$84 Prof. J. S. Townsend and Mr. V. A. Bailey on 



It is difficult to set a limit to the distance between the 

 centres at which the force between the molecule and the 

 electrons would be sufficient to deflect the electron through 

 a small definite angle, but the following definitions may be 

 adopted for the mean free path I and the distance <j which 

 may be called the radius of the molecule as found by these 

 calculations. As the velocity of agitation of the electrons is 



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very large compared with the velocity of agitation of the 

 molecules, the latter may be considered to be at rest. 



If a number of electrons be uniformly distributed in a 

 stream of sectional area its 2 and move with the same velocity 

 in the same direction towards a molecule, the centre of the 

 stream being in line with the centre of the molecule, the 

 average velocity of the electrons after colliding will have" a 

 component in the opposite direction to the original motion 

 if s is very small, as a large proportion of the electrons will 



