24 THE ELECTRIC ARC 



the cathode drop with a voltmeter or galvanometer, a 

 measurement which has been made many times. Probably 

 in Fleming's experiment the pencil was not inserted far 

 enough into the arc to become thoroughly hot. If the 

 pencil was comparatively cold, it would be quite possible 

 to make the observations as they were made by him. 



Measurements of the anode and cathode drops have 

 been made by Lecher, 1 Uppenborn, 2 Luggin, 3 and Fleming. 4 

 But the most accurate work is that given by Mrs. Ayrton, 5 

 who gives 



as the equation for the anode drop when both carbons are 

 solid, where E a is the anode drop, / the current, and I the 

 length of the arc in millimeters. She also gives 



where E c is the cathode drop. When cored carbons are 

 used she found that the potential difference is in general 

 two or three volts lower than when both carbons are solid, 

 and that the cathode drop is about i volt lower. 



The direct measurement of the fall of potential through 

 the gaseous part of the arc cannot be satisfactorily ac- 

 complished. It requires the introduction into the arc of 

 two carbon pencils, and this distorts the arc so much as 

 to make nearly valueless any measurements which may be 



1 Centralbl. f. Elektrot., 10, 48; 1888. Wied. Ann., 33, 609; 1888. 



* Centralbl. f. Elektrot., 10, 102; 1888. 



8 Centralbl. f. Elektrot., 10, 567; 1888. 



4 Proc. Roy. Soc., 27, 118; 1894. 



6 Mrs. Ayrton's "Electric Arc," p. 222. 



