574 Potential Gradient in the Dark Space of Vacuum- lubes. 

 Dark Discharge. 



The gradient as given in the full-line curves in figs. 2, 3, 

 and 4 is single-valued for a cyclic variation of the current, so 

 long as its strength is kept below a certain value. If it rises 

 above that value, the luminous column, under constant current, 

 gradually retreats into a thick disk on the anode, giving the 

 so-called dark discharge. The dotted curves in figs 2 and 4 

 (for 6 m.a.) represent the gradient after this change had 

 been brought about ; the one in fig. 3 represents it during 

 the transition, in which the luminous column extends (as in 

 the full-line curves) from the anode to the intersection of the 

 gradient with the line 11. Alter the dark discharge has set in, 

 the gas requires a certain period to recover the power of 

 transmitting a current by the luminous discharge. In one 

 case the gas was observed to recover its natural luminosity 

 and normal gradient within half an hour after the current was 

 broken; but in general, the longer the period of dark dis- 

 charge, the longer the period required for recovery. Two to 

 three hours was always sufficient to recover both luminosity 

 and gradient. 



With the cathode screened as in fig. 5, the dark discharge 

 could not be produced by double the current (10 to 12 m.a.) 

 usually necessary when the cathode was discharging along 

 the axis of the tube. This, in connexion with the fact that 

 the dark discharge is ushered in, as it were, by a growth of 

 the Faraday dark space, indicates that the transition from 

 the luminous to the dark discharge is at any late largely 

 facilitated by cathode radiation. 



Luminosi'y* 



As already stated, the line 11 marks by its intersection 

 with the potential gradient the limit of the unstriated luminous 

 column. It may be observed in the curves given, that where 

 the gradient falls twenty-five per cent, (roughly estimated) 

 below its constant value in the luminous column, that region 

 is dark. It was also noticed in this connexion, that as the 

 gas recovered from the dark discharge, the luminosity in- 

 creased, as the gradient having risen above 11, approached 

 its normal value. 



Physical Laboratory, 

 University of Nebraska, Lincoln, 

 Aug.' 8, 1900. 



