[ 269 ] 



XXIX. Energy required to Ionize a Molecule by Collision. 

 By J. S. Townsend, Wykeham Professor of Physics, Oxford *. 



IN the theory of ionization o£ gases by collision, it has been 

 shown that the quantity a, representing the number of 

 molecules ionized by one electron in moving through a 

 centimetre of the gas at a pressure of one millimetre, is given 



by an equation of the form a = Ne x for the larger values 

 of the force X. The agreement between the formula and the 

 experimental results was much closer than might have been 

 expected, considering the assumptions which were made in 

 finding the formula. Thus, for example, at the lower values 

 of X, it has been found that the velocity of agitation of the 

 electrons exceeds the velocity acquired under the force. 



If the quantity a represents the number of collisions with 

 molecules in which the velocity of the ion exceeds a certain 

 value V f , the velocity Y f must be obtained on a different 

 principle from that previously adopted, when the force X 

 is smallf. This may be illustrated by taking the values of 

 a. recently obtained by Wheatley % for air corresponding to 

 small values ol the ratio X//>. The velocity of agitation ?f, 

 and the velocity in the direction of the electric force W, for 

 these values of X/p, have also been determined§. 



For air at 1 millimetre pressure the values of a, w, W, and 

 h are given in the following table, k being the factor by 

 which the kinetic energy of the electrons exceeds that of the 

 surrounding molecules. 



X. 



a.. 



MX 10 7 . 



6 



WxlO -6 . 



Jc. 



40 



•019 



9-1 



150 



81 



50 



•055 



101 



173 



100 



70 



•212 



11-3 



220 



125 



90 



•495 



,,2 



25-5 



150 



The velocity of agitation of the molecules may be neglected 

 in comparison with that of the electrons, and the mean free 

 path I of an electron moving in air at 1 millimetre pressure 

 may be taken as *032 centimetre. 



* Communicated by the Author. 



t See ' Theory of Ionization of Gases by Collision,' p. 28. 



I F. W. Wheatley, Phil. Mag. Dec. 1913. 



§ J. S. Townsend' & H. T. Tizard, Proe. Roy. Soc. lxxxviii. p. 330 



(1913). 



