174 THE ELECTRIC ARC 



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changed but little, so that the mean free path in this last 

 case was approximately three hundred times greater than 

 that in the first, while the electric force was only one-half 

 as great. Again, in the data given by Knipp, the change 

 in the electric force was much smaller than that in the 

 pressure. 



It has been suggested by the author that these diffi- 

 culties can be easily explained if we assume that the mole- 

 cules of a gas are much more easily ionized when the 

 temperature of the gas is high than when low. 1 We have 

 already seen that the molecules on the surface of a solid 

 are more easily broken apart if the solid is very hot and 

 in all probability the same thing occurs with gases. This 

 explanation seems the more reasonable since the gas in the 

 arc is always at a high temperature. Even when the 

 pressure of the gas was only 0.5 mm., the temperature of 

 the arc between carbons was sufficiently high to melt 

 platinum. As has been shown there is a difference of 

 opinion regarding the mercury arc, but it seems safe to 

 assume that its temperature is at least high. 



Such an assumption will very easily explain the difficul- 

 ties which have been mentioned. It would, for example, 

 explain the fact that the larger the current flowing through 

 the arc the less the electric force required to produce it, for 

 when the current increases there is an increase in the heat 

 developed in the arc, and if ionization occurs more easily 

 at high temperatures, then the electric force needed to 

 produce the greater amount of ionization may remain 

 nearly constant or even become smaller. 



It would explain the phenomena at different pressures. 

 When the mean free path of the ions is sufficiently large, 



1 Phys. Rev., 19, 134; 1904. 



