10 Wdlisch — Motion of Ions and Electrons through Gases. 



(B) Motion of Free Electrons. 



A number of experiments were undertaken to determine the 

 velocity with which the free electrons moved in an electric field 

 through CO, and H^. Mobility values have already been 

 assigned by Franck^ for the electrons in argon, helium and 

 nitrogen at atmospheric pressure ; the values given were 

 respectively 209, ca. 500, and l^O""^ per sec. per volt per cm. 

 The mobility values were found to be extremely sensitive to 

 the presence of impurities in the gas under consideration, the 

 slightest trace of oxygen, for example, causing a considerable 

 reduction in the value. Recently Hainesf has investigated the 



Fig. 5. 



motion of free electrons in pure nitrogen at atmospheric pres- 

 sure and has obtained a mean value of 367 for the mobility. 



Carbon dioxide appeared especially suitable for experiments 

 in this connection because the electrons were relatively numerous 

 in it and at the same time the density of the gas was sufficiently 

 great to justify the belief that the velocities would not be inor- 

 dinately large and thus incapable of measurement with the 

 apparatus at disposal. Even with the high frequency of 800 

 alternations per second and at the highest practicable pressures 

 of the CO2 it was found that the values of the critical potential 

 (Yp) were considerably less than 10 volts, so that the observa- 



*Franck, Yerli. Deutsch. Phys. Ges., xii, pp. 291, 613, 1910. 

 t Haines, Phil. Mag., vol. xxx, p. 503, 1915. 



