Potentials of Gases observed in Thermionic Valves. -425 



value without, apparently, passing through any of the 

 intermediate values. It' the potential is now gradually 

 lowered there is a marked hysteresis effect, and the general 

 glow suddenly disappears at a potential considerably lower 

 than that at which it appeared. The disappearance of 

 the general glow is accompanied by a sudden and dis- 

 continuous fall in the current. This behaviour is in marked 

 contrast to the behaviour at the critical point. At this 

 latter point there may be a discontinuity in the slope of 

 the curve, but there is certainly no sudden jump in current 

 without passing through intermediate values. Also there is 

 little or no genuine hysteresis, the only observable effect being 

 that the portion of the curve dependent on the 3/2 power of 

 the voltage may be slightly displaced, but not the position 

 of the critical point. This displacement is probably due to 

 some change in the surface of the filament resulting from its 

 bombardment with positive ions. 



The point at which the general glow appears also shows 

 up in ordinary three-electrode valve characteristics as a 

 sudden discontinuity in the value of the current, the occur- 

 rence of which makes the adjustment of some ; ' soft " valves 

 troublesome. 



(6) Summary. — A simple method is described of deter- 

 mining the differences of potential which are necessary to 

 cause positive ions to be produced in soft thermionic valves. 

 This method has been applied to six different gases, viz. : 

 mercury vapour, argon, hydrogen, carbon monoxide, nitrogen, 

 and helium. 



The work was carried out in the Cavendish Laboratory, 

 and the writers wish to express their best thanks to Professor 

 Sir J. J. Thomson. 



Sote added September 3, 1920. 



Since the above was written another valve method of 

 determining ionization potentials has been described by 

 Hodgson and Palmer*. For mercury vapour, nitrogen, 

 and helium these observers obtained results substantially in 

 agreement with those given in the present paper, but for 

 argon a higher value (16*6 voltsj was obtained. 



* Radio Review, vol. i. Aug. 1920, p. 525. 



Phil. Mag. S. 6. Vol. 40. No. 238. Oct. 1920. 2 F 



