THINGS ^ ^ ^:^ .St A BOY SHOULD K ABOUT ELECTRICITY THOMAS m ST. JOHN q3aQ0O0OQffOCQOQ ftp «i oa ooCJiba&BttlKi0iStt*> riitt' C3) Q,C5X'S "5) 1 '^1-3 Cornell 'ITlniversit^^ Xibrar^ OF THE mew l^orl^ State Collecje of aericulture /\.a...^/.M:.'f: (-bl? they are ■^^' ^"'P"'^ ^fted?^--'^°^ nof°u"e°Xr ifr'^ "■port aU cases of book»» nPMr.n la.oQ 7 > Cornell University Library QC 525.S143 Things a boy should know about electric! 3 1924 002 939 704 ^ A BY THE SAME AUTHOR— {Partial List . |_d,^ FUN WITH MAGNETISM. A book and complete outfit oi "^ apparatus for Sixty-One Experiments, Post-paid^ 35 cts. FUN WITH ELECTRICITY. A book and complete outfit of apparatus for Sixty Experiments. Post-paid, 65 cts. FUN WITH PUZZLES. A book, key, and complete outfit for Four Hundred Puzzles. Post-paid. 35 cts. FUN WITH SOAP-BUBBLES. A book and complete outfit of apparatus ior Fancy Bubbles and Films, Post-paid, 35 cts. FUN WITH SHADOWS. A book and complete outfit ot ap- paratus for Shadow Pictures, Pantomimes , etc. Post-paid, 35 cts. FUN WITH PHOTOGRAPHY. A book and complete outfit of apparatus for Amateur Work. Post-paid, 65 cts. FUN WITH CHEMISTRY. A book and complete outfit ol apparatus for Forty-One Experiments. Post-paid, 65 cts. FUN WITH TELEGRAPHY. A book, key, sounder, and wires. Nicely rnounted and very loud. This is a practical learner's outfit. Patented. Post-paid, ^0 cts. With dry bat- tery, /oj/-/(7/rt', 65 cts. ELECTRIC SHOOTING GAME. Simple, fascinating, and absolutely original. Shoots animals by electricity. Patent applied for. Post-paid, 50 cts. HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS. A book containing complete directions for making all kinds of simple apparatus for the study of ele- mentary electricity. Fifth Edition. Post-paid, $1.00. THE STUDY OF ELEMENTARY ELECTRICITY AND MAGNETISM BY EXPERIMENT. This book is designed as a te.xt-book for amateurs, students, and others who wish to take up a systematic course of simple experiments at home or in school. Third Edition. Post-paid, $1.25. THINGS A BOY SHOULD KNOW ABOUT ELEC- TRICITY. This book explains, in simple, straightforward language, many things about electricity; things in which the American bo)' is intensely interested; things he wants to know; things he should know. Fourth Edition. Post-paid, $1.00. REAL ELECTRIC TOY-MAKING FOR BOYS. Contain- ing complete directions for making and using a large num- ber of electrical toys. Over 100 original drawings, diagrams, and full-page plates. Post-paid, $1.00. Anh I'oiii' Bnokse'ler, Stationer, or Toy Dealer for our Boolis, Games, I'ltzzles, Kdiicatiotial Amusements, Etc. C^KTKI-OGUB UPON rePPLICHXION. THOMAS M. ST. JOHN, 848 Ninth Ave., New York. Things A Boy Should Know About Eleftricity BY THOMAS M. ST. JOHN, Met. E. Author of " Fun With Magnetism," " Fun With Electricity, " How Two Boys Made Their Own Electrical Appa- ratus," "The Study of Elementary Electricity and Magnetism by Experiment," etc. FOURTH HLH EDITION NEW YORK THOMAS M. ST. JOHN Publisher '^ '•v^ o <^ -^ Copyright, igoo. By Thomas M St. Tohn. '%, "--. THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY TABLE OF CONTENTS Chapter Page I. About Frictional Electricty, . . . . 7 II. About Magnets and Magnetism ... 21 III. How Electricity is Generated by the Voltaic Cell, 32 IV. Various Voltaic Cells, . . . .36 V. About Push-Buttons. Switches and Binding-Posts, 43 VI. Units and Apparatus for Electrical Measurements, 48 VII. Chemical Effects of the Electric Current, . 58 VIII. How Electroplating and Electrotyping are Done, . 60 IX. The Storage Battery, and How it Works, . 63 X. How Electricity is Generated by Heat, . . 68 XI. Magnetic Effects of the Electric Current, . 71 XII. How Electricity is Generated by Induction, . 77 XIII. How the Induction Coil Works, ... 80 XIV. The Electric Telegraph, and How it Sends Mes- sages, ...... 84 XV. The Electric Bell and Some of its Uses, . . 91 XVI. The Telephone and How it Transmits Speech, 95 XVII. How Electricity is Generated by Dynamos, . loi XVIII. How the Electric Current is Transformed, . 109 XIX. How Electric Currents are Distributed for Use, . 114 XX. How Heat is Produced by the Electric Current, 124 XXI. How Light is Produced by the Incandescent Lamp, 129 XXII. How Light is Produced by the Arc Lamp, . 135 XXIII. X-Rays, and How the Bones of the Human Body are Photographed, ..... 141 XXIV. The Electric Motor, and How it Does Work, . 147 XXV. Electric Cars, Boats and Automobiles, . . 154 XXVI. A Word About Central Stations, . . 162 XXVII. Miscellaneous Uses of Electricity, . . . 165 TO THE READER For the benefit of those who wish to make their own elecftrical apparatus for experimental purposes, references have been made throughout this work to the "Apparatus Book; " b}' this is meant the author's " How Two Boys Made Their Own Eledtrical Apparatus. ' ' For those who wish to take up a course of elementary electrical experiments that can be performed with simple, home-made apparatus, references have been made to " Study; " by this is meant " The Stud}- of Elementary Elecftricitj^ and Magnetism by Experiment." The Author. Things A Boy Should Know About Electricity CHAPTER I. ABOUT FRICTIONAL ELECTKICITT. I. Some Simple Experiments. Have you ever shuffled your feet along over the carpet on a winter's evening and then quickly touched your finger to the nose of an unsuspecting friend ? Did he jump when a bright spark leaped from your finger and struck him fairly on the very tip of his sensitive nasal organ ? Did you ever succeed in proving to the pussy-cat, Fig. i, that something unusual occurs when you thoroughly rub his warm fur with your hand ? Did you notice the bright sparks that passed to your hand when it was held just above the cat's back? You should be able to see, hear, and feel these sparks, especially when the air is dry and you are in a dark room. Did you ever heat a piece of paper before the fire until it was real hot, then lay it upon the table and rub it from end to end with 3'our hand, and finally see it cling to the wall? Fig. I. ABOUT FRICTIONAL ELECTRICITY. Were you ever in a fadlory where there were large belts running rapidly over pulleys or wheels, and where large sparks would jump to j'our hands when held near the belts ? If you have never performed any of the four experi- ments mentioned, j-ou should try them the first time a chance occurs. There are dozens of simple, fascinating experiments that may be performed with this kind of eledlricity. 2. Name. As this variety of eledlricity is made, or generated, by the fridlion of substances upon each other, it is ciXX&A. frinio-)ial elecftricity. It is also called static eledlricity, because it general!}' stands still upon the sur- face of bodies and does not " flow in currents " as easily as some of the other varieties. Static eledlricity may be produced by indudlion as well as \>y fridlion. 3. History. It has been known for over 2,000 3'ears that certain substances adl queerly when rubbed. Amber was the first substance upon which eledlricity was produced hy fric- tion, and as the Greek name for amber is clektron, bodies so affedled were said to be clefl rifled. When a body, like ebonite, is rubbed wdth a flannel cloth, we say that it becomes charged with eleciricity. Just what happens to the ebonite is not clearly under- Fig. 2, ABOUT FRICTIONAL ELECTRICITY. 3r stood. We know, however, that it will attradt light bodies, and then quickly repel them if they be condudtors. Fig. 2 shows a piece of tissue-paper jumping toward a sheet of ebonite that has beeu electrified with a flannel cloth. 4. Conductors and Non-Conductors. Eledtricity can be produced upon glass and ebonite because they do not carr)' or condudl it away. If a piece of iron be rubbed, the elecftricity passes from the iron into the earth as fast as it is generated, because the iron is a con- duHor of eledtricity. Glass is an insulator or non-con- duBor. Fridtional eledtricity resides upon the outside, only, of condu<5tors. A hollow tin box will hold as great a charge as a solid piece of metal having the same out- side size and shape. When fridtional eledtricity passes from one place to another, sparks are produced. Light- ning is caused by the passage of static eledtricity from a cloud to the earth, or from one cloud to another. In this case air forms the condudtor. (For experiments, see "Study," Chapter VII.) 5. Electroscopes. A piece of carbon, pith, or even a small piece of damp tissue-paper will serve as an eledlro- scope to test the presence of static eledtricity. The pith is usually tied to a piece of silk thread which is a non- condudtor. Fig. 3 shows the ordinary form of pith-ball eleSlroscope. The leaf eleilroscope is a very delicate apparatus. Gold- % Fig. 3. ABOUT FRICTIONAL ELECTRICITY. leaf is generally used, but alumiimm-leaf will stand handling and will do for all ordinary purposes. Fig. 4 shows a common form, the glass being used to keep currents of air from the leaves aud at the same time to insulate them from the earth. Eledlroscopes are used to show the presence, relative amount, or kind of static elecTiricity on a body. (See "Study," Chapter XL) 6. Two Kinds of Electrification. It can be shown that the eledlri- fication produced on all bodies by friclion is not the same; for example, that generated with glass and silk is not the same as that made with ebonite and flannel. It has been agreed to call that pro- duced by glass and silk positive, and that by ebonite and flannel nega- tive. The signs + and — are used for positive and negative. 7. Laws of Electrification, (i) Charges of the same kind repel each other; ( 2 ) charges of unlike kinds attracft each other; (3) either kind of a charge attracJts and is attracted bj' a neutral body. 8. Static Electric Machines. In order to produce Fig. 4. ABOUT FRICTIONAL EI,ECTRICITY. II static eledtricity in quantities for experiments, some device is necessarj'. The dcB^'ophorus (e-lec-tropli'-o-rus) is about the sim- plest form of machine. Fig. 5 shows a simple elecSroph- orus in which are two insulators and one condudtor. The ebonite sheet E S is used with a flannel cloth to gen- erate theeledtricity. The metal cover E C is lifted by the insulating handle E R. The cover ' E C is placed upon the thoroughly charged sheet E S, and then it is touched for an instant with the finger, before lifting it by E R. The charge upon E C can then be removed by bringing the hand near it. The bright spark that passes from E C to the hand indicates that E C has discharged itself into the earth. The adtion of the eledlroph- orus depends upon indu(ftion. (For experiments, details of adtion, induced eledtrification, etc., see ' ' The Study of Elementary Eledlricity and Magnetism by Experiment," Chapters VIII. and IX.) The first eleElric viachme consisted of a ball of sulphur fastened to a spindle which could be turned by a crank. By holding the hands or a pad of silk upon the revolving ball, eledtricity was produced. 9. The Cylinder Electric Machine consists, as shown in Fig. 6, of a glass cylinder so mounted that it can be turned by a crank. Fridtion is produced by a pad of leather C, which presses against the cylinder as it turns. Eledtric sparks can be taken from the large ' ' con- dudlors " which are insulated from the earth. The oppo- 12 ABOUT FRICTIONAL EtECTKICITY. Fig. 6. Fig- 7- ABOUT KRICTIONAL ELECTRICITY. 13 site elecftricities uuite with sparks across D and E. If use is to be made of the eledlricity, either the rubber or the prime condutffor must be connected with the ground. In the former case positive eledtricity is obtained; in the latter, negative. 10. The Plate Electrical Machine. Fig. 7 also shows an old form of machine. Such machines are made of circular plates of glass or ebonite, two rubbing pads Fig 8. being usually employed, one on each side of the plate. One operator is seen on an insulated stool (Fig. 7), the elecftricit}^ passing through him before entering the earth by way of the body of the man at the right. II. The Toepler-Holtz Machine, in one form, is shown in Fig. 8. The electricity is produced by the principle of indudlion, and not by mere fricflion. This machine, used in connection with condensers, produces large sparks. 14 ABOUT FRICTIONAL ELECTRICITY. Fig. 9. 12. The Wimshurst Machine is of recent date, and not being- easily affecfted by atmospheric changes, is very useful for ordinary laboratory work. Fig. 9 shows one form of this machine. 13. Influence Machines for Medical Purposes are made in a large variety of forms. A Wimshurst machine is generall}' used as an exciter to charge the plates of the large machine when they lose their charge on account of excessive moisture in the J^Sf atmosphere. Fig. 10 shows a large machine. 14. Uses of Electrical Machines. Static eledlricity has been used for many years in the labora- tor}' for experi- mental purposes, for charging condensers, for medical pur- poses, etc. It is now being used for X-ray work, and considerable ad- vancement has been made within a few years in the construc- tion and efficiency of the machines. Fig. 10. ABOUT FRICTIONAI, ELECTRICITY. 15 With the modern machines large sparks are produced by merely turning a crank, enough eledtricity being pro- duced to imitate a small thunderstorm. The sparks of home-made lightning will jump several inches. Do not think that eledlricitj' is generated in a com- mercial way by static eledlric machines. The pradtical uses of static ele(fi:ricit3- are very few when compared with those of current elecflricity from batteries and dynamos. 15. Condensation of Static Electricity. By means Fig II Fig. 12. of apparatus called condensers, a terrific charge of static elec5lricity may be stored. Fig. 1 1 .shows the most common form of condenser, known as the Leydcn jar. It consists of a glass jar with an inside and outside coating of tin-foil. To charge the jar it is held in the hand so that the out- side coating shall be connecfted with the earth, the sparks i5 ABOUT FRICTIONAI< ELECTRICITY. from an ele(5lric machine being passed to the knob at the top, which is connedted b}- a chain to the inside coating. To discharge the jar, Fig. 12, a conducftor with an insulating handle is placed against the outside coat ; when the other end of the conducftor is swung over towards the knob, a bright spark passes between them. This device is called a dis- charger. Fig. 13 shows a discharge through ether which the spark ignites. 16. The Leyden Bat- tery, Fig. 14, consists of several jars connecfled in such a way that the area of the inner and outer coatings is greatly increased. The bat- tery has a larger ca- pacity than one of its jars. (For Experi- ments in Condensa- tion, see "Study," Chapter X.) 17. Electromotive Force of Static Electricity. Al- though the sparks of static eledtricitv are Fig. 14. large, the quantity of elecftricity is verj' small. It would take thousands of galvanic cells to produce a spark an inch long. While the quantity of static ele(ftricity is ABOUT FRICTlONAIv ELECTRICITY. 17 Fig. 15. small, its potential, or eledlromotive force (E. M. F.), is very high. We say that an ordinary gravity cell has an E. M. F. of a little over one volt. Five such cells joined in the proper way would have an E. M. F. of a little over five volts. You will understand, then, what is meant when we say that the E. M. F. of a lightning flash is millions of volts. 18. Atmospheric Elec- tricity. The air is usuallj' eledtrified, even in clear weather, although its cause is not thoroughly understood. In 1752 it was proved by Benjamin Franklin (Fig. 15), with his famous kite experiment, that atmospheric and fridlional eledlricities are of the same nature. By means of a kite, the string being wet by the rain, he succeeded, during a thunder- storm, in drawing sparks, charg- ing condensers, etc. 19. Lightning may be pro- duced by the passage of eledtricity between clouds, or between a cloud and the earth (Fig. 16), which, with the intervening air, have the effedl of a condenser. When the attracftion between the two elecftrifications gets great enough, a spark passes. When the spark has a zigzag motion it is called chain Fig. 16. I8 ABOUT FRICTIONAL ELECTRICITY. lightning. In hot weather flashes are often seen which light whole clouds, no thunder being heard. This is called heat lightni7ig, and is generallj- considered to be due to distant dis- charges, the light of which is refledted by the clouds. The lightning flash repre- sents billions of volts. 20. Thunder is caused bj- the violent disturbances produced in the air by light- ning. Clouds, hills, etc. produce echoes, which, with the orig- inal sound, make the rolling effect. 21. Lightning- Rods, when well constru(5ted, often pre- vent violent dis- charges. Their point- ed prongs at the top allow the negative eledtricitj' of the earth to pass quietly into the air to neutralize the positive in the cloud above. In case of a discharge, or stroke of lightning, the rods aid in condudting the ele Fig 59- PUSH-BUTTONS, SWITCHES AND BINDING-POSTS. 47 switches, and are used in telegraph and telephone work, in electric light stations, etc., etc. (See Chapter on Central Stations.) Fig. 60 shows a switch used for in- candescent lighting currents. 63. Binding- Posts are used to make connections between two pieces of apparatus, be- tween two or more wires, between a wire and anj' appa- ratus, etc, etc. Tlie}^ allow the wires to be c^uickly fastened or unfast- ened to the apparatus. A large part of the apparatus shown in this book has binding-posts attached. Fig. 61 Fig. 60. Fig. 61. shows a few of the common forms used. (See ' ' Appa- ratus Book," Chapter V., for home-made binding-posts.) CHAPTER VI. UNITS AND APPAEATTJS POR ELECTRICAL MEASURE- MENTS. 64. Electrical Units. In order to measure eledlricity for experimental or commercial purposes, standards or units are just as necessary as the inch or foot for measur- ing distances. 65. Potential ; Electromotive Force. If water in a tall tank be allowed to squirt from two holes, one near the bottom, the other near the top, it is evident that the force of the water that comes from the hole at the bottom will be the greater. The pressure at the bottom is greater than that near the top, because the "head" is greater. When a spark of static eledtricity jumps a long distance, we say that the charge has a high pote7itial ; that is, it has a high elecflrical pressure. Potential, for eledtricity, means the same as pressure, for water. The greater the potential, or cleclromotive force (E.M.F.) of a cell, the greater its power to push a current through wires. (See " Stud}'," § 296 to 305, with experiments.) 66. Unit of E.M.F. ; the Volt.— In speaking of water, we say that its pressure is so many pounds to the square inch, or that it has a fall, or head, of so many feet. We speak of a current as having so many volts; for example, we ^a}' that a wire is carrying a iio-volt current. The volt is the unit of E.M.F. An ordinary gravity cell has an E.M.F. of about one volt. This name was given in honor of Volta. APPARATUS FOR ELECTRICAL MEASUREMENTS. 49 67. Measurement of Electromotive Force. There are several ways by which the E.M.F. of a cell, for example, can be measured. It is usually measured relatively, by com- parison with the E. M. F. of some standard cell. (See "Study," Exp. 140, for measuring the E. M. F. of a cell by comparison Fig. 62. with the two-fluid cell.) Voltmeters are instruments by means of which E.M.F. can be read on a printed scale. They are a variety of galvanometer, and are made with coils of such high resistance, compared with the resistance of a cell or dynamo, that the E. M. F. can be read diredl. The reason for this will be seen b}' referring to Ohm's law ("Study," §356); the resistance is so great that the strength of the cur- rent depends entirely upon the E. M. F. Voltmeters measure eledtrical pressure just as steam gauges measure the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63 shows a voltmeter 50 APPARATUS FOR ELECTRICAI, MEASUREMENTS. with illuminated dial. An eledtrical bulb behind the instrument furnishes light so that the readings can be easily taken. 68. Electrical Resistance. Did 30U ever ride down hill on a hand-sled ? How easily the sled glides over the snow! What happens, though, when j-ou strike a bare place, or a place where some evil-minded person has sprinkled ashes? Does the sled pass easil}- over bare ground or ashes? Snow offers ver}' little resistance to the sled, while ashes offer a great resist- ance. All substances do not allow the eledtric current to pass through them with the same ease. Even the liquid in a cell tends to hold the current back and offiirs in- ternal resistance. The various wires and in.struments conuecfted to a cell offer external resist- Chapter XVIII., for experiments, Fig. 64. ance. (See " Study, etc.) 69. Unit of Resistance; The Ohm is the name given to the unit of resistance. About g ft. g in. of Xo. 30 copper wire, or 39 feet i in. of Xo. 24 copper wire, will make a fairly accurate ohm. Resistance coils, having carefully measured resistances, are made for standards. (See "Apparatus Book," Chapter X\'II., for home-made resistance coils.) Fig. 64 shows a commercial form of a standard resistance coil. The coil is inclosed in a case and has large wires leading APPARATUS FOR ELECTRICAL MEASUREMENTS. 5 1 from its ends for connedlions. Fig. 65 gives an idea of the way in which coils are wound and used with plugs to build up resistance boxes, Fig. 66. 70. Laws of Resistance, i. The resistance of a wire is diredtly proportional to its length, provided its cross-sedtion, material, etc., are uniform. 2. The resistance of a wire is inverselj' proportional to its area of cross-seclion ; or, in other words, inversely propor- tional to the square of its diameter, other things being Fig. 65. equal. 3. The resistance of a wire depends upon its material, as well as upon its length, size, etc. 4. The resistance of a wire increases as its temperature rises. (See "Study," Chapters XVIII. and XIX., for experiments on resistance, its measurement , etc.) 71. Current Strength. The strength of a cur- rent at the end of a circuit depends not only upon the eleclrical pressure, or E. M. F., which drives the current, but also upon the resistance which has to be overcome. Fig. 66. 52 APPARATUS FOR ELECTRICAL MEASUREMENTS. The greater the resistance the weaker the current at the end of its journey. 72. Unit of Current Strength ; The Ampere. A current having an E. M. F. of one volt, pushing its way through a resistance of one ohm, would have a unit of strength, called one ampere. This current, one ampere strong, would deposit, under proper conditions, .0003277 gramme of copper in one second from a solu- tion of copper sulphate. 73. Measurement of Current Strength. A magnetic needle is defledled when a cur- rent passes around it, as in instruments like the galvanometer. The galvanoscope merely in- dicates the presence of a current. Galvanom- eters measure the strength of a current, and the}- are made in many forms, depending upon the nature and strength of the currents to be measured. Galvanometers are standardized, or calibrated, b}' special measurements, or by comparison with some standard in- strument, so that when the deflecftion is a certain number of degrees, the current passing through it is known to be of a certain strength. Fig. 67 shows an astatic galvanometer. Fig. 68 shows a tangent galvanometer, in which the strength of the cur- APPARATUS FOR ELECTRICAI, MEASUREMENTS. 53 rent is proportional to the tangent of the angle of deflec- tion. Fig. 6g shows a D'Arso7ival £-a/va7ioj?ie/er, invfhich a coil of wire is suspended between the poles of a permanent horseshoe mag- net. The lines of force are concen- trated by the iron core of the coil. The two thin suspending wires convej' the current to the coil. A ray of light is refledled from the small mirror and adls as a pointer as in other forms of refledling galvanometers. 74. The Ammeter, Fig. 70, is a form of galvanometer in which the strength of a current, in amperes, can be read. In these the strength of current is proportional to the angular defiedtions. The coils are made with a small resist- ance, so that the current will not be greatly reduced in strength in passing through them. 75. Voltameters measure the strength of a current by chemical means, the quantity of metal de- posited or gas generated being proportional to the time that the current flows and to its strength. In the water voltameter, Fig. 71, the hydrogen and Fig. 69. oxjfgen produced in a 54 APPARATUS FOR PXECTRICAL MEASUREMENTS. given time are measured. (See "Study," Chapter XXI.) The copper voltam- eter measures the amount of copper deposited in a given time by the current. Fig. 72 shows one form. The copper cathode is weighed before and after the current flows. The weight of copper deposited and the time taken are used to calculate the current strength. 76. Unit of Quantity; The Coulomb is the quantity of elecftricit)' given, in one second, by a current having a Fig. 70. Fig. 71. APPARATUS FOR ELECTRICAI< MEASUREMENTS. 55 strength of one ampere. Time is an important ele- ment in consider- ing the work a cur- rent can do. 77. Electrical Horse-powe r; The Watt is the unit of elecflrical power. A current having the strength of one ampere, and an E. M. F. of one volt has a unit of power. 746 watts make one elec- trical horse-power. Watts = amperes X volts. Fig. 73 shows a direcT: reading wattmeter based on the inter- national volt and ampere. They save taking simulta- neous ammeter and voltmeter readings, which are other- wise necessary to get the produdt of volts and am- peres, and are also used on alterna- ting current measurements. There are also forms of watt- meters, Fig. 74, in which the watts are read from dials like those on an ordinary gas-meter, the records being permanent. 56 APPARATUS FOR ELECTRICAL MEASUREMENTS. Fig. 75 shows a voltmeter V, and ammeter A, so placed in the circuit that readings can be taken. D represents a dynamo. A is placed so that the whole current passes through it, while V is placed between the main wires to measure the difference in potential. The produdt of the two readings in volts and amperes gives the number of watts. 78. Chemical Meters also measure the quantity of cur- rent that is used; for example, one may be placed in the cellar to measure the quantity of current used to light the house. Fig. 76 shows a chemical meter, a part of the current passing through a jar containing zinc plates and a solu- tion of zinc sulphate. Metallic zinc is dissolved from one plate and deposited upon the other. The increase in weight shows the amount of chemical acftion which is Fig. 74- Fig. 75- proportional to the ampere hours. Knowing the relation between the quantitj^ of current that can pass through the solution to that which can pass through the meter by APPARATUS FOR KIvECTRICAL MEASUREMENTS. 57 another condudlor, a calculation can be made which will give the current used. A lamp is so arranged that it Fig. 76. automatically lights before the meter gets to the freezing- point; this warms it up to the proper temperature, at which point the light goes out again. CHAPTER VII. CHEMICAL EFFECTS OF THE ELECTRIC CXTERENT. 79. Electrolysis. It has been seen that in the vol- taic cell eledlricity is generated bj' chemical acftion. Sul- phuric acid acfts upon zinc and dissolves it in the cell, hydrogen is produced, etc. When this process is re- versed, that is, when the elecflric current is passed Fig. 77- through some solutions, they are decomposed, or broken up into their constituents. This process is called deHrol- vs!s, and the compound decomposed is the clcHrolyte. (See "Study," §369, etc., with experiments.) Fig. 77 shows how water can be decomposed into its two constituents, hydrogen and oxygen, there being twice as much hydrogen formed as oxygen. Fig. 78 .shows a glass jar in which are placed two metal 53 CHEMICAL EFFECTS OF THE ELECTRIC CURRENT. 59 strips, A and C, these being connecfted with two cells. In this jar may be placed various condudting solutions to be tested. If, for example, we use a solution of copper i> i .^;^'..-r"K" fm • ^m- i \ ft f, 8 r' 1 Fig. 78. sulphate, its chemical formula being Cu SO^, the current will break it up into Cu (copper) and SO^. The Cu will be deposited upon C as the current passes from A to C through the solution. A is called the ayiode, and C the cathode. Fig. 79 shows another form of jar used to study the decomposition of solutions by the elecftric- current. 80. Ions. When a solution is decom- posed into parts by a current, the parts are called the Ions. When copper sulphate (CuSOJ is used, the ions are Cu, which is a metal, and SO,, called an acid radical. Fig^g. When silver nitrate (Ag NO3) is used, Ag and NO3 are the ions. The metal part of the compound goes to the cathode. CHAPTER VIII. HOW ELECTEOPL,A'"ING AND ELECTROTYPING ARE DONE. 8i. Electricity and Chemical Action. We have just seen, Chapter VII., that the eledtric current has the power to decompose certain couipounds when they are in solution. By choosing the right solutions, then, we shall be able to get copper, silver, and other metals set free by eledtrolysis. 82. Electroplating consists in coating substances with metal with the aid of the elecftric current. If we wish to eledlroplate a piece of metal with copper, for example, we can use the arrangement shown in Fig. 78, in which C is the cathode plate to be covered, and A is a copper plate. The two are in a solution of copper sul- phate, and, as explained in § 79, the solution will be decomposed. Copper will be deposited upon C, and the SOj part of the solution will go to the anode A, which it will attack and gradually dissolve. The SO,, acftingupon the copper anode, makes Cu SO^ again, and this keeps the solution at a uniform strength. The amount of copper dissolved from the copper anode equals, nearl}', the amount deposited upon the cathode. The metal is carried in the diredlion of the current. If we wish to plate something with silver or gold, it will be necessary to use a solution of silver or gold for the electrolyte, a plate of metallic silver or gold being used for the anode, as the case may be. 60 ELECTROPLATING AND ELECTROTYPING. 6i Great care is used in cleaning substances to be plated, all dirt and grease being carefully removed. Fig. 80 shows a plating bath in which several articles can be plated at the same time by hanging them upon a metal bar which really forms a part of the cathode. If, for example, we wish to plate knives, spoons, etc., with silver, they would be hung from the bar shown, each being a part of the cathode. The vat would contain a solution of silver, and from the other bar would be hung Fig. 80. a silver plate having a surface about equal to that of the combined knives, etc. Most metals are coated with copper before they are plated with silver or gold. When plating is done on a large scale, a current from a dynamo is used. For experimental purposes a Gravity cell will do very well. (See " Study," § 374 to 380 with experiments.) 83. Electrotyping'. It was observed by De I, ill B'' I,.**-', ■ "^ B 1 *^ Fig. S3. of lead, with raised right-angled ribs on each side, thus forming little depressed squares, or to punch a lead plate full of holes, which squares or holes are then filled with a pasty mixture of red oxide of lead in positive plates, and with litharge in negatives. In a form called the chloride battery, instead of cementing lead oxide paste into or against a lead framing in order to obtain the necessary adtive material, the latter is obtained by a striAly chemical process. 66 THE STORAGE BATTERY, AND HOW IT WORKS. Fig. 82 shows a storage cell with plates, etc., contained in glass jar. Fig. 83 shows a cell of 41 plates, set up in a lead-lined wood tank. Fig. 84 shows three cells joined in series. Many storage cells are used in central eledtric light stations to help the d)-namos during the ' ' rush ' ' hours at night. They are charged during the day when the load on the dynamos is not heavy. Fig. 85 shows another form of storage cell containing a number of plates. 87. The Uses of Storage Batteries are almost Fig. 84. numberless. The current can be used for nearly every- thing for which a constant current is adapted, the follow- ing being some of its applications: Carriage propulsion; eledtric launch propulsion; train lighting; yacht lighting; carriage lighting; bicycle lighting; miners' lamps; den- tal, medical, surgical, and laboratory work; phonographs; kinetoscopes; automaton pianos; sewing-machine motors; fan motors; telegraph; telephone; eledtric bell; eledlric THE STORAGE BATTERY, AND HOW IT WORKS. 67 fire-alarm; heat regulating; railroad switch and signal apparatus. By the installing of a storage plant many natural but small sources of power maj' be utilized in furnishing light and power; sources which otherwise are not available, because not large enough to supply maximum demands. The force of the tides, of small water powers from irri- gating ditches, and even of the wind, come under this heading. As a regulator of pressure, in case of flucftuations in Fig. 85. the load, the value of a storage plant is inestimable. These flucftuations of load are particularly^ noticeable in elecflric railway plants, where the demand is constantly rising and falling, sometimes jumping from almost noth- ing to the maximum, and vice versa, in a few seconds. If for no other reason than the prevention of severe strain on the engines and generators, caused by these flucftuations of demand, a storage plant will be valuable. CHAPTER X. HOW ELECTRICITY IS GENERATED BY HEAT. 88. Thermoelectricity is the name given to elecftricity that is generated by heat. If a strip of iron, I, be con- nected between two strips of copper, C C, these being joined by a copper wire, C W, we shall have an arrange- ment that will generate a current when heated at either of the jundtions between C and I. When it is heated at A the current will flow as shown by arrows, from C to I. If we heat at B, the current will flow in the opposite diredlion through the metals, although it will still go from C to I as before. Such currents are called thermoele^ric currents. Different pairs of metals produce diflferent results. Antimony and bismuth are generallj' used, because the greatest effedl is produced b}- them. If the end of a strip of bismuth be soldered to the end of a similar strip of antimouj', and the free ends be connecfted to a galvanom- eter of low resistance, the presence of a current will be shown when the point of contac5t becomes hotter than the rest of the circuit. The current will flow from bismuth 68 HOW ELECTRICITY IS GENERATED BY HEAT. 69 to antimony across the joint. By cooling the juncflure below the temperature of the rest of the circuit, a current will be produced iu the opposite direcftion to the above. The energy' of the current is kept up by the heat absorbed, just as it is kept up by chemical adlion in the voltaic cell. 89. Peltier Effect. If an eledlric current be passed through pairs of metals, the parts at the jundlion become slightly warmer or cooler than before, depending upon the direcftion of the current. This adtion is really the reverse of that in which currents are produced by heat. 90. Thermopiles. As the E.M.F. of the current produced by a single pair of metals is verj' small, several Fig. 87. pairs are usually joined in series, so that the different currents will help each other by flowing in the same direc- tion. Such combinations are called thermoeledtric piles, or simply thermopiles. Fig 87 shows such an arrangement, in which a large number of elements are placed in a small space. The junctures are so arranged that the alternate ones come together at one side. Fig. 88 shows a thermopile connedted with a galvanom- 70 HOW ELKCTRICITY IS GENERATED BY HEAT. eter. The heat of a match, or the cold of a piece of ice, will produce a current, even if held at some distance from 0«S:^l|p^ Fig. S3. the thermopile. The galvanometer should 1 e a short- coil astatic one. (See "Study," Chapter XXIV., for experiments and home-made thermopile.) CHAPTER XI. MAGNETIC EPFECTS OP THE ELECTRIC CURRENT. 91. Electromagnetism is the name given to magnet- ism that is developed by eledlricity. We have seen that if a magnetic needle be placed in the field of a magnet, its N' pole will point in the direcftion taken by the lines of force as they pass from the N to the S pole of the magnet. 92. Lines of Force about a Wire. When a current Fig. 89. passes through a wire, the magnetic needle placed over or under it tends to take a position at right angles to the wire. Fig. 89 shows such a wire and needle, and how the needle is defledled; it twists right around from its N and S position as soon as the current begins to flow. This shows that the lines of force pass around the wire and not in the direcftion of its length. The needle does not swing entirely perpendicular to the wire, that is, to 72 MAGNETIC EFFECTS OF ELECTRIC CURRENT. the K and W line, because the earth is at the same time pulHng its N pole toward the N. Fig. 90 shows a bent wire through which a current passes from C to Z. If you look along the wire from C toward the points A and B, you will see that iinder the wire the lines of force pass to the left. I,ooking along the wire from Z toward D you will see that the lines of force pass opposite to the above, as the current comes toward j-ou. This is learned by experiment. (See "Study," Exp. 152, § 3S5, etc.) Ride. Hold the right hand with the thumb extended (Fig. 89) and with the fingers pointing in the direction of Fig. 90. Fig. gi. the current, the palm being toward the needle and on the opposite side of the wire from the needle. The north- seeking pole will then be deflecfled in the diredtion in which the thumb points. 93. Current Detectors. As there is a magnetic field about a wire when a current passes through it, and as the magnetic needle is affedled, we have a means of detecting the presence of a current. When the current is strong it is simply necessary' to let it pass once over or under a needle; when it is weak, the wire must pass several times above and below the needle. Fig. 91, to give the needlemotion. (See "Apparatus Book," Chapter XIII., for home-made dete(5tors. ) MAGNETIC EFFECTS OF ELECTRIC CURRENT. 73 Fig. 92. 94. Astatic Needles and Detectors. By arranging two magnetized needles with their poles opposite each other, Fig. 92, an astatic needle is formed. The point- ing-power is almost nothing, although their magnetic fields are retained. This com- bination is u.sed to detedt feeble currents. In the ordinary de- tector, the tendency of the needle to point to the N and S has to be overcome by the magnetic field about the coil before the needle can be moved; but in the astatic detector and galvano- scope this pointing-power is done away with. Fig. 93 shows a simple astatic galvanoscope. Fig. 67 shows an astatic galvanometer for measuring weak currents. 95. Polarity of Coils. When a current of elecftricity passes through a coil of wire, the coil adts very much like a magnet, although no iron enters into its construdlion. The coil becomes magnetized bj' the eledtric cur- rent, lines of force pass from it into the air, etc. Fig. 94 shows a coil connecfted to copper and zinc plates, so arranged with cork that the whole can float in a dish of dilute sulphuric acid. The cur- rent passes as shown bj^ the arrows, and when the N pole of a magnet is brought near the right-hand end, there is a repulsion, showing that that end of the coil has a N pole. Fig- 93- 74 MAGNETIC EFFECTS OF ELECTRIC CURRENT. Rule. When you face the right-hand end of the coil, the current is seen to pass around it in an anti-clockwise diredlion; this produces a N pole. When the current passes in a clockwise direcftion a S pole is produced. 96. Electromagnets. A coil of wire has a stronger field than a straight wire carrying the same current, because each turn adds its field to the fields of the other turns. By having the central part of the coil made of iron, or by having the coil of insulated wire wound upon an iron core, the strength of the mag- netic field of the coil is greatly increased. Lines of force do not pass as readil)' through air as through iron ; in fact, lines of force will go out of their way to go through iron. With a coil of wire the lines of force pass from its N pole through the air on all sides of the coil to its S pole; they then pass through the inside of the coil and through the air back to the N pole. When the resist- ance to their passage through the coil is decreased by the core, the magnetic field is greatly strengthened, and we have an eleBromagjiet. The coil of wire temporarily magnetizes the iron core; it can permanently magnetize a piece of steel used as Fig. 94. MAGNETIC EFFECTS OF EIvECTRIC CURRENT. 75 a core. (See "Study," Chapter XXII., for experi- ments.) 97. Forms of Electromagnets. Fig. 95 shows a straight, or bar elenromagnet. Fig. 96 shows a simple form of horseshoe eleHromagnet . As this form is not easily wound, the coils are generally wound on two separate Fig. 95- cores which are then joined by a yoke. The yoke merely takes the place of the curved part shown in Fig. 96. In Fig. 97 is shown the ordinary form of horseshoe ele(ftromagnet used for all sorts of ele<5trical instruments. (See " Apparatus Book," Chapter IX., for home-made elecftromagnets. ) 98. Yokes and Armatures. In the horseshoe magnet there are two poles to attracft and two to induce. The lines of force pass through the yoke on their way from one core to the other, instead of going through the air. 76 MAGNETIC EFFECTS OF ELECTRIC CURRENT. This reduces the resistance to them. If we had no j'oke we should simply have two straight electromagnets, and the resistance to the Hnes of force would be so great that Fig. 96. Fig. 97- the total strength would be much reduced. Yokes are made of soft iron, as well as the cores and armature. The armature, as with permanent horseshoe magnets, is strongly drawn toward the poles. As soon as the cur- rent ceases to flow, the attraiflion also ceases. Beautiful magnetic figures can be made with horseshoe © Fig. 9S. magnets. Fig. 98 shows that the coils must be joined so that the current can pass around the cores in opposite directions to make unlike poles. (See "Study," Exp. 164 to 173.) CHAPTER XII. HOW ELECTRICITY IS GENERATED BY INDUCTION. 99. Electromagnetic Induction. We have seen that a magnet has the power to adl through space and induce another piece of iron or steel to become a magnet. A charge of static eledlricity can induce a charge upon another condudlor. We have now to see how a curre7tt of eledlricity in one condudtor can induce a current in Fig. gg. Fig. 100. another conducftor, not in any way connedled with the first, and how a magnet and a coil can generate a current. 100. Current from Magnet and Coil. If a bar mag- net, Fig. 99, be suddenly thrust into a hollow coil of wire, a momentary current of eledlricity will be generated in the coil . No current passes when the magnet and coil are still; at least one of them must be in motion. Such a current is said to be induced, and is an i7iverse one when 77 78 ELECTRICITY GENERATED BY INDUCTION. the magnet is inserted, and a direcl one when the magnet is withdrawn from the coil. lOi. Induced Currents and Lines of Force. Per- manent magnets are constantly sending out thousands of lines of force. Fig. loo shows a bar magnet entering a coil of wire; the number of lines of force is increasing, and the induced current passes in an anti-clockwise direc- tion when looking down into the coil along the lines of force. This produces an indirecft current. If an iron core be used in the coil, the induced current will be greatly strengthened. It takes force to move a magnet through the center of a coil, and it is this work that is the source of the induced current. We have, in this simple experiment, the key to the adlion of the dynamo and other elecftrical machines. 102. Current from two Coils. Fig. loi shows two coils of wire, the smaller being connedted to a cell, the larger to a galvanometer. By moving the small coil up ELECTRICITY GENERATED BY INDUCTION. 79 and down inside of the large one, induced currents are generated, first in one direcftion and then in the opposite. We have here two entirely separate circuits, in no way connedted. The primary current comes from the cell, while the secondary current is an induced one. By placing a core in the small coil of Fig. loi, the induced current will be greatly strengthened. It is not necessary to have the two coils so that one or both of them can move. They may be wound on the same core, or otherwise arranged as in the indu(flion coil. (See "Study," Chapter XXV., for experiments on induced currents.) CHAPTER XIII. HOW THE INDUCTION COIL WOEKS. 103. The Coils. We saw, § 102, that an induced current was generated when a current-carrying coil, Fig. loi, was thrust into another coil connedted with a galva- nometer. The galvanometer was used merely to show the presence of the current. The primary coil is the one connedled with the cell ; the other one is called the second- ary coil. When a current suddenh- begins to flow through a coil. Fig. 102. the effedt upon a neighboring coil is the same as that pro- duced by suddenly bringing a magnet near it; and when the current stops, the opposite effedl is produced. It is evident, then, that we can keep the small coil of Fig. loi with its core inside of the large coil, and generate induced currents by merely making and breaking the primary circuit. 80 HOW THE INDUCTION COIL WORKS. 8 1 We maj- consider that when the primary- circuit is closed, the lines of force shoot out through the turns of the secondary coil j ast as they do when a magnet or a current-carrj'ing coil is thrust into it. Upon opening the circuit, the lines of force cease to exist; that is, we may imagine them drawn in again. 104. Construction. Fig. 102 shows one form of home-made indudlion coil, given here merely to explain the adlion and connedlions. Nearly all induction coils have some form of automatic current interrupter, placed in the primary circuit, to rapidly turn the current oS and on. Details of Figs. 102 and 103. Wires 5 and 6 are the ends of the primary coil, while wires 7 and 8 are the terminals of the secondary^ coil. The primary coil is wound on a bolt which ser\-es as the core, and on this coil is wound the secondary which consists of many turns of fine wire. The wires from a battery should be joined to binding- posts W and X, and the handles, from which the shock is felt, to Y and Z. Fig. 103 shows the details of the in- terrupter. If the current from a cell enters at W, it will pass through the primary coil and out at X, after going through 5, R, F, S I, B, E and C. The instant the current passes, the bolt becomes magnetized; this attradls A, which pulls B away from the end of SI, thus auto- maticall}' opening the circuit. B at once springs back to its former position against SI, as A is no longer attradled; 82 HOW THE INDUCTION COIL WORKS. the circuit being closed, the operation is rapidly repeated. A condenser is usually connedled to commercial forms. It is placed under the wood-work and decreases sparking at the interrupter. (See "Apparatus Book," Chapter XI., for home-made indudtion coils.) Fig. 104 shows one form of coil. The battery wires are joined to the binding-posts at the left. The secondary coil ends in two rods, and the spark jumps from one to Fig. 104. the other. The interrupter and a switch are shown at the left. Fig. 105 shows a small coil for medical purposes. A dry cell is placed under the coil and all is included in a neat box. The handles form the terminals of the secondary coil. 105. The Currents. It .should be noted that the current from the cell does not get into the secondary coil. The coils are thoroughly insulated from each other. The secondary current is an induced one, its voltage depending upon the relative number of turns of wire there are HOW THE INDUCTION COIIv WORKS. 83 in the two coils. (See Transformers.) Tlie secondary current is an alternating one; that is, it flows in one diredtion for an instant and then immediately reverses its diredlion. The rapidity of the alternations depends upon the speed of the interrupter. Coils are made that give a secondary current with an enormous voltage; so high, in fadt, that the spark will pass many inches, and otherwise adt like those produced by static eledtric machines. 106. Uses of Induction Coils. Gas-jets can be lighted at a distance with the spark from a coil, by ex- tending wires from the secondary' coil to the jet. Powder can be iired at a distance, and other things performed, when a high voltage current is needed. Its use in medi- cine has been noted. It is largely used in telephone work. Of late, great use has been made of the secondary current in experiments with vacuum-tubes, X-ray work, etc. CHAPTER XIV. THE ELECTRIC TELEGEAPH, AND HOW IT SENDS MESSAGES. 107. The Complete Telegraph Line consists of several instruments, switches, etc. , etc. , but its essential parts are: The Line, or wire, which connedts the dififer- ent stations; the Traiismitter or Key; the Receiver or Soimder, and the Battery or Dynamo. 108. The Line is made of strong copper, iron, or soft steel wire. To keep the current in the line it is insu- lated, generall5r upon poles, by glass insulators. For very short lines two wires can be used, the line wire and the return; but for long lines the earth is used as a return, 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 opened or closed at the will of the operator. An ordinarj^ push- button may be used to turn the current off and on, but it is not so convenient as a key. Fig. 106 shows a side view of a simple key which can be put anywhere in the circuit, one end of the cut wire being attached to X and the other to Y. By mo\'ing the lever C up and down according to a previously arranged set of signals, a current will be allowed to pass to a dis- 84 Fig. 106. THE KLECTRIC TELEGRAPH. 85 tant station. As X and Y are insulated from each other, the current can pass only when C presses against Y. Fig. 107 shows a regular key, with switch, Vv^hich is Fig. 107. used to allow the current to pass through the instrument when receiving a message. no. Telegraph Sounders receive the current from some distant station, and with its eledtromagnet produce sounds that can be translated into messages. Fig. 108 shows simply an electromagnet H, the coil being connedted in series with a key K and a cell D C. The key and D C are shown bj^ a top view. The lever of K does not touch the other metal strap until it is pressed down. A little above the core of H is held a strip of iron, 01 armature I. As soon as the circuit is K. I ~S" m H \By~^ 1 — 'f t'^ ( '^E Fig. 108. •-: " closed at K, the current rushes through the circuit, and the core attracts I making a distinct dick. As soon as K is raised, I springs away from the core, if it has been 86 THE ELKCTRIC TELEGRAPH. properly held. In regular instruments a click is also made when the armature springs back again. The time between the two clicks can be short or long, to represent dots or dashes, which, together with spaces, represent letters. ( For Telegraph Alphabet and complete diredlions for home-made 'keys, sounders, etc. , .see "Apparatus Book," Chapter XIV.) Fig. 109 shows a form of home-made sounder. Fig. 1 10 shows one form of telegraph sounder. Over the poles of the horseshoe elecftromagnet is an armature fixed to a metal bar that can rock up and down. The instant the ?ig. 109. current passes through the coils the armature comes down until a stop-screw strikes firml}' upon the metal frame, making the down click. As soon as the distant THE EI.ECTRIC TELEGRAPH. 87 key is raised, the armature is firmly pulled back and another click is made. The two clicks differ in sound, and can be readily recognized by the operator. 111. Connections for Simple Line. Fig. in shows complete connedlions for a home-made telegraph line. The capital letters are used for the right side, R, and small letters for the left side, L,. Gra-^ity cells, B and b, are used. The sounders, S and s, and the iej's, K and k, are shown by a top view. The broad black lines of S and s represent the armatures which are directly over the electromagnets. The keys have switches, E and e. The two stations, R and L, ma}' be in the same room, or in different houses. The rehcrn wire, R W, passes from the copper of b to the zinc of B. This is important, as the cells must help each other ; that is, they are in series. The line wire, L, W, passes from one station to the other, and the return may be through the wire, R \V, or through the earth; but for short lines a wire is best. 112. Operation of Simple Line. Suppose two boys, R (right) and L (left) have a line. Fig. in shows that R's switch, E, is open, while e is closed. The entire circuit, then, is broken at but one point. As soon as R presses his key, the circuit is closed, and the current from both cells rushes around from B, through K, S, LW, s, k b, R W, and back to B. This makes the armatures of Fig. III. 88 THB ELECTRIC TELEGRAPH. S and s come down with a click at the same time. As soon as the key is raised, the armatures lift and make the up-click. As soon as R has finished, he closes his switch E. As the armatures are then held down, L, knows that R has finished, so he opens his switch e, and answers R. Both E and e are closed when the line is not in use, so that either can open his switch at any time and call up the other. Closed circuit cells must be used for such lines. On very large lines dynamos are used to furnish the current. 113. The Relay. Owing to the large resistance of long telegraph lines, the current is weak when it reaches a distant station, and not strong enough to work an J)C. Fig. 112 ordinary sounder. To get around this, relays are used; these are very delicate instruments that replace the sounder in the line wire circuit. Their coils are usually wound with many turns of fine wire, so that a feeble current will move its nicely adjusted armature. The relay armature merely adls as an automatic key to open and close a local circuit which includes a battery and .sounder. The line current does not enter the sounder; it passes back from the relay to the sending station through the earth. Fig. 112 gives an idea of simple relay connedlions. The key K, and cell D C, represent a distant sending station. E is the eledlromagnet of the relay, and R A is THE ELECTRIC TELEGRAPH. 89 its armature. L W and R W represent the line and return wires. R A will vibrate toward E every time K is pressed, and close the local circuit, which includes a local battery, L, B,- and a sounder. It is evident that as soon as K is pressed the sounder will work with a good strong click, as the local battery can be made as strong as desired. Fig. 113 shows a regular instrument which opens and closes the local circuit at the top of the armature. 114. Ink Writing Registers are frequently used Fig. 113. instead of sounders. Fig. 114 shows a writing register that starts itself promptly at the opening of the circuit, and stops automatical!}' as soon as the circuit returns to its normal condition. A strip of narrow paper is slowly pulled from the reel by the machine, a mark being made upon it every time the armature of an inclosed eledlro- magnet is attracted. When the circuit is simplj' closed for an instant, a short line, representing a dot, is made. Registers are built both .single pen and double pen. In the latter case, as the record of one wire is made with 90 THE ELECTRIC TELEGRAPH. a fine pen, and the other with a coarse pen, they can always be identified. The record being blocked out upon white 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 is indelible, it may be read years afterward. Registers are made for local circuits, for use in connedlion with relays, or for direcft use on main lines, as is usually desirable in fire-alarm circuits. CHAPTER XV. THE ELECTRIC BELL AND SOME OF ITS TJSES. 115. Automatic Current Interrupters are used on most common bells, as well as on inducftion coils, etc. (See§ 104.) Fig. 115 shows a simple form of interrupter. The wire I, from a cell D C, is joined to an iron strip I a short distance from its end. The other wire from D C passes to one end of the eledtro- magnet coil H. The remaining end of H is placed in contadl with I as shown, completing the circuit. As soon as the current passes, I is pulled down and awaj- from the upper wire 2, breaking the circuit. I, being held b}' its left-hand end firmly in the hand, immediately springs back to its former position, closing the circuit again. Fig. 115- This adlion is repeated, the rapidity of the vibrations depending somewhat upon the position of the wires on I. In regular instruments a platinum point is used where 91 92 THE ELKCTRIC BEEL AND SOME OP ITS USES. Fig. 117. the circuit is broken ; this stands the .sparking when the armature vibrates. 116. Electric Bells may be illustrated by referring to Fig. 116, which shows a circuit sim- ilar to that described in § 115, but which also contains a key K, in the circuit. This allows the circuit to be opened and closed at a distance from the vibrating armature. The cir- cuit must not be broken at two places at the same time, so wires should touch at the end of I be- fore pressing K. Upon pressing K the armature I will vibrate rapidl3^ By placing a small bell near the end of the vibrating armature, so that it will be struck b)^ I at each vibration, we should have a simple elecflric bell. This form of eledlric bell is called a trembling bell, on account of its vibrating ar- mature. Fig. 117 shows a form of j;|&S^ trembling bell with cover removed. Fig. 118 shows a siiigle- stroke bell, used for fire-alarms and other signal work. In this the armature is attradled but once each time the current passes. As Fig. n8. THE ELECTRIC BELL AND SOME OE ITS USES. 93 Fig. 119. many taps of the bell can be given as desired by pressing the push-button. Fig. 119 shows a gong for railway crossings, signals, etc. Fig. 120 shows a circuit including cell, push-button, and bell, with extra wire for lengthen- ing the line. Electro-Mechanical Gongs are used to give loud signals for special purposes. The me- chanical device is started by the eledlric current when the armature of the eledlromagnet is attradled. Springs, weights, etc., are used as the 121 shows a small bell of this kind. 117. Magneto Testing Bells, Fig. 122, are really small hand- power djmamos. The armature is made to revolve between the poles of strong permanent magnets, and it is so wound that it gives a cur- rent with a large E. M. F., so that it can ring through the large re- sistance of a long line to test it. Magneto Signal Bells, Fig. 123, are used as generator and bell in connedtion with telephones. The generator, used to ring a bell at a distant station, stands at the bottom of the box. The power. Fig Fig. 120. 94 THE ELECTRIC BELL AND SOME OF ITS USES. bell is fastened to the lid, and receives current from a distant bell. Il8. Electric Buzzers have the same general con- strudtion as eledlric bells; in fadt, you will have a buzzer Fig. 121. Fig. 122. Fig. 124. by removing the bell from an ordinary eledric bell. Buzzers are used in places where the loud sound of a bell would be objedlionable. Fig. 1 24 shows the usual form of buzzers, the cover being removed. CHAPTER XVI. THE TELEPHONE, AND HOW IT TRANSMITS SPEECH. 119. The Telephone is an instrument for reproduc- ing sounds at a distance, and elecftricity is the agent by which this is generally accomplished. The part spoken to is called the transmitter, and the part which gives Fig. 125. Fig. 126. sound out again is called the receiver. Sound itself does not pass over the line. While the same apparatus can be used for both transmitter and receiver, they are generally different in constru(5lion to get the best results. Fig. 127. 120. The Bell or Magneto-transmitter generates its own current, and is, stridtly speaking, a dynamo that 95 96 HOW THE TELEPHONE TRANSMITS SPEECH. is run by the voice. It depends upon indudtion for its adtion. Fig. 125 shows a coil of wire, H, with soft iron core, tlie ends of the wires being conne(5led to a dehcate gal- vanoscope. If one pole of the magnet H M be suddenly Fig. 12S. moved up and down near the core, an alternating current will be generated in the coil, the circuit being completed through the galvanoscope. As H M approaches the core the current will flow in one diredlion, and as H M is withdrawn it will pass in the opposite diredlion. The combination makes a miniature alternating djmamo. If we imagine the soft iron core of H, Fig. 125, taken out, and one pole of HM, or preferably that of a bar magnet stuck through the coil, a feeble current will also be produced by moving the soft iron back and forth near the magnet's pole. This is really what is done in the Bell transmitter, .soft iron in the shape of a thin disc (D, Fig. 126) being made to vibrate by the voice immedi- ately in front of a coil having a per- manent magnet for a core. The disc, or diaphragm, as it is called, is fixed near, but it does not touch, the magnet. It is under a constant strain, being attradted hy the magnet, so its Fig. 129. HOW THE TELEPHONE TRANSMITS SPEECH. 97 slightest movement changes the strength of the magnetic field, causing more or less lines of force to shoot through the turns of the coil and induce a current. The coil con- M ^ Fig. 130. sists of many turns of fine, insulated wire. The current generated is an alternating one, and although exceedingly small can force its way through a long length of wire. Fig. 127 shows a section of a regular transmitter, and Fig. 128 a form of compound magnet frequently used in the transmitter. Fig. 129 shows a transmitter with cords which contain flexible wires. 121. The Re- ceiver, for short lines, may have the same con- strudlion as the Bell transmitter. Fig. 130 shows a diagram of two Bell receivers, either being used as the transmitter and the other as the receiver. As the alternating current goes to the distant receiver, it flies through the coil first in one diredtion and then in the other. This al- Fig. 131. 98 HOW THE TEI.EPHONK TRANSMITS SPEECH. ternately strengthens and weakens the magnetic field near the diaphragm, causing it to vibrate back and forth as the magnet pulls more or less. The receiver dia- phragm repeats the vibrations in the transmitter. / ]X. M^ aaC ^ Fig. 132. Nothing but the induced eledlric current passes over the wires. 122. The Microphone. If a current of eleiftricity be allowed to pass through a circuit like that shown in Fig, 131, which includes a battery, a Bell receiver, and a microphone, any slight sound near the microphone will be greatly magnified in the receiver. The microphone consists of pieces of carbon so fixed that the}' form loose contacTis. An}- slight movement of the carbon causes the resistance to the current to be greatly changed. The rapidly varying resistance allows more or less current to pass, the result being that this pulsating current causes the diaphragm to vibrate. The diaphragm has a con- stantly var3-ing pull upon it when the carbons are in any C K I Fig. 133- way disturbed Ijy the voice, or by the ticking of a watch, etc. This principle has been made use of in carbon transmitters, which are made in a large variety of forms. 123. The Carbon Transmitter does not, in itself, HOW THIi TEI^EPHONJi TRANSMITS SPEECH. 99 generate a current like the magneto-transmitter; it merely produces changes in the strength of a current that flows through it and that comes from some outside source. In Fig. 132, X and Y are two carbon buttons, X being attached to the diaphragm D. Button Y presses gently against X, allow- ing a little current to pass through the circuit which in- cludes a battery, D C, and a re- ceiver, R. When D is caused to vibrate by the voice, X is made to press more or less against Y, and this allows more or less current to pass through the cir- cuit. This direct undulating current changes the pull upon the diaphragm of R, causing it to vibrate and reproduce the original sounds .spoken into the transmitter. In regular lines, of course, a receiver and transmitter are connedled at each end, together with bells, etc. , for signaling. 124. Induction Coils in Telephone Work. As the resistance of long telephone lines is great, a high elecftric- al pressure, or E.M.F., is desired. While the current from one or two cells is sufficient to work the transmitter properly, and cause undulating currents in the short line, it does not have power enough to force its way over a long line. To get around this difBculty, an indudlion coil. Fig. 1 33, Fig. 134- lOO HOW THU TELEPHONE TRANSMITS SPEECH. Fig. 135- is used to transform the battery current, that flows through the carbon transmitter and primary coil, into a current with a high E. M. F. The battery current in the primary coil is undula- ting, but always passes in the same diredlion, making the magnetic field around the core weaker and stronger. This causes an alternating current in the secondar}^ coil and main line. In Fig. 133 P and S rep- resent the primary and second- arjr coils. P is joined in series with a cell and carboti trans- mitter; S is joined to the distant receiver. One end of S can be grounded, the current comple- ting the circuit through the earth and into the receiver through an- other wire entering the earth. 125. Various forms of telephones are shown in Figs. i34> 135, 136. Fig. 134 shows a form of desk tele- phone ; Fig. 135 shows a common form of wall tele- phone ; Fig^ 136 shows head- telephones for switchboard operators. Fig. 136. CHAPTER XVII. HOW ELECTRICITY IS GENERATED BY DYNAMOS. 126. The Dynamo, Dynayno- Electric Machine or Gen- eraioj-, is a machine for converting mechanical energy into an eledtric current, through electromagnetic inducftiou. The dynamo is a machine that will convert .steam power, for example, into an eledtric ctirrent. Stridtly speaking, a djmamo creates elecftrical pressure, or eleiftromotive force, and not eledtricit}^ just as a force-pump creates water-pressure, and not water. They are generally run by steam or water power. 127. Induced Currents. We have already spoken ^SHrrfc^ Fig. 137- H Fig. 13S. about currents being induced by moving a coil of wire in a magnetic field. "We shall now see how this principle is used in the dynamo which is a generator of induced currents. Fig. 137 shows how a current can be generated b}' a bar magnet and a coil of wire. Fig. 138 shows how a current can be generated by a horseshoe magnet and a coil of wire having an iron core. The ends of the coil are lo.-' ELECTRICITY GENERATED BY DYNAMOS. to be connec5led to an astatic galvanoscope; this forms a closed, circuit. The coil may be moved past the magnet, or the magnet past the coil. Fig. 139 shows how a current can be generated bj' two Fig. 139. Fig. 140. coils, H being connedted to an astatic galvanoscope and E to a battery. By suddenh* bringing E toward H or the core of E past that of H, a current is produced. We have in this arrangement the main features of a dynamo. We can reverse the operation, holding E in one position Fig. 141. Fig. 142. and moving H rapidly toward it. In this case H would represent the armature and E the field-magnet. When H is moved toward E, the induced current in H flows in one diredtion, and when H is suddenljr withdrawn from ELECTRICITY GENERATED BY DYNAMOS. 103 (See "Study," Chapter E the current is reversed in H. XXV., for experiments.) 128. Induced Currents by Rotary Motion. The motions of the coils in straight lines are not suit- able for producing currents strong enough for com- mercial purposes. In order to generate currents of considerable strength and pressure, the coils of wire have to be pushed past magnets, oreledtromagnets, with great speed. In the d3'namo the coils are so wound that they can be given a rapid rotarj^ motion as they fl}' past strong eledlro- magnets. In this way the coil can keep on passing the same magnets, in the same direction, as long as force is applied to the shaft that carries them. 129. Field-Magnets; Armature ; Commuta- tor. What we need then, to produce an induced current by a rotarj' motion, is a strong magnetic field, a rotating coil of wire Fig. 144. properly placed in the <> ^ <> ^. I04 EI.ECTRICITY GENERATED BY DYNAMOS. Fig. 145- field, and some means of leading the current from the machine. If a loop of wire, Fig. 14.0, be so arranged on bearings at its ends that it can be made to revolve, a current will flow through it in one diredlion during one-half of the revolu- tion, and in the opposite direc- tion during the other half, it being insulated from all ex- ternal conductors. This agrees with the experiments suggested in § 127, when the current generated in a coil passed in one direction during its motion toicard the strongest part of the field, and in the opposite direcflion when the coil passed out of it. A coil must be cut b}' lines of force to generate a current. A current inside of the machine, as in Fig. 140, would be of no value; it must be led out to external conductors where it can do work. Some sort of sliding contacft is necessarj' to connedl a revolving conductor with outside stationarj- ones. The magnet, called theyfj-A/- magnet, is merely to furnish lines of magnetic force. The one turn of wire represents the simplest form of armature. Fig. 146. EI