86 THE THEORY OF IONIZATION OF GASES 



it might be expected that y would be large compared 

 with a. The experimental evidence therefore shows 

 that y=0. If there is any adhesion of the negative 

 corpuscles to molecules in the circumstances under 

 consideration, the connection must be of a very loose and 

 transitory character. 1 



In the 1906 edition of the treatise on " The Conduc- 

 tion of Electricity through Gases," Professor Sir J. J. 

 Thomson gives another investigation of the sparking 

 potential. 



The sparking potential for parallel plate electrodes is 

 obtained from the condition required to maintain a very 

 small current which does not disturb the uniformity of 

 the electric field, the lowering of the potential for large 

 currents is attributed to the increase of force at the 

 cathode, the effect of the positive ions in ionizing mole- 

 cules of the gas is also taken into consideration, and for 

 the larger distances between the plates this is regarded 

 as the predominating effect of the positive ions. It will 

 be noticed that it was on these principles, as has already 

 been explained, 2 that the theory which gives results in 

 accurate agreement with experimental determinations was 

 originally 3 worked out. 



The general expression for the potential is given by 

 the equation 4 



(a 7 / 



la-y-fid 



7*ciu/t __ e i 



*~d { -^^ - (a-7-/3) 2 +(- 



1 See section 7 above. 



2 Sections 20, 21, and 22. 



8 J. S. Townsend, Electrician, April, 1903; Philosophical Magazine, 

 November, 1903, and March, 1905. 



4 " The Conduction of Electricity through Gases," 1906 edition, 

 pp. 494 and 495. 



