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 porce- 

 lain 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) , . , j , ,, 



ARC LAMP (A), AND FIXED RHEO- Wlre can be included in the cir- 

 STAT (R). cuit at will. The more resis- 



