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LEFT ELECTRICAL UNIT 490 ELECTRIC FURNACE tact with the metallic strip and the positive electricity escapes, leaving the strip charged with negative electricity. Another strip moves into place, and the same process is repeated. In the mean- time, the first strip has moved on toward metal collecting combs, and communi- cates its charge to them. The electricity from combs passes to Leyden jars, where it is stored. The more modern Wimshurst machine resembles the Topler, but consists of two glass plates, each fitted with tin-foil discs, and each made to revolve in the opposite direction to the other. ELECTRICAL UNIT. A coulomb. The quantity of electricity that liberates .000162 grain of hydrogen from water. nace charge, which also acts as the secondary of a transformer; current at high voltage and low amperage is sup- plied to the primary, thus inducing high amperage at low voltage in the second- ary. Electric furnaces can only compete, economically, with the ordinary direct- fired furnace where electric power is generated cheaply by water-power, a^, for instance, at Niagara. They also have the advantage that much higher temperatures can be produced in them, the maximum temperature obtainable in non-electric furnaces being about 2,000° C, whereas, in electric furnaces, a tem- perature of 3,500° C is easily main- tained. There are some processes in ELECTRIC DYNAMO Direct current, lighting and power generator. ELECTRIC BATTERY. See STOR- AGE Battery. ELECTRIC DENSITY, or ELECTRIC THICKNESS, the quantity of electricity found on a given surface. 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