Disappearance of Gas in the Electric Discharge. 595 



value is obtained, no Eurther change occurs even if the 

 thermionic emission is increased 100-fold. There is no 

 evidence that the glow potential depends at all on the ther- 

 mionic emission, so long as it is great enough to give at all 

 a glow potential distinct from the spark potential. 



On the other hand, the glow potential depends greatly on 

 the pressure, as has been indicated already, and on the nature, 

 of the gas. Fig;. 3 shows the variation of Va and V<?' with 



Vg & Vg 

 volt 



1 



; i 











1 

 1 



1 



1 



1 

 1 

 1 

 1 



1 



1 





Vg & Vg' fc - Argon 



1 



1 



\ 



\ 



if 







\ 



| 



D N 



V 





\ 



\ 







v. 







A 





v. 



■ 



* 









< 







the pressure in argon, containing about 5 per cent, of 

 nitrogen. (This gas is chosen because the difference 

 between the rising and falling glow potentials is here 

 measurable even at the highest pressures investigated. In 

 the other gases it becomes inappreciable at pressure above 

 0-02 mm.) It will be seen that V^, Yg\ and the 

 difference between them all increase rapidly with decrease 

 of pressure at the lowest pressures; at the higher they vary 

 but little with the pressure. It is better, therefore, to plot 



