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XCIV. Ihe Motion of Electrons in Argon and in Hydrogen. 

 By J. 8. Townsend, M.A., F U.S., Wykekam Professor 

 of Physics , Oxford^ and V. A- Bailey, M.A., The Queen's 

 College, Oxford *. 



1. rjHRE experiments on the motion of electrons in argon 

 A which we have already published show such re- 

 markable differences between this Pas and nitrogen or 

 hydrogen, that we considered it desirable to make further 

 experiments with argon which had been very completely 

 purified, and to extend the determinations of the velocities 

 over larger ranges of pressures and forces. 



For this purpose it was necessary to construct an apparatus 

 suitable for measuring the velocity in the direction of the 

 electric force, and also the velocity of agitation when the 

 electrons move in a widely diverging stream after passing- 

 through a narrow slit in a metal sheet. 



In order to obtain accurate results it is necessary in all 

 cases that the gas should be free from impurities which tend 

 to form ions. With gases like argon, where the electron 

 loses a very small proportion of its energy in colliding with 

 a molecule, the gas should be free not only from impurities 

 that tend to form ions but also from gases like nitrogen and 

 hydrogen, as the loss of energy of an electron in a collision 

 with a molecule of one of these gases, although small, is 

 large compared with the loss of energy in a collision with 

 argon. Also the effect of such impurities in argon is 

 accentuated by the fact that the probability of a collision 

 between an electron and a molecule is much greater in the 

 other gases than in argon. 



It was found that impurities get into the gases from the 

 materials such as ebonite or elastic cement generally used 

 in the construction of apparatus for measuring velocities, so 

 that in nitrogen or hydrogen the results obtained after the 

 gas had been in the apparatus for a few days were slightly 

 different from those obtained immediately after the gas had 

 been admitted. In the case of argon the effect of these 

 impurities was noticeable after the gas had been in the 

 apparatus for one day. 



2. In order to eliminate the impurities emanating from 

 the apparatus, glass was used instead of ebonite to insulate 

 and fix in position the various electrodes and guard-rings, and 

 the connexions were made through glass capillary tubes 

 instead of ebonite plugs. The capillary tubes were slightly 



* Communicated by the Authors. 

 Phil. Mag. S. 6. Vol. 44. No. 263, Nov. 1922. 3 X 



