Things a boy should know about electricity

Front matter

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Table Of Contents

Chapter Page

I. About Frictional Electricty. . . . . 7

II. About Magnets and Magnetism ... 21

Chapter I. About Frictional Electricity

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 unsuspedling 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 your hand, and finally see it cling to the wall?

ABOUT FRICTION AI. KI^ECTRICITY.

Were you ever in a factory where there were large belts running rapidly over pulleys or wheels, and where large sparks would jump to your hands when held near the belts?

If you have never performed any of the four experiments mentioned, you 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 friclion of substances upon each other, it is Qdll^di frUlional eledlricity. It is also called static electricity, because it generally stands still upon the surface of bodies and does not '' flow in currents " as easily as some of the other varieties. Static eledlricity may be

produced by induction as well as by fridlion. 3. History. It has been known for over 2 ,000 3'ears that certain substances adt queerly when rubbed. Amber was the first substance upon which ele(ftricity was produced by friction, and as the Greek name for amber is elektron, bodies so affedled were said to be eleflrtjied. When a body, like ebonite, is rubbed with a flannel cloth, we say that it becomes charged with electricity. Just what happens to the ebonite is not clearly under-

ABOUT FRICTIONAL EI.ECTRICITY.

sr stood. We know, however, that it will attrac5l 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 been elec?trified with a flannel cloth.

4. Conductors and Non-Conductors. Electricity can be produced upon glass and ebonite because they do not carry or conduct it away. If a piece of iron be rubbed, the eledlricity passes from the iron into the earth as fast as it is generated, because the iron is a condu5lor of eledlricity. Glass is an msidator or no7i-co7t' du5lor. Fridlional eledlricity resides upon the outside, only, of condudlors. A hollow tin box

will hold as great a charge as a solid piece of metal having the same outside size and shape. When fridlional eledlricity passes from one place to another, sparks are produced. Lightning is caused by the passage of static eledlricity from a cloud to the earth, 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 even a small piece of damp tissue-paper will serve as an eledlroscope to test the presence of static eledlricity. The pith is usually tied to a piece of silk thread which is a noncondudlor. Fig. 3 shows the ordinary form of pith-ball electroscope.

The leaf electroscope is a very delicate apparatus. Gold-

Fig. 3- lO

ABOUT FRICTIONAL ELECTRICITY.

leaf is generally used, but aluminum-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 and at the same time to insulate them from the earth.

Eledlroscopes are used to show the presence, relative

amount, or kind of static eledlricity on a body. (See ^' Study," Chapter XL)

6. Two Kinds of Electrification. It can be shown that the eledlrification produced on all bodies by fric5lion 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 produced by glass and silk positive, and that by ebonite and flannel negative. 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 attradl each other; (3) either kind of a charge attradls and is attracted by a neutral body.

8. Static Electric Machines. In order to produce

ABOUT FRICTIONAL ELECTRICITY. II

static eledlricity in quantities for experiments, some device is necessary.

The eleSlrophorus (e-lec-troph'-o-rus) is about the simplest form of machine. Fig. 5 shows a simple eleclrophorus in which are two insulators and one conducT:or. The ebonite sheet E S is used with a flannel cloth to generate theelecftricity. 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 a6lion of the eledlrophorus depends upon inducftion. (For experiments, details of adlion, induced elecftrification, etc., see ' ' The Study of Elementary Elec?tricity and Magnetism by Experiment," Chapters VIII. and IX.)

The first ele5lric viachine 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, eledlricity 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. Fricftion is produced by a pad of leather C, which presses against the cylinder as it turns. Ele6lric sparks can be taken from the large * ' condudlors ' ' which are insulated from the earth. The oppo-

ABOUT FRICTIONAI, EL,ECTRICITY.

Fig. 6.

Fig. 7.

ABOUT FRICTIONAL ELECTRICITY.

site eledlricities unite with sparks across D and E. If use is to be made of the eledlricity, either the rubber or the prime condudlor must be connedled with the ground. In the former case positive eledlricity 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 ele(5lricity 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 eledlricity is produced by the principle of indudlion, and not by mere fridlion. This machine, used in connection with condensers, produces large sparks.

ABOUT FRICTIONAI. EI.ECTRICITY.

12. The Wimshurst Machine is of recent date, and not being easily aflfedled by atm(5spheric 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 generally used as an exciter to charge the plates of the large machine when they lose their charge on account of excessive moisture in the atmosphere. Fig. 10 shows a large machine.

14. Uses of Electrical Machines. Static elecflricity has been used for many years in the laboratory for experimental purposes, for charging condensers, for medical purposes, etc. It is now being used for X-ray work, and considerable advancement has been made within a few years in the construction and efficiency of the machines.

ABOUT FRICTIONAI. EI.ECTRICITY.

With the modern machines large sparks are produced by merely turning a crank, enough eledlricity being produced to imitate a small thunderstorm. The sparks of home-made lightning will jump several inches.

Do not think that elecftricity is generated in a commercial way by static eledlric machines. The prac?tical uses of static eledlricity are very few when compared with those of current elecftricity from batteries and dynamos.

15. Condensation of Static Electricity. By means

Fig. II.

of apparatus called condensers, a terrific charge of static eledlricity may be stored. Fig. 11 shows the most common form of condenser, known as the Leydeii 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 outside coating shall be connected with the earth, the sparks i6

ABOUT FRICTIONAI. K1.ECTRICITY.

from an eledlric machine being passed to the knob at the top, which is connedled by a chain to the inside coating* | To discharge the jar, Fig. 12, a condudlor with an

insulating handle is placed against the outside coat ; when the other end of the condudlor is swung over towards the knob, a bright spark passes between them.

This device is called a discharger. Fig. 13 shows a discharge through ether which the spark ignites.

16. The Leyden Battery, Fig. 14, consists of several jars connedled in such a way that the area of the inner and outer coatings is greatly increased. The battery has a larger capacity than one of its jars. (For Experiments in Condensation, see ''Study,'' Chapter X.)

17. Electromotive Force of Static Electricity. Although the sparks of static eledlricity are large, the quantity of eledlricity is very small. It would take thousands of galvanic cells to produce a spark an inch long. While the quantity of static eledlricity is

ABOUT FRICTIONAL ELECTRICITY.

small, its potential, or elecT:romotive 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 Electricity. The air is usually eledlrified, 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 fricflional electricities are of the same nature. By means of a kite, the string being wet by the rain, he succeeded, during a thunderstorm, in drawing sparks, charging condensers, etc.

19. Lightning may be produced by the passage of elecflricity between clouds, or between a cloud and the earth (Fig. 16), which, with the intervening air, have the effedl of a condenser.

When the attraclion between the two eledlrifications gets great enough, a spark passes. When the spark has a zigzag motion it is called chain i8

ABOUT FRICTIONAIy KI.ECTRICITY.

lightning. In hot weather flashes are often seen which light whole clouds, no thunder being heard. This is called heat lightnings and is generally considered to be

due to distant discharges, the light of which is refledled by the clouds. The lightning flash represents billions of volts.

20. Thunder is caused by the violent disturbances produced in the air by lightning. Clouds, hills, etc. produce echoes^ which, with the original sound, make the rolling effedl.

21. Lightning- Rods, when well construdled, often prevent violent discharges. Their pointed prongs at the top allow the negative eledlricity 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 conducting the eledlricity to the earth. The ends of the rods are placed deep in the earth. Fig. 17.

ABOUT FRICTIONAL ELECTRICITY.

22. St. EImo*s Fire. Elecftrification from the earth is often drawn up from the earth through the masts of ships, Fig 18, to neutralize that in the clouds, and, as it escapes from the points of the masts, light is produced. 23. Aurora Borealis, also called Northern Lights, are

^BOUT FRICTIONAI. ELECTRICITY.

luminous effedls, Fig. 19, often seen in the north. They often occur at the same time with magnetic storms, when telegraph and telephone work ma}- be disturbed. The exac5l cause of this light is not known, but it is thought by many to be due to disturbances in the earth's magnetism caused by the adlion of the sun.

Chapter II. About Magnets And Magnetism

24. Natural Magnets. Hundreds of 3'ears ago it was discovered that a certain ore of iron, called lodestone, had the power of picking up small pieces of iron. It was used to indicate the north and south line, and it was discovered later that small pieces of steel could be permanently magnetized by rubbing them upon the lodestone.

25. Artificial Magnets. Pieces of steel, when magnetized, are called artificial magnets. They are made in many forms. The electromagnet is also . an artificial magnet; this will be treated separately.

26. The Horseshoe Magnet, Fig. 20, is, however, the one with which we are the most familiar. They are alw^ays painted red, but the red paint has nothing to do wdth the magnetism.

The little end-piece is called the keeper, or armature ; it should alw^aA'S be kept in place when the magnet is not in use. The magnet itself is made of steel, w^hile the armature is made of soft iron. Steel retains magnetism for a long time, while soft iron loses it almost instantly. The ends of the magnet are called its poles, and nearly all the strength of the magnet seems to reside at the poles, the curved part having no attradlion for outside bodies. One of the poles of the magnet is marked w4th a line, or with the letter N. This is called the north pole of the magnet, the other being its south pole.

' 27. Bar Magnets are straight magnets. Fig. 21 shows a round bar magnet. The screw in the end is for use in the telephone, described later.

28. Compound Magnets. When several thin steel magnets are riveted together, a compound magnet is formed. These can be made with considerable strength. Fig. 22 shows a compound horseshoe magnet. Fig. 23 shows a form of compound bar magnet used in telephones. The use of the coil of wire will be explained later. A thick piece of steel can not be magnetized through and through. In the compound magnet we have the eflecfl of a thick magnet pradlically magnetized through and through.

29. Magnetic and Diamagnetic Bodies. Iron, and substances containing iron, are the ones most readily attracted by a magnet.

Iron is said to be magnetic. Some substances, like nickel, for example, are visibly attracfled by very strong

ABOUT MAGNETS AND MAGNETISM.

magnets only. Strange as it may seem, some substances

are adlually repelled by strong magnets; these are called

diamagnetic bodies. Brass, copper, zinc, etc., are not

visibly affedled by a magnet.

Magnetism will a6l through

paper, glass, copper, lead,

etc.

30. Making Magnets.

One of the strangest properties that a magnet has is its power to give magnetism to another piece of steel. If a sewing-needle be properly rubbed upon one of the poles of a magnet, it will become strongly magnetized and will retain its magnetism for 3'ears. Strong permanent magnets are made with the aid of electromagnets. Any number of little magnets may be made from a horseshoe magnet without injuring it.

31, Magnetic Needles and Compasses. If a bar magnet be suspended by a string, or floated upon a cork, which can easily be done wdth the magnet made from a sewing-needle. Fig. 24, it will swing around until its poles point north and south. Such an arrangement is called a magnetic needle. In the reg-

ABOUT MAGNETS AND MAGNETISM.

ular compass^ a magnetic needle is supported upon a pivot. Compasses have been used for many centuries by mariners and others. Fig. 25 shows an ordinary pocket compass, and Fig. 26 a form of mariner's compass, in which the small bar magnets are fastened to a card which floats, the whole being so mounted that it keeps a horizontal position, even though the vessel rocks.

Z2. Action of Magnets Upon Each Other. By

making two small sewing-needle magnets, you can easily study the laws of attrac5lion and repulsion. By bringing the two north poles, or the two south poles, near each other, a repulsion will be noticed. Unlike poles attracft each other. The attracftion between a magnet and iron is mutual; that is, each attrac5ls the other. Either pole of a magnet attraAs soft iron.

In magnetizing a needle, either end may be made a north pole at will; in fadl, the poles of a weak magnet can easily be reversed by properly rubbing it upon a stronger magnet.

33. Theory of Magnetism. Each little particle of a piece of steel or iron is supposed to be a magnet, even before it touches a magnet. When these little magnets are thoroughly mixed up in the steel, they pull in all sorts of diredlions upon each other and tend to keep the steel from attra(?ting outside bodies. When a magnet is properly rubbed upon a bar of steel, the north poles of the little molecular magnets of the steel are all made to point in the same diredlion. As the north poles help each other, the whole bar can attracl outside bodies.

By jarring a magnet its molecules are thoroughly shaken up; in fa(5l, most of the magnetism can be knocked out of a weak magnet by hammering it.

34. Retentivity. The power that a piece of steel has to hold magnetism is called 7'etentivity . Different kinds of steel have different retentivities. A sewing-needle of good steel will retain magnetism for years, and it is almost impossible to knock the magnetism out by hammering it. Soft steel has very little retentivit}', because it does not contain much carbon. . Soft iron, which contains less carbon than steel, holds magnetism very poorly; so it is not used for permanent magnets. A little magnetism, however, will remain in the soft iron after it is removed from a magnet. This is called residtial magnetism.

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

ABOUT MAGNETS AND MAGNETISM.

will not attradl red-hot iron. The molecules of a piece of red-hot iron are in such a state of rapid vibration that \hey: refuse to be brought into line by the magnet.

36. Induced Magnetism. A piece of soft iron may be induced to become a magnet by holding it near a magnet, absolute contacft not being necessary.

When the soft iron is removed, again, from the influence of the magnet, its magnetism nearly all disappears. It is said to have temporary magnetism ; it had induced magnetism. If a piece of soft iron be held near the north pole of a magnet, as in Fig. 27, poles will be produced in the soft iron, the one nearest the magnet being the south pole, and the other the north pole.

37. Magnetic Field. If a bar magnet be laid upon the table, and a compass be moved about it, the compass-needle will be attracted by the magnet, and it will point in a different direction for every position given to the compass. This strange

power, called magnetism, reaches out on all sides of a magnet. The magnet may be said to a(5l by indudlion

ABOUT MAGNETS AND MAGNETISM.

upon the compass-needle. The space around the magnet, in which this inducT:ive adlion takes place, is called the magnetic field. Fig. 28 shows some of the positions taken by a compass-needle when moved about on one side of a bar magnet.

38. Magnetic Figures can be made by sprinkling iron

ABOUT MAGNETS AND MAGNETISM.

filings upon a sheet of paper under which is placed a magnet. Fig. 29 shows a magnetic figure made with an ordinary bar magnet. The magnet was placed upon the

table and over this was laid a piece of smooth paper.

Fine iron filings were sifted upon the paper, which was gently tapped so that the filings could arrange themselves. As each particle of iron became a little magnet, by indudlion, its poles were attracted and repelled by the magnet; and when the paper was tapped they swung around to their final positions. Notice that the filings have arranged themselves in lines. These lines show the positions of some of the lines of viagnetic force which surrounded the magnet.

These lines of force pass from the north pole of a magnet through the air on all sides to its south pole.

Fig. 30 shows a magnetic figure made from two bar magnets placed side by side, their unlike poles being next to each other. Fig. 31 shows the magnetic figure of a horseshoe magnet with round poles, the poles being uppermost.

39. The Use of Armatures. A magnet attracts iron most strongly at its poles, because it is at the poles that the greatest number of lines of force pass into the air. Lines of force pass easily through soft iron, which is said to be a good conductor of them. Air is not a good conducfhor of the lines of force; in order, then, for the lines of force to pass from the north pole of a magnet to its south pole, they must overcome this resistance of the air, unless the armature is in place. A magnet will gradually grow weaker w^hen its armature is left off.

40. TerrestrialMagnetism. As the compass-needle points to the north and south, the earth must a(5l like a magnet. There is a place very far north, about a thousand miles from the north pole of the earth, which is called the earth's north magnetic pole. Compass-needles point to this place, and not to the earth's real north pole. You can see, then, that if a compass be taken north of this magnetic pole, its north pole will point south. Lines of force pass from the earth's north magnetic pole through the air on all sides of the earth and enter the earth's south magnetic pole. The compass-needle, in pointing toward the north magnetic pole, merely takes the diredlion of the earth's lines of force, just as the particles of iron filings arrange themselves in the magnetic figures.

41. Declination. As the magnetic needle does not point exadlly to the north, an angle is formed between the true north and south line and the line of the needle. In Fig. 32 the line marked N S is the true north and

ABOUT MAGNETS AND MAGNETISM.

_soutli line. The angle of variation^ or the declination, is the angle A between the line N S and the compass-needle.

42. Dip or Inclination. If a piece of steel be carefully balanced upon a support, and then magnetized, it s^

5 ^ will be found that it will no longer balance. The north pole will dip or point downward. Fig. 33 shows what happens to a needle when it is held in different positions

over a bar magnet. It simph^ takes the diredlions of the lines of force as they pass from the ;iorth to the south pole of the magnet. As the earth's lines of force pass in curves from the north to the south magnetic pole, you can see why the magnetic needle dips, unless its south pole is made heavier than its north. Magnetic needles are balanced after they are magnetized.

Fig. 34 shows a simple form of dipping needle. These are often used by geologists and miners. In the hands of the prospedlor, the miner's compass, or dipping needle, proves a serviceable guide to the discovery and location of magnetic iron ore. In this instrument the magnetic needle is carefully balanced upon a horizontal axis within a graduated circle, and in which the needle will be found to assume a position inclined to the horizon. This angle of deviation is called the inclination or dip, and varies in different latitudes, and even at different times in the same place.

43. The Earth's Inductive Influence. The earth's magnetism a(?ts inductively upon pieces of steel or iron upon its surface. If a piece of steel or iron, like a stove poker, for example, be held in a north and south line with its north end dipping considerably, it will be in the best position for the magnetism of the earth to acl upon it; that is, it wall lie in the diredlion taken by the earth's lines of force. If the poker be struck two or three times with a hammer to shake up its molecules, w^e shall find, upon testing it, that it has become magnetized. By this method we can pound magnetism right out of the air with a hammer. If the magnetized poker be held level, in an east and west diredlion, it wnll no longer be acled upon to advantage by the induflive influence of the earth, and we can easily hammer the magnetism out of it again. (For experiments on magnets and magnetism see ''Study,'' Parti.)

Chapter III. How Electricity Is

GENERATED CELL.

BY THE VOLTAIC

44. Early Experiments. In 1786 Galvani, an Italian physician, made experiments to study the efTedt of static eledlricity upon the nervous excitability of animals, and especially upon the frog. He found that eledlric

machines were not necessary to produce muscular contractions or kicks of the frog's legs, and that they could be produced when two different metals, Fig. 35, like iron and copper, for example, were placed in proper contadl with a nerve and a muscle anc then made to touch each other. Galvan first thought that th( frog generated the eledlricity instead of the metals.

Volta proved that the elecftricity was caused by th* contac5t of the metals. He used the condensing eledlroscope as one means of proving that two dissimilar metals become charged differently when in contacft. Volta also

HOW EI.KCTRICITY IS GENERATED.

carried out his belief by construcfting

what is called a Voltaic Pile. He

thought that by making several pairs

of metals so arranged that all the

little currents would help each other,

a strong current could be generated.

Fig. 36 shows 2, pile, it being made by

placing a pair of zinc and copper discs

in contacft with one another, then laying on the copper disc a piece of

flannel soaked in brine, then on top of

this another pair, etc., etc. By con-

nedling the first zinc and the last

copper, quite a little current was produced. This was a start from which

has been built our present knowledge

of eledlricity . Stridlly speaking,

electricity is not generated by combinations of metals or by cells; they

really keep up a difference of potential,

as will be seen.

45. The Simple Cell. It has been stated that two

different kinds of eledlrifications may be produced by

fridlion; one positive, the other negative. Either can be

produced, at will, by using proper materials. Fig. 37 shows a secftion 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.

34 HOW EI.KCTRICITY IS GENERATED.

dilute sulphuric acid, it can be shown by delicate apparatus that they become charged differently, because the acid adls differently upon the plates. They become charged by chemical adlion, and not by fridlion. The zinc is gradually dissolved, and it is this chemical burning of the zinc that furnishes energy for the eledlric current in the simple cell. The eledlrification, or charge, on the plates tends to flow from the place of higher to the place of lower potential, just as water tends to flow down hill. If a wire be joined to the two metals, a constant current of eledlricity wall flow through it, because the acid continues to ac5l upon the plates. The simple cell is a single-fiidd cell, as but one liquid is used in its construdlion.

45a. Plates and Poles. The metal strips used in voltaic cells are called plates or eleniejits. The one most acfted upon by the acid is called the positive (+) plate. In the simple cell the zinc is the + plate, and the copper the negative (— ) plate. The end of a wire attached to the — plate is called the + pole, or eledlrode. Fig. 37 shows the negative (— ) eledlrode as the end of the wire attached to the + plate.

46. Direction of Current. In the cell the current passes from the zinc to the copper; that is, from the positive to the negative plate, where bubbles of hydrogen gas are deposited. In the wire connedling the plates, the current passes from the copper to the zinc plate. In most cells, carbon takes the place of copper. (See ^ ^ Study, '^ §268.)

47. Local Currents; Amalgamation. Ordinary zinc contains impurities such as carbon, iron, etc., and

HOW EI.ECTRICITY IS GENERATED. 35

when the acid comes in contacft with thevSe, they form with the zinc a small cell. This tends to eat away the zinc without producing useful currents. The little currents in the cell from this cause are called local currents, (See ''Study/* Exp. iii,§273.) This is largely overcome by coating the zinc with mercury. This process is called amalgamatio7i. It makes the zinc a(?t like pure zinc, which is not adled upon by dilute sulphuric acid when the current does not pass. (See '' Study," § 257,