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Things A Boy Should
Know About
Electricity

BY
THOMAS M. ST. JOHN, Met. E.

Author of "Fun With Magnetism," "Fun With Electricity,"
"How Two Boys Made Their Own Electrical Apparatus,"
"The Study of Elementary Electricity
and Magnetism by Experiment," etc.

Copyright, 1900.
By Thomas M. St. John.

THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY

TABLE OF CONTENTS

TO THE READER

For the benefit of those who wish to make their own electrical apparatus for experimental purposes, references have been made throughout this work to the "Apparatus Book;" by this is meant the author's "How Two Boys Made Their Own Electrical 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 Study of Elementary Electricity and Magnetism by Experiment."

The Author.

Things A Boy Should Know About Electricity

CHAPTER I.
ABOUT FRICTIONAL ELECTRICITY.

1. 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?

Fig. 1.

Did you ever succeed in proving to the pussy-cat, Fig. 1, 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?

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 electricity.

2. Name. As this variety of electricity is made, or generated, by the friction of substances upon each other, it is called frictional electricity. 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 electricity may be produced by induction as well as by friction.

Fig. 2.

3. History. It has been known for over 2,000 years that certain substances act queerly when rubbed. Amber was the first substance upon which electricity was produced by friction, and as the Greek name for amber is elektron, bodies so affected were said to be electrified. 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 understood. We know, however, that it will attract light bodies, and then quickly repel them if they be conductors. Fig. 2 shows a piece of tissue-paper jumping toward a sheet of ebonite that has been electrified with a flannel cloth.

Fig. 3.

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 electricity passes from the iron into the earth as fast as it is generated, because the iron is a conductor of electricity. Glass is an insulator or non-conductor. Frictional electricity resides upon the outside, only, of conductors. A hollow tin box will hold as great a charge as a solid piece of metal having the same outside size and shape. When frictional electricity passes from one place to another, sparks are produced. Lightning is caused by the passage of static electricity from a cloud to the earth, or from one cloud to another. In this case air forms the conductor. (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 electroscope to test the presence of static electricity. The pith is usually tied to a piece of silk thread which is a non-conductor. Fig. 3 shows the ordinary form of pith-ball electroscope.

Fig. 4.

The leaf electroscope is a very delicate apparatus. Gold-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.

Electroscopes are used to show the presence, relative amount, or kind of static electricity on a body. (See "Study," Chapter XI.)

6. Two Kinds of Electrification. It can be shown that the electrification produced on all bodies by friction 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. (1) Charges of the same kind repel each other; (2) charges of unlike kinds attract each other; (3) either kind of a charge attracts and is attracted by a neutral body.

8. Static Electric Machines. In order to produce static electricity in quantities for experiments, some device is necessary.

The electrophorus (e-lec-troph´-o-rus) is about the simplest form of machine. Fig. 5 shows a simple electrophorus in which are two insulators and one conductor. The ebonite sheet E S is used with a flannel cloth to generate the electricity. 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 action of the electrophorus depends upon induction. (For experiments, details of action, induced electrification, etc., see "The Study of Elementary Electricity and Magnetism by Experiment," Chapters VIII. and IX.)

Fig. 5.

The first electric machine 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, electricity was produced.

Fig. 6.

Fig. 7.

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. Friction is produced by a pad of leather C, which presses against the cylinder as it turns. Electric sparks can be taken from the large "conductors" which are insulated from the earth. The opposite electricities unite with sparks across D and E. If use is to be made of the electricity, either the rubber or the prime conductor must be connected with the ground. In the former case positive electricity 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 being usually employed, one on each side of the plate. One operator is seen on an insulated stool (Fig. 7), the electricity passing through him before entering the earth by way of the body of the man at the right.

Fig. 8.

11. The Toepler-Holtz Machine, in one form, is shown in Fig. 8. The electricity is produced by the principle of induction, and not by mere friction. This machine, used in connection with condensers, produces large sparks.

Fig. 9.

12. The Wimshurst Machine is of recent date, and not being easily affected 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 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 electricity 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.

Fig. 10.

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

Do not think that electricity is generated in a commercial way by static electric machines. The practical uses of static electricity are very few when compared with those of current electricity from batteries and dynamos.

15. Condensation of Static Electricity. By means of apparatus called condensers, a terrific charge of static electricity may be stored. Fig. 11 shows the most common form of condenser, known as the Leyden 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 from an electric machine being passed to the knob at the top, which is connected by a chain to the inside coating.

To discharge the jar, Fig. 12, a conductor with an insulating handle is placed against the outside coat; when the other end of the conductor 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.

Fig. 13.

16. The Leyden Battery, Fig. 14, consists of several jars connected 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.)

Fig. 14.

Fig. 15.

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

Fig. 16.

19. Lightning may be produced by the passage of electricity between clouds, or between a cloud and the earth (Fig. 16), which, with the intervening air, have the effect of a condenser. When the attraction between the two electrifications gets great enough, a spark passes. When the spark has a zigzag motion it is called chain lightning. In hot weather flashes are often seen which light whole clouds, no thunder being heard. This is called heat lightning, and is generally considered to be due to distant discharges, the light of which is reflected by the clouds. The lightning flash represents billions of volts.

Fig. 17.

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 effect.

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

22. St. Elmo's Fire. Electrification 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.

Fig. 18.

23. Aurora Borealis, also called Northern Lights, are luminous effects, Fig. 19, often seen in the north. They often occur at the same time with magnetic storms, when telegraph and telephone work may be disturbed. The exact 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 action of the sun.

Fig. 19.

CHAPTER II.
ABOUT MAGNETS AND MAGNETISM.

Fig. 20

24. Natural Magnets. Hundreds of years 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 always painted red, but the red paint has nothing to do with the magnetism.

Fig. 21.

Fig. 22.

The little end-piece is called the keeper, or armature; it should always be kept in place when the magnet is not in use. The magnet itself is made of steel, while 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 attraction for outside bodies. One of the poles of the magnet is marked with 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 effect of a thick magnet practically magnetized through and through.

Fig. 23.

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 attracted by very strong magnets only. Strange as it may seem, some substances are actually repelled by strong magnets; these are called diamagnetic bodies. Brass, copper, zinc, etc., are not visibly affected by a magnet. Magnetism will act through paper, glass, copper, lead, etc.

Fig. 24.

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 years. 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.

Fig. 25.

31. Magnetic Needles and Compasses. If a bar magnet be suspended by a string, or floated upon a cork, which can easily be done with 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 regular 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.

Fig. 26.

32. Action of Magnets Upon Each Other. By making two small sewing-needle magnets, you can easily study the laws of attraction and repulsion. By bringing the two north poles, or the two south poles, near each other, a repulsion will be noticed. Unlike poles attract each other. The attraction between a magnet and iron is mutual; that is, each attracts the other. Either pole of a magnet attracts soft iron.

In magnetizing a needle, either end may be made a north pole at will; in fact, 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 directions upon each other and tend to keep the steel from attracting 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 direction. As the north poles help each other, the whole bar can attract outside bodies.

By jarring a magnet its molecules are thoroughly shaken up; in fact, 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 retentivity. 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 retentivity, 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 residual magnetism.

Fig. 27.

35. Heat and Magnetism. Steel will completely lose its magnetism when heated to redness, and a magnet will not attract red-hot iron. The molecules of a piece of red-hot iron are in such a state of rapid vibration that they 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 contact 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.

Fig. 28.

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 act by induction upon the compass-needle. The space around the magnet, in which this inductive action 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 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 induction, 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 magnetic 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. 31.

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 conductor 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 when its armature is left off.

40. Terrestrial Magnetism. As the compass-needle points to the north and south, the earth must act 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 direction 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 exactly 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 south 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 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 simply takes the directions of the lines of force as they pass from the north 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.

Fig. 34 shows a simple form of dipping needle. These are often used by geologists and miners. In the hands of the prospector, 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 acts 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 act upon it; that is, it will lie in the direction 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, we 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 direction, it will no longer be acted upon to advantage by the inductive influence of the earth, and we can easily hammer the magnetism out of it again. (For experiments on magnets and magnetism see "Study," Part I.)

CHAPTER III.
HOW ELECTRICITY IS GENERATED BY THE VOLTAIC CELL.

Fig. 35.

44. Early Experiments. In 1786 Galvani, an Italian physician, made experiments to study the effect of static electricity upon the nervous excitability of animals, and especially upon the frog. He found that electric 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 contact with a nerve and a muscle and then made to touch each other. Galvani first thought that the frog generated the electricity instead of the metals.

Volta proved that the electricity was caused by the contact of the metals. He used the condensing electroscope as one means of proving that two dissimilar metals become charged differently when in contact. Volta also carried out his belief by constructing 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 a pile, it being made by placing a pair of zinc and copper discs in contact 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 connecting 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 electricity. Strictly speaking, electricity is not generated by combinations of metals or by cells; they really keep up a difference of potential, as will be seen.

Fig. 36.

45. The Simple Cell. It has been stated that two different kinds of electrifications may be produced by friction; one positive, the other negative. Either can be produced, at will, by using proper materials. Fig. 37 shows a section of a simple cell; Fig. 38 shows another view. Cu is a piece of copper, and Zn a piece of zinc. When they are placed in dilute sulphuric acid, it can be shown by delicate apparatus that they become charged differently, because the acid acts differently upon the plates. They become charged by chemical action, and not by friction. The zinc is gradually dissolved, and it is this chemical burning of the zinc that furnishes energy for the electric current in the simple cell. The electrification, 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 electricity will flow through it, because the acid continues to act upon the plates. The simple cell is a single-fluid cell, as but one liquid is used in its construction.

45a. Plates and Poles. The metal strips used in voltaic cells are called plates or elements. The one most acted 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 electrode. Fig. 37 shows the negative (-) electrode 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 connecting 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 when the acid comes in contact with these, 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. 111, § 273.) This is largely overcome by coating the zinc with mercury. This process is called amalgamation. It makes the zinc act like pure zinc, which is not acted upon by dilute sulphuric acid when the current does not pass. (See "Study," § 257, 274.)

48. Polarization of Cells. Bubbles of hydrogen gas are formed when zinc is dissolved by an acid. In the ordinary simple cell these bubbles collect on the copper plate, and not on the zinc plate, as might be expected. The hydrogen is not a conductor of electricity, so this film of gas holds the current back. The hydrogen acts like a metal and sets up a current that opposes the zinc to the copper current. Several methods are employed to get rid of the hydrogen. (See "Study," § 278, 279, 280.)

CHAPTER IV.
VARIOUS VOLTAIC CELLS.

49. Single-Fluid and Two-Fluid Cells. The simple cell (§ 45) is a single-fluid cell. The liquid is called the electrolyte, and this must act upon one of the plates; that is, chemical action must take place in order to produce a current. The simple cell polarizes rapidly, so something must be used with the dilute sulphuric acid to destroy the hydrogen bubbles. This is done in the bichromate of potash cell.

In order to get complete depolarization—that is, to keep the carbon plate almost perfectly free from hydrogen, it is necessary to use two-fluid cells, or those to which some solid depolarizer is added to the one fluid.

50. Open and Closed Circuit Cells. If we consider a voltaic cell, the wires attached to it, and perhaps some instrument through which the current passes, we have an electric circuit. When the current passes, the circuit is closed, but when the wire is cut, or in any way disconnected so that the current can not pass, the circuit is open or broken. (See "Study," § 266.)

Open Circuit Cells are those which can give momentary currents at intervals, such as are needed for bells, telephones, etc. These must have plenty of time to rest, as they polarize when the circuit is closed for a long time. The Leclanché and dry cells are the most common open circuit cells.

Closed Circuit Cells. For telegraph lines, motors, etc., where a current is needed for some time, the cell must be of such a nature that it will not polarize quickly; it must give a strong and constant current. The bichromate and gravity cells are examples of this variety. (See "Study," § 286.)

Fig. 39.

51. Bichromate of Potash Cells are very useful for general laboratory work. They are especially useful for operating induction coils, small motors, small incandescent lamps, for heating platinum wires, etc. These cells have an E.M.F. of about 2 volts. Dilute sulphuric acid is used as the exciting fluid, and in this is dissolved the bichromate of potash which keeps the hydrogen bubbles from the carbon plate. (See "Apparatus Book," § 26.) Zinc and carbon are used for the plates, the + pole being the wire attached to the carbon.

Fig. 39 shows one form of bichromate cell. It furnishes a large quantity of current, and as the zinc can be raised from the fluid, it may be kept charged ready for use for many months, and can be set in action any time when required by lowering the zinc into the liquid. Two of these cells will burn a one candle-power miniature incandescent lamp several hours. The carbon is indestructible.

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

52. The Grenet Cell. Fig. 40 is another form of bichromate cell. The carbon plates are left in the fluid constantly. The zinc plate should be raised when the cell is not in use, to keep it from being uselessly dissolved.

53. Plunge Batteries. Two or more cells are often arranged so that their elements can be quickly lowered into the acid solution. Such a combination, Fig. 41, is called a plunge battery. The binding-posts are so arranged that currents of different strengths can be taken from the combination. The two binding-posts on the right of the battery will give the current of one cell; the two binding-posts on the left of the battery will give the current of two cells, and the two end binding-posts will give the current of all three cells. When not in use the elements must always be hung on the hooks and kept out of the solution.

54. Large Plunge Batteries. Fig. 42, are arranged with a winch and a bar above the cells; these afford a ready and convenient means of lifting or lowering the elements and avoiding waste. In the battery shown, Fig. 42, the zincs are 4×6 inches; the carbons have the same dimensions, but there are two carbon plates to each zinc, thus giving double the carbon surface.

Fig. 42.

55. The Fuller Cell, Fig. 43, is another type of bichromate cell, used largely for long-distance telephone service, for telephone exchange and switch service, for running small motors, etc. It consists of a glass jar, a carbon plate, with proper connections, a clay porous cup, containing the zinc, which is made in the form of a cone. A little mercury is placed in the porous cup to keep the zinc well amalgamated. Either bichromate of potash or bichromate of soda can be used as a depolarizer.

Fig. 43.

Fig. 44.

56. The Gravity Cell, sometimes called the bluestone or crowfoot cell, is used largely for telegraph, police, and fire-alarm signal service, laboratory and experimental work, or whenever a closed circuit cell is required. The E.M.F. is about one volt. This is a modified form of the Daniell cell. Fig. 44 shows a home-made gravity cell.

A copper plate is placed at the bottom of the glass jar, and upon this rests a solution of copper sulphate (bluestone). The zinc plate is supported about four inches above the copper, and is surrounded by a solution of zinc sulphate which floats upon the top of the blue solution. An insulated wire reaches from the copper to the top of the cell and forms the positive pole. (See "Apparatus Book," § 11 to 15, for home-made gravity cell, its regulation, etc. For experiments with two-fluid Daniell cell, see "Study," Exp. 113, § 281 to 286.)

Fig. 45.

56a. Bunsen Cells, Fig. 45, are used for motors, small incandescent lamps, etc. A carbon rod is inclosed in a porous cup, on the outside of which is a cylinder of zinc that stands in dilute sulphuric acid, the carbon being in nitric acid.

57. The Leclanché Cell is an open circuit cell. Sal ammoniac is used as the exciting fluid, carbon and zinc being used for plates. Manganese dioxide is used as the depolarizer; this surrounds the carbon plate, the two being either packed together in a porous cup or held together in the form of cakes. The porous cup, or pressed cake, stands in the exciting fluid. The E. M. F. is about 1.5 volts.

Fig. 46.

Fig. 47.

Fig. 48.

Fig. 46 shows a form with porous cup. The binding-post at the top of the carbon plate forms the + electrode, the current leaving the cell at this point.

Fig. 49.

The Gonda Prism Cell (Fig. 47), is a form of Leclanché in which the depolarizer is in the form of a cake.

Fig. 50.

58. Dry Cells are open circuit cells, and can be carried about, although they are moist inside. The + pole is the end of the carbon plate. Zinc is used as the outside case and + plate. Fig. 48 shows the ordinary forms.

Fig. 49 shows a number of dry cells arranged in a box with switch in front, so that the current can be regulated at will.

59. The Edison-Lelande Cells, Fig. 50, are made in several sizes and types. Zinc and copper oxide, which is pressed into plates, form the elements. The exciting fluid consists of a 25 per cent. solution of caustic potash in water. They are designed for both open and closed circuit work.

CHAPTER V.
ABOUT PUSH-BUTTONS, SWITCHES AND BINDING-POSTS.

60. Electrical Connections. In experimental work, as well as in the everyday work of the electrician, electrical connections must constantly be made. One wire must be joined to another, just for a moment, perhaps, or one piece of apparatus must be put in an electric circuit with other apparatus, or the current must be turned on or off from motors, lamps, etc. In order to conveniently and quickly make such connections, apparatus called push-buttons, switches and binding-posts are used.

61. Push-Buttons. The simple act of pressing your finger upon a movable button, or knob, may ring a bell a mile away, or do some other equally wonderful thing. Fig. 51 shows a simple push-button, somewhat like a simple key in construction. If we cut a wire, through which a current is passing, then join one of the free ends to the screw A and the other end to screw C, we shall be able to let the current pass at any instant by pressing the spring B firmly upon A.

Push-buttons are made in all sorts of shapes and sizes. Fig. 52 gives an idea of the general internal construction. The current enters A by one wire, and leaves by another wire as soon as the button is pushed and B is forced down to A. The bottom of the little button rests upon the top of B.

Fig. 53 shows a Table Clamp-Push for use on dining-tables, card-tables, chairs, desks, and other movable furniture. Fig. 54 shows a combination of push-button, speaking-tube, and letter-box used in city apartment houses. Fig. 55 shows an Indicating Push. The buzzer indicates, by the sound, whether the call has been heard; that is, the person called answers back.

Modifications of ordinary push-buttons are used for floor push-buttons, on doors, windows, etc., for burglar-alarms, for turning off or on lights, etc., etc. (See "Apparatus Book," Chapter III., for home-made push-buttons.)

Fig. 55.

62. Switches have a movable bar or plug of metal, moving on a pivot, to make or break a circuit, or transfer a current from one conductor to another.

Fig. 56 shows a single point switch. The current entering the pivoted arm can go no farther when the switch is open, as shown. To close the circuit, the arm is pushed over until it presses down upon the contact-point. For neatness, both wires are joined to the under side of the switch or to binding-posts.

Fig. 56.

Fig. 57 shows a knife switch. Copper blades are pressed down between copper spring clips to close the circuit. The handle is made of insulating material.

Pole-changing switches, Fig. 58, are used for changing or reversing the poles of batteries, etc.

Fig. 59 shows a home-made switch, useful in connection with resistance coils. By joining the ends of the coils A, B, C, D, with the contact-points 1, 2, 3, etc., more or less resistance can be easily thrown in by simply swinging the lever E around to the left or right. If E be turned to 1, the current will be obliged to pass through all the coils A, B, etc., before it can pass out at Y. If E be moved to 3, coils A and B will be cut out of the circuit, thus decreasing the resistance to the current on its way from X to Y. Current regulators are made upon this principle. (See "Apparatus Book," Chapter IV., for home-made switches.)

Fig. 57.

Fig. 58.

Fig. 59.

Switchboards are made containing from two or three to hundreds of 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 incandescent lighting currents.

Fig. 60.

Fig. 61.

63. Binding-Posts are used to make connections between two pieces of apparatus, between two or more wires, between a wire and any apparatus, etc., etc. They allow the wires to be quickly fastened or unfastened to the apparatus. A large part of the apparatus shown in this book has binding-posts attached. Fig. 61 shows a few of the common forms used. (See "Apparatus Book," Chapter V., for home-made binding-posts.)

CHAPTER VI.
UNITS AND APPARATUS FOR ELECTRICAL MEASUREMENTS.

64. Electrical Units. In order to measure electricity for experimental or commercial purposes, standards or units are just as necessary as the inch or foot for measuring 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 electricity jumps a long distance, we say that the charge has a high potential; that is, it has a high electrical pressure. Potential, for electricity, means the same as pressure, for water. The greater the potential, or electromotive force (E.M.F.) of a cell, the greater its power to push a current through wires. (See "Study," § 296 to 305, with experiments.)

66. Unit of E.M.F.; the Volt.—In speaking 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 say that a wire is carrying a 110-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.

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 comparison with the E. M. F. of some standard cell. (See "Study," Exp. 140, for measuring the E. M. F. of a cell by comparison with the two-fluid cell.)

Fig. 62.

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 direct. The reason for this will be seen by referring to Ohm's law ("Study," § 356); the resistance is so great that the strength of the current depends entirely upon the E. M. F.

Fig. 63.

Voltmeters measure electrical pressure just as steam gauges measure the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63 shows a voltmeter with illuminated dial. An electrical bulb behind the instrument furnishes light so that the readings can be easily taken.

68. Electrical Resistance. Did you ever ride down hill on a hand-sled? How easily the sled glides over the snow! What happens, though, when you strike a bare place, or a place where some evil-minded person has sprinkled ashes? Does the sled pass easily over bare ground or ashes? Snow offers very little resistance to the sled, while ashes offer a great resistance.

Fig. 64.

All substances do not allow the electric current to pass through them with the same ease. Even the liquid in a cell tends to hold the current back and offers internal resistance. The various wires and instruments connected to a cell offer external resistance. (See "Study," Chapter XVIII., for experiments, etc.)

69. Unit of Resistance. The Ohm is the name given to the unit of resistance. About 9 ft. 9 in. of No. 30 copper wire, or 39 feet 1 in. of No. 24 copper wire, will make a fairly accurate ohm.

Resistance coils, having carefully measured resistances, are made for standards. (See "Apparatus Book," Chapter XVII., 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 from its ends for connections. 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. 1. The resistance of a wire is directly proportional to its length, provided its cross-section, material, etc., are uniform.

2. The resistance of a wire is inversely proportional to its area of cross-section; or, in other words, inversely proportional to the square of its diameter, other things being equal.

Fig. 65.

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.)

Fig. 66.