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2226. Electrostatic machines have been devised for the purpose of obtaining larger static charges than can be developed by rubbing a glass rod or by the electrophorus. They consist, mainly, of two parts, one for producing and the other for collecting the charges.

There are three important kinds of electrostatic machines—the cylinder, the plate, and the induction machines.

2227. The cylinder machine, as usually constructed, consists of three principal parts: (1) a cylinder of glass revolving upon a horizontal axis; (2) a rubber or cushion of horsehair, to which is attached a long silk flap, and (3) an insulated metallic cylinder called a prime conductor. In Fig. 906 the cushion of horsehair a, covered with a coating of amalgam of zinc, presses against the glass cylinder b from

behind, allowing the silk flap s to rest upon the upper half of the glass. The prime conductor C is provided at one end with a row of fine metallic spikes, and is placed in front of the machine with the row of spikes projecting towards the glass cylinder. When the glass cylinder is revolved, a positive charge is produced upon the glass and a negative charge upon the rubber. The positive charge is carried around upon the glass cylinder, and just before reaching a position opposite the row of spikes it acts inductively upon the prime conductor, attracting a negative charge to the near end and repelling a positive charge to the far end. When the positive charge arrives in front of the row of