Page:Popular Science Monthly Volume 9.djvu/184

164 of the attraction of neutral bodies by our excited tubes. In reality, the attracted bodies are not neutral: they are first electrified by influence, and it is because they are thus electrified that they are attracted.

This is the place to stamp upon your mind the following considerations: Neutral bodies, as just stated, are attracted, because they are really converted into electrified bodies by induction. Suppose a body to be feebly electrified positively, and that you bring your rubbed glass-rod to bear upon the body. You clearly see that the induced negative electricity may be strong enough to mask and overcome the weak positive charge possessed by the body. We should thus have two bodies electrified alike, attracting each other. This is the danger against which I promised to warn yon in Section 10, where the test of attraction was rejected.

We will now apply the principle to explain a very beautiful invention, made known by the celebrated Volta in 1775.

The Electrophorus.—Cut a circle, T (Fig. 15), six inches in diameter, out of sheet-zinc, or out of common tin. Heat it at its



centre by the flame of a spirit-lamp or of a candle. Attach to it there a stick of sealing-wax, H, which, when the metal cools, is to serve as an insulating handle. You have now the lid of the electrophorus. A resinous surface, or what is simpler a sheet of vulcanized India-rubber, P, or even of hot brown paper, will answer for the plate of the electrophorus.

Rub your "plate" with flannel, or whisk it briskly with a fox's brush. It is thereby negatively electrified. Place the "lid" of your electrophorus on the excited surface: it touches it at a few points only. For the most part lid and plate are separated by a film of air.

The excited surface acts by induction across this film upon the lid, attracting its positive and repelling its negative electricity. You have in fact in the lid two layers of electricity, the lower one, which is "bound," positive; the upper one, which is "free," negative. Lift