Page:Electrical Engineering Volume 1.djvu/27

Rh beaten or rubbed with a warm piece of woolen cloth or fur. The disk or cover is then placed upon the cake, touched momentarily with the finger to liberate the free charge, then removed by taking it up by the handle. It is now found to be powerfully electrified with a positive charge; so much so,

indeed, as to yield a considerable spark when the hand is brought near it. The cover may be replaced, touched, and again removed, and will thus yield any number of sparks; the original charge on the resinous plate meanwhile remaining practically as strong as ever.

2225. A static charge of electricity is not usually distributed uniformly over the surface of conducting bodies. Experiments show that there is more electricity on the edges and corners than upon their flatter parts.

The term electric density is used to signify the amount or quantity of electricity residing on a small area of any part of a body, the distribution being supposed to be uniform over that small part of the surface.

The electric density is the quotient arising from dividing the total charge of electricity in units of quantity residing upon the surface of a body, by the area of the surface in square inches. For example, a charge of 240 units of electricity is imparted to a sphere, the surface area of which is 40 square inches; then, the electric density over the surface of the sphere is $240⁄40$ = 6 units of electricity per square inch.