; Electrical Discharge in Helium and Neon. 91 
terminals of a 2000 ohm resistance C’, contact with which could 
be made at 100 different points by a sliding arm D. The current 
was regulated by an adjustable water resistance A. 
Fig. 1. 
The discharge tube F’ was made from a piece of wide glass 
tubing 5°5 cm. in diameter, ground flat at the ends, and closed 
with sealing-wax. Needless to say it was perfectly air-tight. It 
contained three plane aluminium electrodes tightly fitting the 
tube, and held apart at the edges by frames of very thin glass rod 
constructed so that the plates were exactly 15 and 3 cm. away 
from each other. They were held in position by a spring at one 
end. An aperture for the admission of gas and the leads to the 
centre electrode, which were soldered to its edge, was drilled 
opposite the latter, and covered with a dome of glass ground to fit 
the tube. 
The electrodes were very carefully cleaned with glass paper, 
and the middle one used as cathode. The two others could be 
made the anode in turn by means of the tipping key H, the one 
not in use being simultaneously connected to the cathode, which 
was itself at a potential only a few volts removed from that of the 
earth, E. 
An accurate Weston voltmeter G was connected between the 
electrodes, and since its resistance was comparable with that of 
the liquid resistance A, the potential difference across the tube 
could be varied by altering A. The movement of D afforded a 
fine adjustment. A: telephone was included in the circuit to 
enable any intermittency in the discharge to be detected. 
