222 



Mr. Dyke on the Flux of Light from the 



If, however, the arc be made shorter than J in. the efficiency 

 line for the alternating-current arc will be found to gradually 

 approach the line for the continuous-current arc until for an 

 arc-length of -f?, in. they are practically coincident (PI. II.). 



The arc is quite stable at this point, and even in the alter- 

 nating-current case may be supplied at more than 600 watts 

 without causing hissing. 



Any further decrease in the arc-length will result in the 

 alternating-current efficiency line rising above the continuous- 

 current line until at an arc-length of T ] ^j in. they have the 

 position shown in the first curve in PI. II. 



With so small an arc-length, however, the alternating- 

 current arc begins to hiss badly when suppled at a rate 

 greater than 350 watts, although the continuous-current arc is 

 still silent up to 900 watts. 



As far as the author is aware, this is the first case in which 

 an alternating current arc of efficiency greater than the corre- 

 sponding continuous-current arc has been realized in practice, 

 although Dr. Fleming and Mr. Petavel recognized the possi- 

 bility of such a phenomenon in their paper referred to above. 



Why, then, does the alternating-current arc give the same 

 efficiency as the continuous-current arc at this arc-length ? 



This can perhaps be most clearly seen by making some 

 simple approximations with regard to the shape of the carbons, 

 and calculating the length of arc necessary to give the same 

 candle-power with the same power supply. 



Fig. 2. 



1 \ 



D.C. 



A.C. 



Let the carbons be supposed to assume the shape indicated 

 in fig. 2, truncated cores whose heights are equal to their 



