PHOTOMETRY OF THE ELECTRIC ARC 



137 



V 



FIG. 46. 



superimposed on a direct current, changes in the light 



could be observed when the change in the current was only 



3 per cent of the 



total current and 



when there were 



4300 alternations per 



second. 



The alternating- 

 current arc is not 

 so efficient as the 

 one with direct cur- 

 rent. 1 A compari- 

 son of the two for 

 different currents is 

 given in Fig. 47. Matthews gave the candle power per 

 watt for the direct-current enclosed arc as 0.76, for the 

 alternating-current arc without reflector as 0.53 and with 

 reflector as 0.68. 



This form of arc is influenced by length, amount of cur- 

 rent and size of carbons much the same as the direct-current 

 arc is. Thus the watts per candle power become greater 

 as the arc becomes longer. This is shown in Fig. 48, taken 

 from the work of Fleming and Petavel. The candle power 

 is less the larger the diameter of the carbon used and in- 

 creases approximately as the current. 2 



The candle power per watt has been found to depend 

 both on the wave form of the current and on the number 

 of alternations per second. Rossler and Wedding 3 found 



1 Fleming and Petavel, Phil. Mag., (5), 41, 359; 1896. 

 Dyke, Phil. Mag., (6), 10, 216; 1905. 



Elektrot. ZS., 23, 615; 1902. 



2 Blondel and Jigouzo, Elec. World, 29, 232; 1897. 

 1 Elektrot. ZS., 15, 315; 1894. 



