Ionization Velocity for Electrons in Ilel 



mm. 



441 



would not onlv act photoelectricaliy on the metal parts ot 

 the apparatus, but would also be able to ionize any impurities 

 present in the gas*. It is clear that the presence of easily 

 ionizable impurities would thus result in the detection of 

 ionization at the radiation velocity, even when the experiment 

 is performed in an apparatus such as that of Dr. Goucher. 

 In the early experiments with helium-filled valves, which 

 were made at the Royal Holloway College in 1915, evidence 

 of ionization at about 20 volts was always obtained, and it 

 was only during the course of the investigation referred to at 

 the beginning of this paper, that the view that helium can be 

 ionized by electron collisions with this velocity was finally 

 abandoned. Franck and Knipping have also arrived at the 

 conclusion that the earlier experiments of Franck and Hertz 

 measured the minimum radiation velocity and not the 

 minimum ionization velocity of the gas ; and in a recent 

 pa pert they have given values of these two critical velocities 

 in good agreement with those obtained by Horton and Davies. 

 Several forms of apparatus were used in the present expe- 

 riments!, some of these being thermionic valves of the 

 cylindrical pattern, in which the filament was completely 

 enclosed by the grid. In another form two parallel grids 



Fiff. 1. 



D 

 + 



A, Auode ; B, C, Grids ; D, Filament. 



were used, the filament being enclosed below the first, and 

 the anode, a small platinum plate, being situated about 

 1'5 em. beyond the second grid and parallel to it (see fig. 1) . 



* X. Bohr, Phil. Mag. vol. xxx. p. 410(1915). 



t J. Franck and P. Knipping, Phys. Zeits. xx. p. 481 (1919). 



% 1 am indebted to the Government Grant Committee of the Royal 

 Society for the means of purchasing some of the apparatus and materials 

 used in this research. — F. H. 



Phil. Mag. Ser. G. Vol. 40. No. 238. Oct. 1920. 2 G - 



