ILLUMINATION AND PHOTOMETRY 



437 



lamp indicates the position in which it was calibrated. It is 

 customary to use the standard lamp only to calibrate a working 

 standard so that the candlepower of the ultimate standard will 

 not change due to its being in too constant use. 



Standard 

 Lamp 



IVst 



FIG. 392. Connections for photometric test. 



As the candlepower of lamps is very sensitive to changes in 

 voltage, the connections are often made as shown in Fig. 392. 

 Both lights are fed through an adjustable resistance R z . An 

 adjustable resistance R is in series with the standard lamp and 

 another adjustable resistance R\ is in series with the test lamp. 

 A voltmeter is in parallel with each lamp. Both lamp- 

 brought to the desired values of voltage by adjusting roughly R z , 

 and then by separate adjustments of R and AY Any fluctua- 

 tions of line voltage can be taken care of by the resistance Rt t 

 which affects both lamps. Further, any unnoticed change of line 

 volt. -lire affects both lamps approximately to the same 

 the lamps have similar voltagB-candlepower charactenMics. 



200. The Lummer-Brodhun Photometer. The Lummer- 

 Brodhun screen presents two clearly defined elliptical areas, as 



: Lummer-Brodhuo phol 



shown in When :i photometric balance is obtained, 



the two ellip-, into one if tin- lights have the same . 



The operation of thi> BCT" M follows: S a white, 



opaque M Hie liuht coming from 0X16 BOUT06 falls on the 



N i and that coming from the oiln-r BOUJ ide 5f. 



