Fig. 2.—Diagram to illustrate Electro-static Induction.
We can now understand why an electrified body attracts an unelectrified body, as in our pith ball experiments. If we bring a positively charged glass rod near a pith ball, the latter becomes electrified by induction, the side nearer the rod receiving a negative, and the farther side a positive charge. One half of the ball is therefore attracted and the other half repelled, but as the attracted half is the nearer, the attraction is stronger than the repulsion, and so the ball moves towards the rod.
Fig. 3.—The Electrophorus.
[Fig. 3] shows an appliance for obtaining strong charges of electricity by influence or induction. It is called the electrophorus, the name coming from two Greek words, electron, amber, and phero, I yield or bear; and it was devised in 1775 by Volta, an Italian professor of physics. The apparatus consists of a round cake, A, of some resinous material contained in a metal dish, and a round disc of metal, B, of slightly smaller diameter, fitted with an insulating handle. A simple electrophorus may be made by filling with melted sealing-wax the lid of a round tin, the disc being made of a circular piece of copper or brass, a little smaller than the lid, fastened to the end of a stick of sealing-wax. To use the electrophorus, the sealing-wax is electrified negatively by rubbing it with flannel. The metal disc is then placed on the sealing-wax, touched for an instant with the finger, and lifted away. The disc is now found to be electrified positively, and it may be discharged and the process repeated many times without recharging the sealing-wax. The charge on the latter is not used up in the process, but it gradually leaks away, and after a time it has to be renewed.
The theory of the electrophorus is easy to understand from what we have already learnt about influence. When the disc B is placed on the charged cake A, the two surfaces are really in contact at only three or four points, because neither of them is a true plane; so that on the whole the disc and the cake are like A and BC in [Fig. 2], only much closer together. The negative charge on A acts by induction on the disc B, attracting a positive charge to the under side, and repelling a negative charge to the upper side. When the disc is touched, the negative charge on the upper side escapes, but the positive charge remains, being as it were held fast by the attraction of the negative charge on A. If the disc is now raised, the positive charge is no longer bound on the under side, and it therefore spreads over both surfaces, remaining there because its escape is cut off by the insulating handle.
Fig. 4.—Wimshurst Machine.
We may now try to understand the working of influence machines, which are really mechanically worked electrophori. There are various types of such machines, but the one in most general use in this country is that known as the Wimshurst machine, [Fig. 4], and we will therefore confine ourselves to this. It consists of two circular plates of varnished glass or of ebonite, placed close together and so geared that they rotate in opposite directions. On the outer surfaces of the plates are cemented sectors of metal foil, at equal distances apart. Each plate has the same number of sectors, so that at any given moment the sectors on one plate are exactly opposite those on the other. Across the outer surface of each plate is fixed a rod of metal carrying at its ends light tinsel brushes, which are adjusted to touch the sectors as they pass when the plates are rotated. These rods are placed at an angle to each other of from sixty to ninety degrees, and the brushes are called neutralizing brushes. The machine is now complete for generating purposes, but in order to collect the electricity two pairs of insulated metal combs are provided, one pair at each end of the horizontal diameter, with the teeth pointing inward towards the plates, but not touching them. The collecting combs are fitted with adjustable discharging rods terminating in round knobs.