CONTEMPORARY ADVANCES IN PHYSICS 



103 



forward it declines.'^ In Fig. 16 I show three of his V,-\s-p curves for 

 oxygen, in a tube with electrodes 38 mm, apart; they correspond to 

 wave-lengths 9.8, 5.03, 4.32 metres, therefore to frequencies 3.1-10^, 

 6- 10^ 7 • 10^ It is obvious that for any pressure in the range of these 

 experiments, Vs diminishes as v increases; also, that psm as well as Vam 

 diminish with increasing frequency. 



From Townsend's school at Oxford, I will quote some observations 

 of Hay man on helium and neon at pressures ranging from a few mm. 

 to a few tenths of a mm., in cylindrical tubes with external collar 

 electrodes. A curve of Fs-vs-/> for frequency 3.75-10'^ displays a 



:32oo - 



MM Hg 



Fig. 16 — Onset-potential vs. pressure, in rarefied oxygen, for self-sustaining 

 glow-discharge at the indicated wave-lengths of the high-frequency voltage. (Rohde, 

 Ann. d. Phys.) 



minimum. Curves of Fs-vs-i^ slope downwards toward higher fre- 

 quencies over the range from 4.7 • 10^ to 7.5 • 10^ the slope being gentle 

 at pressures above 2 mm. and very rapid at pressures rather lower 

 (at 0.11 mm. there is a drop in a ratio greater than 6 : 1, as the fre- 

 quency is raised from the bottom to the top of the aforesaid interval). 



1* Rohde devotes so much of his attention to the maintaining-potentials (see 

 below) that his allusions to the onset-potential are scanty, and their degree of gener- 

 ality is hard to assess. 



