886 Prof- J. S. Townsend and Mr. V. A. Bailey on 



the velocity is W==— - x-x 0-815, which formula may be 



m 



derived from a more general formula* for the velocity of 

 ions in gases given by Langevin. 



All the quantities in this formula are known except I, 

 e/m being 1*77 x 10 7 the value of this ratio for electrons, and 

 ii = l*15xl0 7 VX, since the mean velocity of agitation of a 



Fig;. 6. 



150 















k 













100 











* 









f / 



* 









50 





/ ** 



f 









«& 















* • / 



If 









Z 

 P 





10 



30 



40 



50 



60 



particle having the same mass as an electron is 1*15 x 10 7 

 centimetres per second when in thermal equilibrium with 

 molecules of a gas at 15° C. 



Taking one millimetre of mercury as the pressure of the 

 gas, the values of I for the different gases as obtained from 

 the above formula are given in Tables IV., V., and VI. 

 The values of I for air have also been calculated and are 

 given in Table VII. 



The values of a given in the tables are obtained from the 

 formula 1=1/ttvo- 2 , v the number of molecules per cubic 

 * Townsend, t Electricity in Gases," p. 180. 



