On the Design of Soft Thermionic Valves. 



471 



plane parallel to that of the anode. Between them was a 

 grid B, made of copper wire and mounted on a thin glass 

 tube, whose lower end was sealed to a float 0. The grid 

 was connected to the source of grid potential by means of 

 the flexible lead L sealed into the glass at D. The float 

 rested on mercury contained in the U-tube E, and the 

 height of the float, and therefore the position of the grid, 

 could be varied by altering the pressure of the air in the 

 other limb of the U-tube. The distance between the planes 

 of the anode and filament was approximately 2 cm., and the 

 grid could move freely in this space, except that it was 

 prevented by a stop from coming* nearer to the filament 

 than about 1 mm. A valve of this type necessarily con- 

 tained mercury vapour at the saturation pressure corre- 

 sponding to room temperature (about '001 mm.) and was thus 



Fig. 1. 



To pami 



fairly soft. The valve was exhausted as thoroughly as 

 possible by a rotary Gaede pump followed by charcoal cooled 

 by liquid air, and was baked during the process of exhausting 

 so far as the presence of the mercury permitted, i. e. to 

 about 350° C. 



Characteristics were plotted for various positions of the 

 grid, the filament current and the temperature of the 

 mercury being kept the same throughout. Some specimen 

 characteristics are shown in fig. 2 (PI. VIII.), for three 

 different distances between grid and filament. The general 

 effect of increasing this distance is to diminish the slope of the 

 anode current-grid voltage characteristic, and to diminish the 



2 12 



