1 70 THE ELECTRIC ARC 



The same reasoning that was given in connection with the 

 cathode drop would apply here. Whether the number of 

 negative ions going to the anode is large or small, a new 

 ion must be formed for each negative ion coming to the 

 anode, and a definite potential difference is needed for this 

 purpose. 



The view was expressed by Sir. J. J. Thomson 1 that the 

 ionization at the anode is caused by the anode being 

 heated to a high temperature and that the anode drop is 

 determined by the amount of energy necessary to keep the 

 anode at this temperature. However, there are many 

 forms of the arc where the anode is not raised to a tem- 

 perature which can be considered high, so that the anode 

 drop would not appear to be determined by such a require- 

 ment. 



Blondel 2 expresses the view that when the anode is 

 vaporized it gives out positive ions. I have not been able 

 to find anything that confirms this view and the fact that 

 the anode drop in a mercury arc is much the same with a 

 mercury anode which is easily vaporized as it is with an 

 iron anode which is not vaporized would seem to make 

 this view untenable. 



Discharge of Ions to Electrodes. We have considered 

 the manner of production of ions at the electrodes, but it is 

 also true that the ions coming to them must in some way 

 lose their charge. Whether the charge is detached from 

 the ion and given to the electrode or is neutralized by 

 some charge coming from the electrode, we do not know, 

 but in either case it is hardly to be expected that the action 

 will take place without loss or gain of energy. That it does 



1 " Conduction of Electricity through Gases," ist ed., p. 424. 



2 Trans. Inter. Elec. Cong., St. Louis, 2, 747, 1904. 



