1. M. Faraday, Experimental Researches in Electricity, Vol. 3, Taylor & Francis, London (1859).
2. V. I. Smirnov, A Course of Higher Mathematics, Vol. 2 [in Russian], GITTL, Moscow (1957).
3. V. S. Vladimirov, Equations of Mathematical Physics [in Russian], Nauka, Moscow (1988).
4. I. Langmuir, The effect of space charge and residual gases on thermiоnic currents in high vacuum, Phys. Rev., 2, Issue 6, 450-486 (1913).
5. I. Langmuir and K. B. Blodgett, Current limited by space charge between coaxial cylinders, Phys. Rev., 22, Issue 4, 347-356 (1923).