e 
96 Mr Watson, Some Experiments on the i 
of gas required is more. This is always to be avoided, as the 
purification of large quantities of gas is a difficult and uncertain. 
operation. The whole of the present experiments were carried out 
with less than 15 c.c. of each gas, 
There is, of course, the objection that the field obtained in this” 
way is not uniform, and indeed, there is about 20 volts difference 
between the minima obtained with the two pairs of electrodes, 
At the same time, as will be seen from the figure, the form of 
curve is not altered, and indeed, if the horizontal scale of the 
upper curve be doubled (the distance of the electrodes for which — 
this was obtained was double that of the other pair) and the 
whole moved downwards, the two nearly coincide. It was intended — 
to make more experiments with the electrodes quite close, but up 
to the present this has not been possible. It seems unlikely, 
however, that the true spark potential for a uniform field between 
infinite planes is more than two or three volts lower than the 
minima already obtained. 
On examining the curves, it will at once be seen that they are 
of a type not previously observed. Below the critical pressure of 
minimum spark potential they are approximately hyperbolic, and 
then for a short distance linear, as is usually the case, but at a 
slightly higher pressure there is a break, and the curves turn over, 
and again become very nearly linear. The same thing occurs in 
the case of neon. With the closer pair of electrodes, this break _ 
begins at a pressure of 10 mm. This would correspond with a 
pressure of 200 mm. in Strutt’s experiments, and is beyond the 
range studied by him. The bend occurred at the same point for 
all samples of gas even when distinctly impure, and so is not 
likely to be caused by impurities. It may however be due to the 
irregularity of the field, but this in turn is improbable, owing to 
the similar shape of the two curves. 
At high pressures there was little difference in the spark 
potential when a small trace of impurity was present, but the 
most minute amount affected to a large degree the depth of the 
dip in the curve and consequently the minimum spark potential. 
The pressures of minimum spark potential were, as nearly as 
could be judged, 2°4 and 2° mm. for the closer pair of electrodes 
in helium and neon respectively. The latter figure is probably 
low, and consequently these pressures are by no means proportional 
to the mean free paths of the ‘molecules of the gases (cf Sir 
J. J. Thomson, Conduction of Electricity through Gases, 2nd 
edition, p. 450) which are in the ratio 1 : 0:704 (Rankine, Roy. 
Soc. Proc. A. 1910, 83, p. 524). 
Since the electrodes in the present case were 20 times the 
distance of those used by Strutt, it might be expected by Paschen’s 
law, that the slope of his curves above the critical pressure would 
