Page:Electrical Engineering Volume 1.djvu/29

 spikes, it will be neutralized by the attracting negative charge from the conductor, leaving the glass in a neutral condition ready to be excited again. A positive charge now remains upon the prime conductor, and can be utilized for other experiments.

2228. The plate machine is similar in all respects to the cylinder machine, with the exception that a glass or ebonite plate is used instead of the glass cylinder, and there are usually two sets of rubbers or cushions instead of one. Each set of cushions is double; that is, it is made in two parts, with the plate revolving between them. One set of cushions is placed at the top of the machine, and the other at the bottom, with silk flaps extending from each over a quadrant of the plate. The charge is collected on two prime conductors connected by a metal rod, and each is provided with a row of fine spikes at one end. They are placed in such a position that the two rows of fine spikes project towards the glass plate at opposite sides of its horizontal diameter. The electrostatic action of the machine is in all respects the same as that of the cylinder machine.

2229. The induction machine differs widely in its action from the two machines previously described. It requires an initial charge from some exterior source to start its action. The initial charge acts inductively across a revolving glass plate and produces other charges; these charges in turn are conveyed by the moving parts to some other point, where they increase the initial charge, or furnish a supply of electricity to a prime conductor.

The two principal machines of this class are the Holtz and the Wimshurst.

2230. It has been shown that opposite charges attract and hold one another; that electricity can not flow through glass, and yet can act across it by induction. If a piece of tin-foil is stuck upon the middle of each face of a thin plate of glass, and one of the pieces is electrified with a positive