1. The Mechanism of Electrical Discharges in Gases of Low Pressure
2. V. ENGEL A. 1955 ,Ionised Gases( Oxford University Press ), p. 223 .
3. Langmuir's Paradox
4. V. GIERKE , G. , OTT , W. , and SCHWJRTZKE , F. , 1961 .Proc. 5th Int. Conf. On fonisation Phenomena in Cases. Vol. 2 , p. 1412 .