8 E. L. Nichols — Alternating Electric Arc. 



series of purple images, the intermediate ones remaining undis- 

 turbed in position, duration and appearance. The intervals of 

 darkness were then estimated to occupy sixth-tenths of each 

 cycle, the discharges, four-tenths ; a ratio which corresponds 

 with that of the duration of positive potential of the ball (as 

 shown in curve) to the total length of a complete cycle. 



The ball and point phenomenon is unquestionably very 

 closely related to a class of effects with which students of static 

 electricity have long been acquainted. One recalls, to begin 

 with, Faraday's experiments with the Leyden jar; in which, of 

 two paths, the spark invariably followed that involving passage 

 from a positive ball to a negative point, in preference to another, 

 through equal air-space, between a negative ball and a positive 

 point.* Wiedemann and Ruhlmann have since shown that 

 between spherical electrodes which differ in diameter, the 

 quantity of electricity necessary to produce a discharge is less 

 when the larger ball is positive than when it is of negative 

 charge,f and Macfarlane has measured the electromotive force 

 which will produce a spark between a point and plate, and has 

 found it to be greater when the point is positive than when it 

 is negative.* 



In view of the experiments described in the present paper, 

 it appears that what is true, in this particular, of the spark 

 from the Leyden jar and the discharge of the Holtz machine, 

 is true also of the alternating current arc. 



Paet II. 



(From experiments made by Mr. F. C. Caldwell.) 



After the completion of the experiments of Messrs. Arch- 

 bold and Teeple, the study of the Ball and Point Phenomenon 

 was taken up under the writer's direction by Mr. F. C. Cald- 

 well ; the chief purpose of the investigation being to test the 

 applicability of the effect to alternate current measurement. § 



Irregularities of action due to rapid changes in the surfaces 

 of the point and ball, by corrosion and disintegration under 

 the arc, finally caused the attempt to be abandoned, for the 

 time being, but Mr. Caldwell in the course of his work, made 

 a large number of observations of the discharge under various 

 conditions. Many of these are of interest in this connection 

 on account of the light which they throw upon the original 

 experiments, and because of the lines of further research which 

 they suggest. 



* Faraday; Experimental Researches, §1493. 



f Wiedemann and Riihlmann : Annalen der Physik und Chemie, cxlv. See 

 also Wiedemann ; Elektricitat, iv, p. 462. 



% Alexander Macfarlane ; Proceedings of the Royal Society of Edinburgh, vol. 

 x, p. 555. 1879-80. 



§ Frank Cary Caldwell : A study of the Alternating Arc between a Ball and 

 Point. Thesis in MS. Library of Cornell University, 1890. 



