Page:Electrical Engineering Volume 1.djvu/26

1458 charge on B will neutralize a + 4 charge on A and leave a + 16 charge, which immediately divides equally between the two balls; that is, a charge of + 8 units remains on each ball when they are separated.

It is found that the effect of this electrostatic induction is greatly increased by placing some other substance, such as glass or paper instead of air, between the two bodies.

2222. The facility with which a body allows electrostatic induction to act across it is called its inductive capacity. The inductive capacity varies with different substances, but almost all non-conductors are better than air.

2223. Any substance which allows electrostatic induction to act across it is termed a dielectric. All dielectrics are non-conductors. Table 73 gives a list of several non-conductors in the order of their inductive capacity values, from which it will be seen that, with two exceptions, air has the lowest inductive capacity.

Glass. Shellac. Sulphur. Ebonite. India-rubber. Petroleum.

Paraffin (solid). Carbonic Acid. Air. Hydrogen. Vacuum.

2224. The electrophorus, Fig. 905, is an instrument devised for the purpose of obtaining an almost unlimited number of static charges of electricity from one single charge, and is based upon the principle of electrostatic induction.

It consists of two main parts: a thin cake of resinous material cast in a round metal dish or pan B, about one foot in diameter; and a round disk A, of slightly smaller diameter, made of metal and provided with a glass handle. In using the electrophorus, the resinous cake must first be