Page:Elementary Text-book of Physics (Anthony, 1897).djvu/313

§ 263] glass plate, mounted so that it can be turned about an axis, and a rubber of leather, coated with a metal amalgam, pressed against it. The rubber is mounted on an insulating support, but, during the operation of the machine, it is usually joined to ground. Diametrically opposite is placed a row of metal points, fixed in a metallic support, constituting what is technically called the comb. The comb is usually joined to an accessory part of the machine presenting an extended metallic surface, called the prime conductor. The prime conductor is carried on an insulating support.

When the plate is turned, an electrical separation is produced by the friction of the rubber, and the rubbed portion of the plate is charged positively. When the charged portion of the plate passes before the comb, an electrical separation occurs in the prime conductor due to the inductive action of the plate, a negative charge passes from the comb to neutralize the positive charge of the plate, and the prime conductor is charged positively. Since accessions are received to the charge of the prime conductor as each portion of the plate passes the comb, it is evident that the potential of the prime conductor will continuously rise, until it is the same as that of the plate, or until a discharge takes place.

The fundamental operations of all induction machines are presented by the action of the electrophorus, an instrument invented by Volta in 1771. It consists of a plate of sulphur or rubber, which rests on a metallic plate, and a metallic disk mounted on an insulating handle. The sulphur is electrified negatively by friction, and the disk, placed upon it and joined to ground, is charged positively by induction. When the ground connection is broken and the disk lifted from the sulphur, its positive charge becomes available. The process is precisely similar to that described in § 256. It may evidently be repeated indefinitely, and the electrophorus may be used as a permanent source of electricity.

It is evident that a charged metallic plate may be substituted for the sulphur in the construction of an electrophorus, provided that the disk be not brought in contact with it, but only near it. A plan by which this is realized, and at the same time an