insulation (ĭnˌsəlāˈshən, ĭnˌsyŏ–) [key], use of materials or devices to inhibit or prevent the conduction of heat or of electricity. Common heat insulators are, fur, feathers, fiberglass, cellulose fibers, stone, wood, and wool; all are poor conductors of heat. The use of asbestos, formerly a common insulating material, has been curtailed due to its implication in lung disease. Industrial furnaces are built of brick, which conducts heat so slowly that a high temperature within is barely apparent in the temperature of the outer surface. Steam pipes and water pipes are commonly insulated with thick wrappings of fiberglass pulp. Since insulators prevent the flow of heat in either direction, refrigerators are commonly constructed with double walls separated by an air space (air being a poor conductor) and lined with some insulating material. The use of double walls or hollow tiles in buildings prevents the entrance of heat and its escape. The very effective insulation in a vacuum bottle results almost entirely from the presence of a vacuum between the double walls of the inner flask. In the conduction of electricity from point to point, the conductor acts as a guide for the electric current and must be insulated at every point of contact with its supports to prevent escape, or leakage, of the current. Dry air is a good insulator, or dielectric, so that conductors used for electric-power transmission require insulating material only at their points of contact with the supporting steel structures. Glass and porcelain are commonly used, molded in bell-shaped forms or in rods made up of several segments. Underground conductors are insulated with dry cotton or pulp, rubber, and bitumen. In electrical apparatus, ebonite is widely used. Some other insulators are paraffin, sulfur, resin, and varnishes. Since wet materials can become conductors, insulation must often be waterproof. Ordinary household wires are commonly insulated by a thin rubber or plastic coating; the electric cables passing between house walls frequently have in addition a metal wrapping. Depending upon the application, the insulating material may also need to be resistant to various types of corrosion resulting from exposure to saltwater, oils, or other influences.