MOTION OF GASEOUS IONS IN STRONG ELECTRIC FIELDS 



229 



the noble gas ions in the parent gas as functions of the parameter a/N. 

 These data have a high field range in which the drift velocity varies as 

 the square root of a/N. This is the variation for a model with constant 

 mean free path, as seen from the dimensional formula (26b). It was 

 indicated furthermore in the Section lA that we have good reason to 

 think of the scattering between an ion and an atom as nearly isotropic.^' 

 These two features characterize uniquely the hard sphere model whose 

 treatment we have just completed. To the extent that they are verified 



Table II 



Dimensionless high-field velocity averages (s, v) for the hard sphere model 

 and mass ratio unity. 



the model is applicable to the experimental data. The formula to apply 

 is (92) in combination with (85) : 



(100) 



fe) = 1-147 \/ ^ 



In the logarithmic plot of {cz) vs a/N the intercept of the straight line of 

 slope 1/2 which fits the high field data thus equals 



1.147 



The values for o- which result from this are shown in Table III. For 

 comparison are shown the corresponding atomic cross section as deter- 

 mined from viscosity data.^* It is interesting to observe that the ratio of 



23 A quantitative discussion of this point for the polarization force will follow 

 in Section IIIB. 



2* Landolt-Bornstein, 1950 edition, Vol. I, part 1, page 325. 



