818 The Mode of Conduction in Gases. 



The values of a and j3 can thus be determined for any 

 force X and pressure p, and it will be seen that when the 

 distance between the plates is further increased, and the 

 point is reached which makes the quantity fle^-PW equal 

 to a, the denominator of the above fraction vanishes, so that 

 « becomes infinite, and a current continues to flow in the 

 gas when the action of the light is stopped. According to 

 the theory, therefore, the sparking potential for the distance 



c^is Xx4 d x being given by the equation d x = p^gl^)* 



When tested experimentally it has been found that the 

 sparking potential for the distance d^ agrees accurately with 

 the theoretical potential in the fifty or sixty cases in which 

 the theory has been tested with various gases at different 

 pressures. All the ions in the discharge are on this theory 

 generated from the molecules of the gas, so that positive and 

 negative ions in equal numbers take part in conducting the 

 electricity. Other theories may possibly be pro| osed which 

 will account equally well for the observed phenomena ; at the 

 present time I know of none that do, as the opinions or views 

 such as those mentioned by Sir Oliver Lodge are founded on 

 very loose arguments which break down immediately they are 

 looked into carefully. 



Sir Oliver Lodge in his recent letter maintains that there 

 is something in his theoryof the action of the " inconspicuous " 

 positive ions, as he has found certain passages with which it 

 is consistent in the volume of 'Recent .Researches ' pub- 

 lished by Professor Sir J. J. Thomson in 1893. No doubt 

 the further one goes back the more likely is one to find 

 something consistent with the properties attributed to the 

 " inconspicuous " positive ions ; but even in 1889 no theory 

 of the kind was generally accepted ; and in a remarkable 

 paper published by Giese (W. Giese, Annalen der Pliysik und 

 Chemie, xxxvii. pp. 576-609, 1889) it was shown most defi- 

 nitely that explanations of the conductivity of gnses founded 

 on the theory that molecules or ions become charged by 

 contact with electrodes were absolutely inconsistent : and it 

 is interesting to note that it was on the principles formulated 

 by Giese to explain the phenomena of the conductivity of 

 flames that much of the modern advance in the theory of the 

 Conductivity of Gases has been made. 



John S. Townsend. 



Electrical Laboratory, Oxford. 

 7th October, 1911. 



