Electrical Discharge in Heliwm and Neon. 93 
t 
“no gas of any kind ever admitted except after freezing in liquid 
air. 
__ Before starting the experiments some helium was admitted to 
the tube and a current run for some weeks, the gas being 
frequently renewed until no change in its spectrum was observed. 
As observed by Strutt (oc. cit.), the minimum spark potential 
is a very delicate criterion of the purity of the gas. It was also 
found in the present experiments that the minimum voltage 
necessary to maintain the current when once started (a quantity 
which will be discussed later) was likewise very sensitive to the 
presence of impurities, and as this was constant over a wide range 
of pressure it afforded a more convenient method of detecting 
their presence. 
Some of the results obtained are given below, p is the pressure 
in mm. of mercury (measured by a McLeod gauge below 9 mm.), 
V the spark potential between the electrodes 1°5 cm. apart, and 
V’ between those 3 cm. apart. 
In the case of helium the figures are for two very pure samples 
of gas. After the first series of readings at comparatively high 
pressures had been taken, the tube was pumped out and more 
helium admitted directly from the charcoal, values for this gas 
being marked with an asterisk. It will be observed that the first 
of these for the nearer pair of electrodes is 180. As a matter of 
fact, immediately the gas had been introduced into the tube, one 
reading of 170 was obtained, and 180 between the other pair of 
electrodes, 180 and 214 being recorded a minute or two later. It 
is difficult to account for this, as the value 184 was found in a 
very similar experiment, when the gas was probably just as pure. 
There may possibly have been some ionising agent in the vicinity 
at the time. Such a low value was never obtained again, and it 
seems probable that the correct figure is not far removed from 184. 
The case of neon is of special interest as at comparatively high 
pressures it allows an electrical discharge to pass far more readily 
than any other gas at the same pressure. This was observed in 
1909 by Dr Collie and the author, and some rough measurements 
were made but not published. Soon afterwards M. Bouty (loc. 
cit.) determined the cohésion drélectrique which is a measure of 
the readiness with which a spark will pass, and found it to be at 
least 6:1, and probably 5:6, the figures for helium and air being 
18:3 and 419, respectively. Hence it seemed not unlikely that 
the minimum spark potential would also be less than that of 
helium, and it was rather surprising to find that this was not the 
case, as may be seen from the figures in the table. Unfortunately 
the experiments are not quite complete, especially at high pressures, 
but the minimum spark potential has, been examined carefully, 
and certainly seems to be higher than that of helium. 
