Analytical Study of the Alternating Current Arc. 359 



Fig. 25. 

 (From 'Electrician,' Dec. 20, 1895, p. 247.) 



5U!I 



True Power in Watts Expended in the Arc. 



amount of mean spherical illumination can be obtained, if that 

 power is supplied in a continuous- current form than if it is 

 supplied in an alternating form. It has been suggested that, 

 with proper carbons and under proper conditions, the alter- 

 nating and continuous-current arc should give the same mean 

 spherical candle-power for the same expenditure of power, as 

 has been shown to hold true in the case of the incandescent 

 lamp by the investigations of Profs. Ayrton and Perry. We 

 think, however, that no a priori reasoning can apply in the 

 case of the arc. It is perfectly clear that, owing to the 

 interval of cooling that elapses, the mean temperature of the 

 two carbons in the case of the alternating-current arc must 

 be less than the temperature of the positive crater of the 

 continuous-current arc, and that therefore the result we have 

 obtained is only w 7 hat might be expected. If the question is 

 asked, how do we account for the difference in efficiency ? the 

 answer must be that the energy absorbed in the case of the 

 alternating-current arc is radiated at a lower temperature, 

 and the two arcs were therefore exactly in the same condition 

 as regards comparison that two incandescent lamps would 

 be, both taking the same total power but worked at different 

 temperatures, and therefore different watts per candle, and 

 therefore giving different mean spherical candle-powers with 

 the same total power expended in them. 



It may be observed in all cases the alternating-current arcs 

 we employed in our experiments took 16 to 17 amperes, and 



