ELECTRIC HEATER. In electric heaters a coil of metal of more or less resistance is wound around a frame and is made the channel of a current sur rounded by insulating material, the heat closed in various forms of receptacle which radiate it. The wire or strip of Metal may be surrounded by air or by a -non-inflammable substance that serves as a conductor. Porcelain, asbestos, enamel, or glass into which the coils or circuits are imbedded are largely in use for fireproof insulation, while some classes of heaters are imbedded in ma terials such as hyposulphite of sodium and crystallized acetate of sodium. There are great varieties in the methods used. In the Parville system heat is engendered by means of rods of metallic powder mixed with fusible clay, com pressed by a force of 2,000 kilograms per square centimeter and baked at a temperature of 1,350° C. The Prome theus heater has a strip of selected metal fused to an enamel covering which receives the heat sent through the strip of metal. In certain types a metallic
paint is fired upon mica strips, which are so grouped as to determine the size of the heater. Electric cars are usually heated by sets of conducting wires coiled round porcelain tubes and connected with the motor. Electric heaters are somewhat too costly for house-heating, but they are of great use in appliances such as flatirons, cooking utensils, and small radiators. The industries devoted to the production of small heaters of this class have grown steadily in recent years. The larger kind are taking their place in certain processes of manufac ture and production as in oil fields, where electric heaters are used to stimu late the heavy petroleum and cause it to flow more easily.