6 E L. Nichols — Alternating Electric Arc, 



results obtained. It was thought that they would give the 

 fluctuations in electromotive force between ball and point, cor- 

 responding in time to the current fluctuations in the resistance 

 R. The indicating galvanometer, however, owing to the very 

 rapid alternations to which the circuit was subjected, was found 

 to possess such high self-induction as to materially influence 

 the result. Strictly speaking, the curves C and D, therefore, 

 give the periodic changes of electromotive force at the termi- 

 nals of the indicator and not those occurring at the ball and 

 point. 



These curves are nevertheless of considerable interest. The 

 curve C shows the character of the cycle when no arc was 

 formed, and D, when the arc was in operation ; C, like A, is 

 approximately a curve of sines. The irregularity at its posi- 

 tive crest, which also appears in D, is probably due to the 

 imperfect performance of the contact brush, and having no 

 bearing upon the phenomena which the curves are intended 

 to elucidate, may be disregarded. Since no current was pass- 

 ing between the ball and point when A and C were taken, 

 they represent the fluctuations in successive portions of the 

 same circuit. The lag, due to self-induction, however, is very 

 marked, amounting to almost 90° of phase. Curve D. which 

 shows the influence of the arc, is especially instructive. The 

 potential rises during the first part of the cycle (ball positive) ; 

 then follows a very sharp oscillation, occupying about one- 

 twentieth of the entire period or 1/4600 of a second of time. 

 The potential then reaches a small positive value which it 

 maintains without fluctuation for at least four-tenths of a com- 

 plete cycle, when it suddenly becomes strongly negative. 



To obtain curves of electromotive force between ball and 

 point directly, a non-inductive resistance was substituted for 

 the indicating galvanometer and the measurements from which 

 curves C and D had been drawn were repeated. Of the two 

 curves thus determined, the one taken when the arc was not 

 playing (E, figure 3), is a sine curve closely coinciding in phase 

 with the simultaneous curve of current (A, figure 2). The 

 corresponding curve F, which was taken while the discharge 

 was passing between the ball and point, is in its essential 

 features of the same character as curve D (figure 2). The 

 interval of uniform positive potential is of the same length and 

 it is coincident with the interval of excess of current which 

 shows itself in the positive branch of curve B. It is note- 

 worthy that this interval of uniform potential, which marks 

 the duration of the arc, occupies in both cases the same portion 

 of the cycle, (between scale-divisions 10 and 19, approximately), 

 although there is otherwise a difference of phase, due to self- 

 induction, amounting to at least four scale divisions. Curve 



