E L. Nichols — Alternating Electric Arc. 



ohms resistance. The galvanometer line was carried to the 

 dynamo, where by means of an instantaneous contact device, 

 the circuit was closed during an interval of exceedingly short 

 duration, once in every revolution. The device consisted of a 

 wooden disk, mounted upon the shaft of the machine. A 

 single bar of brass, on the periphery of the disk, passed under 

 a brush at every turn. This bar was connected metallically 

 with a brass collar on the shaft, and a second brush, bearing 

 upon the collar, completed the circuit. By thus closiug the 

 line through the galvanometer, for an instant, once in a revolu- 

 tion, the electromotive force of the secondary circuit, at that 

 particular point of the cycle for which the contacts were made, 

 could be measured ; and since the brush was adjustable through 

 considerable range, the entire cycle could be explored. 



The arrangement of the entire apparatus is shown in figure 

 1. PC and SC are the primary and secondary coils of the induc- 

 l. torium, jp and h are respect- 



ively the point and ball. R 

 is a non-inductive resistance, 

 g the indicating galvanome- 

 ter, in parallel with the ball 

 and point, r a non-inductive 

 resistance, s a switch by means 

 of which the Thomson galva- 

 nometer could be shunted at 

 will around Ror^. K is the 

 instantaneous contact device, 

 and G is the Thomson galva- 

 nometer. When the Thomson 

 galvanometer was shunted 

 around R, which was placed in 

 the main circuit leading from 

 the induction coil, it served to 

 indicate the current flowing in that circuit during that portion 

 of the cycle for which contact was being made ; when connected 

 in shunt with the galvanometer g, it measured the fall of poten- 

 tial through the coils of the latter instrument. The function 

 of the indicating galvanometer, during this part of the investi- 

 gation, consisted in showing, by the size and direction of its 

 deflection, whether the discharge between the ball and point 

 continued to maintain its proper character. 



The. result of measurements throughout a complete cycle, 

 both when the arc was formed and when it was extinguished, 

 is shown in curves B and A, (figure 2). Curve A is with 

 close approximation a curve of sines, and it indicates the usual 

 fluctuations of current to be looked for in the secondary circuit 

 of an alternating system. Curve B shows the current through- 



