100 Mr Watson, Some Eaperiments on the 
Abnormal forms of Discharge. 
Soon after starting the experiments on spark potential, a 
rather remarkable occurrence was observed. It has been stated 
that to determine the discharge point, the potential was gradually 
raised. On several occasions it was noticed that before the dis- 
charge took place, a glow appeared over the surface of the anode 
and increased in brilliancy as the potential was raised. Suddenly 
‘the discharge would assume its normal form with Aston dark | 
space, Crookes dark space and negative glow. This anode glow | 
was about 5 mm. thick, and although of quite a different order of 
brightness to the normal discharge, was distinctly visible in a well | 
lighted room. It was more conspicuous in neon than in helium, | 
as would be expected from the extreme readiness with which the | 
former gas glows under the least electrical excitation. In helium 
it could not be determined whether the glow appeared suddenly 
or gradually, but im neon it seemed to be produced at a definite 
potential. For pressures above the minimum spark potential 
pressure this occurred at voltages often below the minimum spark 
potential itself, and always below the normal spark potential. 
For instance, in one case with helium at 4 mm. pressure, the 
glow appeared at a potential difference of 185 volts, while the 
true spark potential was 205, while in another with neon at 
33 mm. the glow started at 165 volts and the spark potential was 
267. Below the critical pressure, the voltage at which the glow 
formed seemed identical with the spark potential at that- 
pressure. 
It must be emphasised that this glow discharge is not a part 
of the normal discharge occurring at a voltage insufficient to 
produce the latter, such as takes place from a Wehnelt cathode 
or in the discharge from points (Cond. of Hlectricity through 
Gases, 2nd edn. pp. 479, 513), but an entirely different and 
alternative form of discharge. It was quite impossible in any 
experiment to determine a priort which form would make its 
appearance, but at low pressures the glow seemed to be the more 
stable form. : 
If the glow appeared, the potential could be raised far above 
the normal spark potential without any change occurring, but if— 
the rise was sufficient, then the ordinary discharge would appear, 
although the potential at which this happened was by no means — 
definite. Thus, in a sample of neon, at 10 mm. pressure, the 
glow formed at 199 volts, and the normal discharge appeared at 
voltages varying from 272 to 330. The spark potential was 200. 
Owing to these circumstances the measurement of the spark — 
potential was a matter of some difficulty. It was found, however, — 
that if the normal discharge was once obtained, it tended to : 
