Electric Arc between a Ball and Point. 133 



between the ball and point, and the precise position in the 

 cycle occupied by the latter was thus readily determined. 



Closer study of the two paths of discharge showed that the 

 oblique arc left the very apex of the point, swinging out 

 laterally into its path ; also that the normal arc, on approach- 

 ing the point, avoided the apex and entered the wire from the 

 side, never passing in at the point itself. 



From these observations it appears : — 



1. That the discharge from the ball (positive) leaves the 

 latter in a direction normal to the surface, but that it enters 

 the other terminal at some distance from the apex. 



2. That the discharge from the point (positive) leaves the 

 very apex of the latter, but is deflected into a course nearly 

 45° from the axis and reaches the ball obliquely at some point 

 on its side. 



Taking these observations into consideration, the explana- 

 tion of the cessation of the discharge from the point at the 

 critical distance and of the establishment of the " one way " 

 arc, follows at once. The two paths of discharge differ in 

 length, and for a given electromotive force the maximum 

 striking distance is sooner reached in the case of the oblique 

 than of the normal arc, so that the latter continues to pass 

 at greater distances (of ball and point) than the former. 



Further inspection of the images of the two arcs in the 

 revolving mirror revealed another curious fact. The mirror 

 was set up with its axis of revolution parallel to the common 

 axis of the ball and point. The image of an instantaneous 

 spark following the line of the normal arc would therefore be 

 a line parallel to the axis of the mirror. Since the duration of 

 the discharge was nearly *001 second, this linear image was 

 expanded into a broad rectangular band. The image of any 

 oblique discharge would in general be an oblique parallelogram. 

 The image of the discharge from point to ball, however, was 

 not of that form. It appeared rather as a warped surface the 

 form of which could be explained only by supposing that the 

 discharge at first followed the normal path to the ball, and was 

 gradually displaced as the cycle progressed, until it reached 

 its extreme position at 45° to the axis, just before the rapture 

 of the arc. 



Definite results were obtained only while the platinum sur- 

 face was new and bright. The region where the normal arc 

 left the ball soon became tarnished and corroded and there 

 was an increasing tendency on the part of the oblique arc to 

 leave its own path and join the other. 



When the ball was supplanted by a platinum wire, 1 millim. 



