70 THE ELECTRIC ARC 



there is an excess of current flowing from the carbon. This 

 called attention again to the phenomenon which had been 

 noticed by Grove and further experiments have shown that 

 in general when the electrodes are of different substances, 

 the current flows more easily in one direction than in the 

 other. 1 For example, it flows more easily from any metal 

 to carbon than in the reverse direction; more easily from 

 cored carbon to solid than from solid to cored; and more 

 N easily from a carbon that is easily kept hot to one that 

 ; , is not, as a rotating carbon disk, than in the opposite 

 direction. 



The explanation of this is as follows: The conductivity 

 of the arc depends on the kind of vapor in the arc and 

 also upon the ease with which the cathode can be kept at 

 a high temperature, but does not depend to any large ex- 

 tent on the temperature of the anode. If the anode gives 

 off a conducting vapor when heated, this vapor will affect 

 the conductivity, but the anode does not directly affect the 

 conductivity to more than a very small extent. These 

 facts will be discussed more fully at a later point, but may 

 be accepted for the present in explaining the phenomena 

 under discussion. 



If we apply these facts to the arc between iron and car- 

 bon, for example, we find that the current flows more 

 easily from iron to carbon; first, because the conduc- 

 tivity of the iron vapor is greater than that of the carbon 

 vapor, and there is more of this vapor sent into the arc 



1 See also the following : 



Gold, Wien. Sitzungsber., 104, 2A, 814; 1895. 

 Pettinelli, Rend. Ace. Lind., (5), 5, 118 and 136; 1896. 

 Von Lang, Wied. Ann., 63, 191; 1897. 

 Cassuto, Phys. ZS., 5, 263; 1904. 



