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

BENJAMIN FRANKLIN (1706-1790)

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 and a negative charge a deficiency in a certain normal amount of an all-pervading electrical fluid

Such a conductor would, as a whole, show no charge of either positive or negative electricity. But as soon as a body charged, for example, positively (Fig. 224) is brought near such a conductor, the free negative electrons are attracted to the near end, leaving behind them the positively charged but immovable atoms. 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 2,095,000,000 of them in one of the units defined in § 280. Every electrical charge consists of an

exact number of these ultimate electrical atoms. (See opposite p. 479.)

A

B

FIG. 225. Obtaining a plus and a minus charge by induction

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, and 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 produced upon a charged electroscope brought into their vicinity, the leaves falling together when it is brought near one and spreading apart when brought near the other.

We see, therefore, that if we cut in two, or separate into two parts, a conductor while it is under the influence of an electrical 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 C the negative electrons moved out of A into B, which act made A positive and B negative.

a+

B

-b

Let the insulated conductor B (Fig. 226) be touched at a by the finger while a positively charged rod C 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 insulated 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 b 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.

286. Charging the electroscope by induction. Let a glass rod which has been rubbed with silk be brought near the knob of the electroscope. The leaves at once diverge (Fig. 227 (1)). Let the

knob be touched with the finger while the rod is held in place. The leaves will fall together (Fig. 227 (2)). Let the finger be removed (Fig. 227 (3)), and then the rod. The leaves will fly apart again (Fig. 227 (4)).

XXX

(1)

(2)

(3)

(4)

The electroscope is now charged by induction; and as the charge on the glass rod is +, the charge on the electroscope must be. If this conclusion is tested by bringing the charged glass rod near the electroscope, the leaves will fall together as the rod approaches the knob.

This proves that

FIG. 227. Charging the electroscope by induction

the charge on the electroscope is. If the empty neutral hand approaches the knob, the leaves will fall. Explain.

287. Plus and minus electricities always appear simultaneously and in equal amounts. Let an ebonite rod be completely discharged by passing it quickly through a Bunsen flame. Let a flannel cap having a silk thread attached be slipped over the rod, as in Fig. 228, and twisted rapidly round a number of times. When rod and cap together are held near a charged electroscope, no effect will be observed; but if the cap is pulled off, it will be found to be positively charged, and the rod will be found to have a negative charge.

FIG. 228. Plus and minus electricities always developed in equal amounts

Since the two together produce no effect, the experiment shows that the plus and minus charges were equal in amount. This experiment confirms the view already brought forward in connection with induction, that electrification always consists in a separation of plus and minus charges that already exist in equal amounts within the bodies in which the electrification is developed.