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CH. XIII] RHEOSTATS AND OTHER BALLAST 523
§ 725. Getting rid of the heat developed. — As much heat is developed in the rheostat, it is necessary to so arrange the coils of wire, etc., forming it, that the heat can easily escape, otherwise the wire might get so hot that it would melt. Provision for carrying away the heat then is of prime importance. For example, a large iron telegraph wire would get red hot in the air if it were used for 100 amperes, while a much smaller wire if immersed in water would carry the current easily on account of the rapid dissipation of the heat in the water.
Ordinarily the resistance wire is in coils, and these are hung on non-conductors in such a way that there is free circulation of air around and through the coils to carry off the heat.
Sometimes the wire or strips of metal serving for the resistance are imbedded in porcelain, and a considerable surface of the porcelain being exposed to the air, the heat readily escapes. This is often the method with the rheostats used for dimming the lights in theaters (theater dimmers). (See fig. 183, 186 for a theater dimmer used as a rheostat).
In fig. 198 is a small rheostat with the metal in a helical coil and wound around a porcelain core. This rheostat is for the small arc lamp to be used on the house lighting system, and restricts the current to 4-6 amperes.
§ 726. Fixed rheostat. — This is a rheostat in which the entire amount of resistance wire must be traversed whenever the current is on, the amperage of the current is then practically constant. For example in using the arc lamp if the rheostat is designed for 1 5 amperes, that current must always be used. The fixed rheostat is
best adapted for any place where many use the same apparatus (fig. 280).
§ 727. Adjustable rheostat— The adjustment consists of an arrangement by which a greater
or less length of the resistance FIG. 280. CIRCUIT WITH DYNAMO (G) • 1 • i 1 1 • ,1
ARC LAMP (4), AND FIXED RHEO W11"6 can be included in the cirSTAT (R). cuit at will. The more resis