436 



ALTERNATING CURRENTS 



surrounding portion of the disc, when viewed from either side. 

 When this occurs, a photometrical balance is obtained. 



In order that the observer may view both sides of the screen 

 simultaneously, two mirrors M, Fig. 391, are set at an angle and 

 reflect the light in the manner shown by the dotted lines. 



The screen S is moved until the screen looks the same on 

 both sides, and the distances I and li are read. Let E be the 

 candlepower of the candle or standard and EI the candlepower 

 of the test lamp. Remembering that light intensity varies 

 inversely as the square of the distance. 



C- o 



FIG. 391. Bunsen photometer. 



The candlepower of the test lamp 



If a standard candle is used, E equals 1.0. 



If the two lights have different color, the two sides of the 

 screen will never appear alike and only an approximate balance 

 can be obtained. The position of balance is to a considerable 

 extent determined by the personal equation of the observer. 



Because of the unreliability of candles, standard incandescent 

 lamps are used. These may be obtained from the Bureau of 

 Standards. Such lamps, when used at the voltage at which they 

 are calibrated, are very accurate standards, An arrow on the 



