782 Prof. W. G. Duffield and Miss M. D. Waller on the 



. The present set of experiments was undertaken for the 

 purpose of testing the behaviour of the alternating- current 

 arc chiefly as regards the above-mentioned features. 



Since in an alternating-current arc each pole acts for half 

 a period as anode and half a period as cathode, it seemed 

 most likely that the loss from each pole would be intermediate 

 between those from the anode and cathode of a direct-current 

 arc using the same current. This we have put to the test 

 and shown to be the case; indeed, the loss in the case of the 

 alternating current approximates a little more closely than 

 might be expected to the mean of the anode and cathode 

 rates of carbon consumption. 



In putting the matter briefly upon record, it is only 

 necessary to say that the previous method of experimenting 

 was adopted (loc. cit.), the special precautions being the 

 burning to shape of the poles before the first weighing, and 

 the prevention of absorption of moisture from the atmosphere. 



The results are best shown diagrammatically. Diagram 1 

 shows the relationship between the loss per coulomb and the 

 arc length for different currents. The curves possess the 

 general shape of those obtained with the direct-current arc, 

 rising at first rapidly, but ultimately reaching a nearly 

 stationary value. They resemble the anode rather than the 

 cathode curves in that they do not all diverge from a common 

 point upon the vertical axis. Even in the case of the 

 shortest possible arc the consumption of carbon per coulomb 

 exceeds 3*1 x 10~ 5 gm., the theoretical limit; so it is clear 

 that in all cases there is subsidiary as well as essential carbon 

 loss. 



Diagram 2 shows the loss from either pole of an alternating- 

 current arc using 8 amperes, compared with the individual 

 losses from the anode and cathode of a direct-current arc 

 carrying the same current, and with their mean value. The 

 latter agrees well with the alternating-current curve ; but is a 

 little higher when the arc-gap is long. For currents of 4 

 and 2 amperes the agreement is not so good, though of the 

 same order of magnitude, the alternating-current consumption 

 being definitely less than the mean value in the direct-current 

 curves. 



It thus appears that during the half-period when any 

 given pole is acting as anode, it does not play a passive role 

 like that which was, by artificial means, induced in a rotating 

 anode, but that there is time for the pole to assume to a large 

 extent the condition of the anode in the direct-current arc, 

 though there is no crater ; it certainly gets hotter than the 

 cathode, and is able to lose a considerable amount of material. 



