1 84 THE ELECTRIC ARC 



negative ions in the iron vapor do not require so large a 

 potential difference in order to ionize the atoms at the 

 surface of the anode. 



The anode drop in the carbon arc decreases when the 

 current increases. As was the case with the cathode drop 

 this may be due to an increase in the temperature of the 

 anode or to more rapid vaporization of the carbon. Those 

 who have made determinations of the temperature of the 

 anode differ as to whether this temperature depends on 

 the amount of current flowing or not (p. 45), so that we can 

 not tell which of these two phenomena should be expected 

 to follow when there is an increase in the current. Either 

 of them would be followed by a lowering of the anode drop. 



The cause of the increase of the anode drop when the 

 pressure of the gas is increased is no doubt the same as 

 the corresponding increase in the cathode drop. The 

 greater the pressure the faster the heat is taken away by 

 molecules of the gas and the more heat must be produced 

 by the current. Again, the greater the number of mole- 

 cules in the neighborhood of the anode the more the ions 

 passing to and from it will be retarded. 



Variations in the Electric Force through the Arc. - 

 The electric force through the arc is changed very much 

 when the kind of gas is changed, as when a salt is intro- 

 duced into the arc or when the anode becomes sufficiently 

 hot to send vapor into the arc. This force also depends on 

 the kind of gas in which the arc is placed. Thus Stark, 

 Retschinsky and Schaposchnikoff found that the electric 

 force between either copper or carbon electrodes was greater 

 when the carbons were in hydrogen than when they were 

 in carbon dioxide. 



The cause of the change in some cases is the greater ease 



