Design of Soft Thermionic Valves. 479 



YI. Methods of estimating Gas Pressure in Valines. — In 

 the manufacture of soft valves it is very important to have a 

 simple method of estimating the pressure of the gas before 

 sealing off the valves from the pump. A very convenient 

 method, which is accurate enough for the purpose, is to 

 measure the width of the Crookes's dark space in a small 

 vacuum-tube attached to the apparatus for the purpose. 

 The vacuum-tube employed was provided with an aluminium 

 disk as cathode, and a small aluminium rod in a side tube as 

 anode. By experiment the relation between the pressure 

 and the width of the dark space was determined for nitrogen, 

 .argon, helium, hydrogen, and carbon monoxide. The width 

 of the cathode dark space depends on the current density 

 through the tube *, but a pressure indication of sufficient 

 accuracy can be obtained if the diameter of the cathode is 

 specified, and an induction-coil of definite size is used. 



This method of estimating gas pressures is much simpler 

 for manufacturing purposes than the McLeod gauge method, 

 and has the advantage of not introducing mercury vapour 

 into the apparatus. It is necessary that mercury vapour 

 should be excluded since it ionizes at about 10*5 volts and a 

 small trace of mercury vapour has a large influence on the 

 characteristics. If evacuation is performed by means of a 

 rotary oil pump and the dark space method of estimating 

 pressures is employed, it is not necessary to introduce the 

 complication of a liquid air-trap to keep back mercury 

 vapour. 



The dark space method is, of course, no longer available 

 after a valve has been sealed off from the pump, and it is 

 very useful to have another pressure test which is applicable 

 to sealed valves. For this purpose use was made of the 

 phenomenon known to the staff of H.M. Signal School as 

 *■* backlash." When the difference of potential between the 

 anode and the negative end of the filament becomes suffi- 

 ciently great positive ions are produced by collision, and, if 

 under these conditions the potential of the grid is made a 

 little negative to the negative end of the filament, the grid 

 will pick up positive ions and the grid current will be 

 reversed. The magnitude of this backlash current depends 

 on the anode and grid potentials and on the pressure and 

 nature of the gas, and it forms a very simple and convenient 

 method of estimating pressures in sealed valves. The pro- 

 cedure usually adopted was to make the grid have a definite 

 negative potential (usually — 2 volts) and then increase the 



* Aston, Proc. Roy. Soc. 1907, 1011. 



