<P> The first rubber - insulated cables for USA building wiring were introduced in 1922 with US patent 1458803, Burley, Harry & Rooney, Henry, "Insulated electric wire", issued 1923 - 06 - 12, assigned to Boston Insulated Wire And Cable . These were two or more solid copper electrical wires with rubber insulation, plus woven cotton cloth over each conductor for protection of the insulation, with an overall woven jacket, usually impregnated with tar as a protection from moisture . Waxed paper was used as a filler and separator . </P> <P> Over time, rubber - insulated cables become brittle because of exposure to atmospheric oxygen, so they must be handled with care and are usually replaced during renovations . When switches, socket outlets or light fixtures are replaced, the mere act of tightening connections may cause hardened insulation to flake off the conductors . Rubber insulation further inside the cable often is in better condition than the insulation exposed at connections, due to reduced exposure to oxygen . </P> <P> The sulphur in vulcanised rubber insulation attacked bare copper wire so the conductors were tinned to prevent this . The conductors reverted to being bare when rubber ceased to be used . </P> <P> About 1950, PVC insulation and jackets were introduced, especially for residential wiring . About the same time, single conductors with a thinner PVC insulation and a thin nylon jacket (e.g. US Type THN, THHN, etc .) became common . </P>

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