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Page:Popular Science Monthly Volume 9.djvu/48

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30
THE POPULAR SCIENCE MONTHLY.

LESSONS IN ELECTRICITY.[1]

HOLIDAY LECTURES AT THE ROYAL INSTITUTION.

By Prof. TYNDALL, F. R. S.
II.

SECTION 8. Electrics and Non-Electrics.—For a long period, bodies were divided into electrics and non-electrics, the former deemed capable of being electrified, the latter not. Thus the amber of the ancients, and the spars, gems, fossils, stones, glasses, and resins, operated on by Dr. Gilbert, were electrics, while all the metals were non-electrics. We must now determine the true meaning of this distinction.

Take in succession a ball of brass, of wood coated with tin-foil, a lead bullet, and an apple, in the hand, and strike them briskly with silk, flannel, or the fox's brush; none of them will attract the balanced lath (Fig. 4), or show any other symptom of electric excitement. All of them, therefore, would have been once called non-electrics.

But suspend them in succession by a string of silk held in the hand, and strike them again; every one of them will now attract the lath.

Reflect upon the meaning of this experiment. We have introduced an insulator—the silk string—between the hand and the body struck, and we find that by its introduction the non-electric has been converted into an electric.

The meaning is obvious. When held in the hand, though electricity was developed in each case by the friction, it passed immediately through the hand and body to the earth. This transfer being prevented by the silk, the electricity, once excited, is retained, and the attraction of the lath is the consequence.

In like manner, a brass tube, held in the hand and struck with a fox's brush, shows no attractive power; but when a stick of sealing-wax, ebonite, or gutta-percha, is thrust into the tube as a handle, the striking of the tube at once develops the power of attraction.

And now you see, more clearly than you did at first, the meaning of the experiment with the heated foolscap and India-rubber. Paper and wood always imbibe a certain amount of moisture from the air. When the rubber was passed over the cold paper, electricity was excited, but the paper, being rendered a conductor by its moisture, allowed the electricity to pass away.

Prove all things. Lay your cold foolscap on a cold board, supported by warm dry tumblers; pass your India-rubber over the paper; lift it by a loop of silk, for if you touch it it will discharge itself.

  1. A course of six lectures, with simple experiments in frictional electricity, before juvenile audiences during the Christmas holidays.