136 



THE ELECTRIC ARC 



FIG. 45. 



giving properties are concerned is that with the latter the 

 greater part of the light comes from only one of the ter- 

 minals, namely the anode, while with the 

 former, both terminals give the same 

 amount of light, the anode alternating from 

 one to the other terminal. The distribu- 

 tion of light is, accordingly, that shown in 

 Fig. 45- 



With ordinary frequencies the light from 

 the alternating-current arc appears to the 

 eye to be continuous, while with smaller 

 frequencies the flickering becomes noticeable. 

 Thus Georges 1 found that with 60 periods 

 per second the flickering was not notice- 

 able, while with 40 periods per second it was. very dis- 

 agreeable. 



While this is the appearance presented to the eye, 

 the actual amount of light changes very appreciably dur- 

 ing an alternation, the candle power corresponding much 

 more closely to the current flowing at each instant than 

 might be expected when one remembers that the crater 

 must be heated up and cooled off with each alternation. 

 This is shown by curves in Fig. 46. In this figure the curve 

 marked a represents the potential difference, b the current, 

 c the watts, and d the candle power. During the first part 

 of the alternation the photometer was illuminated by the 

 light of the anode. During the second part this carbon 

 had become the cathode and was giving much less light. 2 

 Duddell 3 found that when an alternating current was 



1 Elektrot. ZS., 16, 548; 1895. 



2 See also Leonard, Eel. Elec., 42, 241; 1905. 

 1 Lond., Elec., 46, 270; 1900. 



