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. [Illustration: FIG. 4.--Wimshurst Machine.]