THEORY OF THE ELECTRIC ARC l8l 



An illustration of the effect produced by changing some 

 of the conditions is shown when different metals are used 

 for the cathode of an arc in a vacuum. As has been stated, 

 it is difficult to maintain such an arc when the metal used 

 for the cathode has a high melting point, and in general the 

 higher the melting point of the cathode metal, the greater 

 the cathode drop which is necessary. The difficulty here 

 is not altogether due to the heat being conducted away 

 from the hot point of the cathode, for the thermal conduc- 

 tivity of platinum and lead are nearly the same, yet lead 

 can easily be made the cathode, while platinum can not 

 be so used except with high voltages. The thermal con- 

 ductivity of zinc is greater than that of iron, yet zinc may 

 be the cathode of an arc in a vacuum, while iron can not. 



Whether the difficulty is due to the hot point on the 

 cathode being too cool to give off electrons in abundance, 

 or to a loading of these ions with a condensing vapor we 

 can not at present say. That the vapor does in some cases 

 condense on the negative ions is known to be a fact and it 

 may well be the cause of the difficulty here. 



Whatever its cause, it is apparently obviated when there 

 is a chance for chemical action at the cathode. Thus, it is 

 easy to maintain an arc with the oxide of a metal for the 

 cathode, and also with metals in air, and with the greater 

 number of them in nitrogen. There is no great difficulty in 

 maintaining one between graphite or carbon terminals in 

 hydrogen, while it is very difficult to do so between copper 

 or iron terminals in that gas. 



Apparently it is easier to maintain an arc when there 

 is either chemical action or ebullition at the cathode, as 

 if ionization could occur more easily when physical or chem- 

 ical change is occurring. Even with carbon in hydrogen 



