Electric Arc between a Ball and Point. 131 



tial of the ball (as shown in the 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 involv- 

 ing passage from a positive ball to a negative point, in pre- 

 ference 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 negative f ; and Macfarlane has measured the electro- 

 motive 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 J. 



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. 



Part II. § 



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 connexion 

 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. 



t Wiedemann and Riihlmann, Annalen der Physik und Cheniie, cxlv. 

 See also Wiedemann, fflektrwiliit, iv. p. 462. 



% Alexander Macfarlane, Proceedings of the Roval Society of Edin- 

 burgh, vol. x. p. 555 (1879-80). 



§ From experiments made by Mr, F. C. Caldwell. 



|| Frank Cary Caldwell, " A study of the Alternating Arc between a 

 Ball and Point." Thesis in MS. Library of Cornell University (1890). 



