ARC IN AIR BETWEEN CARBON ELECTRODES 



drop in potential at the surface between a gas and a solid 

 when a current passes from one to the other. Such a 

 drop here might introduce a large error. But this objec- 

 tion disappears when an electrometer is used instead of a 

 voltmeter and it has been shown l that the two instruments 

 give the same readings when used to measure the potential 

 in the arc. It, therefore, seems safe to assume that in this 



60 



50 



40 



30 



20 



10 



ANODE 



DISTANCE FROM ANODE 

 FlG. Q. 



a,TH 



CATHODE 



case the exploring carbon becomes so hot and the vapor of 

 the arc is such a good conductor that there is no appreci- 

 able drop in potential between the pencil and the gas 

 about it. 



This fact is not in agreement with an experiment of 

 Fleming's. 2 He was not able to find any current flowing 

 through a circuit from an exploring pencil to the cathode. 

 If this were correct, it would not be possible to measure 



1 Phys. Rev., 19, 119; 1904. 



8 Proc. Roy. Soc., 47, 123; 1890. 



