Conductors and Non-conductors of Electricity. When an electrified body is placed upon a metallic stand, so that it is in metallic connection with the earth, all traces of electrification disappear; but if placed upon supports of glass or ebonite, its charge is still retained, the body then being said to be insulated. In the former case, the electric charge having passed to the ground through the metallic support, the metal is termed a conductor of electricity; in the latter, the glass or ebonite is termed a non-conductor or insulator, since by its means the charge is prevented from leaving the body. This distinction is somewhat artificial, since no substance is known to be either a perfect conductor or an absolute insulator. The substances in frequent use as conductors and non-conductors may be arranged in the following order of their conductivity, beginning with the best: Silver, copper, gold, brass, zinc, platinum, iron, tin, lead, mercury, German-silver, graphite, red phosphorus, mineral acids, saline solutions, sea-water, pure water, alcohol, wood, ice, vegetable oils, lime, chalk, camphor, porcelain, wool, hair, silk, glass, wax, sulphur, resin, amber, gutta-percha, shellac, paraffin, ebonite, air and other gases.
It is found that the efficiency of a non-conductor, when used as an insulator in electrostatic experiments, depends very largely on the state of its surface. In a damp atmosphere, glass becomes coated with a thin film of moisture which considerably lessens its insulating power. Hence these experiments succeed better in dry weather; otherwise the apparatus requires to be heated, or the glass insulators employed to be coated with shellac varnish, upon which moisture does not so readily deposit. In metals, conducting power decreases when the temperature is raised; on the other hand, glass, wax, sulphur, and some other bodies which are extremely bad conductors at ordinary temperatures, conduct very much better when raised to a sufficiently high temperature. It has been observed that a series of metals arranged in order of their electrical conducting powers, exhibits the same order as when similarly arranged for their thermal conducting powers; in other words, metals which conduct electricity well, conduct heat well. Also, that when two specimens of the same metal differ in electric conductivity, they differ in thermal conductivity, and in the same way. See LIGHTNING CONDUCTOR.