be found to have become a magnet with its upper end an S pole, like the north pole of the earth, and its lower end an N pole. If the rod is reversed and tapped again with the hammer, its magnetism will be reversed. If held in an east-and-west position and tapped, it will become demagnetized, as will be shown by the fact that either end of it will attract either end of a compass needle. In some respects a soft-iron rod is more satisfactory for this experiment than a steel rod, on account of the smaller retentivity.

QUESTIONS AND PROBLEMS

1. Make a diagram to show the general shape of the lines of force between unlike poles of two bar magnets; between like poles.

2. Devise an experiment which will show that a piece of iron attracts a magnet just as truly as the magnet attracts the iron.

3. In testing a needle with a magnet to see if the needle is magnetized why must you get repulsion before you can be sure it is inagnetized?

4. A nail lies with its head near the N pole of a bar magnet. Diagram the nail and magnet, and draw from the N pole through the nail a closed curve to represent one line of force.

5. Explain, on the basis of induced magnetization, the process by which a magnet attracts a piece of soft iron.

6. Do the facts of induction suggest to you any reason why a horseshoe magnet retains its magnetism better when a bar of soft iron (a keeper, or armature) is placed across its poles than when it is not so treated? (See Fig. 218.)

7. Why should the needle used in the experiment of § 276 be placed east and west, when adjusting for neutral equilibrium, before it is magnetized?

8. How would an ordinary compass needle act if placed over one of the earth's magnetic poles? How would a dipping needle act at these points?

9. Why are the tops of steam radiators S magnetic poles, as proved by their invariable repulsion of the S pole of a compass?

10. Give two proofs that the earth is a magnet.

11. A magnetic pole of 80 units' strength is 20 cm. distant from a similar pole of 30 units' strength. Find the force between them.

CHAPTER XIII

STATIC ELECTRICITY

GENERAL FACTS OF ELECTRIFICATION

278. Electrification by friction. If a piece of hard rubber or a stick of sealing wax is rubbed with flannel or cat's fur and then brought near some dry pith balls, bits of paper, or other light bodies, these bodies are found to jump toward the rod. This sort of attraction, so familiar to us from the behavior of our hair in winter when we comb it with a rubber comb, was observed as early as 600 B. C., when Thales of Greece commented upon the fact that rubbed amber draws to itself threads and other light objects. It was not, however, until A. D. 1600 that Dr. William Gilbert, physician to Queen Elizabeth, and sometimes called the father of the modern science of electricity and magnetism, discovered that the effect could be produced by rubbing together a great variety of other substances besides amber and silk, such, for example, as glass and silk, sealing wax and flannel, hard rubber and cat's fur, etc.

Gilbert (see opposite p. 222) named the effect which was produced upon these various substances by friction electrification, after the Greek name electron, meaning "amber." Thus, a body which, like rubbed amber, has been endowed with the property of attracting light bodies is said to have been electrified, or to have been given a charge of electricity. In this statement nothing whatever is said about the nature of electricity. We simply define an electrically charged body as one which has been put into the condition in which it acts toward light bodies like the rubbed amber or the rubbed sealing wax. To

this day we do not know with certainty what the nature of electricity is, but we are fairly familiar with the laws which govern its action. The following sections deal with these laws.

279. Positive and negative electricity. Let a pith ball suspended by a silk thread, as in Fig. 221, be touched to a glass rod which has been rubbed with silk; the ball will thus be put into the condition in which it is strongly repelled by this rod.

Next let a stick of sealing wax or an ebonite rod which has been rubbed with cat's fur or flannel be brought near the charged ball. It will be found that it is not repelled but, on the contrary, is very strongly attracted. Similarly, if the pith ball has touched the sealing wax so that it is repelled by it, it is found to be strongly attracted by the glass rod. Again, two pith balls both of which have been in contact with the glass rod are found to repel each other, while pith balls one of which has been in contact with the glass rod and the other with the sealing wax attract each other.

FIG. 221. Pith-ball electroscope

Evidently, then, the electrifications which are imparted to glass by rubbing it with silk and to sealing wax by rubbing it with flannel are opposite in the sense that an electrified body that is attracted by one is repelled by the other. We say, therefore, that there are two kinds of electrification, and we arbitrarily call one positive and the other negative. Thus, a positively electrified body is one which acts with respect to other electrified bodies like a glass rod which has been rubbed with silk, and a negatively electrified body is one which acts like a piece of sealing wax which has been rubbed with flannel. These facts and definitions may be stated in the following general law: Electrical charges of like kind repel each other, while charges of unlike kind attract each other. The forces of attraction or repulsion are found, like those of gravitation and magnetism, to decrease as the square of the distance increases.

280. Measurement of electrical quantities. The fact of attraction and repulsion is taken as the basis for the definition and measurement of so-called quantities of electricity. Thus, a small charged body is said to contain 1 unit of electricity when it will repel an exactly equal and similar charge placed 1 centimeter away with a force of 1 dyne. The number of units of electricity on any charged body is then measured by the force which it exerts upon a unit charge placed at a given distance from it; for example, a charge which at a distance of 10 centimeters repels a unit charge with a force of 1 dyne contains 100 units of electricity, for this means that at a distance of 1 centimeter it would repel the unit charge with a force of 100 dynes (see § 279).

E

Let an electroscope E

B

281. Conductors and nonconductors. (Fig. 222), consisting of a pair of gold leaves a and b, suspended from an insulated metal rod r and protected from air currents by a case J, be connected with the metal ball B by means of a wire. Now let an ebonite rod be electrified and rubbed over B. The immediate divergence of the gold leaves will show that a portion of the electric charge placed upon B has been carried by the wire to the gold leaves, where it causes them to diverge in accordance with the law that bodies charged with the same kind of electricity repel each other.

a

FIG. 222. Illustrating conduction

Let the experiment be repeated when E and B are connected with a thread of silk or a long rod of wood instead of the metal wire. No divergence of the leaves will be observed. If a moistened thread connects E and B, the leaves will be seen to diverge slowly when the ball B is charged, showing that a charge is carried slowly by the moist thread.

These experiments make it clear that while electric charges pass with perfect readiness from one point to another in a wire, they are quite unable to pass along dry silk or wood, and pass with difficulty along moist silk. We are therefore accustomed to divide substances into two classes, conductors and nonconductors, or insulators, according to their ability to transmit

electrical charges from point to point. Thus, metals and solutions of salts and acids in water are all conductors of electricity, while glass, porcelain, rubber, mica, shellac, wood, silk, vaseline, turpentine, paraffin, and oils are insulators. No hard-and-fast line, however, can be drawn between conductors and nonconductors, since all so-called insulators conduct to some slight extent, while the so-called conductors differ greatly in the facility with which they transmit charges.

The fact of conduction brings out sharply one of the most essential distinctions between electricity and magnetism. Magnetic poles exist only in iron and steel, while electrical charges may be communicated to any body whatever, provided it is insulated. These charges pass over conductors and can be transferred by contact from one body to any other, while magnetic poles remain fixed in position and are wholly uninfluenced by contact with other bodies, unless these bodies themselves are magnets.

282. Electrostatic induction. Let the ebonite rod be electrified by friction and slowly brought toward

FIG. 223. Illustrating induction

the knob of the gold-leaf electroscope (Fig. 223). The leaves will be seen to diverge, even though the rod does not approach to within a foot of the electroscope.

This makes it clear that the mere influence which an electric charge exerts upon a conductor placed in its neighborhood is able to produce electrification in that conductor. This method of producing electrification is called electrostatic induction.

As soon as the charged rod is removed, the leaves will be seen to collapse completely. This shows that this form of electrification is only a temporary phenomenon which is due simply to the presence of the charged body in the neighborhood.