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ELECTRICAL CONDUCTORS 69
will conduct current, or by both wires coming into contact with damp ground.
Various materials offer varying resistance to the pas sage of current. (26) In fact, so far as is known there is no substance through which current cannot be forced if the voltage is high enough. There are, however, substances through which only minute current will flow unless the voltage be above, and in some cases far above, the voltage used for ordinary commercial work. Some of these substances are considered as insulating materials and are used as such. At the head of the list stand, in the order named, (27) glass, porcelain, mica and rubber.
Various natural substances such as marble, mica, slate and asbestos are excellent insulating materials for ordinary commercial voltages up to 220 volts. There are also many insulating compounds, their composition often being a trade secret. In practice such compounds are used to saturate cloth, or other material, which afterward are used in many forms for insulation purposes.
"R. C."
(28) The initial "RC" or R.C. stands for "rubber covered." Rubber covered wire is first coated with tin. It is then coated with either pure rubber or a rubber compound, over which is placed one or more protective coverings of cotton braid which has been impregnated with insulating compound. The tin coating is to prevent the sulphur present in rubber from attaching the copper.
(29) Rubber is a highly satisfactory insulation material but it is easily injured by heat or oil. Conse quently under no circumstances should rubber covered wires be overloaded. Overloading — that is, increased temperature — hardens the rubber, renders it brittle and destroys its insulating value. (30) Because of this deficiency rubber-covered wires arc rated lower in amperage than are wires of similar diameter and com