Relative Ionization Potentials of Gases. 413 



It has been urged against the idea of a value of force not 

 varying inversely as the square of the distance that the scat- 

 tering of the a. particles indicates a force of this type. The 

 scattering of the a particles, however, practically all takes 

 place at distances from the centre very small compared with 

 the distance of the electrons which scatter the light or affect 

 the emission of visible light ; and it is quite possible that the 

 law should be the inverse square both at distances which are 

 very large compared with c and again at distances which 

 are very small compared with it, and yet be quite of a 

 different character when r is comparable with c — in fact, 

 the law expressed by the value of <f> given above is at both 

 very great and very small distances that of the inverse 

 square. 



XL VII. On the Relative Ionization Potentials of Gases as 

 observed in Thermionic Valves. By G. Stead, M.A., and 

 B. S. Gossling, M.A* 



Preliminary Note. — The observations described in this 

 paper were made during the summer of 1917, in connexion 

 with the improvement of thermionic valves of a type in 

 which the action was in considerable measure dependent on 

 ionization. They are now published with the permission o£ 

 the Admiralty. 



(1) Method employed. — Some of the earlier methods f of 

 determining the ionization potentials of gases have been the 

 subject of criticism as indicating, not the appearance of 

 positive ions, but some other effect such as photo-electric 

 emission of electrons from the electrodes whose intended 

 function was to collect the positive ions when formed. In 

 the experiments here described the method used was similar 

 to that employed by BazzoniJ, and consists in observing the 

 effects of the presence of gas on the form of curves repre- 

 senting the variation of the current leaving an incandescent 

 cathode with change in the potential applied to the collecting 

 electrodes. When no gas is present, the chief factor deter- 

 mining the value of the space-current is the modification of 

 the collecting field due to the applied potential by the addition 



* Communicated by Professor Sir J. J. Thomson. 

 f E. y. Frauck and Hertz, Deutsch. Phys. Ges. vol. XV. (1913) ; 

 Pawlow, Proc. Roy. Soc. vol. xe. p. 398 (1914). 

 | Phil. Mag. vol. xxxii. p. 566 (1916). 



