PHOTOMETRY OF THE ELECTRIC ARC 



135 



have ever been used commercially, and there is little object 

 in getting their candle power for any other purpose. 



The candle power of the mercury arc has been examined 

 by Kuch and Retschinsky. 1 Their measurements were 

 made with a mercury arc in a quartz tube approximately 

 27 cm. long. The comparisons were made with standard 



1.0 



.8 



-.4 



400 



600 800 

 WATTS 



1000 1200 



FIG. 44. 



incandescent lamps by means of a flicker photometer. 

 They found the intensity of light with a current of 1.9 

 amperes and voltage of 27 volts to be 58 candles (66 hefner 

 units). With 3.85 amperes and 304 volts the candle power 

 was 6270. In Fig. 44 the relation is shown between the 

 watts used and the watts per hefner unit. 



They found also that the energy of the ultra-violet light 

 per watt of power supplied also passed through a maximum 

 and decreased in a manner similar to that of the visible rays. 



Alternating-current Arcs. The chief difference between 

 the alternating- and direct-current arcs as far as light- 



1 Ann. d. Phys., 20, 568; 1906, and 22, 852; 1907. 



