This experiment shows that the mere influence which an electric charge exerts upon a conductor placed in its vicinity is able to produce electrification in that conductor. This method of producing electrification is called electrostatic induction.
As soon as the charged rod is removed the leaves will collapse, indicating that this form of electrification is only a temporary phenomenon which is due simply to the presence of the charged body in the neighborhood.
Nature of the Induced Charge.—This is shown by the experiment illustrated in fig. 17.
Let a metal ball A be charged by rubbing it with a charged rod, and let it then be brought near an insulated metal cylinder B which is provided with pith balls on strips of paper C, D, E, as shown.
The divergence of C and E will show that the ends of B have received electrical charges because of the presence of A, while the failure of D 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 E.
From these experiments, the conclusion is 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 opposite sign.
The Electrophorus.—This is a simple and ingenious instrument, invented by Volta in 1775 for the purpose of procuring, by the principle of induction, an unlimited number of charges of electricity from one single charge.
It consists of two parts, as shown in fig. 19, a round cake of resinous material B, cast in a metal dish or “sole” about one foot in diameter, and a round disc A, of slightly smaller diameter made of metal or of wood covered with tinfoil, and provided with a glass handle. Shellac, or sealing wax, or a mixture of resin shellac and Venice turpentine, may be used to make the cake.
