E L. Nichols — Alternating Electric Arc. 11 



by a separate wire, — the intention being if possible to separate 

 the two phases of current and conduct them over different 

 wires, — it was found that the arc from the point (positive) 

 always entered the central wire, never being diverted to the 

 ring, although the intervening air-space was less than a milli- 

 meter. The returning arc, however, would sometimes leave 

 the ring and sometimes the wire, and a galvanometer placed 

 in the circuit between the ring and the induction coil, showed 

 a large deflection, such as would be caused by a flow of current 

 towards the ring. This result seems to be in accordance with 

 the conclusion reached from the inspection of the images in 

 the revolving mirror : namely that the arc from the point 

 always formed first along the shortest path. Deflection from 

 that path in the case of the wire and ring, would probably be 

 hindered by the insulating medium which intervened. When, 

 finally, a cluster of points were opposed to the single point, it 

 was found that the discharge from the latter was always along 

 a single path, whereas the return arc from the cluster (positive) 

 often followed several paths. 



It had been noted by Messrs. Archbold and Teeple, that the 

 platinum point used in their experiments, which was red hot 

 while the arc was passing in both directions, became white hot 

 during the " one way " stage. The heating effects at the 

 surface of the brass ball were not discernible, but when Mr. 

 Caldwell substituted a thin sheet of platinum for the ball, this 

 became incandescent under the action of the arcs. The spot 

 where the oblique arc (point positive) impinged upon the foil 

 became white hot, while that at which the normal arc (ball 

 positive) left the foil was barely red hot. In this respect then, 

 the discharges act like the ordinary sparks of the influence 

 machine or induction coil, which, as has been shown by 

 Despretz, Foggendorf, Naccari and other observers, heat the 

 negative electrode to a higher degree than the positive one. 

 This action is in marked contradistinction to that of the con- 

 tinuous current arc, the positive terminal of which takes the 

 higher temperature. 



In this brief account of Mr. Caldwell's experiments, I have 

 omitted to mention many of the observations recorded by him. 

 He had occasion in the course of his investigation, to study the 

 discharge under a variety of conditions, and found that when 

 liquid surfaces (mercury or water) were used in place of the 

 ball ; also that when hydrogen, carbon-dioxide or illuminating 

 gas were substituted for air, as a dielectric, the ball and point 

 effect, more or less modified, could still be obtained. The 

 investigation of these points, although it has already led to 

 some results of significance, is as yet very incomplete. 



Physical Laboratory of Cornell University : 

 September, 1890. 



