T H E 



LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



MARCH 1905. 



XXIX. A Theory of the Variation of the Potential required to 

 maintain a Current in a Gas. By John S. Townsend, 

 M.A., Wykeham Professor of Physics, Oxford*. 



1. rriHE researches t which have been made on the 

 _L ionization of a gas by the motion of positive and 

 negative ions, show that a satisfactory explanation of the 

 sparking potentials between two parallel plate-electrodes is 

 obtained on the supposition that all the ions are produced by 

 collisions from the molecules of the gas. The calculations 

 which lead to the various conclusions are very simple, as the 

 theory rests upon the results of experiments made with small 

 currents between the plates so that the uniformity of the 

 electric field is not disturbed. 



The results of the experiments may also be applied to 

 exp-am a large number of the variations of the electric field 

 which ensue when a steady current is maintained in the gas. 



Although the experimental results show that these fields 

 of force are widely different under different circumstances, 

 still the number of hypotheses and theories which have been 

 proposed in order to explain them appear to be superfluous. 

 It ia to be expected that there is a close connexion between 

 many of these phenomena; and it is desirable, if possible, to 

 obtain a simple theory which will, . afford a satisfactory 

 explanation of the various predominating features and at the 

 same time show how they are related to one another. 



* Communicated bv the Author. 



t J. S. Townsend/ Phil. Mag. November 1903 ; J. S. Townsend & 

 H. E. Hurst, Phil. Mag. December 1904. 



Phil. Mag. S. 6. Vol. 9. No. 51. March 1905. U 



