480 Mr. G. Stead on the 



anode potential until the backlash current rose to a specified 

 value (generally 1 microampere). The anode voltage required 

 for this purpose increases as the pressure is diminished, and the 

 relation between these two quantities may be determined by 

 experiment for any given gas. It is evident that the required, 

 anode voltage will be related in some way to the ionization 

 potential of the gas, but the relation between the two is by 

 no means a simple one. Experiments have been carried out 

 for nitrogen, helium, argon, hydrogen, and carbon monoxide, 

 and for grid potentials of — 1"5, — 2*0, and — 2'5 volts.. 

 The results for a grid potential of —2 volts are shown in 

 fig. 9 (PL VIII.). 



VII. Cooling of Filaments by surrounding Gase*. — When 

 a filament is surrounded by a gaseous atmosphere the power 

 required to maintain it at a given temperature is greater 

 than when the filament is in vacuo, owing to loss of heat by 

 conduction and convection through the gas, as well as by 

 radiation. It was necessary to know whether this effect 

 would be appreciable at pressures likely to be used in valves.. 

 The method adopted was to measure the current required to 

 maintain the filament at a given resistance, first in vacuo and 

 then when surrounded by gases at various pressures. From 

 these measurements it was possible to plot curves showing 

 the number of watts taken by the filament at different gas 

 pressures. Measurements of this kind were made for nitro- 

 gen and helium, and the results are shown in fig. 10 

 (PI. VIII.). It was found that in nitrogen at a pressure of 

 L in in of mercury the watts required were only 10 per cent, 

 more than in a good vacuum, whilst at the pressure employed 

 in the B.2 valve (06 mm.) the additional watts amounted to 

 1 percent. The effect is thus of little importance in this latter 

 case. The cooling due to helium is appreciably larger, 

 amounting to about 13 per cent, at a pressure of 1 mm., and 

 20 p^r cent, at 4 mm. In hydrogen it is still larger, but no- 

 systematic observations were made. It will be observed that, 

 both for helium and nitrogen, as the pressure increases the 

 curves become gradually less steep up to a certain point, and 

 then the steepness begins to increase again rather suddenly.. 

 This effect was always observed and appears to be genuine 

 and not due to accidental causes. It seems likely that the 

 upward turn of the curve occurs when convection in the gas 

 begins to be appreciable. 



VIII. Objections to Nitrogen atmospheres. — As already 

 mentioned, nitrogen had to be abandoned as an atmosphere- 



