Mobility of the Negative Ion. 



451 



the mobility of the negative ion attains a maximum value 

 and begins to diminish. This phenomenon could be pre- 

 dicted from Langevin's formula for the mobility of a charged 

 particle whose mass is small compared with that of a mole- 



to e I 

 cule : -^ = — . - , where e and m are the charge and the 



X 



m 



mass of the particle, I its free path, and u its velocity of 

 agitation. The mass associated with the negative ion 

 gradually diminishes with increase of electric force, and at 

 this stage of evolution of an ion its mobility increases with 

 the force. When, finally, the electronic state is reached by 

 the ion, its mobility follows the above equation, and since 

 the velocity of agitation of an electron increases with the 

 electric force *, its mobility begins to decrease j. 



K* 5. 















7 











■vT 



6 











^^ 



5 

























1 





)w/ 





^\e 



"^~~~«£^ 



3 











50n-^>- 



2 







' 







w+ 





m + 





~r ' 



2Q0 



400, 



Electric Force in 



This decrease in the mobility of the negative ion is shown 

 in curves I., II., III., and IV. (fig. 5), for pressures of 200, 

 100, 50, and 10 mm. respectively. The curves III. (+) and 

 IV.(+) show the mobility of the positive ions at 50 and 10 mm. 



* J. S. Townsend and H. T. Tizard, Proc. Roy. Soc. A, lxxxviii. 

 p 336 (1913). 



t A full discussion of this question will be found in J. S. Townsend's- 

 book, ' Electricity in Gases.' Oxford, 1915. 



li 



