Design of Soft Thermionic Valves. 477 



One of the five valves was finally selected as giving the 

 most satisfactory results in the circuits employed. This 

 was a cylindrical valve having a grid made of v\ ire of 

 diameter 04 mm. coiled into a helix of mean diameter 

 4*5 mm. and having 5*5 turns per cm. Full characteristics 

 were drawn for this val\e for a range of pressures from 

 0*5 mm. to "0045 mm. Finally a pressure of about 06 mm. 

 was selected as giving the most satisfactory characteristics. 

 This valve was the Naval valve designated by the number R2» 



For reasons which will appear later (see Section VIII. 

 below) nitrogen did not prove a satisfactory gas for valve 

 purposes, and the next gas tried was argon, which was 

 chosen as being the most easily obtained inert gas. Experi- 

 ments on oscillation ranges in argon were carried out for 

 pressures between 0'25 and '042 mm. The results given by 

 argon were very similar to those obtained when nitrogen 

 was employed, but it was soon found that there was a fatal 

 objection to the use of argon in valves. When the filament 

 of an argon-filled valve w T as heated up there was no indication 

 of a change of pressure, or of filament resistance, after 

 3i hours, but as soon as a potential of about 20 volts was 

 applied to the anode the filament resistance began to rise 

 rapidly, and after 30 hours had increased by nearly 50 per 

 cent., although there was still no alteration in pressure. It 

 appears, therefore, that tungsten filaments disintegrate rapidly 

 in argon und^r the conditions which occur in a valve during 

 operation, and it seems likely that this disintegration is due 

 to the bombardment of the filament by the very heavy 

 positive ions of argon. 



Argon having proved impracticable helium was next tried. 

 This gas should share with argon the advantage of not being- 

 absorbed by the hot filament, without possessing the disadvan- 

 tage of causing disintegration of the filament. Experiments 

 on oscillation ranges in helium were carried out for pressures 

 between 4 mm. and *042 mm. The results are shown for 

 four different pressures in fig. 7 (PI. VIII.) Full charac- 

 teristics for helium were also drawn, and these are 

 reproduced in fig. 8 for two different pressures. These 

 characteristics, instead of being drawn in the usual manner, 

 represent the results in a very convenient way suggested by 

 B. S. Gossling. In this method anode potentials arc plotted 

 as ordinates and grid potentials as abscissa?, and each curve 

 represents a lino of constant anode (or grid) current. The 

 curves are therefore of the nature of contours. A general 

 discussion of these contour characteristics has been given 

 bv Gossling *. 



* Journal I. E. F. he. cff. 



