Potentials of Gases observed in Thermionic Valve*. 42'.) 



results obtained were not in agreement with the values 

 generally accepted at that time. Thus the ionization 



potentials of hydrogen and nitrogen, as measured by the 

 Franck and Hertz method, came out at about 11 volts and 

 8 volts respectively, and for mercury vapour opinion was 

 divided between 4*9 volts and about 10*5 volts. By a 

 modification of the Franck and Hertz method, designed to 

 eliminate photo-electric effects, Davis and Goucher * have 

 established the fact that no appreciable ionization occurs in 

 mercury vapour under 10 volts. In the same paper Davis 

 and Goucher came to the conclusion that ionization occurs in 

 hydrogen at about 11 volts, but that a second type of ioni- 

 zation sets in at about 1 5*8 volts. Shortly after the appear- 

 ance of Davis and Goucher's paper Bishop f published an 

 account of some experiments on hydrogen in which he 

 confirmed Davis and Goucher's result that a second (and 

 stronger) ionization appears in hydrogen at between 15 and 

 16 volts. The higher value agrees with the value of the 

 ionization potential of hydrogen as determined by the 

 method described in the present paper. No trace ot ioni- 

 zation at 11 volts, however, appears in a valve containing 

 hydrogen. 



The only measurements of the ionization potential of 

 nitrogen which are in agreement with the value here given 

 (17*2 volts] are those made by Davis and Goucher J, and 

 more recently by H. D. Smyth §. These observers have 

 come to the conclusion that the true ionization potential of 

 nitrogen is between 17 and 18 volts, and not about 8 volts, 

 as had been previously supposed. 



The value 12*5 volts for argon is in agreement with the 

 result given by McLennan in the Physical Review for July 

 1917, but very recently Horton and Davies |,' have come to 

 the conclusion that ionization does not occur in argon until 

 about 15 volts, and Rentschler H gives the value 17 volts. 

 It is very difficult to reconcile this with the behaviour of 

 argon in valves,, in which it is hardly possible to avoid the 

 conclusion that strong ionization occurs at about 12*5 volts. 

 The result (20'8 volts) obtained for the ionization potential 



* Physical Review, August 1917, p. 101. 



t Phys. Rev., September 1917, p. 244. 



t Phys. Rev., January 1919. 



§ Phys. Rev. xjv. p. 409 (1919). 



II Proc. Roy. Soc. vol. xcvii. A. p. 1 (1920). 



H Phys. Rev. xiv. p. 503 (1919). 



