Electric Arc icitli Varying Power- Supply. 



219 



When dealing with arcs o£ some 2000 mean spherical 

 candle-power, it was necessary to bring the balancing-lamp 

 very close to the photometer. 



In this case it was necessary to apply a correction to the 



Fig. 1. 





scale-reading of the distance d, due to the anode subtended 



O 7 CD 



by the filament of the balancing-lamp becoming applicable. 



If I is the length of the filament, and i the brightness per 

 unit-length. 



Then, considering a flat-loop lamp at a distance d from the 

 photometer (see fig. 1), we have : — 



Intensity of illumination due to 2 legs of filament 



= 2{ 2 ;f\&costf)eostf/(^) 2 } 



tan '2J d 



1 I 

 — 1 2/ d 



d |_ tdn + cP + {il yJ 



