1. Thomson, J. J., and Thomson, G. P., Conduction of Electricity through Gases, 466 (Camb. Univ. Press, 1928).
2. Theobald, J. K., J. App. Phys., 123 (1953).
3. Bradbury, N. E., Phys. Rev., 40, 980 (1932).
4. Chaudhri, R. M., and Ullah, Rahmat, Nature, 188, 131 (1960).
5. Ostrowski, Kazimeirs W., and Turek, Leszek, Acta Physica Polonica, 17, 97 (1958). Ostrowski, Kazimeirz W., ibid., 18, 231 (1959).