1. Hershey, A. V., Phys. Rev., 56, 908 (1939).
2. Loeb, L. B., “Fundamental Processes of Electrical Discharge in Gases” 551 (New York: Wiley, 1947).
3. Charlton, E. E., and Cooper, F. S., Gen. Elec. Rev., 40, 438 (1937).
4. Warren, J. W., Hopwood, W., and Craggs, J. D., Proc. Phys. Soc., 63, 180 (1950).