Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/90

 50 Certain liquids, such as naphtha, turpentine, and some oils, are insulators, but inferior to most of the solid insulators. The resistance of most substances, except the metals, and selenium and carbon, seems to diminish as the temperature rises.

Specific Inductive Capacity.

52.] All bodies whose insulating power is such that when they are placed between two conductors at different potentials the electromotive force acting on them does not immediately distribute their electricity so as to reduce the potential to a constant value, are called by Faraday Dielectrics.

Faraday discovered that the capacity of an accumulator depends on the nature of the insulating medium between the two conductors, as well as on the dimensions and relative position of the conductors themselves. By substituting other insulating media for air as the dielectric of the accumulator, without altering it in any other respect, he found that when air and other gases were employed as the insulating medium the capacity of the accumulator remained the same, but that when shell-lac, sulphur, glass, &c., were substituted for air, the capacity was increased in a ratio which was different for each substance.

The ratio of the capacity of an accumulator formed of any dielectric medium to the capacity of an accumulator of the same form and dimensions filled with air, was named by Faraday the Specific Inductive Capacity of the dielectric medium. It is equal to unity for air and other gases at all pressures, and probably at all temperatures, and it is greater than unity for all other liquid or solid dielectrics which have been examined.

If the dielectric is not a good insulator, it is difficult to measure its inductive capacity, because the accumulator will not hold a charge for a sufficient time to allow it to be measured; but it is certain that inductive capacity is a property not confined to good insulators, and it is probable that it exists in all bodies.

Absorption of Electricity.

53.] It is found that when an accumulator is formed of certain dielectrics, the following phenomena occur. When the accumulator has been for some time electrified and is then suddenly discharged and again insulated, it becomes recharged