THEORY OF THE ELECTRIC ARC 183 



also be that the increase in the number of molecules impedes 

 the movement of the ions and thus makes necessary a 

 greater drop in potential. 



Variations in the Anode Drop. So long as the anode is 

 kept cool, it may be changed without producing any 

 appreciable effect in the anode drop. When it is heated 

 two effects may occur. The first of these was pointed out 

 by Stark, Retschinsky and Schaposchnikoff 1 who called 

 attention to the fact that when a metal becomes very hot 

 it tends to send out negative ions, and if the metal is the 

 anode, these ions must be driven back again into the metal. 

 It would, therefore, be necessary to have a larger anode 

 drop when the metal is hot than when it is cool (p. 97). 



In the second place the conditions become quite different 

 when the anode becomes sufficiently hot to vaporize. This 

 is especially true, if the vapor which the anode sends into 

 the arc is different from the gas existing there when the 

 anode is cool. The anode drop may then change by a 

 very appreciable amount. Such a change no doubt occurs 

 in the iron arc when it passes from the first to the second 

 stage (p. 65). 



The anode drop is also very greatly affected by any other 

 change in the kind of vapor in the arc. As we have seen 

 (p. 60), the introduction of a salt, such as sodium carbonate, 

 causes the anode drop to be very much lower. Again, if 

 the cathode is changed so that a different vapor comes from 

 it into the arc, the anode drop is changed. Thus I found 

 that with cored carbon for both anode and cathode the 

 anode drop was 38 volts. With the same current and 

 length of arc and the same anode, but with iron for the 

 cathode the anode drop was 14 volts. Apparently the 



1 Ann. d. Phys., 18, 219; 1905. 



