ATION BY POSITIVE IONS 45 



and a given quantity of gas between the plates the 

 current is independent of the distance I. This follows 

 immediately from elementary considerations, for when 

 the number of molecules between the plates is constant 

 an ion makes a fixed number of collisions, and the fall 

 of potential along each path depends only on the 

 potential difference between the plates. Under these 

 circumstances the total number of ions generated by 

 positive and negative ions must be independent of the 

 distance between the plates. 



19. Curves representing ftfp as a function of X/p; com- 

 parison of effects produced by positive and negative ions. 



The values of {B/p in terms of X/p for the different 

 gases are shown by the curves figure 9. The numbers 

 obtained for ft in argon and nitrogen are practically the 

 same, so that a single curve is given for the values of 

 /3/p for these two gases. 1 



It will be observed that for the same force and 

 pressure ft is much less than a in any gas. .It can be 



1 The values of Q/p for helium are probably not as accurate as 

 those for other gases. They have been deduced from the values of 

 a and a few determinations of the sparking potential obtained for 

 pure helium (E. W. B. Gill and F. B. Pidduck, Philosophical 

 Magazine, August, 1908). When the sparking potential Y is 

 known for a given pressure p and distance S between the plates, 



TT V 



then a= , and the corresponding value of a can be found from 

 pS p 



the curve, giving afp in terms of X/p. The value of # may then 



be deduced from the equation a 3e ^ =o. The presence 

 of impurities in helium gives rise to comparatively large errors in 

 the values of a and #, and it is intended to make further experiments 

 in which particular care will be taken to maintain the helium free 

 from impurities. 



