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by the electrons during this time can be measured from the avalanche photographs. Using the observed drift velocities the corresponding values of E/Po are calculated (Po is the final cloud-chamber pressure corrected to 296'K and, hence, corresponds to the argon gas density used by other observers). These results along with results of other observers are shown in Fig. 3, The present data indicate that in the range of E/Po values used (7.9 to 10.4 V/cm mm Hg) ke is independent of E/8 sand has a value of, :3. 04 X 10' cm'/V sec mm Hg.Herring' measured drift velocities with E/Ps values up to 6 V/cm riun of Hg. Herring's data is in excellent agreement with Nielsen's~data over Nielsen's range of E/Pp, i.e. , E/P'o values of less than four. Nielsen's data is usually considered to give the best drift velocity in-. formation for electrons in pure argon. The present mobility measurements appear to agree quite well with this data if its range is extended, although they are from 10 to 15% higher than the mobilities measured by Krrett. ' The higher mobilities measured by Riemann' ' P. Herring, Compt. Rend. 217, 75 (1943). ' R. A. Nielsen, Phys. Rev. 49, 338 (1936).

The data points in Fig. 2

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