THEORY OF THE ELECTRIC ARC 175 



ionization will be produced without high temperature and 

 we then have the arc at lower temperatures. As the mean 

 free path becomes less, the average velocity of the ions 

 becomes less, and in order to have ionization, the tem- 

 perature must be raised. At still higher pressures the 

 mean velocity of the ions becomes very small and the tem- 

 perature is the determining factor. Then it makes little 

 difference what the mean free path of the ions may be, if 

 only the temperature is sufficiently high. 



This view is also in harmony with the facts observed by 

 Pollak. The mercury vapor above the arc was found by 

 him to be but slightly conducting when the current passing 

 through it was small. The conductivity was found to 

 become very large when the current was increased, so that 

 a considerable amount of heat was developed. 



But it should be added that the assumption that the 

 ultra-violet light causes the molecules to become more 

 easily ionized by impact would also explain any difficulties. 

 We not only have high temperatures, but also ultra-violet 

 light in all forms of the arc. This is especially true of the 

 mercury arc, where ions are formed with exceptional ease. 

 Thus, when the current increases, the amount of this light 

 increases, which could account for the greater ease of 

 ionization with larger currents. With increase in pressure 

 there is an increase in the voltage, the amount of energy 

 used in the arc increases, and no doubt the amount of ultra- 

 violet light also becomes greater, which would account for 

 the ionization by impact regardless of the small difference 

 of potential through which the electrons move. 



Thus, either of these explanations would seem reasonable 

 and no fact is known to the writer which would enable one 

 to decide between them. It is indeed altogether probable 



