Analytical Study of the Alternating Current Arc. 349 



The second portion of the experimental work here recorded 

 consisted in an investigation of the efficiency of the alternating- 

 current arc as a light-giving agent, when compared with the 

 continuous-current arc taking up the same mean power. The 

 question in all its generality, whether alternating-current arcs 

 are less efficient as light-giving agents for a given amount of 

 power expended in them than continuous-current arcs absorb- 

 ing the same power, is not one which we have attempted to 

 settle, for the simple reason that a far more extended series of 

 experiments would be necessary before it would be possible to 

 say whether an alternating-current arc can or cannot be made 

 to give the same mean spherical candle-power as a continuous- 

 current arc absorbing the same mean power. Limitations of 

 time compelled us to reduce our investigation to one definite 

 problem. Taking alternating-current arcs, formed with the 

 length, voltage, and the current and the carbons as described 

 below, and taking continuous-current arcs also with the length, 

 current, and voltage and carbons most usually employed in 

 practice, we have investigated the relative magnitude of the 

 mean spherical candle-power produced by these arcs for equal 

 expenditure of powers in the arcs varying from 200 to 600 

 watts. This, it will be observed, is an investigation which has 

 nothing whatever to do with the relative mechanical or elec- 

 trical efficiency or excellence of the arc-lamp mechanisms, but 

 it is purely a physical measurement made of the two arcs them- 

 selves under the conditions which are found to obtain in 

 practice. In order to settle this question, an alternating- 

 current arc lamp was taken with good regulating qualities ; 

 it was furnished with cored carbons 15 mm. in diameter, and 

 being placed under the conditions above described, in which 

 the power supplied to the arc could be regulated and mea- 

 sured, a series of observations was made of the mean spherical 

 candle-power of the arc for different powers expended in the 

 arc and for two frequencies of 83'3 and 50 — -. A similar set 

 of experiments was made with the continuous-current arc, 

 using in one case the same carbons — 15 mm. carbons, both 

 cored — and in the other case the positive carbon 15 mm. cored 

 and a negative carbon 9 mm. solid, These sizes were chosen 

 because the 15 mm. cored and 9 mm. solid sizes are those that 

 are frequently employed in continuous-current arc lighting. 

 The arrangements for obtaining the mean spherical candle- 

 power consisted of a mirror — the coefficient of reflexion of 

 which had been determined — which was employed to reflect 

 the light from the arc proceeding in different directions to the 

 horizon into the photometer, the ray always falling upon the 

 mirror at an incidence of 45 . The standard of comparison 



Phil. Mag. S. 5. Vol. 41. No. 251. April 1896. 2 B 



