40 THE ELECTRIC ARC 



This explanation can be tested by changing in some way 

 the temperature of the electrodes. The greatest change in 

 the residual E.M.F. may be expected when the anode is 

 cooled, for under ordinary conditions this is hotter than 

 the cathode, and when cooled we should expect the E.M.F. 

 to be diminished or reversed. Such a test was made by 

 myself. 1 The anode was cooled by three different methods. 

 In the first the anode was a hollow carbon through which 

 water could be passed. In the second a carbon arc was 

 placed in a vacuum, where the anode does not become 

 heated as much as in air. With both of these methods the 

 residual E.M.F. was much smaller than when an ordinary 

 arc between carbons in air is used. 



In the third method mercury was used for the anode 

 with the arc in a vacuum. Here the temperature of the 

 anode was lower than in either of the other cases and the 

 residual E.M.F. was reversed. These results were all in 

 harmony with the idea that the residual E.M.F. of the arc 

 is due to the difference in temperature of the terminals. 



Temperature of the Arc. There is at present no method 

 of determining the exact temperature of the arc. The 

 temperature of the vapor is not even known approximately. 

 All that one can say about the vapor is that the most re- 

 fractory substances may be vaporized in it. Since this is 

 so, the temperature of the vapor must certainly be hotter 

 than that of the carbon electrodes. Moissan 2 found that 

 with large currents more refractory elements were vaporized 

 than with small, and concluded that with such currents 

 higher temperatures were reached. This cannot be consid- 

 ered proven, since the greater power to vaporize may have 



1 Phys. Rev., 30, 311; 1910. 



2 C. R., 116, 1429; 1893, and 119, 776; 1894. 



