounted by passing a nail through the holes in the ends and through the holes1 ,a and ,b1 in the field and then riveting the ends over4 ,assemble the motor as shown in ,figure #bee4 ,if desirable1 a small pulley may be fitted to the shaft and the motor used to run small mechanical toys4 ,if it is properly constructed1 two or three dry cells will furnish sufficient current to run the motor at high speed4 ,,chapter ,,xvii ,,dynamos ,there is perhaps no other electrical device entering into the young experimenter's domain requiring the careful workmanship and tool facilities that the dynamo does4 ,in order to construct a practical working dynamo it would be necessary to have at hand a lathe for turning the castings4 ,rather than describe a machine which comparatively few of my readers would be able to build1 ,i have explained below how it is possible to so alter an old telephone magneto that it may be made to serve as a small dynamo4 ,telephone magnetos1 also sometimes called hand generators1 are used in many telephone systems to supply the current which rings the telephone bell at the other end4 ,the magneto is placed in a small box on the telephone1 only the handle being exposed4 ,in order to make a call the handle is given several brisk turns before raising the receiver4 ,when the handle is turned the moving parts of the generator revolve and produce a current of electricity which goes forth over the line and rings the bell at the other end4 ,illustration3 ,fig4 #bea--;,a ,telephone ,magneto4 ,telephone magnetos are gradually being discarded in all the large telephone systems1 a method known as central energy1 in which the current for ringing bells is supplied from the central office1 taking their place4 ,for that reason1 there are a great many telephone magnetos to be found in second-hand shops and at electrical houses1 where they can be purchased for a fractional part of the original cost4 ,fifty cents will buy a first-class second-hand telephone magneto4 ,the author saw a pile of telephones as large as a haystack1 each telephone containing a magneto1 in the back yard of a second-hand shop1 and the owner would have been glad to sell the complete instruments for fifty cents each4 ,before explaining how to reconstruct such a machine1 it is best to impress upon the reader that a careful study of the principles of the dynamo is well worth the time spent4 ,almost any book on physics or electricity1 or even the encyclopedia1 will be found to contain a description of this wonderful machine that supplies the power for running the trolley cars1 electric lights1 etc41 in fact all of the electricity in use to-day with the exception of that generated by batteries for telegraph and telephone lines4 ,it will be remembered that if a bar magnet is suddenly plunged into a hollow coil of wire1 a momentary electric current will be generated in the coil4 ,the current is easily detected by means of an instrument called a galvanometer4 ,the space in the vicinity of a magnet is filled with a peculiar invisible force called magnetism4 ,the magnetism flows along a certain path1 passing through the magnet itself and then spreading out in curved lines4 ,if a sheet of paper is laid over a magnet and a few iron filings are sprinkled on the paper1 they will follow the magnetic lines of force4 ,when the magnet is plunged into the hollow coil1 the lines of force flow through the turns of wire1 or are said to cut them4 ,whenever lines of force cut a coil of wire and they are in motion1 electricity is produced4 ,it does not matter whether the coil is slipped over the magnet or the magnet is plunged into the coil1 a current will be produced as long as they are in motion4 ,as soon as the magnet or the coil stops moving the current stops4 ,by arranging a coil of wire between the poles of a horse-shoe magnet so that it can be made to revolve1 the motion can be made continuous and the current of electricity maintained4 ,figure #beb shows such an arrangement4 ,some means of connection with the coil of wire must be established so that the current can be led off4 ,if two metal rings are connected to the ends of the coil1 connection can be made by little strips of metal called brushes rubbing against the rings4 ,this scheme is the principle of the telephone magneto and the basis of all dynamos4 ,illustration3 ,fig4 #beb4--,the ,principle of the ,alternator and the ,direct-,current ,dynamo4 ,in the telephone magneto1 more than one horseshoe magnet is usually provided4 ,the coil of wire revolves between the poles of the magnets4 ,the coil is wound around an iron frame and together they are called the armature4 ,the end of the armature shaft is fitted with a small spur gear meshing with a larger gear bearing a crank1 so that when the crank is turned the motion is multiplied and the armature is caused to revolve rapidly4 ,one end of the coil or armature winding is connected to a small brass pin4 ,this pin connects with a second pin set in the end of the shaft in an insulating brush of hard rubber4 ,the other terminal of the coil is connected to the armature itself4 ,thus connection can be had to the coil by connecting a wire to the frame of the machine and to the insulated pin4 ,illustration3 ,fig4 #bec4--;,details of the ,armature1 ,commutator1 and ,brushes4 ,the armature of a magneto is usually wound with a very fine silk insulated wire1 about ,no4 #cf ,b4 ,s4 gauge in size4 ,this should be carefully removed and wound upon a spool for future use4 ,replace the wire with some ordinary cotton-covered magnet wire1 about ,no4 #bd or #be ,b4 ,s4 gauge1 winding it on very carefully and smoothly4 ,connect one end of the winding to the pin leading to the insulated pin by soldering it4 ,this pin is the one at the end of the shaft opposite to that one to which the spur gear is fastened4 ,connect the other end of the wire to the pin at the same end of the shaft as the gear4 ,this pin is grounded1 that is1 connected to the frame4 ,an ordinary telephone magneto gives a very high voltage current4 ,the voltage may vary from twenty-five to several hundred1 depending upon how fast the machine is run4 ,this is due to the fact that the armature winding is composed of a very large number of turns of wire4 ,the more turns that are placed on the armature1 the higher its voltage will be4 ,the current or amperage of a large telephone magneto wound with a large number of turns of fine wire is very low4 ,too low in fact to be used for anything except ringing a bell or testing4 ,winding the armature with fewer turns of large wire reduces the voltage and increases the amperage so that the current will light a small lamp or may be used for other purposes4 ,the winding does not change the principle of the magneto1 it merely changes its amperage and voltage4 ,the magneto may be mounted on a wooden base-board and screwed to a table1 so that the handle may be turned without inconvenience4 ,a small strip of copper1 called a brush1 should be fastened to the base with a screw and brought to bear against the end of the insulated pin4 ,the brush should be connected to a binding-post with a piece of wire4 ,a second wire leading to a binding-post should be connected to the frame of the magneto4 ,when the handle is turned rapidly1 currents may be drawn from the two binding-posts4 ,the current is of the kind known as alternating1 that is to say1 it flows first in one direction1 then reverses and flows in the other4 ,in order to make the machine give direct current1 it must be fitted with a commutator4 ,this is somewhat difficult with some magnetos but the following plan may be carried out in most cases4 ,cut a small fiber circle or disk about one inch in diameter from sheet fiber three-sixteenths of an inch thick4 ,cut a small hole in the center1 just large enough so that the fiber will slip very lightly over the end of the shaft from which the insulated pin projects4 ,two small commutator sections similar to that shown in ,figure #bec must be cut from sheet-brass or sheet-copper4 ,the three long ears shown in the drawing are bent back around the fiber and squeezed down flat with a pair of pincers so that they grip the fiber very tightly and will not slip4 ,one ear on one section should be bent over the back down to the hole1 where it will connect with the shaft4 ,the other section of the commutator is connected to the insulated pin by a drop of solder4 ,in this manner1 one end of the winding is connected to one section of the commutator and the other end to the other section4 ,the commutator should fit tightly on the end of the shaft so that it will not twist4 ,the dividing line between the section should be parallel to a line drawn to the axis of the actual armature coil4 ,when the iron parts of the armature are nearest the poles of the horseshoe magnets in their revolution1 the slot in the commutator should be horizontal4 ,when the magnet is provided with a commutator1 it may also be run as a motor by connecting it to a battery4 ,in order to operate it either as a dynamo or a motor1 however1 it must first be fitted with a pair of brushes4 ,they are shown in detail in ,figure #bec4 ,they are made from two small strips of sheet-copper bent as shown and mounted on a small wooden block4 ,they must be adjusted to bear against the commutator so that when the dividing line between the two sections is horizontal1 the upper brush bears against the upper section and the lower brush against the lower section4 ,the two brushes form the terminals of the machine4 ,they should be connected to binding-posts4 ,illustration3 ,fig4 #bed4--,the ,complete ,generator4 ,in order to operate the dynamo properly and obtain sufficient current from it to operate a couple of small incandescent lamps1 it will have to be provided with a pulley mounted on the end of the shaft after the gear wheel has been removed4 ,the dynamo may then be driven at high speed by connecting it to a sewing-machine with a belt1 or the back wheel of a bicycle from which the tire has been removed4 ,the completed dynamo is shown in ,figure #bed4 ,the voltage and amperage of the dynamo will depend upon the machine in question1 not only upon the size of the wire but also upon the size of the machine1 the speed at which it is run1 and the strength of the horseshoe magnets4 ,it is impossible to tell just what the current will be until it is tested and tried4 ,a #aj-,watt ,dynamo ,probably few experimenters fully understand how almost impossible it is to construct a dynamo1 worthy of the name as such1 without resort to materials and methods employed in the commercial manufacture of such machines4 ,practical telegraph instruments1 telephones1 etc41 can be constructed out of all sorts of odds and ends1 but in order to make a real dynamo it is necessary to use certain materials for which nothing can be substituted4 ,the field magnets must be soft gray cast-iron except in special instances4 ,the wire used throughout must be of good quality and must be new4 ,the necessity for good workmanship in even the smallest detail cannot be overestimated4 ,poor workmanship always results in inefficient working4 ,no dynamo will give its stated output continuously and safely unless the materials and workmanship are up to a high standard4 ,since castings must be used as field magnets1 a pattern is necessary to form the mould for the casting4 ,pattern work is something requiring skill and knowledge usually beyond the average experimenter4 ,a lathe is necessary in order to bore or tunnel the space between the ends of the field magnet into which the armature fits4 ,it may be possible for several boys to club together and have a pattern made by a pattern-maker for building a dynamo4 ,then by using the lathe in some convenient shop or manual training school secure a field magnet and armature for a really practical small dynamo4 ,illustration3 ,fig4 #bee4--;,details of the ,field ,casting4 ,for these reasons1 ,i have described below a small dynamo of about ten watts output1 the castings for which can be purchased from many electrical dealers with all machine work done at an extremely low price4 ,the field magnet shown in ,figure #bee is drawn to scale and represents the best proportions for a small overtype dynamo of ten to fifteen' watts output4 ,the dimensions are so clearly shown by the drawings that further comment in that respect is unnecessary4 ,the armature is of the type known as the ,siemen's ,h4 ,it is the simplest type of armature it is possible to make1 which is a feature of prime importance to the beginner at dynamo construction1 although it is not the most efficient form from the electrical standpoint4 ,the armature in this case is also a casting and therefore a pattern is required4 ,illustration3 ,fig4 #bef4--;,details of the ,armature ,casting4 ,the patterns for both the field and the armature are of the same size and shape as shown in ,figures #bee and #bef4 ,they are made of wood1 and are finished by rubbing with fine sandpaper until perfectly smooth and then given a coat of shellac4 ,the parts are also given a slight draft1 that is1 a taper toward one side1 so that the pattern may be withdrawn from the mould4 ,the patterns are turned over to a foundry1 where they are carefully packed in a box1 called a flask1 full of moulder's sand4 ,when the patterns are properly withdrawn1 they will leave a perfect impression of themselves behind in the sand4 ,the mould is then closed up and poured full of molten iron4 ,when the iron has cooled the castings are finished except for cleaning and boring4 ,the shaft is a piece of steel rod1 three-sixteenths of an inch in diameter1 and four and one-half inches in length4 ,the portion of the field into which the armature fits is bored out to a diameter of one and five-sixteenth inches4 ,considerable care is necessary in performing this operation in order not to break the field magnet apart by taking too heavy a cut4 ,illustration3 ,fig4 #beg4--;,details of the ,commutator4 ,the armature should be turned down to a diameter of one and one-quarter inches or one-sixteenth of an inch smaller than the tunnel in which it revolves between the field magnets4 ,the center of the armature is bored out to fit the shaft4 ,figure #beg shows a two-part commutator for fitting to an armature of the ,siemen's ,h type4 ,it consists of a short piece of brass tubing fitted on a fiber core and split length-wise on two opposite sides1 so that each part is insulated from the other4 ,the fiber is drilled with a hole to fit tightly on the shaft4 ,it is then placed in a lathe and turned down until a suitable piece of brass tube can be driven on easily4 ,two lines are then marked along the tube diametrically opposite4 ,a short distance away from each of these lines1 and on each side of them1 bore two small holes to receive very small wood screws4 ,the screws should be counter-sunk4 ,it is very important that none of the screws should go into the fiber core far enough to touch the shaft4 ,the commutator may then be split along each of the lines between the screws with a hacksaw4 ,the saw-cut should be continued right through the brass and slightly into the insulating core4 ,the space between the sections of the commutator should be fitted with well-fitting slips of fiber1 glued in4 ,the commutator should now be trued up and made perfectly smooth4 ,illustration3 ,fig4 #beh4--;,diagram showing how to connect the ,armature ,winding to the ,commutator4 ,the commutator is provided with a small brass machine screw threaded into each section near the edge as shown in ,figure #beg4 ,these screws are to receive the ends of the armature winding and so facilitate connections4 ,the commutator1 shaft and armature are assembled as shown in ,figure #beh4 ,the armature may be held to the shaft by a small set screw or a pin4 ,the commutator should fit on the shaft very tightly so that it will not slip or twist4 ,every part of the armature and shaft touched by the armature winding must be insulated with paper which has been soaked in shellac until soft4 ,the armature must be left to dry before winding4 ,the armature should next be wound with ,no4 #bj ,b4 ,s4 gauge single-cotton-covered magnet wire4 ,sufficient wire should be put on to fill up the winding space completely4 ,care should be taken1 however1 not to put on too much wire or it will interfere with the field magnets and the armature cannot revolve4 ,after winding the armature1 test it carefully to see that the wire is thoroughly insulated from the iron4 ,illustration3 ,fig4 #bei4--;,details of the ,wooden ,base4 ,if the insulation is correct1 paint the whole armature with thick shellac varnish and bake it in a warm oven to set the shellac4 ,figure #beh is a diagram showing how the winding is made and connected4 ,it is wound about the armature1 always in the same direction1 just as if the armature were an ordinary electro-magnet4 ,the ends of the winding are each connected to one of the commutator sections by scraping the wire and placing it under the screws4 ,the winding space in the field magnet should be shellacked1 and insulated with brown paper by wrapping the core with a strip of paper and covering the bobbin ends with circular pieces made in two halves4 ,the field magnet is wound full of ,no4 #bj ,b4 ,s4 gauge single-cotton-covered wire4 ,the wire should be put on in smooth1 even layers and the winding space completely filled up4 ,illustration3 ,fig4 #bfj4--,the ,pulley and ,bearings4 ,the base for the dynamo is a piece of hard wood1 five inches long1 four inches wide1 and five-eighths of an inch thick4 ,the bearings are small brass castings of the dimensions shown in ,figure #bfj4 ,it is necessary first to make a wooden pattern and send it to the foundry for the castings4 ,the bearings are fastened to the projecting arms on the field casting by means of machine screws eight-thirty-seconds of an inch in thickness4 ,the field magnet should not be screwed down on to the base until the armature runs easily and truly in the tunnel4 ,the brushes are made from thin gauge sheet-copper according to the shape and dimensions shown in ,figure #bfa4 ,they are bent at right angles and mounted on the base on either side of the commutator with small round-headed wood screws4 ,the completed dynamo is shown in ,figure #bfb4 ,one end of the shaft is provided with a small pulley to accommodate a small leather belt4 ,illustration3 ,fig4 #bfa4--,the ,brushes4 ,the dynamo is connected as a shunt machine1 that is1 one terminal of the field magnet is connected to one of the brushes1 and the other terminal to the other brush4 ,a wire is then led from each of the brushes to a binding-post4 ,a shunt dynamo will only generate when run in a certain direction4 ,in order to make it generate when run in the opposite direction1 it is necessary to reverse the field connections4 ,the dynamo just described should have an output of from #aj to #ae watts and deliver about #f volts and #a #c_/d to #b #a_/b amperes4 ,in order to secure current from the dynamo it will first be necessary to magnetize the field by connecting it to several batteries4 ,illustration3 ,,the ,,junior ,,dynamo ,,mounted ,,on ,a ,,long ,,wooden ,,base ,,and ,,belted ,,to ,a ,,grooved ,,wheel ,,fitted ,,with ,a ,,crank ,,so ,,that ,,the ,,dynamo ,,can ,,be ,,run ,,at ,,high ,,speed ,,by ,,hand ,,power4 ,,the ,,illustration ,,also ,,shows ,a ,,small ,,incandescent ,,lamp ,,connected ,,to ,,the ,,dynamo ,,so ,,that ,,when ,,the ,,crank ,,is ,,turned ,,the ,,lamp ,,will ,,light4 ,it will be found that the dynamo will also operate as a very efficient little motor1 but that on account of having a two-pole armature it must be started by giving the shaft a twist4 ,illustration3 ,fig4 #bfb4--;,complete ,dynamo4 ,it can be used as a generator for lighting small lamps1 electro-plating1 etc41 but cannot be used for recharging storage cells on account of having a two-pole armature4 ,the dynamo may be driven with a small water motor or from the driving-wheel of a sewing-machine4 ,before the machine will generate as a dynamo1 it must be connected to a battery and run as a motor4 ,this will give the field the residual magnetism which is necessary before it can produce current itself4 ,,chapter ,,xviii ,,an ,,electric ,,railway ,no toys loom up before the mind of the average boy with more appeal to his love of adventure than do railway cars and trains4 ,in ,england1 the construction and operation of miniature railways is the hobby not only of boys but of grown men1 and on a scale that is hardly appreciated in this country4 ,the height of ambition of many boys is not only to own a miniature railway system but to build one4 ,for some unknown reason1 none of the boys' papers or books have heretofore given any information on this interesting subject4 ,the car shown in ,figure #bfc is such that it can be easily built by any boy willing to exercise the necessary care and patience in its construction4 ,the first operation is to cut out the floor of the car4 ,this is a rectangular piece of hard wood1 eight inches long1 three and one-quarter inches wide and one-half of an inch thick4 ,its exact shape and dimensions are shown in ,figure #bfd4 ,the rectangular hole cut in the floor permits the belt which drives the wheels to pass down from the counter-shaft to the axle4 ,illustration3 ,fig4 #bfc4--;,complete ,electric ,railway operated by ,dry ,cells4 ,note how the ,wires from the ,battery are connected to the ,rails by means of the ,wooden ,conductors illustrated in ,figure #bgg4 ,the two pieces forming the wheel-bearings are cut out of sheet-brass according to the shape and dimensions shown in ,figure #bfe4 ,the brass should be one-sixteenth of an inch thick4 ,the two projecting pieces at the top are bent over at right angles so that they can be mounted on the under side of the car floor by small screws passing through the holes4 ,the holes which form the bearings for the ends of the axles upon which the wheels are mounted should be three inches apart4 ,the bearings cannot be placed in position on the under side of the car floor until the wheels and axles are ready1 but when this work is done1 care should be taken to see that they line up and come exactly opposite to each other4 ,illustration3 ,fig4 #bfd4--;,details of the ,floor of the ,car4 ,the wheels themselves cannot be made by the young experimenter unless he has a lathe4 ,they are flanged wheels1 one and one-eighth inches in diameter1 and are turned from cast iron or brass4 ,such wheels can be purchased ready made1 or it may be possible to obtain from some broken toy a set which will prove suitable4 ,illustration3 ,fig4 #bfe4--;,details of the ,bearing which supports the ,wheel and ,axle4 ,each shaft is composed of two pieces of ,bessemer rod held together by a short piece of fiber rod having a hole in each end into which one end of each piece of iron rod is driven4 ,the wheels fit tightly on the other end of each of these pieces4 ,they should be spaced so as to run on rails two inches apart4 ,illustration3 ,fig4 #bff4--,the ,wheels and ,axle4 ,the purpose of the fiber rod is to insulate the halves of the axle from each other4 ,the electric current which operates the car is carried by the two rails which form the track1 and if the axles were made in one piece or the halves joined together so as to form an electrical connection1 the battery furnishing the current would be short-circuited1 because the current would pass along the two rails and across the axles instead of through the motor4 ,one pair of wheels are fitted with a grooved pulley one inch in diameter4 ,it is hardly necessary to say that the wheels and axles should be perfectly aligned1 and should run true4 ,illustration3 ,fig4 #bfg4--,the ,motor4 ,the motor used to drive the car will prove more satisfactory if purchased ready made4 ,a self-starting three-pole motor similar to that shown in ,figure #bfg will serve very nicely4 ,the wooden base should be removed and the motor screwed down firmly to the floor of the car as in ,figure #bfh4 ,one terminal of the motor is connected to one of the bearings1 and the other terminal to the other bearing4 ,the motor is belted to a countershaft so that it will have sufficient power to move the car4 ,it cannot be directly connected or belted to the axle1 because the speed of a small motor is so high that it has comparatively little turning power or torque4 ,the speed must be reduced and the torque increased before it will drive the car4 ,the countershaft consists of two grooved pulleys mounted upon an axle running in two bearings mounted upon the floor of the car4 ,the bearings are made from a strip of heavy sheet-brass1 bent at right angles and fastened to the car floor with small screws4 ,the large pulley1 ,a1 is one inch and one half in diameter and the small pulley1 ,b1 is five-sixteenths of an inch in diameter4 ,the countershaft is mounted in such a position that a belt may be run from the small pulley1 ,b1 to the pulley mounted on the axle of one pair of wheels4 ,a belt is also run from the small pulley on the motor to the large pulley1 ,a1 on the countershaft4 ,the pulleys must all be carefully lined up so that the belts will run in their grooves without danger of slipping out4 ,illustration3 ,fig4 #bfh4--,the ,complete ,truck of the ,car without the ,body4 ,the shield on the platform at each end of the car is made of sheet-iron or tin4 ,two small projections on the bottom are bent over at right angles and used to secure the shields in position by driving a small tack through them into the floor of the car4 ,the steps on either side of each platform are also made by bending strips of sheet-iron or tin and fastening them to the car with small nails or tacks4 ,the coupler consists of a strip of tin having a small hook soldered to the end so that a trail car may be attached if desirable4 ,illustration3 ,fig4 #bfi4--,pattern for the ,sides and ,ends of the ,car4 ,the car is now ready for testing1 and when held in the hand so that the wheels are free to run1 two cells of dry battery should be found all that is necessary to drive them at a fair rate of speed4 ,the two wires leading from the battery should be connected to the bearings1 one wire leading to each bearing4 ,it will require more than two cells1 however1 to drive the wheels properly when the car is on the track1 ,all moving parts should run freely and smoothly4 ,the car may be used just as it is1 but if fitted with a body and a top it will present a much more realistic appearance4 ,the sides and ends of the car body are made of sheet-iron or tin4 ,figure #bfi shows the pattern and dimensions for these parts4 ,they may be made from one piece of metal eighteen and one-half inches long and three and three-quarters inches wide4 ,the doors and windows are cut out with a pair of tin-snips4 ,the small projections along the top are bent down at right angles and the roof is fastened to them4 ,the dotted lines indicate the places for bending these projections and also the sides and ends of the car4 ,illustration3 ,fig4 #bgj4--,the ,roof of the ,car4 ,the roof is made in two pieces4 ,it also is sheet-iron or tin4 ,the roof proper is eight inches long and four inches wide4 ,it has a hole five and one-half inches long and one and three-quarters inches wide cut in the center4 ,a number of small projections are left and bent upward to support the deck and to form imitation ventilators4 ,the deck is six inches long and two and one-quarter inches in width4 ,it is placed in position on the roof and fastened by soldering4 ,the roof is fastened to the sides and ends of the car by soldering4 ,it must be bent slightly to conform with the curve at the top of the front and the rear of the car4 ,illustration3 ,fig4 #bga4--,the ,completed ,car4 ,the car when completed will appear as in ,figure #bga4 ,the track is made of smooth spring steel1 one-half inch wide and either ,no4 #bj or ,no4 #bb gauge in thickness4 ,illustration3 ,fig4 #bgb4-;,details of a ,wooden ,tie4 ,the wooden ties are three and one-half inches long1 three-quarters of an inch wide and three-eighths of an inch thick4 ,each tie has two saw-cuts1 exactly two inches apart across the top face4 ,this part of the work is best performed in a miter-box so that the cuts will be perfectly square across the ties4 ,a saw should be used which will make a cut of such a size that the steel track will fit tightly into it4 ,the distance between the two rails of the track1 or the gauge1 as it is called1 is two inches4 ,illustration3 ,fig4 #bgc4-;,arrangement of ,track4 ,the track is assembled as in ,figure #bgc4 ,the spring steel is forced into the saw-cuts in the ties by tapping with a light wooden mallet4 ,the ties should be spaced along the track about three inches apart4 ,the work of laying the track must be very carefully done so that the car wheels will not bind at any spot4 ,curves should not be too sharp1 or the car will not pass around4 ,the track may be laid out in a number of different shapes1 some of which are shown in ,figure #bgd4 ,illustration3 ,fig4 #bgd4--,three ,different ,patterns for laying out the ,track4 ,a circle is the easiest form of track to make4 ,in laying out a circle or any sort of curved track1 the outside rail must necessarily be made longer than the inside one4 ,the oval shape is a very good form to give the track in a great many cases1 especially where it is desirable for the car to have a longer path than that afforded by a circle4 ,illustration3 ,fig4 #bge4--;,details of the ,base of the ,cross-over4 ,in order to make a figure-eight out of the track1 a crossing1 or cross-over1 as it is sometimes called1 will be required4 ,this is shown in ,figure #bge4 ,a cross-over permits two tracks to cross each other without interference4 ,it consists of a wooden base1 eight inches square and three-eighths of an inch thick4 ,four saw-cuts1 each pair exactly parallel1 and two inches apart1 are made at right angles to each other across the top surface of the base1 as shown in the illustration4 ,the track used on the cross-over is semi-hard hoop-brass1 one-half of an inch wide and of the same gauge as the steel track4 ,the brass is more easily bent than the steel and is used for that reason1 it being practically impossible to bend the steel track at right angles without snapping it4 ,four pieces of the brass1 each five inches long1 are bent at right angles exactly in the center4 ,four short pieces1 each one and one-half inches long1 will also be required4 ,illustration3 ,fig4 #bgf4--,the ,completed ,cross-over4 ,the cross-over is assembled as shown in ,figure #bgf4 ,the strips marked ,d are strips of very thin sheet-brass or copper4 ,the purpose of these strips is to connect the ends of the track on the cross-over to the ends of the track forming the figure-eight so that the cross-over will not be a dead section1 that is1 a section of track where the car cannot get any current4 ,the long strips1 bent at right angles to each other and marked ,a1 ,a1 ,b1 ,b1 in the illustration1 are forced into the saw-cuts in the base over the strips marked ,d4 ,the small pieces1 ,c1 ,c1 ,c1 ,c1 are placed in between the long strips1 leaving a space between so that the flanges of the car wheels can pass4 ,the pieces1 ,c1 ,c1 ,c1 ,c1 should form a square open at the corners4 ,the two long strips1 ,a1 ,a1 should be at opposite corners diagonally across the square4 ,b and ,b should occupy the same relative position at the other corners4 ,a and ,a are connected together and ,b and ,b are connected together by wires passing on the under side of the base4 ,the ends of the track forming the figure-eight are forced into the saw-cuts at the edges of the base so that they form a good electrical connection with the small strips marked ,d4 ,it is quite necessary to use care in arranging a figure-eight track1 or there will be danger of short-circuiting the batteries4 ,the outside rails of the figure-eight1 distinguished by the letter ,b in the illustration1 should be connected together by the cross-over4 ,the inside rails1 marked ,a1 should also be connected together by the cross-over4 ,in order to make a good mechanical and electrical connection between the ends of the rails when two or more sections of track are used in laying out the system1 it is necessary to either solder the rails together or else use a connector such as that shown in ,figure #bgg4 ,this consists of a small block of wood having a saw-cut across its upper face and a piece of thin sheet-brass set into the cut4 ,the two rails are placed with their ends abutting and one of these connectors slipped up from beneath and forced on the rails4 ,the piece of thin brass set into the wooden block serves to make an electrical connection between the two rails and also to hold them firmly in position4 ,a small screw and a washer placed outside the track and passing through the brass strip will allow a battery wire to be conveniently attached4 ,illustration3 ,fig4 #bgg4--;,a ,connector for joining the ,ends of the ,rails4 ,the steel rails should be occasionally wiped with machine oil or vaseline to prevent rusting1 and also to allow the car to run more freely wherever the flanges of the wheels rub against the rails in passing around a curve4 ,four dry cells or three cells of storage battery should be sufficient to operate the car properly4 ,if it is desirable1 a small rheostat may be included in the battery circuit1 so that the speed of the car can be varied at will4 ,the motor and the wheels should be carefully oiled so that they will run without friction4 ,the belts should not be so tight that they cause friction or so loose that they allow the motor to slip1 but should be so adjusted that the motor runs freely and transmits its power to the wheels4 ,the car may be made reversible by fitting with a small current reverser1 but unless the reverser is carefully made the danger of loss of power through poor contacts is quite considerable4 ,if the car is fitted with a reverser the handle should be arranged to project from the car in a convenient place where it can be easily reached by the fingers and the car sent back or forth at will4 ,a railway system such as this can be elaborated and extended by adding more than one car to the line or such features as bridges and stations4 ,illustration3 ,fig4 #bgh4--;,a ,bumper for preventing the ,car from leaving the ,rails4 ,the ends of a blind section of track1 that is1 a straight piece of track not part of a circle or curve so that the car can return1 should be fitted with a track bumper1 to prevent the car leaving the rails4 ,illustration3 ,fig4 #bgi4--;,a ,design for a ,railway ,bridge4 ,no dimensions are given in ,figures #bgi and #bhj1 showing designs for a bridge and a station1 because they are best left to be determined by the scale upon which the railway system is to be extended4 ,illustration3 ,fig4 #bhj4--;,a ,design for a ,railway ,station4 ,both the bridge and the station are very simple4 ,the bridge is built entirely of wood1 with the exception of the steel rails4 ,the station may be made out of thin wood1 such as cigar-box wood4 ,the doors1 windows1 etc41 may be painted on the walls4 ,if this is carefully done1 it will give a very realistic appearance to your station4 ,,chapter ,,xix ,,miniature ,,lighting ,miniature lighting is a field of many interesting possibilities for the young experimenter4 ,any labor expended along this line will result in something far more useful from a practical standpoint than almost any of the other things described in this book4 ,miniature lights1 operated from batteries1 may be used in various ways2 to light dark corners1 hallways1 or other places where a light is often temporarily wanted without the accompanying danger and nuisance of matches or kerosene lamps4 ,miniature lighting has only been made practical by the tungsten filament lamp4 ,the filament1 or wire inside the globe1 which becomes hot and emits the light when the current is turned on1 is made of tungsten in a tungsten lamp4 ,in the earlier lamps1 it was made of carbon4 ,the carbon lamp is now seldom used and is highly inefficient when compared to the tungsten4 99,a ,carbon ,lamp99 consumes about three and one-half watts of current for each candle-power of light1 whereas a small tungsten lamp uses only about one watt per candle-power small tungsten lamp uses only about one watt per candle-power4 ,the tungsten lamp is therefore three times as efficient as a carbon lamp1 and when used on a battery of equal voltage it is possible to obtain the same amount of light with one-third of the current that would be required by a carbon lamp4 ,illustration3 ,fig4 #bha4--,miniature ,carbon ,battery ,lamp4 ,carbon lamps similar to that shown in ,figure #bha are made in a number of different voltages4 ,the lowest voltage that it is practically possible to make a carbon lamp for is three and one-half4 ,a three-and-one-half volt carbon lamp is designed to be operated on small dry cells such as flashlight batteries4 ,the ,e4 ,m4 ,f4 of a dry cell is about one and one-half volts1 but when three small dry cells of the flashlight type are connected in series and used to operate a lamp1 their voltage drops1 and the available ,e4 ,m4 ,f4 is only about three and one-half volts4 ,four-volt carbon lamps are intended to be operated on large dry batteries or wet cells because they do not lose their voltage as quickly as small dry cells4 ,the table below gives the voltage and candle-power of the various small carbon lamps which are carried in stock by most electrical dealers or supply houses3 99,,miniature ,,carbon ,,battery ,,lamps99 #c4e volts for flashlight batteries #d volts4 #b candle-power #e4e volts for flashlight batteries #f volts4 #b candle-power #f volts4 #d candle-power #h volts4 #d candle-power #aj volts4 #f candle-power 99,tungsten ,lamps99 are made for voltages as low as one and one-half1 and will light on one cell of dry battery4 ,the range of voltages is quite wide and varied4 ,a few of the most common sizes are given below3 99,,miniature ,,tungsten ,,battery ,,lamps99 #a4e volts4 for one dry cell #b4e volts4 for two-cell flashlight battery #b4h volts4 for two-cell flashlight battery #c4e volts4 for three-cell flashlight battery #c4h volts4 for three-cell flashlight battery #d volts4 #d candle-power #f volts4 #b candle-power #f volts4 #d candle-power #f volts4 #f candle-power #f volts4 #h-aj-ab-af-bj-bd candle-power ,illustration3 ,fig4 #bhb4--,miniature ,tungsten ,battery ,lamp4 ,to find the approximate amount of current drawn from a battery by a tungsten lamp1 divide the candle-power by the voltage and the result will be the current in amperes4 ,for example1 a #f v4 #b c4 p4 lamp will require1 #b divided by #f1 or one-third of an ampere4 ,six-volt tungsten lamp giving a light greater than six candle-power are only used on storage batteries and are employed principally for automobile lighting4 ,the filament of a tungsten lamp is much longer than that of a carbon lamp and is usually in the form of a spiral or helix1 as shown in ,figure #bhb4 ,the bases of battery lamps1 the base being the lower portion of the lamp1 which is made of brass and fits into a socket or receptacle1 are made in three different styles3 miniature1 candelabra1 and ,ediswan4 ,illustration3 ,fig4 #bhc4--,lamps fitted respectively with ,miniature1 ,candelabra1 and ,ediswan ,bases4 ,the miniature and candelabra bases have a threaded brass shell on the outside and a small brass contact-button on the bottom4 ,they are similar except in respect to size4 ,the miniature base is smaller than the candelabra4 ,the ,ediswan base is a plain brass shell having two pins on the side and two contacts on the bottom4 ,this type of base is only used in this country on automobiles4 ,the miniature and the candelabra bases are standard for battery lighting4 ,the miniature base has many advantages over the candelabra for the young experimenter1 and should be adopted in making any of the apparatus described in this chapter4 ,these three bases are shown in ,figure #bhc4 ,illustration3 ,fig4 #bhd4--,miniature ,flat-,base ,porcelain ,receptacle4 ,in order to form a good electrical connection between the lamp and the power wires some sort of a receptacle or socket is necessary4 ,the most common arrangement for this purpose is the miniature flat-base porcelain receptacle shown in ,figure #bhd4 ,this type of receptacle is used in places where it can be permanently fastened in position with two small screws4 ,illustration3 ,fig4 #bhe4--,weather-proof and ,pin-,sockets4 ,the devices shown in ,figure #bhe are known respectively as a porcelain weather-proof socket and a pin-socket4 ,sockets similar to the weather-proof socket are also made of wood4 ,the weather-proof sockets are used in places where the light is to be exposed out-of-doors1 as for instance on a porch4 ,the small metal parts are sealed in the porcelain and entirely protected4 ,the pin-sockets and the wooden sockets are used principally on ,christmas trees or in decorative outfits where lamps are hung in festoons4 ,the flat-base receptacle1 the pin-socket1 and also the wooden socket will be found very useful in making the apparatus described farther on in this chapter4 99,the ,wires99 used to carry the current in a miniature lighting system may be of the sort known as annunciator or office wire if the wires are to be run entirely indoors4 ,the wire should not be smaller than ,no4 #af ,b4 ,s4 gauge4 ,when the wires are run outdoors1 on a porch1 or in some other place exposed to the weather1 the wire used should be rubber-covered4 ,hanging lights or lights intended to be adjustable should be connected with flexible conductor4 ,this is made of a number of very fine wires braided together and insulated with silk4 ,the wires used in a lighting system should not in any case be longer than it is necessary to have them4 ,when a battery is connected to a system of wires it is found that the voltage at the end of the wires is much lower than at a point near the battery4 ,this is called voltage drop1 and is much greater as the wires grow longer4 ,a light placed at the end of two very long wires will not burn as brightly as it would if connected to the same battery by means of short wires4 99,switches99 can be made by following the suggestions given in ,chapter ,,vii4 ,suitable switches can be purchased for a few cents at a most any electrical house and will prove very much neater and efficient4 ,they should preferably be of any of the types shown in ,figure #bhf4 99,the ,batteries99 used for miniature lighting may be made up of storage cells1 dry cells or carbon cylinder cells4 ,storage cells will prove the most satisfactory1 provided that the experimenter has some convenient means of recharging them or of having them recharged4 ,storage cells will be found of especial value wherever it is desirable to operate several lights from one battery4 ,carbon cylinder cells are only suitable where one cell is to be operated at a time4 ,if more than one is used1 the battery is liable to become polarized and the lamps will not burn brightly4 ,carbon cylinder batteries are very inexpensive to renew1 and will be found the cheapest method of lighting a small tungsten lamp4 ,illustration3 ,fig4 #bhf4--,types of ,battery ,switches suitable for ,miniature ,lighting4 ,if lamps requiring more than two amperes are to be operated on dry cells1 the latter should be connected in series-multiple1 as shown in ,figure #fi4 ,two sets of dry cells connected in series-multiple will give more than twice the service of a single set4 ,lamps may be connected either in multiple or in series1 provided that the proper voltages of both battery and lamps are used4 ,when they are to be connected in multiple1 the voltage of the lamps should be the same as that of the battery4 ,when they are to be used in series1 the voltage of the lamps multiplied by the number used should equal the voltage of the battery4 ,for example1 suppose that you wish to use a number of six-volt lamps on a six-volt storage battery4 ,in that case they must be connected in multiple4 ,but if it should be that the lamps are only two-volt lamps and you wish to operate three of them on a six-volt battery you will have to place them in series4 ,illustration3 ,fig4 #bhg4--;,how ,lamps are ,connected in ,multiple4 ,illustration3 ,fig4 #bhh4--;,how ,lamps are ,connected in ,series4 ,it is sometimes desirable to arrange a lamp and two switches so that it can be turned off or on from either switch independently of the other4 ,this is called three-way wiring1 and is a very convenient method of arranging a light in a hallway4 ,if one switch is placed at the top of a stairway and the other switch at the bottom1 a person can pass upstairs or downstairs1 light the lamp ahead1 and turn it out as he passes the last switch1 no matter in which direction the previous user of the light may have gone4 ,the switches are two-point switches1 and the circuit should be arranged as in ,figure #bhi4 ,the switch-levers should always rest on one of the contacts and never be left between1 as shown in the drawing4 ,illustration3 ,fig4 #bhi4--,three-way ,wiring ,diagram4 ,the ,light may be turned off or on from either ,switch4 ,they are represented that way in the illustration in order not to conceal the contacts4 ,small brackets made of brass and similar to that shown in ,figure #bij are for sale at many electrical supply houses1 and will add a very realistic appearance to a miniature lighting plant4 ,illustration3 ,fig4 #bij4--;,a ,lamp ,bracket for ,miniature ,lighting4 ,brackets may be constructed after the plan shown in ,figure #bia4 ,a wooden socket or a pin-socket is mounted on the end of a small piece of brass tubing which has been bent into the shape shown in the illustration4 ,the other end of the tube is set into a wooden block so that the bracket may be mounted on the wall4 ,the wires from the socket lead through the brass tube and through the back or top of the block4 ,illustration3 ,fig4 #bia4--;,a ,home-made ,bracket4 ,hanging lights may be arranged by fitting a wooden socket and a lamp with a reflector as shown in ,figure #bif4 ,the reflector consists of a circular piece of tin or sheet-aluminum having a hole in the center large enough to pass the base of a miniature lamp4 ,the circle is then cut along a straight line from the circumference to the center4 ,if the edges are pulled together and lapped the circular sheet of metal will take on a concave shape and form a shade or reflector which will throw the light downwards4 ,the overlapping edges of the reflector should be soldered or riveted together4 ,the reflector is slipped over the base of the lamp1 a small rubber or felt washer having been placed over the base next to the glass bulb so that the reflector will not break the lamp4 ,the lamp is then screwed into a socket and allowed to hang downwards from a flexible conductor4 ,illustration3 ,fig4 #bib4--;,a ,hanging ,lamp4 ,a very pretty effect can be secured by drilling the edges of a reflector full of small holes about three-sixteenths of an inch apart and then hanging short strings of beads from the holes4 ,the beads should form a hanging fringe around the edge of the reflector1 and if they are of glass1 a pleasing brilliancy is produced4 ,figure #bic shows how to make the reflector4 ,illustration3 ,fig4 #bic4--;,how the ,reflector is made4 ,the batteries for a miniature lighting plant may be located in a closet1 under a stairway1 or in some other out-of-the-way place4 ,wires from there may be extended to various parts of the house1 such as hallways1 closets1 the cellar stairs1 over a shaving-mirror in the bath-room or in any dark corner where a light is often temporarily needed4 ,the wires can be run behind picture-mouldings or along the surbase and be almost entirely concealed4 ,illustration3 ,fig4 #bid4--;,a ,three-,cell ,dry ,battery for use in ,hand-,lanterns1 etc4 99,small ,batteries99 consisting of three small dry cells enclosed in cardboard box1 as shown in ,figure #bid1 are on the market1 and may be bought at prices ranging from thirty to forty cents1 depending upon the size and the maker4 ,one of the most convenient and practical sizes of this type of battery has the dimensions shown in the illustration1 and with its aid it is possible to construct a number of very useful electrical novelties and household articles in the shape of portable electric lamps1 etc4 ,these batteries are quite small and are only intended to operate very small lamps4 ,only one lamp should be used on each battery at a time1 and it should not be allowed to burn long4 ,some of these batteries will give ten to fourteen hours of intermittent service but if allowed to burn continuously would only light the lamp for about five hours at the most4 ,it is much the better plan to use them only for a few minutes at a time1 and then turn the light off and allow the battery to recuperate4 99,an ,electric ,hand-,lantern99 is a very convenient device which is quite simple to make4 ,it consists of a wooden box large enough to receive a three-cell battery1 such as that shown in ,figure #bie4 ,the back of the box should open and close on hinges and be fastened with a hook so that the battery may be easily removed for renewal4 ,illustration3 ,fig4 #bie4--;,an ,electric ,hand-,lantern4 ,a three-and-one-half-volt tungsten lamp is mounted on the front of the lantern and connected with the battery and a switch so that the light can be turned on and off at will4 ,the switch may be placed at the top of the box so that the fingers of the same hand used to carry the lantern may be used to turn the light on and off4 ,the lantern is fitted with a leather strap at the top1 to be convenient for carrying4 99,the ,ruby ,lantern99 shown in ,figure #bif is somewhat similar in arrangement to the lantern just described1 which may be used both as a hand-lantern and a ruby light for developing photographs4 ,illustration3 ,fig4 #bif4--;,an ,electric ,ruby ,lantern4 ,it consists of a wooden box to hold a three-cell dry battery1 and is provided with a handle so that it may be easily carried4 ,a switch by which to turn the lamp on and off is mounted on the side of the box4 ,the light is furnished by a three-and-one-half-volt tungsten lamp mounted on the front of an inclined wooden board arranged as shown in the illustration so as to throw the light downward4 ,the sides and bottom of the box are grooved near the front edges so that a piece of ruby glass may be inserted4 ,ruby glass for this purpose may be purchased at almost any store dealing in photographers' supplies4 ,the top is provided with a shield which is fastened in position by means of four small hooks after the glass is in place4 ,the shield is used in order to prevent any white light from escaping through the crack between the glass and the top of the box4 ,a ruby lamp of this sort must be made absolutely light-tight so that the only light emitted is that which passes through the ruby glass4 ,if any white light escapes it is liable to fog and spoil any pictures in process of development4 ,illustration3 ,fig4 #big4--,the ,electric ,ruby ,lamp with ,glass and ,shield ,removed4 ,by removing the ruby glass and the shield1 as shown in ,figure #big1 the light is changed into a hand-lantern4 ,the back of the box should be made removable so that the battery can be replaced when worn out4 99,a ,night-,light99 arranged to shine on the face of the clock so that the time may be easily told during the night without inconvenience is shown in ,figure #bih4 ,illustration3 ,fig4 #bih4--;,an ,electric ,night-,light for telling the ,time during the ,night4 ,it consists of a flat wooden box containing a three-cell dry battery and having a small three-and-one-half-volt tungsten lamp mounted on the top in the front with room for a clock to stand behind4 ,the battery and the lamp are connected to a switch so that the light may be turned on and off4 ,by attaching a long flexible wire and a push-button of the pear-push type it is possible to place the light on a table and run the wire with the push-button attached over to the bed so that one may see the time during the night without getting up4 ,the bottom of the box should be made removable so that a new battery may be inserted when the old one is worn out4 99,the ,watch-,light99 is in many ways similar to the clock light just described--but is smaller4 ,it consists of a box just large enough to receive a three-cell flashlight battery4 ,a piece of brass rod is bent into the form of a hook or crane from which to suspend the watch4 ,illustration3 ,fig4 #bii4--;,a ,watch-,light4 ,the light is supplied by a three-and-one-half-volt tungsten flashlight bulb mounted on the top of the box in front of the watch4 ,if desirable1 the light may be fitted with a small shade or reflector so that it shines only on the dial and not in the eyes4 ,the figures on the face of the timepiece can then be seen much more plainly4 ,the lamp is mounted in a small wooden socket or a pin-socket passing through a hole in the top of the box1 so that the wires are concealed4 ,a small push-button is located in one of the forward corners of the box1 so that when it is pressed the lamp will light4 ,two small binding-posts mounted at the lower right-hand corner of the box are connected directly across the terminals of the switch1 so that a flexible wire and a push-button can be connected1 and the light operated from a distance4 99,an ,electric ,scarf-,pin99 can be made by almost any boy who is skillful with a pocket-knife4 ,the material from which the pin is made may be a piece of bone1 ivory1 or meerschaum4 ,it is carved into shape with the sharp point of a penknife and may be made to represent a skull1 dog's head1 an owl1 or some other simple figure4 ,the inside is hollowed out to receive a pea lamp4 ,pea lamps with a cord and a plug attached as shown in ,figure #cjj may be purchased from almost any electrical supply house4 ,the lamp is a miniature carbon bulb about one-eighth of an inch in diameter4 ,the eyes1 nose1 and mouth of the figure are pierced with small holes1 so that when the lamp is lighted the light will show through the holes4 ,the figure should be carved down thin enough to be translucent and light up nicely4 ,illustration3 ,fig4 #cjj4--;,a ,pea ,lamp attached to a ,flexible ,wire and a ,plug4 ,a large pin is cemented or otherwise fastened to the back of the figure so that it can be placed on the necktie or the lapel of the coat4 ,the lamp is removed from the socket of an electric flashlight and the plug attached to the pea lamp screwed into its place4 ,the pea lamp is inserted inside the figure and bound in place with some silk thread4 ,then when the button is pressed on the flashlight case1 the pin will light up and tiny beams of light will shoot out from the eyes1 nose1 and mouth of the figure4 ,illustration3 ,fig4 #cja4--;,four ,steps in ,carving a ,skull ,scarf-,pin4 #a4 ,the ,bone4 #b4 ,hole drilled in ,base4 #c4 ,roughed out4 #d4 ,finished4 ,the drawings in ,figure #cja show how to carve a skull scarf-pin4 ,it is made from a cylindrical piece of bone about five-eighths of an inch long and three-eighths of an inch in diameter4 ,the first operation is to drill a hole three-eighths of an inch deep into the bottom4 ,the hole should be large enough in diameter to pass the pea lamp4 ,illustration3 ,fig4 #cjb4--,the ,completed ,pin ready to be connected to a ,battery by removing the ,lamp from a ,flashlight and screwing the ,plug into its ,place4 ,then carve the eyes and nose and teeth4 ,the drawings will give a good idea of the steps in this part of the work4 ,next round off the top of the skull4 ,bore a small hole in the back to receive the pin4 ,put the light inside of the skull1 and after it is bound in position the scarf-pin is finished4 ,,chapter ,,xx ,,miscellaneous ,,electrical ,,apparatus ,,how ,,electricity ,,may ,,be ,,generated ,,from ,,heat ,for the past century there has been on the part of many scientists and inventors a constant endeavor to harness the sunlight4 ,the power which streams down every day to our planet is incalculable4 ,the energy consumed in the sun and thrown off in the form of heat is so great that it makes any earthly thing seem infinitesimal4 ,we can only feel the heat from a large fire a few feet away1 yet the scorching summer heat travels #ij1jjj1jjj miles before it reaches us1 and even then our planet is receiving only the smallest fractional part of the total amount radiated4 ,dr4 ,langley of the ,smithsonian ,institute estimated that all the coal in the ,state of ,pennsylvania would be used by the sun in a fraction of a second if it were sent up there to supply energy4 ,perhaps1 some day in the future1 electric locomotives will haul their steel cars swiftly from city to city by means of electricity1 generated with sun power4 ,perhaps energy from the same source will heat our dwellings and furnish us light and power4 ,this is not an idle dream1 but may some day be an actuality4 ,it has already been carried out to some extent4 ,a ,massachusetts inventor has succeeded in making a device for generating electricity from sun energy4 ,the apparatus consists of a large frame1 in appearance very much like a window4 ,the glass panes are made of violet glass1 behind which are many hundred little metallic plugs4 ,the sun's heat1 imprisoned by the violet glass1 acts on the plugs to produce electricity4 ,one of these generators exposed to the sun for ten hours will charge a storage battery and produce enough current to run #cj large tungsten lamps for three days4 ,illustration3 ,fig4 #cjc4--;,how the ,copper ,wires 7,c7 and the ,silver ,wires 7,i7 are twisted together in ,pairs4 ,the principle upon which the apparatus works was discovered by a scientist named ,seebeck1 in #ahbb4 ,he succeeded in producing a current of electricity by heating the points of contact between two dissimilar metals4 ,any boy can make a similar apparatus1 which1 while not giving enough current for any practical purpose1 will serve as an exceedingly interesting and instructive experiment4 ,cut forty or fifty pieces of ,no4 #af ,b4 ,s4 gauge ,german silver wire into five-inch pieces4 ,cut an equal number of similar pieces of copper wire1 and twist each ,german silver wire firmly together with one of copper so as to form a zig-zag arrangement as in ,figure #cjc4 ,illustration3 ,fig4 #cjd4--,wooden ,ring4 ,next make two wooden rings about four inches in diameter by cutting them out of a pine board4 ,place the wires on one of the rings in the manner shown in ,figure #cje4 ,place the second ring on top and clamp it down by means of two or three screws4 ,illustration3 ,fig4 #cje4--;,complete ,thermopile4 ,an ,alcohol ,lamp should be lighted and placed so that the ,flame heats the ,inside ,ends of the ,wires in the ,center of the ,wooden ,ring4 ,the inner junctures of the wires must not touch each other4 ,the outer ends should be bent out straight and be spaced equidistantly4 ,the ring should be supported by three iron rods or legs4 ,the two terminals of the thermopile as the instrument is called1 should be connected to binding-posts4 ,place a small alcohol lamp or ,bunsen burner in the center1 so that the flame will play on the inner junctures of the wires4 ,a thermopile of the size and type just described will deliver a considerable amount of electrical energy when the inside terminals are good and hot and the outside terminals fairly good4 ,the current may be very easily detected by connecting the terminals to a telephone receiver or galvanometer4 ,by making several thermopiles and connecting them in parallel1 sufficient current can be obtained to light a small lamp4 ,,how ,,to ,,make ,a ,,reflectoscope ,a reflectoscope is a very simple form of a magic lantern with which it is possible to show pictures from post-cards1 photographs1 etc4 ,the ordinary magic lantern requires a transparent lantern slide1 but the reflectoscope will make pictures from almost anything4 ,the picture post-cards or the photographs that you have collected during your vacation may be thrown on a screen and magnified to three or four feet in diameter4 ,illustrations clipped from a magazine or newspaper or an original sketch or painting will likewise show just as well4 ,everything is projected in its actual colors4 ,if you put your watch in the back of the lantern1 with the wheels and works exposed1 it will show all the metallic colors and the parts in motion4 ,illustration3 ,fig4 #cjf4--;,a ,reflectoscope4 ,the reflectoscope1 shown in ,figure #cjf1 consists of a rectangular box nine inches long1 six inches wide1 and six inches high outside4 ,it may be built of sheet-iron or tin1 but is most easily made from wood4 ,boards three-eighths of an inch thick are heavy enough4 ,the methods of making an ordinary box are too simple to need description4 ,the box or case in this instance1 however1 must be carefully made and be light-tight1 that is1 as explained before1 it must not contain any cracks or small holes which will allow light to escape if a lamp is placed inside4 ,a round hole from two and one-half to three inches in diameter is cut in the center of one of the faces of the box4 ,the exact diameter cannot be given here because it will be determined by the lens which the experimenter is able to secure for his reflectoscope4 ,only one lens is required4 ,it must be of the double-convex variety1 and be from two and one-half to three inches in diameter4 ,a lens is very easily secured from an old bicycle lantern4 ,it should be of clear glass4 ,illustration3 ,fig4 #cjg4--;,how the ,lens is ,arranged and ,mounted4 ,a tube1 six inches long and of the proper diameter to fit tightly around the lens1 must be made by rolling up a piece of sheet-tin and soldering the edges together4 ,this tube is the one labeled movable tube in the illustrations4 ,a second tube1 three inches long and of the proper diameter to just slip over the first tube1 must also be made4 ,a flat ring cut from stiff sheet-brass is soldered around the outside of this second tube1 so that it may be fastened to the front of the case by three or four small screws in the manner shown4 ,the hole in the front of the box should be only large enough to receive the tube4 ,the lens is held in position near one end of the movable tube by two strong wire rings4 ,these rings should be made of wire that is heavy and rather springy1 so that they will tend to open against the sides of the tube4 ,it is a good plan to solder one of them in position1 so that it cannot move1 and then put in the lens4 ,after the lens is in position1 the second ring should be put in and pushed down against the lens4 ,do not attempt to put the lens in1 however1 until you are sure that the metal has cooled again after soldering1 or it will be liable to crack4 ,illustration3 ,fig4 #cjh4--;,a ,view of the ,reflectoscope from the ,rear1 showing the ,door1 etc4 ,the back of the box contains a small hinged door about four inches high and five and one-half inches long4 ,the pictures that it is desired to project on the screen are held against this door by two small brass clips1 as shown in ,figure #cjh4 ,illustration3 ,fig4 #cji4--;,a ,view of the ,reflectoscope with the ,cover removed1 showing the ,arrangement of the ,lamps1 etc4 ,the light for the reflectoscope is most conveniently made by two #af-;candle-power electric incandescent lamps4 ,figure #cji shows a view of the inside of the box with the cover removed1 looking directly down4 ,the lamps fit into ordinary flat-base porcelain receptacles1 such as that shown in ,figure #caj4 ,two of these receptacles are required1 one for each lamp4 ,they cost about ten cents each4 ,illustration3 ,fig4 #caj4--;,a ,socket for holding the ,lamp4 ,the reflectors are made of tin1 bent as shown in ,figure #caa4 ,they are fastened in position behind the lamps by four small tabs4 ,it is possible to fit a reflectoscope with gas or oil lamp to supply the light1 but in that case the box will have to be made much larger1 and provided with chimneys to carry off the hot air4 ,the interior of the reflectoscope must be painted a dead black by using a paint made by mixing lampblack and turpentine4 ,the interior also includes the inside of the tin tubes4 ,the electric current is led into the lamps with a piece of flexible lamp-cord passing through a small hole in the case4 ,an attachment-plug is fitted to the other end of the cord1 so that it may be screwed into any convenient lamp-socket4 ,illustration3 ,fig4 #caa4--,the ,tin ,reflector4 ,the pictures should be shown in a dark room and projected on a smooth white sheet4 ,they are placed under the spring clips on the little door and the door closed4 ,the movable tube is then slid back and forth until the picture on the screen becomes clear and distinct4 ,the lantern may be improved considerably by using tungsten lamps of #bb c4 p4 each in place of ordinary c4 p4 carbon filament lamps4 ,if four small feet1 one at each corner1 are attached to the bottom of the case1 its appearance will be much improved4 ,very large pictures will tend to appear a little blurred at the corners4 ,this is due to the lens and cannot be easily remedied4 ,,how ,,to ,,reduce ,,the #aaj-v4 ,,current ,,so ,,that ,,it ,,may ,,be ,,used ,,for ,,experimenting ,oftentimes it is desirable to operate small electrical devices from the #aaj-v4 lighting or power circuits4 ,alternating current can be reduced to the proper voltage by means of a small step-down transformer1 such as that described in ,chapter ,,xiii4 ,direct current may be reduced by means of a resistance4 ,the most suitable form of resistance for the young experimenter to use is a lamp bank4 ,a lamp bank consists of a number of lamps connected in parallel1 and arranged so that any device may be connected in series with it4 ,the lamps are set in sockets of the type known as flat-base porcelain receptacles1 such as that shown in ,figure #caj1 mounted in a row upon a board and connected as shown in ,fig4 #cab4 ,the current from the power line enters through a switch and a fuse and then passes through the lamps before it reaches the device it is desired to operate4 ,the switch is for the purpose of shutting the current on and off1 while the fuse will blow in case too much current flows in the circuit4 ,the amount of current that passes through the circuit may be accurately controlled by the size and number of lamps used in the bank4 ,the lamps may be screwed in or out and the current altered by one-quarter of an ampere at a time if desirable4 ,the lamps should be of the same voltage as the line upon which they are to be used4 ,each #h-;candle-power1 #aaj-v4 carbon lamp used will permit one-quarter of an ampere to pass4 ,each #af-;candle-power1 #aaj-v4 lamp will pass approximately one-half an ampere4 ,a #cb-;candle-power lamp of the same voltage will permit one ampere to flow in the circuit4 ,illustration3 ,fig #cab4--,top ,view of ,lamp ,bank1 showing how the ,circuit is arranged4 ,a and ,b are the ,posts to which should be connected any ,device it#8s desirable to operate4 ,,an ,,induction ,,motor 99,an ,induction ,motor99 is a motor in which the currents in the armature windings are induced4 ,an induction motor runs without any brushes1 and the current from the power line is connected only to the field4 ,the field might be likened to the primary of a transformer4 ,the currents in the armature then constitute a secondary winding in which currents are induced in the same manner as in a transformer4 ,an induction motor will operate only on alternating current4 ,a small motor such as that shown in ,figure #bfg1 and having a three-pole armature1 is the best type to use in making an experimental induction motor4 ,remove the brushes from the motor and bind a piece of bare copper wire around the commutator so that it short-circuits the segments4 ,a source of alternating current should then be connected to the terminals of the field coil4 ,if you have a step-down transformer1 use it for this purpose1 but otherwise connect it in series with a lamp bank such as that just described4 ,place a switch in the circuit so that the current may be turned on and off4 ,wind a string around the end of the armature shaft so that it may be revolved at high speed by pulling the string in somewhat the same manner that you would spin a top4 ,when all is ready1 give the string a sharp pull and immediately close the switch so that the alternating current flows into the field4 ,if this is done properly1 the motor will continue to run at high speed1 and furnish power if desirable4 ,most of the alternating-current motors in every-day use for furnishing power for various purposes are induction motors4 ,they are1 however1 self-starting1 and provided with a hollow armature1 which contains a centrifugal governor4 ,when the motor is at rest or just starting1 four brushes press against the commutator and divide the armature coils into four groups4 ,after the motor has attained the proper speed1 the governor is thrown out by centrifugal force and pushes the brushes away from the commutator1 short-circuiting all the sections and making each coil a complete circuit of itself4 ,,electro-,,plating ,water containing chemicals such as sulphate of copper1 sulphuric acid1 nitrate of nickel1 nitrate of silver1 or other metallic salts is a good conductor of electricity4 ,such a liquid is known as an electrolyte4 ,it has been explained in ,chapter ,,iv that chemical action may be used to produce electricity and that in the case of a cell such as that invented by ,volta1 the zinc electrode gradually wastes away and finally enters into solution in the sulphuric acid4 ,it is possible exactly to reverse this action and to produce what is known as electrolysis4 ,if an electrolyte in which a metal has been dissolved is properly arranged so that a current of electricity may be passed through the solution1 the metal will plate out1 or appear again upon one of the electrodes4 ,electrolysis makes possible electro-plating and thousands of other exceedingly valuable and interesting chemical processes4 ,more than one-half of all the copper produced in the world is produced electrolytically4 ,practically all plating with gold1 silver1 copper and nickel is accomplished with the aid of electricity4 ,these operations are carried out on a very large scale in the various factories1 but it is possible to reproduce them in any boy's workshop or laboratory1 with very simple equipment4 ,the proper chemicals1 a tank1 and a battery are the only apparatus required4 ,the current must be supplied by storage cells or a bichromate battery because the work will require five or six amperes for quite a long period4 ,a small rectangular glass jar will make a first class tank to hold the electrolyte4 ,the simplest electro-plating process1 and the one that the experimenter should start with is copper-plating4 ,fill the tank three-quarters full of pure water and then drop in some crystals of copper-sulphate until the liquid has a deep blue color and will dissolve no more4 ,obtain two copper rods and lay them across the tank4 ,cut two pieces of sheet copper having a tongue at each of two corners so that they can be hung in the solution1 as shown in ,figure #cac4 ,hang both of the sheets from one of the copper rods4 ,connect this rod to the positive pole of the battery4 ,these sheets are known as the anodes4 ,then if a piece of carbon1 or some metallic object is hung from the other rod and connected to the negative pole of the battery1 the electro-plating will commence4 ,the apparatus should be allowed to run for about half an hour and then the object hung from the rod connected to the negative pole of the battery should be lifted out and examined4 ,it will be found thickly coated with copper4 ,it is absolutely necessary to have the poles of the battery connected in the manner stated1 or no deposit of copper will take place4 ,objects which are to be electro-plated must be free from all traces of oil or grease and absolutely clean in every respect1 or the plating will not be uniform1 because it will not stick to dirty spots4 ,illustration3 ,fig4 #cac4--;,a ,glass ,jar arranged to serve as an ,electro-,plating ,tank4 ,such articles as keys1 key-rings1 tools1 etc41 can be prevented from rusting by coating with nickel4 ,nickel-plating is very similar to copper-plating4 ,instead1 however1 of having two copper sheets suspended from the rod connected to the positive pole of the battery1 they must be made of nickel4 ,the electrolyte is composed of one part of nickel-sulphate dissolved in twenty parts of water to which one part of sodium-bisulphate is added4 ,this mixture is placed in the tank instead of the copper-sulphate4 ,the objects to be plated are hung from the copper rod connected to the negative pole of the battery4 ,when the nickel-plated articles are removed from the bath they will have a dull1 white color known as white nickel4 ,when white nickel is polished with a cloth wheel revolving at high speed1 and known as a buffing-wheel1 it will assume a high luster4 ,,how ,,to ,,make ,a ,,rheostat ,it is often desirable to regulate the amount of current passing through a small lamp1 motor1 or other electrical device operated by a battery4 ,this is accomplished by inserting resistance into the circuit4 ,a rheostat is an arrangement for quickly altering the amount of resistance at will4 ,a simple rheostat is easily made by fitting a five-point switch such as that shown in ,figure #ie with several coils of ,german-silver resistance wire4 ,german silver has much more resistance than copper wire1 and is used1 therefore1 because less will be required1 and it will occupy a smaller space4 ,a five-point switch will serve satisfactorily in making a rheostat1 but if a finer graduation of the resistance is desired it will be necessary to use one having more points4 ,two lines of small wire nails are driven around the outside of the points1 and a ,german-silver wire of ,no4 #bd ,b4 ,s4 gauge wound in zig-zag fashion around the nails from one point to the other4 ,illustration3 ,fig4 #cad4--;,a ,rheostat4 ,the rheostat is placed in series with any device it is desirable to control4 ,when the handle is on the point to the extreme left1 the rheostat offers no resistance to the current4 ,when the lever is placed on the second point1 the current has to traverse the first section of the ,german-silver wire and will be appreciably affected4 ,moving the handle to the right will increase the resistance4 ,if the rheostat is connected to a motor1 the speed can be increased or decreased by moving the lever back and forth4 ,in the same manner1 the light from a small incandescent lamp may be dimmed or increased4 ,a ,,current ,,reverser ,,or ,,pole-,,changing ,,switch ,a pole-changing or current reversing switch is useful to the experimenter4 ,for example1 if connected to a small motor1 the motor can be made to run in either direction at will4 ,a motor with a permanent magnet field can be reversed by merely changing the wires from the battery so that the current flows through the circuit in the opposite direction4 ,if the motor is provided with a field winding1 however1 the only way that it can be made to run either way is by reversing the field4 ,this is best accomplished with a pole-changing switch4 ,such a switch may be made by following the same general method of construction as that outlined on pages #ajg and #ajh1 but making it according to the design shown in ,figure #cae4 ,motors such as those illustrated can be made to reverse by connecting to a pole-changing switch in the proper manner4 ,the two outside points or contacts 7marked ,d and ,d7 should both be connected to one of the brushes on the motor4 ,the middle contact1 ,c1 is connected to the other brush4 ,one terminal of the field is connected to the battery4 ,the other terminal of the field is connected to the lever1 ,a4 ,b connects to the other terminal of the battery4 ,illustration3 ,fig4 #cae4--;,a ,pole-,changing ,switch or ,current ,reverser4 ,the ,connecting ,strip is pivoted so that the ,handle will operate both the ,levers1 ,a and ,b4 ,when the switch handle is pushed to the left1 the lever ,a should rest on the left-hand contact1 ,d4 ,the lever ,b should make contact with ,c4 ,the motor will then run in one direction4 ,if the handle is pushed to the right so that the levers ,a and ,b make contact respectively with ,c and ,d 7right-hand71 the motor will reverse and run in the opposite direction4 ,a ,,complete ,,wireless ,,receiving ,,set ,many experimenters may wish to build a wireless receiving set which is permanently connected and in which the instruments are so mounted that they are readily portable and may be easily shifted from one place to another without having to disturb a number of wires4 ,the receiving set shown in ,figure #caf is made up of some of the separate instruments described in ,chapter ,,xiv1 and illustrates the general plan which may be followed in arranging an outfit in this manner4 ,illustration3 ,,complete ,,receiving ,,set1 ,,consisting ,,of ,,double ,,slider ,,tuning ,,coil1 ,,detector ,,and ,,fixed ,,condenser4 ,illustration3 ,,complete ,,receiving ,,set1 ,,consisting ,,of ,a ,,loose ,,coupler ,,in ,,place ,,of ,,the ,,tuning ,,coil1 ,,detector ,,and ,,fixed ,,condenser4 ,the base is of wood1 and is nine inches long1 seven inches wide1 and one-half of an inch thick4 ,a double-slider tuning coil1 similar to that shown in ,figure #bjc1 is fastened to the back part of the base by two small wood-screws passing upwards through the base into the tuner heads4 ,illustration3 ,fig4 #caf4 ,a ,complete ,wireless ,receiving ,outfit4 ,the fixed condenser is enclosed in a rectangular wooden block which is hollowed out underneath to receive it and then screwed down to the base in the forward right-hand corner4 ,the detector is mounted in the forward left-hand part of the base1 and in the illustration is shown as being similar to that in ,figure #baj4 ,any type of detector may1 however1 be substituted4 ,the tuning coil may be replaced by a loose coupler if desirable1 but in that case the base will have to be made larger4 ,the telephone receivers are connected to two binding-posts mounted alongside the detector4 ,the circuit shown in ,figure #bah is the one which should be followed in wiring the set4 ,the wires which connect the various instruments should be passed through holes and along the under side of the base so that they are concealed4 ,,how ,,to ,,build ,a ,,tesla ,,high-,,frequency ,,coil ,a ,tesla high-frequency coil or transformer opens a field of wonderful possibilities for the amateur experimenter4 ,innumerable weird and fascinating experiments can be performed with its aid4 ,when a ,leyden jar or a condenser discharges through a coil of wire1 the spark which can be seen does not consist simply of a single spark passing in one direction1 as it appears to the eye1 but in reality is a number of separate sparks alternately passing in opposite directions4 ,they move so rapidly that the eye is unable to distinguish them4 ,the time during which the spark appears to pass may only be a fraction of a second1 but during that short period the current may have oscillated back and forth several thousand times4 ,if the discharge from such a ,leyden jar or a condenser is passed through a coil of wire acting as a primary1 and the primary is provided with a secondary coil containing a larger number of turns1 the secondary will produce a peculiar current known as high-frequency electricity4 ,high-frequency currents reverse their direction of flow or alternate from one hundred thousand to one million times a second4 ,illustration3 ,fig4 #cag4--;,illustrating the ,principle of the ,tesla ,coil4 ,a ,leyden ,jar discharges through the ,primary ,coil and a ,high-,frequency ,spark is produced at the ,secondary4 ,high-frequency currents possess many curious properties4 ,they travel only on the surface of wires and conductors4 ,a hollow tube is just as good a conductor for high-frequency currents as a solid rod of the same diameter4 ,high-frequency currents do not produce a shock4 ,if you hold a piece of metal in your hand you can take the shock from a high-frequency coil throwing a spark two or three feet long with scarcely any sensation save that of a slight warmth4 ,the ,tesla coil described below is of a size best adapted for use with a two-inch or three-inch spark coil1 or a small high-potential wireless transformer4 ,the purpose of the spark coil or the transformer is to charge the ,leyden jars or condenser which discharge through the primary of the ,tesla coil4 ,illustration3 ,fig4 #cah4--;,details of the ,wooden ,rings used as the ,primary ,heads4 ,if the young experimenter wishes to make a ,tesla coil which will be suited to a smaller spark coil1 for instance1 one capable of giving a one-inch spark1 the dimensions of the ,tesla coil herein described can be cut exactly in half4 ,instead of making the secondary twelve inches long and three inches in diameter1 make it six inches long and one and one-half inches in diameter1 etc4 99,the ,primary99 consists of eight turns of ,no4 #aj ,b4 ,s4 gauge copper wire wound around a drum4 ,the heads of the drum are wooden rings1 seven inches in diameter and one-half inch thick4 ,a circular hole four and one-half inches in diameter is cut in the center of each of the heads4 ,illustration3 ,fig4 #cai4--;,details of the ,cross ,bars which support the ,primary ,winding4 ,the cross bars are two and one-half inches long1 three-quarters of an inch thick and one-half of an inch wide4 ,six cross bars are required4 ,they are spaced at equal distances around the rings and fastened by means of a brass screw passing through the ring4 ,when the drum is completed it should resemble a squirrel cage4 ,small grooves are cut in the cross bars to accommodate the wire4 ,the wires should pass around the drum in the form of a spiral and be spaced about five-sixteenths of an inch apart4 ,the ends of the wire should be fastened to binding-posts mounted on the heads4 99,the ,secondary99 is a single layer of ,no4 #bf ,b4 ,s4 silk- or cotton-covered wire wound over a cardboard tube1 twelve inches long and three inches in diameter4 ,the tube should be dried in an oven and then given a thick coat of shellac1 both inside and out1 before it is used4 ,this treatment will prevent it from shrinkage and avoid the possibility of having to rewind the tube in case the wire should become loose4 ,illustration3 ,fig4 #cbj4--,the ,secondary ,head4 ,the secondary is fitted with two circular wooden heads just large enough to fit tightly into the tube1 having a half-inch flange1 and an outside diameter of three and seven-eighths inches4 99,the ,base99 of the coil is fifteen inches long and six inches wide and is made of wood4 ,the coil is assembled by placing the primary across the base and exactly in the center4 ,two long wood-screws passing through the base and into the primary heads will hold it firmly in position4 ,the secondary is passed through the center of the primary and supported in that position by two hard rubber supports1 four inches high1 seven-eighths of an inch wide and one-half of an inch thick4 ,a brass wood-screw is passed through the top part of each of the supports into the secondary heads so that a line drawn through the axis of the secondary will coincide with a similar line drawn through the axis of the primary4 ,illustration3 ,a ,,complete ,,coherer ,,outfit ,,as ,,described ,,on ,,page #bgd4 ,illustration3 ,,the ,,tesla ,,high ,,frequency ,,coil4 ,the supports are made of hard rubber instead of wood1 because the rubber has a greater insulating value than the wood4 ,high-frequency currents are very hard to insulate1 and wood does not usually offer sufficient insulation4 ,a brass rod1 five inches long and having a small brass ball at one end1 is mounted on the top of each of the hard-rubber supports4 ,the ends of the secondary winding are connected to the brass rods4 ,illustration3 ,fig4 #cba4--;,end ,view of the ,complete ,tesla ,coil4 ,the lower end of each of the hard-rubber supports is fastened to the base by means of a screw passing through the base into the support4 ,in order to operate the ,tesla coil1 the primary should be connected in series with a condenser and a spark-gap as shown in ,figure #cbd4 ,the condenser may consist of a number of ,leyden jars or of several glass plates coated with tinfoil4 ,it is impossible to determine the number required ahead of time1 because the length of the connecting wires1 the spark-gap1 etc41 will have considerable influence upon the amount of condenser required4 ,the condenser is connected directly across the secondary terminals of the spark coil4 ,when the spark coil is connected to a battery and set into operation1 a snappy1 white spark should jump across the spark-gap4 ,if the hand is brought close to one of the secondary terminals of the ,tesla coil1 a small reddish-purple spark will jump out to meet the finger4 ,illustration3 ,fig4 #cbb4--,the ,complete ,tesla ,coil4 ,adjusting the spark-gap by changing its length and also altering the number of ,leyden jars of condenser plates will probably increase the length of the high-frequency spark4 ,it may be possible also to lengthen the spark by disconnecting one of the wires from the primary binding-posts on the ,tesla coil and connecting the wire directly to one of any one of the turns forming the primary4 ,in this way the number of turns in the primary is changed and the circuit is tuned in the same way that wireless apparatus is tuned by changing the number of turns in the tuning coil or helix4 ,illustration3 ,fig #cbc4--,showing how a ,glass-,plate ,condenser is built up of ,alternate ,sheets of ,tinfoil and ,glass4 ,the weird beauty of a ,tesla coil is only evident when it is operated in the dark4 ,the two wires leading from the secondary to the brass rods and the ball on the ends of the rods will give forth a peculiar brush discharge4 ,if you take a piece of metal in your hand and hold it near one of the secondary terminals1 the brushing will increase4 ,if you hold your hand near enough1 a spark will jump on to the metal and into your body without your feeling the slightest sensation4 ,if one of the secondary terminals of the ,tesla coil is grounded by means of a wire connecting it to the primary1 the brushing at the other terminal will increase considerably4 ,make two rings out of copper wire4 ,one of them should be six inches in diameter and the other one four inches in diameter4 ,place the small ring inside the large one and connect them to the secondary terminals4 ,the two circles should be arranged so as to be concentric1 that is1 so that they have a common center4 ,the space between the two coils will be filled with a pretty brush discharge when the coil is in operation4 ,illustration3 ,fig4 #cbd4--;,a ,diagram showing the ,proper ,method of ,connecting a ,tesla ,coil4 ,there are so many other experiments which may be performed with a ,tesla coil that it is impossible even to think of describing them here1 and the young experimenter wishing to continue the work further is advised to go to some library and consult the works of ,nikola ,tesla1 wherein such experiments are fully explained4 ,,conclusion ,unless the average boy has materially changed his habits1 in recent years1 it matters not what the preface of a book may contain1 for it will be unceremoniously skipped with hardly more than a passing glance4 ,with this in mind1 the author has tried to steal a march on you1 and instead of writing a longer preface1 and including some material which might properly belong in that place1 has added it here in the nature of a conclusion1 thinking that you would be more likely to read it last than first4 ,some time ago1 when in search for something that might be described in this book1 ,i thought of some old boxes into which my things had been packed when ,i had dismantled my workshop before going away to college4 ,they had been undisturbed for a number of years and ,i had almost forgotten where they had been put4 ,at last a large box was unearthed from amongst a lot of dusty furniture put away in the attic4 ,i pried the cover off and took the things out one by one and laid them on the floor4 ,here were galvanometers1 microphones1 switches1 telegraph keys1 sounders1 relays1 and other things too numerous to mention4 ,they had all been constructed so long ago that ,i was considerably amused and interested in the manner in which bolts1 screws1 pieces of curtain rod1 sheet-iron1 brass1 and other things had been taken to form various parts of the instruments4 ,the binding-posts had almost in every case seen service as such on dry cells before they came into my hands4 ,the only parts that it had been necessary to buy were a few round-headed brass screws and the wire which formed the magnets4 ,in several instances1 the latter were made so that they might be easily removed and mounted upon another instrument4 ,the magnets on the telegraph sounder could be removed and fitted to form part of an electric engine or motor4 ,one particular thing which struck me very forcibly was the lack of finish and the crudeness which most of the instruments showed4 ,of course it was impossible to avoid the clumsy appearance which the metal parts possessed1 since they were not originally made for the part that they were playing1 but ,i wished that ,i had taken a little more care to true up things properly or to smooth and varnish the wood1 or that ,i had removed the tool-marks and dents from the metal work by a little filing4 ,if ,i had done so1 ,i should now be distinctly proud of my work4 ,that is not to say that ,i am in the least ashamed of it1 for my old traps certainly served their purpose well1 even if they were not ornamental and were better back in their box4 ,perhaps ,i might be excused for failing in this part of the work through lack of proper tools1 and also because at that time there were no magazines or books published which explained how to do such things1 and when ,i built my first tuning coils and detectors nothing on that subject had ever been published4 ,i had to work out such problems for myself1 and gave more thought to the principles upon which the instruments operated than to their actual construction4 ,the boys who read this book have the advantage of instructions showing how to build apparatus that has actually been built and tested4 ,you know what size of wire to use and will not have to find it out for yourself4 ,for that reason you ought to be able to give more time to the construction of such things4 ,the purpose of this conclusion is simply a plea for better work4 ,the ,american boy is usually careless in this regard4 ,he often commences to build something and then1 growing tired before it is finished1 lays it aside only to forget it and undertake something else4 ,finish whatever you undertake4 ,the principle is a good one4 ,remember also that care with the little details is what insures success in the whole4 ,if in carrying out your work1 you get an idea1 do not hesitate to try it4 ,a good idea never refused to be developed4 ,it is not necessary to stick absolutely to the directions that ,i have given4 ,they will insure success if followed1 but if you think you can make an improvement1 do so4 ,of course1 such a book as this cannot1 in the nature of things1 be exhaustive1 nor is it desirable1 in one sense1 that it should be4 ,i have tried to write a book which1 considered as a whole1 would prove to be exhaustive only in that it treats of almost every phase of practical electricity4 ,the principle in mind has been to produce a work which would stimulate the inventive faculties in boys1 and to guide them until face to face with those practical emergencies in which no book can be of any assistance but which must be overcome by common sense and the exercise of personal ingenuity4 ,the book is not as free from technical terms or phrases1 as it lay in my power to make it1 because certain of those terms have a value and an every-day use which are a benefit to the young experimenter who understands them4 ,any one subject treated in the various chapters of the ,boy ,electrician may be developed far beyond that point to which ,i have taken it4 ,the railroad system could be fitted with electric signals1 drawbridges1 and a number of other devices4 ,many new ideas suggest themselves to the ready-witted ,american boy4 ,i shall always be pleased to hear from any boy who builds any of the apparatus ,i have described1 and1 if possible1 to receive photographs of the work4 ,i should be glad to be of any assistance to such a lad1 but remember that some of the drawings and text in this book required many hours even to complete a small portion1 and therefore please do not write to ask how to build other apparatus not described herein4 ,and1 as the future years bring new inventions and discoveries1 no one now knows but that1 some day1 perhaps ,i will write another ,boy ,electrician4 ,,the ,,end4