424 Messrs. Stead and Grossling on Relative Ionization 



of helium is in agreement with that of most other observers*, 

 except Horton and Daviesf, who have recently found a higher 

 value, as in the case of argon. Richardson and Bazzoni % 

 deduced a value of 29 volts from a consideration of the 

 ultra-violet spectrum of helium. 



As regards carbon monoxide, the value, 15 volts, here 

 given at present lacks confirmation by other observers. 

 Hughes and Dixon§, employing a method of the Franck and 

 Hertz type, obtained the value 7*2 volts. It seems probable 

 that, as in the case of nitrogen, this low value represents a 

 photo-electric effect, rather than the true ionization potential. 

 At any rate there is no measurable ionization, in a valve 

 containing carbon monoxide, below about 15 volts. 



It may be of interest to note that the increase in slope of 

 the current-voltage curve which occurs at the critical point 

 is not accompanied by any visible radiation. The conditions 

 for observing a faint glow between the electrodes are, 

 however, not good, on account of the glare from the filament. 

 All workers with soft valves are familiar with the fact that 

 when the anode voltage exceeds a certain value a general 

 glow suddenly fills the bulb, but the appearance of this 

 general glow must on no account be confused with the 

 critical point which has been taken by the writers to denote 

 the formation of positive ions by collision. The general glow 

 sets in at a much higher voltage than the critical point — 

 usually not less than twice the voltage corresponding to the 

 critical point. Moreover, whilst the position of the critical 

 point is independent of the form of the electrodes and of the 

 gas pressure, and nearly independent of the filament tempe- 

 rature, the potential at which the general glow appears 

 depends very much on all these factors. Thus, in an 

 audion type valve, with plane anodes and grids, the general 

 glow occurs much more readily than in a cylindrical type 

 valve, and the appearance of the general glow depends so 

 much on the gas pressure that a rough estimate of the 

 softness of a valve may be made by observing the voltage 

 at which the glow appears. Again, if a current-voltage 

 curve is plotted up to a potential at which the general 

 glow appears, there is at this point a complete discontinuity, 

 the current suddenly jumping to many times its previous 



* McLennan, Phys. Rev., July 1917, p. S4. Bazzoni, loc. cit., and 

 others. 



t Proc. Roy. Soc. vol. xcvii. A, p. 408. 

 % Phil. Mag. xxxiv.p. 285 (1917). 

 § Phys. Rev., November 1917. 



