ELECTRIC FURNACE. Electric furnaces may be broadly classified into (1) Resistance furnaces, in which heat is produced by the passage of an electric current through a poor conductor, (2) Arc furnaces, in which the heat of the electric arc is used, (3) Induction fur naces, which are a modified form of the resistance furnace, and in which the heat is generated by an induced current. In this type of furnace the resistance element is frequently formed by the fur which these extremely high tempera tures are necessary, as, for instance, in the manufacture of carborundum. In this process, a core of granular carbon, or coke, is placed between electrodes, and around the core is packed a mixture of coke and sand. The electric current flows through the core and raises the temperature of the sand and coke mix ture to such a point that combination takes place with the formation of car borundum.
The induction type of furnace is largely used in the steel industry, one of the best known examples being that designed by F. A. Kjellin. In this fur nace, current at 3,000 volts is supplied to the primary, inducing a current of 30,000 amperes in the metallic charge, which forms the secondary.
Electric furnaces are also used in the manufacture of aluminum, calcium car bide, phosphorus, various iron alloys, and in the production of artificial gra phite.
In the laboratory, small electric fur naces are in common use. In these a current is passed through a high resist ance wire, made of nichrome or some similar alloy, which is wound around the furnace, the latter being made of fire clay. The whole is inclosed in in sulating material to prevent loss of heat by radiation.