CONDUCTORS AND NON-CONDUCTORS OF ELECTRICITY. If a rod of metal be made to touch the prime conductor of an electrical machine immediately after the plate has ceased to rotate, every trace of electricity immediately disappears. But if the rod were of shellac, little or no diminution would be perceptible in the electrical excitement of the conductor. The metal in this case leads away the electricity into the body of the exper imenter, and thence into the ground, where it becomes lost, and it receives in conse quence the name of a conductor. The shellac, for the opposite reason, is called a non conductor. Different substances are found to possess the power of conducting electricity in very different degrees. The following series classifies the more common substances according to their conducting powers, beginning with the best, and ending with the worst conductors: Conductors—The metals, graphite, sea-water, spying-water, rain-water. Semi-conductors—Alcohol and ether, drywood, marble, paper, straw, icc at 32' F. Non conductors—Dry metallic oxides, fatty oils, ice at-13' F., phosphorus, lime, chalk, caoutchouc, camphor, porcelain, leather, dry paper, feathers, hair, wool, silk, gems, glass, agate, wax, sulphur, resin, amber, shellac.
The arrangement into conductors, semi-conductors, and non-conductors, is made with reference to frictional electricity, or electricity of a high tension. The substances which are semi-conductors for frictional electricity are found to be almost, if not altogether, non-conducting for the electricity of the galvanic battery, which is too feeble to force a passage through them. The metals, which appear to be all nearly alike conducting for frictional electricity, offer widely differing resistances to the transmission of the galvanic current. By experiments made with galvanic electricity, it is found that the more ordi
nary metals stand thus, as regards their powers of conduction, beginning as before with the best conductor: Silver, gold, copper, brass, zinc, iron, platinum, tin, nickel, lead, German-silver, mercury. An increase of temperature has in the metals the effect of lessening the conducting power, whilst in almost all other substances it has an opposite effect. becomes conducting at a red heat, and so do wax, sulphur, amber, and shellac, when fused.
When a conductor is placed on non-conducting supports, so as to prevent the elec tricity communicated to it from passing into the ground, it is said to be insulated.. The usual insulating material employed in the construction of electrical apparatus is glass, which, though not so perfect a non-conductor as the others lower in the scale, far exceeds them in hardness and durability. In a damp atmosphere, glass becomes coated with a thin layer of moisture, which very considerably diminishes its insulating power. Hence arises the necessity in certain states of weather of heating so as to dry all elec trical apparatus previous to use. This imperfection is very much lessened by covering the glass with shellac varnish.
The very fact that a conductor may be insulated, indicates that the air is a non conductor. Dry air possesses this property in a high degree, while moist air renders insulation for any length of time impossible.