482 On the Design of Soft Thermionic Valves. 



of 20 volts was applied to the anode the pink colour o£ the 

 discharge began to niter, and after 24 hours there was no 

 trace of pink colour left. The spectrum then showed carbon 

 monoxide bands strongly, one or two faint nitrogen bands, 

 and argon lines as before. 



It seems clear from these experiment that under the 

 ordinary working conditions of a valve, nitrogen is absorbed 

 and electrode gas given out. Furthermore, the evolution of 

 electrode gas is at least in part a purely thermal effect 

 whilst the absorption of nitrogen is apparently electro- 

 chemical rather than chemical, since it does not take place 

 unless there is a potential on the anode. Theie seems reason 

 to believe that no absorption of nitrogen tales place unless 

 the difference of potential between the anode and the negative 

 end of the filament exceeds the ionization potential of 

 nitrogen (i. e , about 17* 5 volts) so that probably tungsten 

 at a temperature of 2300° K does not combine with nitrogen 

 molecuh s, but is capable of uniting with nitrogen presented 

 in the form of rapidly moving positive ions. This is in agree- 

 ment with the result previously obtained by Langmuir *-. 



IX. Summary — An account is given of experiments which 

 led up to the design of a satisfactory soft valve for Naval 

 uses. The chief points considered are : — 



(1) The effect on the valve characteristics of the position 

 of the grid with respect to the filament, and of the close- 

 ness of the grid structure. 



(2) The effect of the pressure and nature of the gas on the 

 valve characteristics and on the production of oscillations. 

 Nitrogen, argon, and helium were studied from this point of 

 view, and it was found that there were serious objections to 

 the first two gases, but that helium was very satisfactory. 



(3) A method of estimating the pressure of the gas in a 

 sealed valve is given, and the effect of nitrogen and helium 

 in cooling the filament by conduction and convection is 

 considered. 



In conclusion the writer wishes to express his sincere 

 thanks to Professor Sir J. J. Thomson for his unfailing 

 kindness, and his interest in the work. 



Oarendinh Laboratory, 



Cambridge, 



August 1920. 



* Pbp. Rev. ii. p. 450 (1913), and Jour. Amer. Chem. Soc. xxx v. 

 p. 948 0913). 



