134 Alternating Electric Arc between a Ball and Point. 



in diameter, with rounded tip, the object being to force the 

 two discharges into a common path, it was found that the arc 

 from the point (positive) avoided the end of the wire alto- 

 gether, and struck in upon the cylindrical surface beyond. 

 When the end of this wire was surrounded by an insulated 

 platinum ring which was connected with the terminal of the 

 induction-coil 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 centre wire, never 

 being diverted to the ring, although the intervening air-space 

 was less than a millimetre. 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 deflexion, 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. Deflexion 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 dis- 

 charge 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 ar^s. 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, Poggendorf, 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 



