E L. Nichols — Alternating Electric Arc, 7 



C for instance, reaches its maximum in the neighborhood of 

 scale-division 8, curve E at scale-division 12. The exclusion of 

 the coils of the indicating galvanometer from the circuit, 

 reduced this difference of phase to a small quantity, and it 

 suppressed altogether the remarkable oscillation of electro- 

 motive force (see curve D) which in all preceding experiments 

 had introduced the formation of the arc. 



3. 



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The results exhibited graphically in these six curves, afford 

 abundant verification of the theory of the ball and point phe- 

 nomenon, given in a previous paragraph ; and they establish 

 the fact that in the secondary circuit of a transformer, such as 

 that made use of in these experiments, the striking distance 

 from ball to point exceeds that from point t ball. It follows 

 that whenever the space between the ball and point is less than 

 the former and greater than the latter distance, discharge will 

 occur only during that portion of each alternation for which 

 the ball is positive, and that under such circumstances, a gal- 

 vanometer placed in the circuit will show a constant deflection. 



Complete corroboration of the foregoing conclusion, was 

 obtained by studying the image of the arc in a revolving mirror. 

 With a discharge of less than the critical length, the discharge 

 was seen to consist of two distinct sets of sparks, all of the 

 same duration but differing in color. Each alternate discharge 

 was purple, the intermediate ones being of a greenish cast. 

 The spark-images were everywhere equi-distant and their dura- 

 tion was about four times as great as the intervening intervals 

 of darkness. The extension of the sparking distance beyond 

 the critical point, resulted in the complete suppression of the 



