phenomena, and a body that is capable of attracting others in this way is said to be electrified.

Demonstration. - Make a wire stirrup, as in Fig. 294, and suspend in it a wooden rod two or three feet long. Suspend this by à silk thread from a support and present, near one end, an electrified glass rod. The wooden rod will be at once attracted. Take out the wooden rod, suspend the glass rod, and present the end of the wooden rod; and now the glass rod moves. Suspend both rods, and they move toward each other until they touch.

FIG. 294

This shows that the action that takes place between an electrified and an unelectrified body is mutual.

341. Two Kinds of Electrification.

We have seen that

the glass rod, the sealing wax, and the ebonite rod all attract other bodies when electrified. Their action upon one another

ELECTRIFIED GLASS ROD.

ELGLASS ROD

FIG. 295

may be seen in the following:

Demonstration. Electrify a glass rod and suspend it in the wire stirrup. Bring a wooden rod near it, and it will be attracted.

Bring an electrified ebonite rod near it, and it will be attracted more

than before. Now electrify a second glass rod and bring it near the end of the suspended rod, and repulsion takes place. Suspend

the electrified sealing wax in the stirrup and hold near it a second electrified stick of sealing wax, and repulsion takes place. Hold near it an electrified glass rod, and it is attracted. Hold near it an electrified ebonite rod, and it is repelled.

We find that the electrified glass rod and sealing wax have apparently the same effect upon an unelectrified body, but act differently upon one that is electrified. When this difference was first observed, the kind of electrification produced on a glass rod by rubbing with silk was called vitreous electricity, and that produced on sealing wax by rubbing it with flannel, resinous electricity. These are now called positive or + and negative or -, respectively. For the sake of convenience, these states of electrification will be spoken of as positive and negative electricity.

Ex

342. Action of Electrified Bodies upon Each Other. periment has established the following general law concerning electrified bodies:

Bodies charged with like electricities repel each other, and those charged with unlike electricities attract each other.

Compare this with the first law of magnetism (§ 322).

Demonstration. - Cut out a number of pith balls with a penknife and roll them in the hands. Use the pith of corn, elder, or, better still, burdock. Suspend one of them from a bent glass rod by a fine silk thread. Electrify a straight glass rod and bring it near the ball, which will at once be attracted, cling to the rod for an instant, and then fly away charged with a + charge. Rub different bodies, such as a dry paper, a rubber comb, etc., first with silk and then with flannel. How are those charged that repel the ball? If a body attracts a charged pith ball, is it a proof that the body is charged?

343. Measure of Electrical Attraction and Repulsion. If two minute bodies with equal charges of like electricities

when one centimeter apart in air, repel each other with a force of one dyne, each charge of electricity is called a unit charge. If one of these unit charges remained the same, the other would need to be increased to 10 units in order to increase the force of repulsion to 10 dynes. It is found that the force of electrical repulsion or attraction between any two small electrified bodies varies directly as the product of the charges, and inversely as the square of the distance between their centers. Hence

in which ƒ is the force in dynes, Q and q the electrical charges of the bodies, and d the distance between their centers in centimeters. By reference to the law

of mutual action (§ 342) it will be seen that the + sign means repulsion and the sign attraction.

344. Electroscopes.-Any instrument by means of which we can determine whether bodies are charged or not is an electroscope. A pith ball arranged as in Fig. 296 is a pith-ball electroscope. A common form is the gold-leaf electroscope,

FIG. 296

which consists of a glass jar, through the wooden stopper of which passes a brass rod terminating in a ball on the outside, and having two long, narrow leaves of some thin metal, as goldfoil, attached to the inner end. Whenever the ball is touched with a charged body the leaves receive a part of the same charge and diverge in accordance with the law of repulsion (§ 342), as shown in Fig. 297.

FIG. 297

FIG. 298

For convenience in using an electroscope, a proof plane may be made by cementing a thin metal disk to the end of a rubber penholder (Fig. 298). Put this plane in contact with any charged body, remove it, and quickly touch the knob of an electroscope with it. Rub a glass rod with silk, touch it with the proof plane, and then bring the plane near the knob of the electroscope. If the leaves diverge still more, they show that the first body was charged positively. If the leaves fall together somewhat, it is probable that the body was charged negatively; but as the repulsion of the leaves is the only sure test, the proof plane must be charged negatively from sealing wax and again brought near the electroscope. If the leaves now diverge, the first body was charged negatively.

345. Conductors and Insulators. Demonstration.

- Wind

one end of a copper wire a meter long around the knob of an electroscope, and attach a

brass ball to the other end. Rest it on a glass support. Charge the proof plane from a charged body and touch the ball with it. The leaves of the electroscope will instantly di

FIG. 299

verge. Discharge the electroscope by touching it with the finger. Replace the wire by a silk thread, and no effect will be seen when the ball is charged. Replace the silk thread by a damp cotton thread, and the leaves will diverge gradually when the ball is charged.

Bodies like the wire, which carry electrical charges readily, are called conductors; while those like silk, which carry them

with difficulty, are called insulators, nonconductors, or dielectrics. There are no substances that are perfect conductors, neither are there any that are perfect insulators; but the following table gives what are usually classed as conductors and insulators, all arranged in the order of their conductivity.

The conductivity of bodies depends upon their physical condition, temperature and moisture having a decided effect. Glass becomes a conductor at 200° C. Air under normal pressure is a good insulator, rarefied air is a poor one. Pure water is a very poor conductor, but is rendered a good conductor by the addition of a little salt or a few drops of acid. Whether bodies are conductors or not also depends upon the character of the electrical charge. A single layer of cotton is insulation between two wires carrying the current for an electric bell, but is no protection whatever against the sparks from a charged glass rod.

346. Friction Develops Both Kinds of Electricity.-Demonstration. Rub a glass rod and an ebonite rod together, much as a mower whets his scythe. Test each by bringing it near the knob of an electroscope. The glass will be found to be charged positively and the ebonite will show an equal negative charge.

Tests carefully made by rubbing various substances together show not only that both kinds of electricity are produced, but also that the opposite charges generated are equal in amount. For instance, the charge generated upon a glass rod is the same in quantity as the charge generated upon the silk by which it is rubbed.