Disappearance of Gas in the Electric Discharge. (501 



Some Consequences of the Observations of the 

 Glow Potential. 



9. Before observations of a different nature are described it 

 will l)e well to point our in what manner the glow potential, 

 and in particular the difference between the rising- and 

 falling glow potentials, may affect the changes that are the 

 main subject of investigation. 



It will be seen later that there are circumstances in which 

 gas disappears in the discharge if V is greater than Yg and 

 the glow is developed, but not if there is no glow. Suppose, 

 then, that in these circumstances we are raising- V so slowlv 

 that the gas has time to disappear under the discharge. If 

 the initial pressure is represented by A (fig. 3) and V is 

 raised from zero along- the line NA, then, when A is 

 reached, the glow starts. The gas begins to disappear and 

 the pressure to fall ; but the glow will not cease immediately. 

 It will continue, while the pressure falls along the line AB, 

 until JB is reached and the potential is no longer able to 

 maintain the discharge. It" the potential is raised once 

 more along BC, the glow will not start again until G is 

 reached, and till C is reached there will be no disappearance 

 of gas. But when G is reached the gas disappears once- 

 more, and the pressure falls along GD until the glow ceases 

 at D. It will not start again till V rises to E, and so on. 



In suitable circumstances this process can be easily traced 

 experimentally. As the potential is raised slowly, the glow 

 in the lamp flickers in a way familiar to all who have 

 watched the " cleaning-up " of a lamp; and by measuring 

 the pressure every time the glow ceases, it can be established 

 that the points lie as they should on the full and dotted 

 curves of fig. 3. The nicker also appears when the 

 potential is kept constant, if gas is slowly leaking into the 

 lamp from the walls or along a narrow connecting tube. 

 The glow appears when the pressure rises to the point at 

 which Y is equal to Yg; it falls until V is equal to Yg', and 

 the glow does not start again till the pressure has risen 

 once more. 



A further consequence of the difference between Yg and 

 Yg' should be noted. The potential available for causing 

 the discharge which produces the disappearance of the gas 

 may be limited — for example, in an ordinary lamp it is 

 limited to that which can be applied to the ends of the 

 filament without burning it out. If the maximum potential 

 is ( J (fig. 3), then so long as the initial pressure in the lamp 

 is greater than that corresponding to C, the lamp can be 

 Phil. Mag. Ser. 6. Vol. 40. No. 239. Nov. 1920. 2 R 



