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LXVIII. The Current- Density at the Cathode in the Electric 

 Discharge in Air. By Hakold A. Wilson, B.A., D.Sc. 

 (Lond.), Clerk- Maxwell Student, Felloiv of Trinity College ; 

 Cambridge *. 



THE experiments described in this paper were undertaken 

 with the object of measuring the current per square 

 centimetre on the cathode, in the electric discharge in air at 

 low pressures. It is well known that the negative glow at 

 pressures of about one millimetre is confined to a definite 

 area on the cathode, and that this area covered by the glow 

 increases with the current through the tube. Wehnelt (Ann. 

 der Physik, No. 2, 1902) has shown that the discharge from 

 the cathode is confined to the area covered by the glow and 

 that the current-density is uniform throughout this area. So 

 long as the cathode is not entirely covered by the glow the 

 fall of potential between the glow and the cathode remains 

 independent of the current, but when the cathode is entirely 

 covered the " cathode drop " increases with the current. 

 This fact leads one to expect the area covered by the glov r to 

 be proportional to the current through the tube, and the 

 experiments now to be described show that this is the case. 



The method employed was very simple ; the cathode used 

 consisted of a straight wire fixed along the axis of the dis- 

 charge-tube and the length of the wire covered by the glow 

 was measured and also the current through the tube. 



The discharge-tube used is shown in the figure. It con- 

 sisted of a glass tube, AB, about 3 cms. in diameter and 



Fiff. 1. 



TO PC/MR ETC 



5v 



4- 



« 



A 



30 cms. long, having an aluminium disk electrode A sealed 

 in at one end, and a narrow tube BG joined on at the other. 

 This narrow tube served to support a glass tube E which 

 carried the cathode WW. The glass tube E was fastened in 

 with sealing-wax at C. so that it could be easily taken out 

 when it was desired to try a new cathode. 



The discharge-tube was connected to a Toepler pump and 

 McLeod gauge and to bulbs containing phosphorus pentoxide. 



The discharge was passed from the disk A to the cathode 

 wire, and it was found that the glow appeared on the end of 



* Communicated by Prof. J. J. Thomson. 





