1. R. N. Jackson and K. E. Johnson, Advances in Electronics and Electron Physics, edited by L. Marton (Academic, New York, 1974), p. 191.
2. B. Welber (private communication).
3. F. M. Penning and C. C. J. Addink, Physica (Utr.) 1, 1007 (1934).PHYSAG0031-8914
4. J. M. Meek and J. D. Craggs, Electrical Breakdown of Gases (Clarendon, Oxford, 1953), Chap. 2.
5. S. M. Frouws, Proceedings of the Third International Conference on Ionization Phenomena in Gases, Venice, 1957 (North‐Holland, Amsterdam, 1958), p. 341.