52 THE ELECTRIC ARC 



The current between a hot carbon used as cathode and 

 a cooler one used as anode with different temperatures and 

 different potential differences has been examined by Pollock 

 and Ranchaud. 1 They studied this with especial reference 

 to the conditions under which the current changes from a 

 small, non-luminous discharge to the arc. The hot carbon 

 was heated by passing an electric current through it, and its 

 temperature was determined by a Holborn-Kurlbaum opti- 

 cal pyrometer. The anode became heated somewhat by 

 the radiation from the cathode and its temperature was 

 determined by a thermojunction. 



VOLTS BETWEEN CARBONS 

 FIG. 13. 



Some of their results are given in Fig. 13. The first 

 number above each curve gives the temperature of the 

 cathode and the second one that of the anode. The 

 dotted parts of the curve are extrapolated. The crosses 

 "x" indicate the points at which the discharge became an 



arc. 



1 Phil. Mag., (6), 17, 366; 1909. 



