1. Langmuir, I., and Tonks, L., Phys. Rev., 33, 195 (1929).
2. Bitó, J., Magyar Fizikai Folyóirat, 10, 303 (1962).
3. Lakatos, G., and Bitó, J., Acta Phys., 13, 2, 193 (1961).
4. Lakatos, G., and Bitó, J., paper given at the second Czech. Conference on Electronics and Vacuum Physics, 1962, Czechosl. J. Phys. (in the press).
5. Lakatos, G., and Bitó, J., Zh. T. F., 32, 902 (1962).