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283. Nature of electrification produced by induction. Let a metal ball A (Fig. 224) be strongly charged by rubbing it with a charged rod, and let it then be brought near an insulated* metal body B which is provided with pith balls or strips of paper a, b, c, as shown. The divergence of a and c will show that the ends of B have received electrical charges because of the presence of A, while the failure of b to diverge will show that the middle of B is uncharged. Further, the rod which charged A will be found to repel c but to attract a.
FIG. 224. Nature of induced charges
We conclude, therefore, that when a conductor is brought near a charged body, the end away from the inducing charge is electrified with the same kind of electricity as that on the inducing body, while the end toward the inducing body receives electricity of the opposite kind.
284. The electron theory of electricity. The atoms of all substances are now known to contain as constituents both positive and negative electricity, the latter existing in the form of minute corpuscles, or electrons, each of which has a mass
1 of that of the hydrogen atom. These electrons are probably grouped in some way about the positive electricity as a nucleus. The sum of the negative charges of these electrons is supposed to be just equal to the positive charge of the nucleus, so that in its normal condition the whole atom is neutral, or uncharged. But in conductors electrons are continually getting loose from the atoms and reëntering other atoms, so that at any given instant there are in every conductor a number of free negative electrons and a corresponding number of atoms which have lost electrons and which are therefore positively charged. Such a conductor would, as a whole, show no charge of either positive or negative electricity.
* Sulphur is practically a perfect insulator in all weathers, wet or dry. Metal conductors of almost any shape resting upon pieces of sulphur will serve the purposes of this experiment in summer or winter.
But as soon as a body charged, for example, positively (Fig. 224) is brought near such a conductor, the negatively charged electrons are attracted to the near end, leaving behind them the positively charged atoms, which are not free to move from their positions. On the other hand, if a negatively charged body is brought near the conductor, the negative electrons stream away and the near end is left with the immovable plus atoms. As soon as the inducing charge is removed, the conductor becomes neutral again, because the little negative corpuscles return to their former positions under the influence of the attraction of the positive atoms. This is the present-day picture of the mechanism of electrification by induction.
The charge of one electron is called the elementary electrical charge. Its value has recently been accurately measured. There are 2.095 billion of them in one of the units defined in § 280. Every electrical charge consists of an exact number of these ultimate electrical atoms.
285. Charging by induction. Let two metal balls or two eggshells, A and B, which have been gilded or covered with tin foil be suspended by silk threads and touched together, as in Fig. 225. Let a positively charged body C be brought near them. As described above, A and B will at once exhibit evidences of electrification; that is, A will repel a positively charged pith ball, while B will attract it. If C is removed while A and B are still in contact, the separated charges reunite and A and B cease to exhibit electrification. But if A and B are separated from each other while C is in place, A will be found to remain positively charged and B negatively charged. This may be proved either by the attractions and repulsions which they show for charged rods brought near them or by the effects which they produce upon a charged electroscope brought into their vicinity, the leaves of the latter falling together when it is brought near one and spreading farther apart when brought near the other.
FIG. 225. Obtaining a plus and a minus charge by induction
Celebrated American statesman, philosopher, and scientist; born at Boston, the sixteenth child of poor parents; printer and publisher by occupation; pursued scientific studies in electricity as a diversion rather than as a profession; first proved that the two coats of a Leyden jar are oppositely charged; introduced the terms positive and negative electricity; proved the identity of lightning and frictional electricity by flying a kite in a thunderstorm and drawing sparks from the insulated lower end of the kite string; invented the lightning rod; originated the one-fluid theory of electricity which regarded a positive charge as indicating an excess, a negative charge a deficiency, in a certain normal amount of an all-pervading electrical fluid
In June, 1752, Franklin demonstrated the identity of the electric spark and lightning. To prevent his kite from being torn in the rain he made it of a silk handkerchief. The lower end of the kite string and a silk ribbon were tied to the ring of a key, and, to prevent any charge that might appear upon the string and the key from escaping through his body to the earth, he held the kite by grasping the insulating silk ribbon. Standing under a shed to keep the ribbon dry, Franklin, by presenting his knuckle to the key, obtained sparks similar to those produced by his electric machine. With these sparks he charged his Leyden jar and used it to give a shock. Indeed, he performed with lightning all the experiments which he had previously performed with sparks from his frictional machine. The experiment is dangerous and should not be attempted by inexperienced persons
We see, therefore, that if we cut in two, or separate into two parts, a conductor while it is under the influence of an electric charge, we obtain two permanently charged bodies, the remoter part having a charge of the same sign as that of the inducing charge, and the near part having a charge of unlike sign. Under the influence of the positive charge on the negative electrons moved out of A into B, which act made A positive and B negative.
Let the conductor B (Fig. 226) be touched at a by the finger while a charged rod is near it. Then let the finger be removed and after it the rod C. If now a negatively charged pith ball is brought near B, it will be repelled, showing that B has become negatively charged. In this experiment the body of the experimenter corresponds to the egg A of the preceding experiment, and removing the finger from B corresponds to separating the two eggshells. Let the last experiment be repeated with only this modification, that B is touched at b rather than at a. When B is again tested with the pith ball, it will still be found to have a negative charge, exactly as when the finger was touched at a.
FIG. 226. A body charged by induction has a charge of sign opposite to that of the inducing charge
We conclude, therefore, that no matter where the body B is touched, the sign of the charge left upon it is always opposite to that of the inducing charge. This is because the negative electricity, that is, the electrons, can under no circumstances escape from so long as C is present, for they are bound by the attraction of the positive charge on C. Indeed, the final negative charge on B is due merely to the fact that the positive charge on C pulls electrons into B from the finger, no matter where B is touched. In the same way, if C had been negative, it would have pushed electrons off from B through the finger and thus have left B positively charged.