{ Slate { Oils { Porcelain { Dry leather { Dry paper { Wool { Silk { Sealing wax { Sulphur Insulators, or { Resin so-called { Gutta-percha non-conductors. { Shellac { Ebonite { Mica { Jet { Amber { Paraffin wax { Glass (varies with quality) { _Dry_ air. [Illustration: FIGS. 76 to 78.--Various covered wires. fig. 76, single; fig. 77, duplex; fig. 78, automobile high tension cable.] The earth is a good conductor; much difficulty is frequently experienced by the wires making contact with some substance that will conduct the electricity to the earth. This is called “grounding.” [Illustration: FIGS. 79 to 81.--Standard porcelain insulators. Fig. 79, tube type; figs. 80 and 81, grooved insulators.] =Mode of Transmission.=--The exact nature of electricity is not known, yet the laws governing its action, under various conditions are well understood, just as the laws of gravitation are known, although the constitution of gravity cannot be defined. Electricity, though not a substance, can be associated with matter, and its transmission requires energy. While it is neither a gas nor a liquid, its behavior sometimes is similar to that of a fluid so that it is said to “flow” through a conductor. This expression of flowing does not really mean that there is an actual movement in the wire, similar to the flow of water in a pipe, but is a convenient expression for the phenomena involved. =Effect of Heat.=--The conducting power of bodies is affected in different ways by their temperature. In the metals it is diminished by elevation of temperature; but in all other bodies, and especially in liquids, it is augmented. Some substances which are insulators in the solid state, become conductors when fused. Sir H. Davy found that glass raised to a red heat became a conductor; and that sealing wax, pitch, amber, shellac, sulphur, and wax, became conductors when liquefied by heat.