Electric cooking appliances, like the heaters just described, depend upon the heating of resistance wires or strips of metal. The familiar electric kettle has a double bottom, and in the cavity thus formed is placed the resistance material, protected by strips of mica, a mineral substance very largely used in electrical appliances of all kinds on account of its splendid insulating qualities. Electric irons are constructed in much the same way as kettles, and sometimes they are used with stands which cut off the current automatically when the iron is laid down upon them, so that waste and overheating are prevented. There are also a great many varieties of electric ovens, grillers, hot-plates, water-heaters, glue-pots, and foot and bed warmers. These of course differ greatly in construction, but as they all work on the same principle there is no need to describe them. Electric hot-plates are used in an interesting way in Glasgow, to enable the police on night duty to have a hot supper. The plates are fitted to street telephone signal boxes situated at points where a number of beats join. By switching on current from the public mains the policemen are able to warm their food and tea, and a supper interval of twenty minutes is allowed. Even policemen are sometimes absent-minded, and to avoid the waste of current and overheating of the plate that would result if a “bobby” forgot to switch off, an arrangement is provided which automatically switches off the current when the plate is not in use. [Illustration: FIG. 23.--Diagram to illustrate principle of Electric Furnace.] We must turn now to electric heating on a much larger scale, in the electric furnaces used for industrial purposes. The dazzling brilliance of the light from the electric arc lamp is due to the intense heat of the stream of vaporized carbon particles between the carbon rods, the temperature of this stream being roughly about 5400° F. This great heat is made use of in various industries in the electric arc furnace. Fig. 23 is a diagram of a simple furnace of this kind. A is a vertical carbon rod which can be raised or lowered, and B is a bed of carbon, forming the bottom of the furnace, and acting as a second rod. A is lowered until it touches B, the current, either continuous or alternating, is switched on, and A is then raised. The arc is thus struck between A and B, and the material contained in the furnace is subjected to intense heat. When the proper stage is reached the contents of the furnace are drawn off at C, and fresh material is fed in from above, so that if desired the process may be kept going continuously. Besides the electric arc furnace there are also resistance furnaces, in which the heat is produced by the resistance of a conductor to a current passing through it. This conductor may be the actual substance to be heated, or some other resisting material placed close to it.