﻿1048 Prof. Townsend and Mr. Bailey on the 



where the velocities u are about 12 x 10 7 cm. per sec, the 

 mean free paths obtained by the above formula may be 

 20 or 30 per cent, above their correct values. More accurate 

 determinations of .the mean free paths in argon for these 

 velocities of agitation may be deduced from the mean free 

 paths in a mixture of argon and hydrogen ; and it is of 

 interest to compare the values of I obtained by the two 

 methods. 



12. The simplest method of finding the mean free paths 

 in argon when the velocity of agitation is less than 

 115x 10 7 cm. per second, is to find the mean free paths in 

 a mixture of hydrogen and argon and also in pure hydrogen, 

 and to calculate the mean free paths in pure argon from the 

 two sets of measurements. The velocity of agitation is 

 controlled mainly by the hydrogen ; and as there is so little 

 loss of energy in the collisions with molecules of argon, the 

 principal direct effect of the argon is to reduce the mean 

 free paths of the electrons, and therefore to reduce the 

 velocity in the direction of the electric force. In order to 

 produce any measurable effect on the velocities of the 

 electrons in hydrogen, it is necessary to add a large quantity 

 of argon to it. In some previous experiments* it was found 

 that when the partial pressure of the argon is four times that 

 of the hydrogen, the velocities in the mixture were not more 

 than 10 per cent, lower than the velocities under the same 

 forces in the hydrogen alone. 



These observations show directly that the mean free path 

 in argon for certain velocities of agitation of: the electron 

 must b8 of the order of fifty times the mean free paths in 

 hydrogen at the same pressure. As no accurate conclusions 

 could be deduced from experiments where the velocities 

 differed by only a few per cent., the experiments were 

 repeated, using much larger quantities of argon. 



The velocities W in the direction of the electric force for 

 a mixture containing argon at a partial pressure twenty-four 

 times that of the hydrogen are given in fig. 6, the values 

 of Z/p being the ratio of the electric force to the partial 

 pressure p of the hydrogen. Thus, taking p = l, the curves 

 show that with a force of two volts per centimetre the 

 velocity of the electrons in pure hydrogen at a millimetre 

 pressure is reduced from 16 x 10 5 to 11*7 X 10 5 cm. per second 

 by adding argon to bring the total pressure up to 25 milli- 

 metres. The mean velocity of agitation is only reduced by 

 1 or 2 per cent, by the argon, so that under these conditions 



* Phil. Mag. June 1922. 



