440 



ALTERNATING CURRENTS 



ment may be determined by an ammeter connected in series with 

 the lamp. A later method, which depends upon the wheatstone 

 bridge principle, has been developed and is illustrated by Fig. 395. 

 The lamp is one arm of a wheatstone bridge. A, C and D, made 

 of materials with zero temperature coefficient, form the other 

 three arms. The resistance of a tungsten filament is very sensi- 

 tive to changes of temperature and therefore to changes of cur- 

 rent. For this reason the bridge will balance only for one value 

 of total current, which is controlled by the resistance R. The 

 slider S allows small adjustments to be made. Instead of the 

 galvanometer, G, a telephone receiver is often used, the balance 

 being determined by interrupting the telephone circuit. If the 

 system is out of balance a click is heard in the receiver. This 

 equipment eliminates the weight of either an ammeter or a 

 galvanometer. 



202. The Rousseau Diagram. The mean horizontal candle- 

 power of an incandescent lamp may be determined by rotating 



FIG. 396. Rousseau diagram. 



the lamp and at the same time measuring its candlepower in a 

 horizontal direction. The speed of rotation must be sufficiently 

 high so that flicker will not prevent the obtaining of an accurate 

 balance. Too high a speed distorts the filament by centrifugal 

 force. The rotator is usually so designed that the lamp can be 

 turned through any vertical angle between and 180. If 

 the candlepower is measured at various angles between and 

 180 and the corresponding values of candlepower are laid off on 



