[ 563 J 



LIV. On the Potential Gradient in the so-called Faraday 

 Dark Space of Vacuum-Tubes as Related to Conditions 

 Prevalent at the Cathode. By Clarence A. Skinner, 

 Ph.PJ., Adjunct-Professor of Physics in the University of 

 Nebraska *. 



IN a former paper f results were given showing that, on 

 sending a current of electricity through nitrogen at 

 about one millimetre pressure, the space of strikingly low 

 potential gradient near the anode increases in extent as the 

 drop of potential at the anode augments. This was attri- 

 buted to a disturbance sent out from the anode. At the 

 same time it was suggested that the low gradient of the 

 Faraday dark space was likewise due to the "drop" at 

 the cathode. 



The present paper contains primarily the results of a series 

 of experiments, made for the purpose of studying the relation 

 existing between the potential gradient of this dark space 

 and the conditions at the cathode. The results, as will be 

 seen, trace the lower gradient (compared with that in the 

 luminous column) to cathode radiation. 



Measurements of the gradient in the Faraday dark space 

 were first made by Hittorf J, and recently more exhaustive 

 measurements have been made by Graham §. The latter 

 studied it' in nitrogen at a definite distance from the cathode, 

 by measuring the difference of potential between two parallel 

 wires inserted respectively in two successive cross-sections of 

 the discharge-tube. His results gave the variation (at the 

 position noted) of the gradient with the current, at various 

 gas-pressures — the whole, however, connected with a con- 

 siderable cathode "drop.'" Graham also obtained by means 

 of moveable wires the general form of the gradient throughout 

 the dark space — not, however, in its relation to the cathode, 

 with which this paper deals. 



Apparatus. 



The method used was the second adopted by Graham. 

 Fig. 1 shows the discharge-tube sealed to an open top baro- 

 meter-tube b, through the mercury column of which rises 

 one arm of a U-tube carrying the exploring-wires, while 

 the other arm passes to the outside through the reservoir 

 below (not shown). By this means the exploring-wires are 



* Communicated by the Author. 



t C. A. Skinner, Wied. Ann. lxviii. p. 752 (1899). 



% W. Hittorf, Wied. Ann. xx. p. 705 (1883). 



§ W. P. Graham, Wied. Ann. Ixiv. p. 49 (1898). 



