﻿to maintain a Current in a Gas .at Low Pressui 



*es. 



215 



The potential of the battery was in all cases considerably in 

 excess of the potential at the electrodes, and a large resistance 

 was used to adjust the current, for reasons which are explained 

 below. (See Part II. A.) 



Results. 



The curves for the pressures above the critical pressure 

 (fig. 3) show clearly that the potential required to maintain a 

 current is less than the sparking potential which is indicated 

 on each curve (Sp. V.). It was found that the curves are the 

 same for zinc, aluminium, or silver electrodes, those given 

 being the average of a number obtained with zinc. The 

 currents in the figure are expressed as amperes per square 

 centimetre of the electrode, the distance between the plates 

 being 1 cm. 



Fie. 3. 



VOITS 



A 









/btrs 



B 







350 



Spl 



'=4/3 



'n 







350 



Sp. 



V* 4/9 



■4-.6/m> 



?t. 





3d5 













32*, 



V 































300 









i 





5 1 

 4*|0" 5 = 



1 

 Amp pe? 



5 



' Sf. C77U. 





C 



j 



r|-4x I0" s 



i 



= Amp.; 







oer spent 



5 









2 7 5 



TS 



Spl/ 



c 



--3/9 



• 27 ?7Z 771, 



















S£Q- 

















5 10 (5 



(x f.4- */o~ s = /??7?p. per s<7. cm. 



At Pressures above the Critical Pressure. 



The galvanometer used was not sensitive enough to detect 

 smaller currents than those indicated : but the tendency of 

 the curve is shown in each case, and the sparking potential is 

 given. This drop soon reaches a limit, and for current 

 densities larger than , „ \ „ f , of an ampere per sq. cm., the 



10*000 



potential may be taken as practically constant as the current 



