things a boy should know about electricity 1000004810

7/30/2019 Things a Boy Should Know About Electricity 1000004810

1/196


2/196


3/196


4/196
http://www.forgottenbooks.org/redirect.php?where=fb&pibn=1000004810http://www.forgottenbooks.org/redirect.php?where=it&pibn=1000004810http://www.forgottenbooks.org/redirect.php?where=es&pibn=1000004810http://www.forgottenbooks.org/redirect.php?where=fr&pibn=1000004810http://www.forgottenbooks.org/redirect.php?where=de&pibn=1000004810http://www.forgottenbooks.org/redirect.php?where=co.uk&pibn=1000004810http://www.forgottenbooks.org/redirect.php?where=com&pibn=1000004810


5/196


6/196


7/196


8/196


9/196

THINGS A BOY SHOULD KNOWABOUT ELECTRICITY

TABLE OF CONTENTSChapter Page

I. About Frictional Electricty, . . . . 7II. About Magnets and Magnetism ... 21

III. How Electricityis Generated by the Voltaic Cell, 32IV. Various Voltaic Cells, . . . -36V. About Push-Buttons. Switches and Binding-Posts, 43

VI. Units and Apparatus for Electrical Measurements, 48VII. Chemical Eflfects of the Electric Current, . 58VIII. How Electroplating and Electrotyping are Done, . 60

IX. The Storage Battery, and How it Works, 63X. How Electricityis Generated by Heat, . . 68

XI. Magnetic Eflfects of the Electric Current, . 71XII. How Electricityis Generated by Induction, . 77XIII. How the Induction Coil Works, ... 80XIV. The Electric Telegraph, and How it Sends Mes-ages,

...... 84XV. The Electric Bell and Some of its Uses, . . 91

XVL The Telephone and How it Transmits Speech, 95XVII. How Electricityis Generated by Dynamos, . loi

XVIII. How the Electric Current is Transformed, . 109XIX. How Electric Currents are Distributed for Use, . 114XX. How Heat is Produced by the Electric Current. 124

XXI. How Light is Produced by the Incandescent Lamp, 129XXII. How Light is Produced by the Arc Lamp, . 135

XXIII. X-Rays, and How the Bones of the Human Bodyare Photographed, . . . . .141

XXIV. The Electric Motor, and How it Does Work, . 147XXV. Electric Cars, Boats and Automobiles, . . 154

XXVI. A Word About Central Stations, . 162XXVII. Miscellaneous Uses of Electricity, . .165


10/196


11/196

Things A Boy Should Know AboutElectricity

CHAPTER I.ABOUT FBICTIONAIi EIjECTBICITT.

I. Some Simple Experiments. Have you evershuffled your feet along over the carpet on a winter'sevening and then quickly touched your finger to thenose of an unsuspedling friend? Didhe jump when a bright spark leapedfrom your finger and struck him fairlyon the very tip of his sensitive nasalorgan ?

Did you ever succeed in proving tothe pussy-cat, Fig. i, that somethingunusual occurs when you thoroughlyrub his warm fur with your hand ? Didyou notice the bright sparks that passedto your hand when it was held justabovethe cat's back? You should be able tosee. hear, and feel these sparks, especiallywhen the airis dry and you are in a dark room.

Did you ever heat a piece of paper before the fire untilit was real hot, then lay it upon the table and rub it fromend to end with your hand, and finallysee it cling to thewaU?

Fig. I.


12/196

8 ABOUT FRICTION AI^ KI^KCTRICITY.

Were you ever in a fadlory where there were largebelts running rapidlyover pulleysor wheels, and wherelargesparkswould jump to your hands when held nearthe belts?

If you have never performed any of the four experi-entsmentioned, you should try them the first time a

chance occurs. There are dozens of simple,fascinatingexperiments that may be performed with this kind ofeledbricity.

2. Name. As this variety of eledlricitys made, orgenerated,by the fricSlionof substances upon each other,it is called frillionalledlricity.It is also called staticeledlricity,ecause it generallystands still upon the sur-ace

of bodies and does not ' * flow in currents ' * as easilyas some of the' other varieties. Static eledlricityay be

produced by indudlionas well as by fridlion.3. History. It has

been known for over2 ,000 years that certainsubstances a"Sl queerlywhen rubbed. Amberwas the first substanceupon which eledlricitywas produced by fric-ion,

and as the Greekname for amber is

elektron,bodies so affedled were said to be ele5lrijied.When a body, like ebonite, is rubbed with a fiannelcloth, we say that it becomes charged with ele5lricity.Just what happens to the ebonite is not clearlyunder-


13/196

"^^nt

ABOUT PRICTIONAt "I,ECTRICITY. 9

Stood. We know, however, that it will attradl lightbodies,and then quicklyrepelthem ifthey be condudlors.Fig. 2 shows a pieceof tissue-paperjumping toward asheet of ebonite that has been eledlrified with a flanneldoth.

4. Conductors and Non-Conductors. Ele"5tricitycan be produced upon glassand ebonite because they donot carry or condudl it away. If a piece of iron berubbed, the eledlricitypasses from the iron into theearth as fast as it is generated,because the iron is a con-duElor of eleiSlricity.lass is an insulator or non-con-du5lor. Fri(5tional eledlricityesides upon the outside,only, of condu(5tors. A hollow tin boxwill hold as great a charge as a solidpieceof metal having the same out-ide

size and shape. When fridlionaleledlricityasses from one place toanother,sparks are produced. Light-ing

is caused by the passage ofstatic eleAricityrom a cloud to theearth, or from one cloud to another.In this case air forms the condudlor.(For experiments, see ' * Study, * *Chapter VII.)

5. Electroscopes. A piece of carbon, pith,or evena small pieceof damp tissue-paperill serve as an eledlro-scope to test the presence of static electricity.he pithis usuallytied to a pieceof silk thread which is a non-onductor.

Fig. 3 shows the ordinary form of pith-balleleSlroscope,

The Uaf eUHroscopeis a very delicate apparatus. Gold-

"r

Fig. 3.


14/196


15/196

ABOUT FRICTION AI, KLECTRICITY. II

static eledlricityin quantitiesfor experiments, somedevice is necessary.

The ele5lrophoruse-lec-troph'-o-rus)s about the sim-lestform of machine. Fig. 5 shows a simpleeledbroph-

orus in which are two insulators and one conductor.The ebonite sheet E S is used with a flannel cloth to gen-rate

theeledlricity.The metal cover E C islifted by theinsulatingandle E R. The cover E C is placed uponthe thoroughlycharged sheet E S, and then it is touchedfor an instant with the finger,before liftingt by E R.The charge upon E C can then beremoved by bringingthe hand nearit. The brightspark that passesfrom E C to the hand indicates thatE C has dischargeditself into theearth. The adlion of the elecSlroph-orus depends upon indudlion. (Forexperiments, details of adlion,induced eledhification,etc., see**The Study of Elementary Eledlricitynd Magnetismby Experiment,''haptersVIII. and IX.)

The firstle6lricmachine consisted of a ball of sulphurfastened to a spindlewhich could be turned by a crank.By holdingthe hands or a pad of silk upon the revolvingball,eledlricityas produced.

9. The Cylinder Electric Machine consists,asshown in Fig.6, of a glasscylindef so mounted that itcan be turned by a crank. Fri(5lion is produced by apad of leather C, which presses againstthe cylinderas itturns. Eledlric sparkscan be taken from the large* * con-dudtors ' * which are insulated from the earth. The oppo-


16/196

12 ABOUT FRICTIONAI, KI*BCTRICITY.

Fig. 6.

Fig. 7.


17/196


18/196


19/196

ABOUT FRICTlONAI" EI.KCTRICITY. 15

With the modem machines largesparks are producedby merely turning a crank, enough eledlricityeing pro-uced

to imitate a small thunderstorm. The sparks ofhome-made lightningwill jump several inches.

Do not think that eledlricitys generated in a com-ercialway by static eledlric machines. The practical

.uses of static eledlricityre very few when comparedwith those of current eledlricityfrom batteries anddynamos.

15. Condensation of Static Electricity. By means

Fig. II. Fig. 12.

of apparatus called condensers y a terrific charge of staticeledlricitymay be stored. Fig. 11 shows the mostcommon form of condenser, known as the Leyden jar.It consists of a glassjarwith an inside and outside coatingof tin-foil.

To chargethe jarit is held in the hand so that the out-idecoatingshall be connedled with the earth, the sparks


20/196


21/196

ABOUT FRICTIONAI. ELECTRICITY. 17

Fig. 15.

small, its potential,r eledlromotive force (E. M. F.),is very high. We say that an ordinary gravitycell hasan E. M. F. of a little over one volt. Five such cellsjoined in the proper waywould have an E. M. F. of alittle over five volts. Youwill understand, then, whatis meant when we say that theE. M. F. of a lightningflashis millions of volts.

18. Atmospheric Elec-ricity.The air is usually

eledlrified,even in clearweather, although its cause isnot thoroughly understood.In 1752 it was proved byBenjamin Franklin (Fig. 15), with his famous kiteexperiment,that atmosphericand fridlional eledlricitiesare of the same nature. By means of a kite,the stringbeing wet by the rain, he succeeded, during a thunder-torm,

in drawing sparks,charg-ngcondensers,etc.

19. Lightning may be pro-ucedby the passage of eledlricity

between clouds, or between acloud and the earth (Fig. 16),which, with the interveningair,have the effedl of a condenser.When the attraction between

the two eledlrifications gets great enough, a sparkpasses.When the spark has a zigzag motion it is called chain

Fig. i6.


22/196

i8 ABOUT FRICTIONAI. ELECTRICITY.

lightning. In hot weather flashes are often seen whichlightwhole clouds, no thunder being heard. This iscalled heat lightning,and is generallyconsidered to be

due to distant dis-harges,the light of

which is refledled bythe clouds. Thelightningflash repre-ents

billionsof volts.20. Thunder is

caused by the violentdisturbances producedin the air by light-ing.

Clouds, hills,etc. produce echoes,which, with the orig-nal

sound, make therollingeffedl.

21. Lightning-Rods, when wellconstructed,often pre-ent

violent dis-harges.Their point-d

prongs at the topallow the negativeeledlricityf the earth

to pass quietlyinto the air to neutralize the positiventhe cloud above. In case of a discharge,r stroke oflightning,he rods aid in condudling the eledlricityothe earth. The ends of the rods are placeddeep in theearth.Fig. 17.

^^^^^^^^^

Fig. 17.


23/196
http://www.forgottenbooks.org/in.php?btn=4&pibn=1000004810&from=pdfhttp://www.forgottenbooks.org/in.php?btn=3&pibn=1000004810&from=pdfhttp://www.forgottenbooks.org/in.php?btn=2&pibn=1000004810&from=pdfhttp://www.forgottenbooks.org/in.php?btn=1&pibn=1000004810&from=pdf


24/196

20 BOUT FRICTIONAI. ELECTRICITY.

Fig. 19.

luminous effedls,ig. 19, often seen in the north. Theyoften occur at the same time with magneticstorms, whentelegraphand telephonework may be disturbed. Theexadl cause of this lightis not known, but it is thoughtby many to be due to disturbances in the earth's mag-etism

caused by the adlion of the sun.


25/196

CHAPTER II.

ABOUT MAGNETS AND MAGNETISM.

24. Natural Magnets. Hundreds of years ago itwas discovered that a certain ore of iron, called lodestone,had the power of picking up small piecesof iron. It wasused to indicate the north and south line,and it was dis-overed

later that small pieces of steel could be perma-entlymagnetized by rubbing them upon the lodestone.

25. Artificial Magnets. Pieces of steel,when mag-etized,are called artificial magnets. They are made in

many forms. The eledlromagnet is also an artificialmagnet; this will be treated separately.

26. The Horseshoe Magnet, Fig. 20, is,however,the one with which we are the most familiar.They are always painted red, but the red painthas nothing to do with the magnetism.

The little end-piece is called the keeper orarmature; it should always be kept in placewhen the magnet is not in use. The magnetitself is made of steel, while the armature is

p. ^^made of soft iron. Steel retains magnetismfor a long time, while soft iron loses it almost instantly.The ends of the magnet are called its poleSy and nearlyall the strength of the magnet seems to reside at thepoles,the curved part having no attradlion for outsidebodies. One of the poles of the magnet is marked witha line, or with the letter N. This is called the northpole of the magnet, the other being its south pole.


26/196

22 ABOUT MAGNETS AND MAGNETISM.

27. Bar Magnets are straightmagnets. Pig. 21

Fig. 21.

shows a round bar magnet. The screw in the end is foruse in the telephone,described later.

28. Compound Magnets. When several thin steelmagnets are riveted together,a com-ound

magnet is formed. These can bemade with considerable strength. Fig.22 shows a compound horseshoe magnet.Fig. 23 shows a form of compound barmagnet used in telephones. The use ofthe coil of wire will be explainedlater.A thick pieceof steel can not be magnet-zed

through and through. In the com-oundmagnet we have the effedl of a

thick magnet pradlicallymagnetizedthrough and through.

29. Magnetic and DiamagneticBodies. Iron, and substances contain-ngiron,are the ones most readilyattracted by a magnet.Fig. 22.

Fig. 23.Iron is said to be magnetic. Some substances, likenickel,for example, are visiblyattradled by very strong


27/196

ABOUT MAGNETS AND MAGNETISM. 23

Fig. 24.

magnets only. Strange as it may seem, some substancesare adluallyrepelledby strong magnets; these are calleddiamagneticbodies. Brass, copper, zinc, etc., are notvisiblyaffe"5led by a magnet.Magnetism will adl throughpaper, glass, copper, lead,etc.

30. Making Magnets.One of the strangest proper-ies

that a magnet has is its power to give magnetism toanother pieceof steel. If a sewing-needlebe properlyrubbed upon one of the poles of a magnet, it will be-ome

stronglymagnetizedand will retain its mag-etism

for years. Strongpermanent magnets aremade with the aid ofelectromagnets. Anynumber of littlemagnetsmay be made from a horse-hoe

magnet without in-uringit.31. Magnetic Needles

and Compasses. If abar magnet be suspendedby a string,or floatedupon a cork, which caneasily be done with the

magnet made from a sewing-needle.Fig. 24, it willswing around until its polespointnorth and south. Suchan arrangement is called a magneticneedle. In the reg-

Fig. 25.


28/196


ular compass,a magnetic needle issupportedupon a pivot.Compasses have been used for many centuries by mar-ners

and others. Fig. 25 shows an ordinary pocketcompass, and Fig. 26 a form of mariner's compass, inwhich the small bar magnets are fastened to a cardwhich floats,the whole being so mounted that it keeps ahorizontal position,ven though the vessel rocks.

Fig. 26.

32. Action of Magnets Upon Each Other. Bymaking two small sewing-needlemagnets, you can easilystudy the laws of attradlion and repulsion. By bringingthe two north poles,or the two south poles,near eachother,a repulsionwill be noticed. Unlike poles attradleach other. The attraction between a magnet and ironis mutual; that is,each attradls the other. Either poleof a magnet attradls soft iron.


29/196

ABOUT MAGNETS AND MAGNETISM. 25

In magnetizing a needle, either end may be made anorth pole at will;in fadl,the poles of a weak magnetcan easily be reversed by properlyrubbing it upon astronger magnet.

33. Theory of Magnetism. Each littleparticlef apieceof steel or iron is supposed to be a magnet, evenbefore it touches a magnet. When these little magnetsare thoroughly mixed up in the steel,they pull in allsorts of diredlions upon each other and tend to keep thesteel from attradlingoutside bodies. When a magnet isproperlyrubbed upon a bar of steel,the north polesof thelittlemolecular magnets of the steel are all made to pointin the same dire"5lion. As the north poles help eachother, the whole bar can attradl outside bodies.

By jarring a magnet its molecules are thoroughlyshaken up; in fadl, most of the magnetism can beknocked out of a weak magnet by hammering it.

34. Retentivity The power that a pieceof steel hasto hold magnetism is called retentivity.Different kindsof steel have different retentivities. A sewing-needleofgood steel will retain magnetism for years, and it isalmost impossibleto knock the magnetism out byhammering it. Soft steel has very little retentivity,because it does not contain much carbon. Soft iron,which contains less carbon than steel,holds magnetismvery poorly;so it is not used for permanent magnets.A littlemagnetism, however, will remain in the soft ironafter it is removed from a magnet. This iscalled residualmagnetism,

35. Heat and Magnetism. Steel will completelylose its magnetism when heated to redness,and a magnet


30/196


will not attradl red-hot iron. The molecules of a pieceof red-hot iron are in such a state of rapidvibration thatthey refuse to be brought into line by the magnet.

36. Induced Magnet-sm.A pieceof soft iron

may be induced to becomea magnet by holding itnear a magnet, absolutecontadl not beingnecessary.When the soft iron is re-oved,

again,from the in-luenceof the magnet, its

magnetism nearly all dis-ppears.It is said to have

temporary magnetism ; ithad induced magnetism. Ifa pieceof soft iron be heldnear the north pole of amagnet, as in Fig. 27,

poleswill be produced in the soft iron,the one nearestthe magnet being the south pole,and the other the northpole.

37. Magnetic Field. If a bar magnet be laid uponthe table,and a compass be moved about it,the compass-needle will be attracted bythe magnet, and it will pointin a different diredlion forevery positiongiven to thecompass. This strangepower, called magnetism, reaches out on all sides of amagnet. The magnet may be said to adl by indudlion

Fig. 27.

yj^s./Fig. 28.


31/196


32/196


filingsUpon a sheet of paper under which is placed amagnet. Fig. 29 shows a magnetic figuremade with anordinarybar magnet. The magnet was placedupon the

table and over this was laida piece of smooth paper.Fine iron filingsere siftedupon the paper, which wasgently tapped so that thefilingsould arrange them-elves.

As each particlefiron became a littlemagnet,by indudlion,its poleswereattradled and repelledbythe magnet; and when thepaper was tapped theyswung around to their finalpositions.Notice that thefilingsave arrangedthem-elves

in lines. These linesshow the positionsof someof the lines ofmagneticforcewhich surrounded themagnet.

These lines of force passfrom the north pole of amagnet through the air onall sides to its south pole.Fig.30 shows a magneticfiguremade firom two bar

magnets placed side by side, their unlike poles beingnext to each other. Fig. 31 shows the magnetic figure

Fig. 31.


33/196


34/196


south line. The angleofvariation,or the declination,isthe angle A between the line N S and the compass-needle.

42. Dip or Inclination. If a pieceof steel be care-ullybalanred upon a support, and then magnetized, it

Fig- 33-will be found that it will no longerbalance. The northpolewill dip or point downward. Fig. 33 shows whathappens to a needle when it is held in different positionsover a bar magnet. It

simplytakes the dire"5lionsof the lines of force asthey pass from the northto the south pole of themagnet. As the earth'slines of force pass in curvesfrom the north to the southmagnetic pole, you cansee why the magneticneedle dips, unless itssouth poleis made heavierthan itsnorth. Magnetic

needles are balanced after they are magnetized.Fig. 34 shows a simpleform of dippingneedle. These

are often used by geologistsnd miners. In the hands

Fig. 34-


35/196

ABOUT MAGNBTS AND MAGNETISM. 3 1

of the prospe(5lor,he miner's compass, or dippingneedle,proves a serviceable guide to the discovery andlocation of magnetic iron ore. In this instrument themagnetic needle is carefullybalanced upon a horizontalaxis within a graduated circle,and in which the needlewill be found to assume a positionnclined to the horizon.This angleof deviation is called the inclination or dip,and varies in diflFerent latitudes,and even at differenttimes in the same place.

43. The Earth's Inductive Influence. The earth'smagnetism adls indudlivelyupon piecesof steel or ironupon its surface. If a pieceof steel or iron,like a stovepoker, for example, be held in a north and south linewith its north end dipping considerably,it will be in thebest positionfor the magnetism of the earth to adl uponit;that is,it will lie in the diredlion taken by the earth'slines of force. If the poker be struck two or three timeswith a hammer to shake up its molecules,we shall find,upon testingit,that it has become magnetized. By thismethod we can pound magnetism rightout of the air witha hammer. If the magnetized poker be held level,in aneast and west direAion, it will no longer be adled upon toadvantage by the indu"5live influence of the earth, andwe can easilyhammer the magnetism out of it again.(For experiments on magnets and magnetism see'^ Study,''Parti.)


36/196

CHAPTER III.

HOW EliECTBIOITT IS QENEBATED BT THE VOLTAICCELL.

44. Early Experiments. In 1786 Galvani, anItalian physician, made experiments to study the eflfe"5lofstatic eledlricityupon the nervous excitabilityof animals,and especially upon the frog. He found that eledfaic

machines were not

necessary to producemuscular contrac-ionsor kicks of the

frog'slegs, and thatthey could be pro-uced

when two dif-erentmetals, Fig.

35, like iron andcopper, for example,were placed in propercontadl with a nerveand a muscle andthen made to toucheach other. Galvanifirst thought that the

fi^oggenerated the ele"5tricitynstead of the metals.Volta proved that the eledlricity was caused by the

contadl of the metals. He used the condensing ele"5lro-scope as one means of proving that two dissimilar metalsbecome charged differentlywhen in contadl. Volta also

3a

Fig. 35.


37/196

HOW ELECTRICITY IS GENERATED. 33

carried out his belief by construdlingwhat is called a Voltaic Pile, Hethought that by making several pairsof metals so arranged that all thelittle currents would help each other,a strong current could be generated.Fig. 36 shows a pile,t being made byplacing pairof zinc and copper discsin conta"5l with one another, then lay-ng

on the copper disc a piece offlannel soaked in brine,then on top ofthis another pair,etc., etc. By con-nedling the first zinc and the lastcopper, quitea little current was pro-uced.

This was a start from whichhas been built our present knowledgeof ele"5lricity Stridllyspeaking,electricitys not generatedby com- ,binations of metals or by cells;theyreallykeep up a difference of potential,as will be seen.

45. The Simple Cell. It has been stated that twodifferent kinds of ele"5lrifications may be produced byfridlion;ne positive,he other negative. Either can beproduced,at will,by using proper mate-ials.

Fig. 37 shows asedlion of a simple cell;Fig.38 shows another view.Cu is a piece of copper,and Zn a piece of zinc.

Fig. 37. When they are placed in Fig. 38.

Fig. 36.


38/196

34 HOW ELECTRICITY IS GENERATED.

dilute sulphuricacid,it can be shown by delicate appa-atusthat they become charged diflFerently,ecause the

acid adls differentlypon the plates. They becomecharged by chemical adlion,and not by fridlion. Thezinc isgraduallydissolved,and it is this chemical burningof the zinc that furnishes energy for the ele"5lriccurrentin the simple cell. The ele"5lrification,r charge, onthe platestends to flow from the place of higher to theplaceof lower potential,ustas water tends to flow downhill. If a wire be joinedto the two metals, a constantcurrent of eledlricityill flow through it,because theacid continues to a"5lupon the plates. The simplecellis a single-fluidell,as but one liquidis used in its con-strudlion.

45a. Plates and Poles. The metal stripsused involtaic cells are called platesor elements. The one mostadled upon by the acid is called the positive(+) plate.In the simplecell the zinc is the -f plate,and the copperthe negative(") plate. The end of a wire attached tothe " plateis called the + pole,or eledlrode. Fig. 37shows the negative(" ) ele"ftrode as the end of the wireattached to the + plate.46. Direction of Current. In the cell the current

passes from the zinc to the copper; that is,from the posi-iveto the negativeplate,where bubbles of hydrogen

gas are deposited. In the wire connedlingthe plates,the current passes from the copper to the zinc plate. Inmost cells,carbon takes the place of copper. (See** Study, ^' "268.)

47. Local Currents; Amalgamation. Ordinaryzinc contains impuritiesuch as carbon, iron, etc., and


39/196


40/196

CHAPTER IV.

VABIOXJS VOLTAIC CELIiS.

49. Single-Fluid and Two-Fluid Cells. The sim-lecell (" 45) is a single-fluidcell. The liquid is called

the ele6lrolyte,nd this must a"5l upon one of the plates;that is,chemical adlion must take place in order to pro-uce

a current. The simple cell polarizes rapidly, sosomething must be used with the dilute sulphuric acid todestroy the hydrogen bubbles. This is done in thebichromate of potash cell.

In order to get complete depolarization that is, tokeep the carbon plate almost perfedlly free from hydro-en,

it is necessary to use two-fluid cells,or those towhich some solid depolarizer is added to the one fluid.

50. Open and Closed Circuit Cells. If we considera voltaic cell, the wires attached to it,and perhaps someinstrument through which the current passes, we have aneleBric circuit. When the current passes, the circuit isclosed

ybut when the wire is cut, or in any way discon-

nedled so that the current can not pass, the circuit isopen or broken, (See ** Study," " 266.)

Open Circuit Cells are those which can give momentarycurrents at intervals, such as are needed for bells, tele-hones,

etc. These must have plenty of time to rest, asthey polarizewhen the circuit is closed for a long time.The Leclanchk and dry cells are the most common opencircuit cells.

Closed Circuit Cells, For telegraph lines, motors, etc.,36


41/196

VARIOUS VOLTAIC CELLS. 37

where a current is needed for some time,the cell must beof such a nature that it will not polarizequickly;it mustgive a strong and constant current. The bichromate andgravitycellsare examples of this variety. (See * * Study, ' *"286.)

SI. Bichromate of Potash Cells are very useful forgenerallaboratorywork. They are especiallyseful foroperating indudlion coils,smallmotors, small incandescent lamps,for heating platinum wires, etc.These cells have an E.M.F. ofabout 2 volts. Dilute sulphuricacid is used as the excitingfluid,and in this is dissolved the bi-hromate

of potash which keepsthe hydrogen bubbles from thecarbon plate. (See '* ApparatusBook,'* " 26.) Zinc and carbonare used for the plates,the +pole being the wire attached tothe carbon.

^^Z' 39 shows one form of bi-hromatecell. It furnishes a largequantityof current,

and as the zinc can be raised from the fluid,it may bekept charged ready for use for many months, and can beset in adlion any time when requiredby lowering thezinc into the liquid. Two of these cells will burn a onecandle-powerminiature incandescent lamp several hours.The carbon is indestru"5lible.

Note. For various forms of home-made cells,see ** ApparatusBook," Chapter I.,and for battery fluids see Chapter II.


42/196

38 VARIOUS VOLTAIC CKI*I"S.

52. The Grenet Cell. Fig. 40 is another form ofbichromate cell. The carbon platesare left in the fluidconstantly. The zinc plateshould be raised when thecell is not in use, to keep itfrom being uselesslyissolved.

Fig. 40. Fig. 41.53. Plunge Batteries. Two or more cells are often

arranged so that their elements can be quicklyloweredinto the acid solution. Such a combination, Fig.41, iscalled a plunge battery.The binding-postsre so arrangedthat currents of different strengthscan be taken from thecombination. The two binding-postsn the rightof thebattery will give the current of one cell;the two binding-posts on the left of the battery will give the current oftwo cells,and the two end binding-postswill give thecurrent of all three cells. When not in use the elementsmust always be hung on the hooks and kept out of thesolution.


43/196

VARIOUS VOI.TAIC CKLI^. 39

54. Large Plunge Batteries,Fig. 42, are arrangedwith a winch and a bar above the cells;these afford aready and convenient means of liftingr loweringtheelements and avoiding waste. In the battery shown,Fig. 42, the zincs are 4x6 inches; the carbons have the

Fig. 42.same dimensions, but there are two carbon platesto eachzinc,thus givingdouble the carbon surface.

55. The Fuller Cell, Fig. 43, is another type ofbichromate cell,used largelyfor long-distanceelephoneservice, for telephone exchangeand switch ser\ace, for runningsmall motors, etc. It consists of aglass jar, a carbon plate, withproper connedlions,a clay porouscup, containingthe zinc,which ismade in the form of a cone. Alittle mercury is placed in theporous cup to keep the zinc wellamalgamiated. Either bichromateof potashor bichromate of soda canbe used as a depolarizer. Fig. 43.


44/196

40 VARIOUS VOLTAIC CEI.LS.

Fig. 44.

56. The Gravity Cell,sometimes called the bluestoneor crowfootcell,is used largelyfor telegraph,police,nd

^*-^n^ fire-alarm signal service, laboratorj'^and experimentalwork, or whenevera closed circuit cell is required. TheE.M.F. is about one volt. This is amodified form of the Daniell cell. Fig.44 shows a home-made gravitycell.

A copper plate is placed at thebottom of the glassjar,and upon thisrests a solution of copper sulphate(bluestone). The zinc plateis sup-orted

about four inches above thecopper, and is surrounded by a solu-ion

of zinc sulphate which floats upon the top of theblue solution. An insulated wire reaches from the copperto the top of the cell and formsthe positivepole. (See * *Appara-us

Book," " II to 15, for home-adegravity cell,its regulation,

etc. For experiments with two-fluid Daniell cell,ee ** Study,"E^tp.113, " 281 to 286.)56a. Bunsen Cells,Fig.45, are

used for motors, small incandescentlamps, etc. A carbon rod is in-losed

in a porous cup, on theoutside of which is a cylinder ofzinc that stands in dilute sul-huric

acid,the carbon being innitric acid. Fig. 45.

Digitizedy


45/196

VARIOUS VOLTAIC CELLS. 41

57. The Leclanche Cell is an open circuit cell. Salammoniac is used as the excitingfluid,carbon and zincbeing used for plates. Manganese dioxide is used as thedepolarizer;his surrounds thecarbon plate,the two beingeither packed together in a

Fig- 46. Fig. 47.porous cup or held togetherin the form of cakes. Theporous cup, or pressedcake, stands in the excitingfluid.The E. M. F. is about 1.5 volts.

fTRAOE MAR**)

^HE MESCO,

Fig. 48.


46/196

42 VARIOUS VOI.TAIC CELI^.

Fig. 49.

Fig.46 shows a form with porous cup. The binding-post at the top of the carbon plate forms the + elec-rode,

the currentleavingthe cell at thispoint.

The Gonda PrismCell (Fig. 47), is aform of Leclanch6 inwhich the depolarizeris in the form of acake.

58. Dry Cells areopen circuit cells,ndcan be carried about,although they are

moist inside. The -h pole is the end of the carbon plate.Zinc is used as the outside case and -h plate. Fig.48shows the ordinary forms.

Fig. 49 shows a number ofdry cells arranged in a boxwith switch in front,so that thecurrent can be regulatedat will.

59. The Edison-LelandeCells, Fig. 50, are made inseveral sizes and types. Zincand copper oxide, which ispressed into plates,form theelements. The exciting fluidconsists of a 25 per cent, solu-ion

of caustic potash in water.They are designed for both open and closed circuit work.

Fig. 50.


47/196


48/196


49/196

PUSH-BUTTONS, SWITCHES AND BINDING-POSTS. 45

Fig. 55.

** Apparatus Book/'Chapter III., for home-ade

push-buttons.)62. Switches have a

movable bar or plug ofmetal, moving on a pivot,to make or break a circuit,or transfer a current fromone condudlor to another.

Fig. 56 shows a singlepoint switch. The cur-ent

enteringthe pivotedarm can go no fartherwhen the switch is open,as shown. To close thecircuit,the arm is pushedover until it presses down upon the contadl-point.Forneatness, both wires are joinedto the under side of theswitch or to binding-posts.Fig. 57 shows a knife switch. Copper blades are

presseddown between copper spring clipsto close thecircuit. The handle ismade of insulatinga-erial.

Pole - ch d ng ingswitches^Fig. 58, areused for changing orreversing the poles ofbatteries,etc.

Fig- 59 shows ahome-made switch, use-


50/196

46 PUSH-BUTTONS, SWITCHES AND BINDING-POSTS.

Fig. 57.current will beobliged to passthrough all thecoils A, B, etc.,before it can passout at Y. If Ebe moved to 3,coils A and B willbe cut out of the circuit,thus decreasing the resist-nce

to the current on itsway from X to Y. Cur-ent

regulators are madeupon this principle.(See'* Apparatus Book," Chap-er

IV., for home-madeswitches. )

Switchboards are madecontaining from two orthree to hundreds of

ful in connedlion withresistance coils. By join-ng

the ends of the coilsA, B, C, D, with thecontadl-pointsi, 2, 3,etc. , more or less resist-nce

can be easilythrowniu by simply swingingthe lever E around tothe left or right. IfEbe turnnd to i, the

yn :b c J3Fig 59-


51/196

PUSH-BUTTONS, SWITCHES AND BINDING-POSTS. 47

switches,and are used in telegraphand telephonework,in ele"5lric light stations,etc., etc. (See Chapter onCentral Stations.) Fig. 60 shows a switch used for in-andescent

lightingcurrents.63. Binding-

Posts are used tomake connedlionsbetween two piecesof apparatus, be-ween

two or morewires, between awire and any appa-atus,

etc., etc.They allow thewires to be quickly Fig, 60.fastened or unfast-ned

to the apparatus. A large part of the apparatusshown in this book has binding-poststtached. Fig. 61

Fig. 61.

shows a few of the common forms used. (See * ' Appa-atusBook,'* Chapter V., for home-made binding-posts.)


52/196

CHAPTER VI.

UNITS AND APPABATUS FOR ELECTBICAL MEASTTBE-MENTS.

64. Electrical Units. In order to measure eledlricityfor experimental or commercial purposes, standards orunits are just as necessary as the inch or foot for measur-ng

distances.65. Potential ; Electromotive Force. If water in

a tall tank be allowed to squirt from two holes, one nearthe bottom, the other near the top, it is evident that theforce of the water that comes from the hole at the bottomwill be the greater. The pressure at the bottom is greaterthan that near the top, because the '' head " is greater.

When a spark of static eledlricityjumps a long distance,we say that the charge has a high potential; that is,ithas a high eledlrical pressure. Potential, for eledlricity,means the same as pressure, for water. The greater thepotential, or ele^lromotive force (E.M.F.) of a cell,thegreater its power to push a current through wires. (See** Study," " 296 to 305, with experiments.)

66. Unit of E.M.F.; the Volt." In speaking ofwater, we say that its pressure is so many pounds to thesquare inch, or that it has a fall,or head, of so manyfeet. We speak of a current as having so many volts;for example, we say that a wire is carrying a no- voltcurrent. The volt is the unit of E.M.F. An ordinarygravity cell has an E.M.F. of about one volt. Thisname was given in honor of Volta.

48


53/196

APPARATUS FOR EI"ECTRICAI" MEASUREMENTS. 49

Fig. 62.

67. Measurement of Electromotive Force. Thereare several ways by which the E.M.F. of a cell,forexample, can bemeasured. It isusually measuredrelativelyy com-arison

with theE. M. F. of somestandard cell

. (See**Study," Exp.140, for measuringthe E. M. F. of acell by comparisonwith the two-fluid cell.)

Voltmeters are instruments by means of which E.M.F.can be read on a printedscale. They are a varietyofgalvanometer, and are made with coils of such highresistance,compared with the resistance of a cell or

dynamo, that the E. M. F.can be read diredl. Thereason for this will be seenby referringo Ohm's lawC' Study," "356); theresistance is so great thatthe strength of the cur-ent

depends entirelyuponthe E. M. F.

Voltmeters measure^^* ^' eledlrical pressure just as

steam gauges measure the pressure of steam. Fig. 62shows one form of voltmeter. Fig. 63 shows a voltmeter


54/196


55/196


56/196

52 APPARATUS FOR EI^ECTRICAI, MKASURKMKlfTS.

The greater the resistance the weaker the current at theend of its journey.

72. Unit of Current Strength ; The Ampere. Acurrent having an E. M. F. of one volt pushing its way-through a resistance of one ohm^ would have a unit ofstrength,called one ampere. This current, one amperestrong, would deposit,nder proper conditions,.0003277

gramme of copper inone second from a solu-ion

of copper sulphate.73. Measurement

of Current Strength.A magnetic needle isdefledled when a cur-ent

passes around it,as in instruments likethe galvanometer. Thegalvanoscopemerely in-icates

the presence ofa current. Galvanom-ters

measure thestrength of a current,

and they are made in many forms, depending upon thenature and strength of the currents to be measured.Galvanometers are standardized,or calibrated,by specialmeasurements, or by comparison with some standard in-trument,

so that when the defledlion is a certain numberof degrees,the current passing through it is known tobe of a certain strength.

Fig.67 shows an astatic galvanometer. Fig. 68 showsa tangentgalvanometer,in which the strength of the cur-

Digitizedy


57/196

APPARATUS FOR ELECTRICAI, MEASUREMENTS. 53

Fig. 68.

rent is proportionalo the tangent of the angle of deflec-ion.Fig.69 shows a D^ Arsonval galvanometer in which

a coil of wire is suspendedbetween thepoles of a permanent horseshoe mag-et.

The lines of force are concen-ratedby the iron core of the coil.

The two thin suspendingwires conveythe current to the coil. A ray of lightis refle"5ledfrom the vsmall mirror andadls as a pointeras in other forms ofrefledlingalvanometers.

74. The Ammeter, Fig. 70, is aform of galvanometer in which the strength of a current,in amperes, can be read. In these the strengthof currentis proportionalo the angular defledlions. The coils are

made with a small resist-nce,so* that the current

will not be greatlyreducedin strength in passingthrough them.

75. Voltametersmeasure the strengthof acurrent by chemical means,the quantityof metal de-osited

or gas generatedbeing proportionalto thetime that the current flowsand to its strength. Inthe water voltameter,Fig.71, the hydrogen and

Fig. 69. oxygen produced in a


58/196

54 APPARATUS FOR KI.KCTRICAI. MEASUREMENTS.

given time aremeasured. (See'Study,'' ChapterXXI.)

The copper voltam-termeasures the

amount of copperdepositedin a giventime by the current.Fig. 72 shows oneform. The coppercathode is weighed

before and after the current flows. The weight ofcopper depositedand the time taken are used to calculatethe current strength.76. Unit of Quantity;The Coulomb is the quantity

of eledlricityiven,in one second,by a current having a

Fig. 70.

Fig. 71.


59/196

APPARATUS FOR ELECTRICAL MEASUREMENTS. 55

Strength of oneampere. Time isan important ele-ent

in consider-ngthe work a cur-entcan do.

77. ElectricalHorse-power;The Watt is theunit of eledlricalpower. A current ^*^* 72-having the strength of one ampere, and an E. M. F. ofone volt has a unit of power. 746 watts make one elec-rical

horse-power. Watts = amperes X volts. Fig. 73shows a diredl reading wattmeter based on the inter-ational

volt and ampere. They save taking simulta-eousammeter and voltmeter readings,which are other-ise

necessary toget the produdlof volts and am-eres,

and are alsoused on altema-ting currentmeasurements.

There are alsoforms of watt-eters,

Fig. 74,in which the wattsare read from

Fig. 73. dials like those onan ordinary gas-meter, the records beingpermanent.


60/196

56 APPARATUS FOR EI.KCTRICAI. MEASUREMENTS.

Fig. 75 shows a voltmeter V, and ammeter A, so placedin the circuit that readingscan be taken. D representsa dynamo. A is placedso that the whole current passes

through it,while V is placedbetween the main wires tomeasure the difference inpotential.The produdlof thetwo readings in volts andamperes gives the number ofwatts.78. Chemical Meters also

measure the quantityof cur-entthat is used; for example,

one may be placedin the cellarto measure the quantity of current used to lightthehouse.Fig. 76 shows a chemical meter, a part of the current

passingthrough a jar containingzinc platesand a solu-ionof zinc sulphate. Metallic zinc is dissolved from

one plateand depositedupon the other. The increase inweight shows the amount of chemical adlion which is

Fig. 74.

Fig. 75.

proportionalo the ampere hours. " Knowing the relationbetween the quantityof current that can pass throughthe solution to that which can pass through the meter by


61/196

APPARATUS FOR ELKCTRICAI. MEASUREMENTS. 57

another condudlor, a calculation can be made which willgive the current used. A lamp is so arranged that it

Fig. 76.

automaticallylightsbefore the meter gets to the freezing-point;this warms it up to the proper temperature, atwhich pointthe lightgoes out again.


62/196

CHAPTER VII.CHEMICAL EFFECTS OF THE ELECTRIC CUBEENT.

79. Electrolysis. It has been seen that in the vol-aiccell dedlricity is generated by chemical adlion. Sul-huricacid adls upon zinc and dissolves it in the cell,

hydrogen is produced, etc. When this process is re-ersed,that is, when the eledlric current is passed

Fig. 77.through some solutions,they are decomposed, or brokenup into their constituents. This process is called eleHrol-ysisy and the compound decomposed is the electrolyte,(See '* Study," " 369, etc., with experiments.)Fig. 77 shows how water can be decomposed into its

two constituents, hydrogen and oxygen, there beingtwice as much hydrogen formed as oxygen.

Fig. 78 shows a glassjar in which are placed two metal58


63/196


64/196

CHAPTER VIII.

HOW ELECTROPLATING AND ELECTBOTYPING ABE DONE.

8i. Electricity and Chemical Action. We havejust seen, Chapter VII., that the eledlric current has thepower to decompose certain compounds when they are insolution. By choosing the right solutions, then, we shallbe able to get copper, silver, and other metals set free byelec5lrolysis.

82. Electroplating consists in coating substanceswith metal with the aid of the eleAric current. If wewish to eledlroplate a piece of metal with copper, forexample, we can use the arrangement shown in Fig. 78,in which C is the cathode plate to be covered, and A is acopper plate. The two are in a solution of copper sul-hate,

and, as explained in " 79, the solution will bedecomposed. Copper will be deposited upon C, and theSO4 part of the solution will go to the anode A, which itwill attack and gradually dissolve. The SO^ , adling uponthe copper anode, makes Cu SO^ again, and this keeps thesolution at a uniform strength. The amount of copperdissolved from the copper anode equals, nearly, theamount deposited upon the cathode. The metal is carriedin the direction of the current.

" If we wish to plate something with silver or gold, itwill be necessary to use a solution of silver or gold forthe eledlrolyte, a plate of metallic silver or gold beingused for the anode, as the case may be.

60


65/196


66/196

62 EI.ECTROPI.ATING AND KI.KCTROTYPING.

making a copy in metal of a wood-cut, page of type, etc.A mould or impressionof the type or coin is firstmade in wax, or other suitable material. These mouldsare, of course, the reverse of the original,nd as they donot condudl eleAricity,have to be coated with graphite.This thin coating lines the mould with condudling

.material so that the current can get to every part of themould. These are then hung upon the cathode in a bathof copper sulphate as described in "82. The eledlriccurrent which passes through the vat depositsa thinlayerof metallic copper next to the graphite. When thiscopper gets thick enough, the wax is melted away fromit,leavinga thin shell of copper, the side next to thegraphitebeing exac5llyalike in shape to the type, butmade of

copper.These thin

coppersheets are too thin

to stand the pressure necessary on printingpresses, sothey are strengthenedby backing them with soft metalwhich fillsevery crevice,making solid platesabout % in.thick. These plates or eleSlrotypesre used to printfrom, the originaltype beingused to set up another page.


67/196

CHAPTER IX.

THE STORAGE BATTEB7, AND HOW IT WOBKS.

84. Polarization. It has been stated that a simplecell polarizesrapidly on account of hydrogen bubbles thatform upon the copper plate. They tend to send a currentin the opposite diredlion to that of the main current,which is thereby weakened.

85. Electromotive Force of Polarization. It hasbeen shown, Fig. 71, that water can be decomposed by

the eledlric current.Hydrogen andoxygen have astrong attrac5lion orchemical affinityforeach other, or theywould not unite toform water. Thisattradlion has to beovercome before the

water can be decomposed. As soon as the decomposingcurrent ceases to flow, the gases formed try to rush to-ether

again; in fact,if the water voltameter be discon-neAed from the cells and conne"5led with a galvanoscope,the presence of a current will be shown. This voltam-ter

will give a current with an E. M. F. of nearly 1.5volts; so it is evident that we must have a current with ahigher voltage than this to decompose water. This

63


68/196

64 I'HE STORAGE BATTERY, AND HOW IT WORKS.

E. M. F., due to polarization,s called the E. M. F. ofpolarization.

86. Secondary or Storage Batteries, also calledaccumulators, do not reallystore eledlricity.They mustbe charged by a current before they can give out anyeledlricity.Chemical changes are produced in the stor-ge

cells by the charging current justas they are in vol-ameters,eledlroplatingolutions,etc. ; so it is potential

chemical energy that is reallystored. When the new prod-udls are allowed to go backto their originalstate, byjoining the eledlrodes of thecharged cell, current is pro-uced.

Fig. 8 1 shows two leadplates, and B, immersed indilute sulphuric acid, andconnedted with two ordinarycells. A strong current willpass through the liquidbe-ween

A and B at first,ut itwill quickly become weaker, as chemical changes takeplace in the liquid. This may be shown by a galva-ometer

put in the circuit before beginning the experi-ent.By disconnedlingthe wires from the cells and

joiningthem to the galvanometer, it will be shown thata current comes from the lead plates.This arrangementmay be called a simplestorage cell. Regular storage cellsare charged with the current from a dynamo. (See"Study," Exp. 151.)

Fig. 82.


69/196

THE STORAGE BATTERY, AND HOW IT WORKS. 65The first storage cells were made of plainlead plates,

rolled up in such a way that they were close to eachother, but did not touch. These were placed in dilutesulphuricacid. They were charged in alternate direc-ions

several times,until the lead became properly adledupon, at which time the cell would furnish a current.

A great improvement was made in 1 88 1, by Faure, whocoated the plateswith red lead.

The method now generallypracticedis to cast a frame

Fig. 83.of lead,with raised right-angledribs on each side,thusforming littledepressedsquares, or to punch a lead platefull of holes,which squares or holes are then filledwitha pasty mixture of red oxide of lead in positiveplates,and with lithargein negatives. In a form called thechloride battery, instead of cementing lead oxide pasteinto or against a lead framing in order to obtain thenecessary adlive material, the latter is obtained by astridllychemical process.


70/196


71/196


72/196


73/196

HOW EI.ECTRICITY IS GENERATED BY HEAT. 69to antimony across the joint. By coolingthe junAurebelow the temperature of the rest of the circuit, currentwill be produced in the oppositediredlion to the above.The energy of the current iskept up by the heat absorbed,just as it is kept up by chemical adlion in the voltaiccell.89. Peltier Effect. If an eledlric current be passed

through pairsof metals, the parts at the jundlionbecomeslightlywarmer or cooler than before,depending uponthe dire"5lion of the current. This adlion is reallythereverse of that in which currents are produced by heat.

90. Thermopiles. As the E.M.F. of the currentproduced by a singlepairof metals is very small,several

Fig. 87.

pairs are usually joined in series,o that the diflFerentcurrents will help each other by flowingin the same direc-ion.

Such combinations are called thermoeledlric piles,or simply thermopiles.

Fig 87 shows such an arrangement, in which a largenumber of elements are placed in a small space. Thejundturesare so arranged that the alternate ones cometogetherat one side.

Fig. 88 shows a thermopileconnedled with a galvanom-


74/196

70 HOW EI.KCTRICITY IS GENERATED BY HEAT.

eter. The heat of a match, or the cold of a pieceof ice,will produce a current, even if held at some distance from

Fig. 88.

the thermopile. The galvanometer should be a short-coil astatic one. (See ''Study," Chapter XXIV., forexperimentsand home-made thermopile.)


75/196


76/196

72 MAGNETIC EFFECTS OF EI.ECTRIC CURRENT.

the E and W line,because the earth is at the same timepullingits N poletoward the N.Fig. 90 shows a bent wire through which a current

passes from C to Z. If you look along the wire from Ctoward the pointsA and B, you will see that under thewire the lines of force pass to the left. I^ookingalongthe wire from Z toward D you will see that the lines offorce pass oppositeto the above, as the current comestoward you. This is learned by experiment. (See''Study,''Exp. 152, " 385, etc.)

Rule. Hold the right hand with the thumb extended(Fig.89) and with the fingerspointingin the diredlion of

Fig. 91.the current, the palm being toward the needle and onthe oppositeside of the wire from the needle. The north-seeking pole will then be defledled in the diredtion inwhich the thumb points.

93. Current Detectors. As there is a magneticfieldabout a wire when a current passes through it,and as themagnetic needle is affedled,e have a means of dete"5lingthe presence of a current. When the current is strong itis simply necessary to let it pass once over or under aneedle; when it is weak, the wire must pass severaltimes above and below the needle, Fig. 91, to give theneedle motion. (See "Apparatus Book/* ChapterXIII.,for home-made dete"5lors.)


77/196

MAGNETIC EFFECTS OF EI.ECTRIC CURRENT. 73

Fig. 92.

94. Astatic Needles and Detectors. By arrangingtwo magnetized needles with their poles oppositeeachother, Fig. 92, an astatic needle is formed. The point-ng-power

is almost nothing, although their magneticfields are retained. This com-bin^on is used to dete"5l feeblecurrents. In the ordinary de-tedlor,the tendency of the needleto pointto the N and S has to beovercome by the magnetic fieldabout the coil before the needlecan be moved; but in the astatic dete^or and galvano-scopethis pointing-poweris done away with. Fig. 93shows a simple astatic galvanoscope. Fig. 67 shows anastatic galvanometer for measuring weak currents.95. Polarityof Coils. When a current of eledtricity

passes through a coil of wire, thecoil adls very much like a magnet,although no iron enters into itsconstrudtion. The coil becomesmagnetized by the eledlric cur-ent,

lines of force pass from itinto the air,etc. Fig. 94 shows acoil connedted to copper and zincplates,o arranged with cork thatthe whole can float in a dish ofdilute sulphuric acid. The cur-ent

passes as shown by thearrows, and when the N pole of a magnet is broughtnear the right-handend, there is a repulsion,showingthat that end of the coil has a N pole.

Fig. 93.


78/196

74 MAGNETIC BFFBCTS OF BI"BCTRIC CURRENT.

Ride, When you face the right-handend of the coil,the current is seen to pass around it in an anti-clockwisediredlion;this produces a N pole. When the currentpasses in a clockwise direction a S poleis produced.96. Electromagnets.

A coil of wire has a strongerfield than a straight wirecarrying the same current,because each turn adds itsfield to the fields of theother turns. By having thecentral part of the coilmade of iron,or by havingthe coil of insulated wirewound upon an iron core^ ^c*the strength of the mag-etic

field of the coil isgreatlyincreased. .

Lines of force do notpass as readilythrough air Fig. 94.as through iron; in fact,lines of force will go out of their way to go throughiron. With a coil of wire the lines of force pass from itsN pole through the air on all sides of the coil to its Spole;they then pass through the inside of the coil andthrough the air back to the N pole. When the resist-nce

to their passage through the coil is decreased by thecore, the magnetic field is greatlystrengthened,and wehave an eleSlromagnet.

The coil of wire temporarilymagnetizes the iron core;it can permanently magnetize a piece of steel used as


79/196


80/196


81/196


82/196


83/196

EI.BCTRICITY GENERATED BY INDUCTION. 79

and down inside of the large one, induced currents aregenerated,firstin one diredlion and then in the opposite.We have here two entirelyseparate circuits,n no wayconnedled. The primary current comes from the cell,while the secondaryurrent isan induced one. By placinga core in the small coil of Fig. 10 1, the induced currentwill be greatlystrengthened.

It is not necessary to have the two coils so that one orboth of them can move. They may be wound on thesame core, or otherwise arranged as in the indudlion coil.(See ^' Study,** Chapter XXV., for experiments oninduced currents.)


84/196

CHAPTER XIII.HOW THE INDXJOnON COIIi WOBKS.

103. The Coils. We saw, " 102, that an inducedcurrent was generated when a current-carrying coil,Fig.1 01, was thrust into another coil connedled with a galva-ometer.

The galvanometer was used merely to show thepresence of the current. The primary coil is the oneconnedled with the cell; the other one is called \}ci!tecond-ry

coil.When a current suddenly begins to flow through a coil.

Fig. 102.

the effedl upon a neighboring coil is the same as that pro-ucedby suddenly bringing a magnet near it; and when

the current stops, the opposite effedl is produced. It isevident, then, that we can keep the small coil of Fig. loiwith its core inside of the large coil,and generate inducedcurrents by merely making and breaking the primarycircuit.

80


85/196


86/196

82 HOW THE INDUCTION COII. WORKS.

the circuit being closed, the operation is rapidlyrepeated.

A condenser is usually connedled to commercial forms.It is placedunder the wood- work and decreases sparkingat the interrupter.(See ''Apparatus Book/' ChapterXI., for home-made indudlion coils.)Fig. 104 shows one form of coil. The batterywires

are joinedto the binding-postst the left. The secondarycoil ends in two rods, and the spark jumps from one to

Fig. 104.

the other. The interrupterand a switch are shown atthe left.Fig. 105 shows a small coil for medical purposes. A

dry cell is placed under the coil and all is included ina neat box. The handles form the terminals of thesecondary coil.

105. The Currents. It should be noted that thecurrent from the cell does not get into the secondary coil.The coils are thoroughlyinsulated from each other. Thesecondarycurrent is an induced one, itsvoltagedependingupon the relative number of turns of wire there are


87/196


88/196

CHAPTER XIV.

THE BLECTBIO TELEGRAPH, AND HOW IT SBin"SMESSAGES.

107. The Complete Telegraph Line consists ofseveral instruments, switches, etc., etc., but its essentialparts are: The Line, or wire, which connedls the diflfer-ent stations; the Transmitter or Key ; the Receiver orSounder, and the Battery or Dynamo.

108. The Line is made of strong copper, iron, or softsteel wire. To keep the current in the line it is insu-ated,

generally upon poles, by glass insulators. Forvery short lines two wirescan be used, the line wireand the return; but for longlines the earth is used as areturn, a wire from each

end being joined to large metal plates sunk in the earth.109. Telegraph Keys are merely instruments by

which the circuit can be conveniently and rapidly openedor closed at the will of the operator. An ordinary push-utton

may be used to turn the current off and on, but itis not so convenient as a key.

Fig. 106 shows a side view of a simple key which C".nbe put anywhere in the circuit, one end of the cut wirebeing attached to X and the other to Y. By moving thelever C up and down according to a previously arrangedset of signals, a current will be allowed to pass to a dis-

8i

Fig. 106.


89/196


90/196

86 THK ELECTRIC TELEGRAPH.

properlyheld. In regular instruments a click is alsomade when the armature springsback again.

The time between the two clicks can be short or long,to representdots or dashes^which, togetherwith spaces^represent letters. (ForTelegraph Alphabet andcomplete diredlions forhome-made keys, sounders,etc., see *' ApparatusBook,*' Chapter XIV.)

Fig. 109 shows a form ofhome-made sounder. Fig.

1 10 shows one form of telegraphsounder. Over the polesof the horseshoe eledlromagnetis an armature fixed to ametal bar that can rock up and down. The instant the

Fig. 109.

current passes through the coils the armature comesdown until a stop-screw strikes firmlyupon the metalframe, making the down click. As soon as the distant


91/196

THE EtECTRIC TEI.EGRAPH. 87key is raised,the armature is firmly pulledback andanother click is made. The two clicks diflferin sound,and can be readilyrecognized by the operator.

111. Connections for Simple Line. Fig.1 11 showscomplete connexions for a home-made telegraphline.The capitaletters are used for the rightside,R, andsmall letters for the left side,L. Gravity cells,B and b,are used. The sounders,S and s, and the keys,K and k,are shown by a top view. The broad black lines of S ands represent the armatures which are diredllyover theeledlromagnets.The keys have switches,E and e.

The two stations,R and L, may be in the same room,or in different houses.The return wire, R W,passes from the copperof b to the zinc of B.This isimportant,as thecells must help eachother; that is,they arein series. The line wire,It W, passes from onestation to the other,and the return may be through thewire, R W, or through the earth; but for short lines awire is best.

1 12, Operation of Simple Line. Suppose two boys,R (right)nd 1, (left)ave a line. Fig. 11 1 shows thatR's switch, E, is open, while e is closed. The entirecircuit,hen, is broken at but one point. As soon as Rpresses his key, the circuit is closed,and the current fromboth cells rushes around from B, through K, S, lyW, s,k, b, R W, and back to B. This makes the armatures of


92/196


93/196

THE EI*KCTRIC TEI*EGRAPH. 89its armature. L W and R W represent the line andreturn wires. R A will vibrate toward E every time Kis pressed,and close the local circuit,hich includes alocal battery,ly B, and a sounder. It is evident that assoon as K is pressed the sounder will work with a goodstrong click,as the local battery can be made as strongas desired.

Fig. 113 shows a regular instrument which opens andcloses the local circuit at the top of the armature.

114. Ink Writing Registers are frequentlyused

Fig. 113.

instead of sounders. Fig. 114 shows a writingregisterthat starts itself promptly at the opening of the circuit,and stops automaticallyas soon as the circuit returns toits normal condition. A stripof narrow paper is slowlypulledfrom the reel by the machine, a mark being madeupon it every time the armature of an inclosed ele"Slro-magnet is attracted. When the circuit is simply closedfor an instant,a short line,representing dot^is made.Registersare built both singlepen and double pen.

In the latter case, as the record of one wire is made with


94/196

90 THE EI^ECTRIC TELEGRAPH.

a fine pen, and the other with a coarse pen, they canalways be identified. The record beingblocked out uponwhite tape in solid black color,in a series of clean-cut

Fig. 114.

dots and dashes, it can be read at a glance,and as it isindelible,t may be read years afterward. Registersremade for local circuits,or use in connedlion with relays,or for diredl use on main lines,as is usually desirable infire-alarm circuits.


95/196


96/196

92 THE KI^KCTRIC BKI.I. AND SOMB OP ITS USES.

Fig. 117.

the circuit is broken ; this standsthe sparkingwhen the armaturevibrates.

116. Electric Bells may beillustrated by referringto Fig.116, which shows a circuit sim-lar

to that described in " 115,but which also contains a keyK, in the circuit. This allowsthe circuit to be opened andclosed at a distance from thevibrating armature. The cir-uit

must not be broken at twoplacesat the same time, so wiresshould touch at the end of I be-ore

pressingK. Upon pressingK the armature I willvibrate rapidly. By placinga small bell near the end ofthe vibratingarmature, sothat it will be struck by I ateach vibration, we shouldhave a simple elecftric bell.This form of eledlric bell iscalled a trembling bell, onaccount of its vibratingar-ature.

Fig. 117 shows a form oftrembling bell with coverremoved. Fig. 118 shows asingle-strokeell, used forfire-alarms and other signalwork. In this the armatureis attradled but once each time the current passes. As

Fig. 118.


97/196

THB ElyECTRIC BEl.Iy AND SOME OF ITS USES. 93

many taps of the bell can begiven as desired by pressing

.the push-button. Fig. 119shows a gong for railwaycrossings,signals,etc. Fig.120 shows a circuit includingcell,push-button, and bell,with extra wire for lengthen-ng

the line.Electro- Mechanical Gongs are

used to give loud signalsforspecial purposes. The me-hanical

device is started bythe eleAric current when thearmature of the elecflromagnetis attracted. Springs,weights, etc., are used as thepower. Fig. 121 shows a small bell of this kind.

117. Magneto Testing Bells,Fig. 122, are reallysmall hand-power dynamos. The armature ismade to revolve between the polesof strong permanent magnets, andit is so wound that it gives a cur-ent

with a largeE. M. F., so thatit can ringthrough the large re-istance

of a long line to test it.Magneto Signal Bells

^Fig. 123,

are used as generator and bell inFig. 120. connedlion with telephones. The

generator, used to ring a bell at adistant station, stands at the bottom of the box. The


98/196

94 ^HB ElvECTRIC BEI*I" AND SOME OF ITS USES.

bell is fastened to the lid,and receives current from adistant bell.

Xi8. Electric Buzzers have the same general con-struiftionas ele"ftricbells;in facft,ou will have a buzzer

Fig. 121. Fig. 122.

Fig. 124.by removing the bell from an ordinary eledlric bell.Buzzers are used in placeswhere the loud sound of a bellwould be objedlionable.Fig. 124 shows the usual formof buzzers,the cover being removed.


99/196

CHAPTER XVI.

THE TELEPHONE, AND HOW IT TBANSllOTS SPEECH.

119. The Telephone is an instrument for reproduc-ngsounds at a distance, and eledlricityis the agent by

which this is generally accomplished. The part spokento is called the transmitter, and the part which gives

Fig. 125. Fig. 126.

sound out again is called the receiver, Sound itself doesnot pass over the line. While the same apparatus can beused for both transmitter and receiver,they are generallydifferent in construdlion to get the best results.

Fig. 127.

120. The Bell or Magneto-transmitter generatesits own current, and is,stricftlypeaking, a dynamo that

95


100/196

96 HOW THE TEI*EPHONE TRANSMITS SPEECH.is run by the voice. It depends upon induction for itsadlion.Fig. 125 shows a coil of wire, H, with soft iron core,

the ends of the wires being conneAed to a delicate gal-vanoscope. If one poleof the magnet H M be suddenly

Fig. 128.

moved up and down near the core, an alternatingurrentwill be generatedin the coil,the circuit beingcompletedthrough the galvanoscope. As H M approachesthe corethe current will flow in one direcflion,nd as HM iswithdrawn it will pass in the oppositedirection. Thecombination makes a miniature alternatingdynamo.

If we imagine the soft iron core of H, Fig. 125, takenout, and one pole of H M, or preferablythat of a barmagnet stuck through the coil,a feeble current will also

be produced by moving the soft ironback and forth near the magnet's pole.^This is reallywhat is done in theBell transmitter,soft iron in the shapeof a thin disc (D, Fig. 126) beingmade to vibrate by the voice immedi-tely

in front of a coil having a per-anentmagnet for a core. The disc,

or diaphragm, as it is called,is fixednear, but it does not touch, the magnet. It is under aconstant strain,being attradled by the magnet, so its

Fig- 129.


101/196

HOW THE TKI.EPHONB TRANSMITS SPEECH. 97

slightestovement changes the strengthof the magneticfield,causingmore or less lines of force to shoot throughthe turns of the coil and induce a current. The coil con-

IF^ /A,"IFig. 130.

sists of many turns of fine,insulated wire. The currentgeneratedis an alternatingne, and althoughexceedinglysmall can force its way through a long lengthof wire.Fig. 127 shows a section of a regular transmitter,and

Fig. 128 a form of compound magnet frequentlysed inthe transmitter.Fig. 129 showsa transmitterwith cords whichcontain flexiblewires.

121. The Re-eiver,for short

lines,may havethe same con-truction

as theBell transmitter.Fig. 130 showsa diagramof twoBell receivers,ither being used as the transmitter andthe other as the receiver. As the alternatingurrentgoes to the distant receiver,it flies through the coilfirst in one diredlion and then in the other. This al-

Fig. 131.


102/196

98 HOW THE TEI^EPHONK TRANSMITS SPEECH.

temately strengthensand weakens the magnetic fieldnear the diaphragm, causingit to vibrate back and forthas the magnet pullsmore or less. The receiver dia-hragm

repeats the vibrations in the transmitter.

leEZZ^^^IFig. 132.

Nothing but the induced eledlric current passes over thewires.122. The Microphone. If a current of eledlricitye

allowed to pass through a circuit like that shown in Fig.131, which includes a battery, a Bell receiver,and amicrophone,any slightsound near the microphone willbe greatlymagnified in the receiver. The microphoneconsists of piecesof carbon so fixed that they form loosecontadls. Any slightovement of the carbon causes theresistance to the current to be greatlychanged. Therapidlyvarying resistance allows more or less current topass, the result being that this pulsatingcurrent causesthe diaphragm to vibrate. The diaphragm has a con-tantlyvaryingpullupon it when the carbons are in any

C ^Fig. 133.

way disturbed by the voice,or by the tickingof a watch,etc. This principleas been made use of in carbontransmitters,hich are made in a largevarietyof forms.

123. The Carbon Transmitter does not, in itself,


103/196


104/196

lOO HOW THE TKI^EPHONB TRANSMITS SPEECH.

Fig. 135.

is used to transform the batterycurrent, that flows through thecarbon transmitter and primarycoil,into a current with a highE. M. F. The battery currentin the primary coil is undula-ing,

but always passes in thesame direction, making themagnetic field around the coreweaker and stronger. Thiscauses an alternating currentin the secondary coil and mainline. In Fig. 133 P and S rep-esent

the primary and second-rycoils. P is joinedin serieswith a cell and carbon trans-itter;

S is joinedto the distantreceiver. One end of S can begrounded, the current comple-ing

the circuit through the earthand into the receiver through an-ther

wire enteringthe earth.125. Various forms of

telephonesare shown in Figs.i34" 135, 136. Fig. 134shows a form of desk tele-hone

; Fig. 135 shows acommon form of wall tele-hone

; Fig. 1 36 shows head-telephones for switchboardoperators. Fig. 136.


105/196

CHAPTER XVII.

HOW BLECTBICITY IS GENEBATED BY DYNAMOS.

126. The Dynamo, Dynamo- Electric Machine or Gen-rator,is a machine for converting mechanical energy into

an eleAric current, through eledlromagnetic induction.The dynamo is a machine that will convert steam power,for example, into an eledlric current. Stridllyspeaking,a dynamo creates eledlrical pressure, or electromotiveforce, and not eledlricity,just as a force-pump createswater-pressure, and not water. They are generally runby steam or water power.

127. Induced Currents. We have already spoken

Fig. 137. Fig. 138.about currents being induced by moving a coil of wire ina magnetic field. We shall now see how this principleis used in the djmamo which is a generator of inducedcurrents.

Fig. 1 37 shows how a current can be generated by abar magnet and a coil of wire. Fig. 138 shows how acurrent can be generated by a horseshoe magnet and acofl of wire having an iron core. The ends of the coil are


106/196

lO.? ELECTRICITY GENERATED BY DYNAMOS.

to be connedled to an astatic galvanoscope;this forms aclosed circuit. The coil may be moved past the magnet,or the magnet past the coil.Fig. 139 shows how a current can be generated by two

J"H

Fig. 139. Fig. 140.coils,H being connected to an astatic galvanoscopeandK to a battery. By suddenly bringingE toward H orthe core of E past that of H, a current is produced. Wehave in this arrangement the main features of a dynamo.We can reverse the operation,holdingE in one position

Fig. 141. Fig. 142.

and moving H rapidlytoward it. In this case H wouldrepresent the armature and E the field-magnet.WhenH is moved toward E, the induced current in H flows inone diredlion,and when H is suddenly withdrawn from


107/196

ELECTRICITY GENERATED BY DYNAMOS. 103

Fig. 143.

E the current is reversed in H. (See '' Study/' ChapterXXV., for experiments.)

128. Induced Currentsby Rotary Motion. Themotions of the coils instraightlines are not suit-ble

for producing currentsstrong enough for com-ercial

purposes. In orderto generate ' currents ofconsiderable strength andpressure, the coils of wirehave to be pushed pastmagnets, oreledlromagnets,with great speed. In thedynamo the coils are so wound that they can be givena rapid rotary motion as they fly past strong electro-agnets.

In this way thecoil can keep on passingthe same magnets, in thesame diredlion,as long asforce is appliedto the shaftthat carries them.

129. Field-Magnets ;Armature ; Commuta-or.

What we need then,to produce an inducedcurrent by a rotary motion,is a strong magnetic field,a rotating coil of wire

Fig. 144. properly placed in the

6 c!)"i)(j)


108/196

I04 ELECTRICITY GENERATED BY DYNAMOS.

Fig. 145.

field,and some means of leading the current from themachine.

If a loop of wire, Fig. 140, be so arranged on bearingsat its ends that it can be madeto revolve,a current will flowthrough it in one directionduring one-half of the revolu-ion,

and in the oppositedirec-ionduring the other half , it

being insulated from all ex-ernalconductors. This

agrees with the experimentssuggested in "127, when thecurrent generated in a coilpassedin one diredlion duringits motion toward the strongest part of the field,and

in the oppositediredlion when the coil passed out ofit. A coil must be cut by lines of force to generate acurrent. A currentinside of the machine,as in Fig. 140, wouldbe of no value; it mustbe led out to externalcondudlors where itcan do work. Somesort of slidingcontadlis necessary to connedla revolvingcondudlorwith outside stationarynes. The magnet, called ihftfield-magnet^ is merely to furnish lines of magnetic force. Theone turn of wire represents the simplestform of armature.

Fig. 146.


109/196

ELECTRICITY GENERATED BY DYNAMOS. 105

Fig. 141 shows the ends of a coil joinedto two rings,X,Y, insulated from each other,and rotatingwith the coil.The two stationarypiecesof carbon, A, B, called brushes,press againstthe rings,and to these are joinedwires,which completethe circuit,nd which lead out where thecurrent can do work. The arrows show the diredlion ofthe current during one-half of a revolution. The rings

Fig. 147.

form a colle6lor,nd this arrangement givesan alternatingcurrent.

In Fig. 142 the ends of the coil are joinedto the twohalves of a cylinder. These halves,X and Y, are insu-ated

from each other,and from the axis. The currentflows from X onto the brush A, through some externalcircuit,o do the work, and thence back through brushB onto Y. By the time that Y gets around to A, thediredlion of the current in the loophas reversed,so thatit passes toward Y, but it still enters the outside circuit


110/196

io6 ELECTRICITY GENERATED BY DYNAMOS.

through A, because Y is then in contadl with A. Thisdevice is called a commutator ^ and it allows a constant ordirect current to leave the machine.

In regularmachines, the field-magnetsre dedlromag-nets, the whole or a part of the current from the dynamopassingaround them on its way out, to excite them andmake a powerfulfield between the poles. To lessen theresistance to the lines of force on their way from the N to

Fig. 148.

the S pole of the field-magnets,he armature coils arewound on an iron core; this greatlyincreases the strengthof the field,s the lines of force have to jump across buttwo small air-gaps. There are many loops of wire onregulararmatures, and many segments to the commuta-or,carefullyinsulated from each other,each gettingitscurrent from the coil attached to it.

130. Types of Dynamos. While there is an almostendless number of different makes and shapesof dynamos,


111/196


112/196

I08 ELECTRICITY GENERATED BY DYNAMOS.

machine is carried around the field-magnetores throughmany turns of fine wire. The compound wound dynamois really a combination of the two methods justgiven.In separately-excitedynamos, the current from a sepa-ate

machine is used to excite the field-magnets.132. Various Machines. Fig. 145 shows a hand

power dynamo which produces a current for experimentalwork. Fig. 146 shows a magneto-eledlricalgeneratorwhich produces a current for medical use. Figs. 147,148 show forms of dynamos, and Fig. 149 shows how arclamps are connedled in series to dynamos.


113/196


114/196

no THK EI^ECTRIC CURRENT TRANSFORMED.

core; in fact,an indudlion coil may be considered a trans-ormer,but in this a diredl current has to be interrupted.

If the secondary coil has loo times as many turns of wireas the primary, a current of loo volts can be taken fromthe secondary coil when the primary current is but ivolt; but the strength(amperes)of this new current willbe but one-hundredth that of the primary current.

By using the coil of fine wire as the primary, we canlower the voltage and increase the strengthin the sameproportion.Fig. 150 shows about the simplestform of transformer

Fig. 151.

with a solid iron core, on which are wound two coils,theone, P, being the primary,and the other,S, the secondary .Fig. 151 shows the general appearance of one make oftransformer. The operationof this apparatus, as alreadymentioned, is to reduce the high pressure alternatingcurrent sent out over the condudlors from the dynamo,to a potentialat which it can be employed with conveni-nce

and safety,for illumination and other purposes.


115/196

7/30/2019 Things a Boy Should Know About Electric

things a boy should know about electricity 1000004810

Documents