ELECTRICAL MACHINERY 



491 



FIG. 369 COMMERCIAL SWITCHBOARD 



disintegrate the carbon and cause it practically to boil ; 

 this boiling throws off a vapor which is a conductor of 

 electricity and as a consequence conducts the current 

 across the gap. The temperature of the arc at its hottest 

 point is about 3,500 C., which is about twice the tem- 

 perature required to melt platinum, the most refractory 

 of metals. 



Arc lamps are rated according to the watts consumed. 

 They generally range from 6 X 45 = 270 watts to 10 X 45 

 = 450 watts. About 12 per cent of the energy supplied to 

 an arc light really appears as light; the rest goes to 

 produce the heat evolved, 



