Electrical Discharge in Helium and Neon. 103 
Fatigue of the Electrodes. 
Some experiments were carried out to determine the effect on 
the electrodes of prolonged passage of the current. Tube no. 3 
with a large sodium and aluminium electrode was originally 
made for this purpose. No convenient source of direct current 
being available it was connected to the 200 volt alternating 
mains with one carbon lamp in series. The current passing was 
jth amp. and was sufficient to cover only about half the cathode 
with glow. This glow, however, gradually spread, and at the end 
of 44 hours covered the whole electrode. The tube was about as 
hot as an ordinary carbon filament lamp. After about 10 hours 
further running, most of the discharge took place from the 
sodium, although there was still a certain amount from the 
aluminium. The experiment was continued, but no further 
changes occurred. The initial voltage across the tube was 143, 
corresponding to a maximum of 200, and this rose to 154, the 
glow also diminishing greatly in intensity. 
From this it may readily be seen that after some time, 
current passed less easily from a given portion of the electrode, 
and moved towards an unused portion. Finally when the whole 
of the aluminium was “exhausted” the discharge started from 
the sodium which was slightly oxidised and evidently less 
favourable to its passage than fresh aluminium. 
A peculiar effect was observed with this tube. When the 
current was cut off with a one-pole switch which disconnected the 
large sheet electrode, the gas round the wire electrode continued 
to glow. Presumably the leakage across the switch or its 
capacity allowed sufficient current to pass to produce luminosity. 
This is another striking demonstration of the minute currents 
which suffice to show their presence in neon, and a tube of this 
nature might well be used in testing for leaks. 
Fatigue was especially noticeable in the tube no. 5 with 
carbon electrodes. For instance, after running about 30 muilli- 
amperes through one electrode at 2:1 mm. pressure, the spark 
potential rose from 400 to 530, and the running potential from 
255 to 300. The tube then completely rectified a 500 volt 
alternating current. In this case a large amount of gas was 
evolved, and this appears to facilitate the passage of the current. 
If this is the case, a new aluminium electrode which evolves 
much hydrogen might be expected to give rise to a lower 
cathode fall than an old one as was observed to be the case in 
one experiment. 
Experiments were made later on a small tube with two sheet 
aluminium electrodes in the same plane, and filled with neon at 
5 mm. pressure. This was connected to a 240 volt main through 
