Electrical Discharge in Heliwm and Neon. 95 
figures at higher pressures for the slightly impure gas are not very 
far wrong. In the case of the second sample, the constancy of 
the spark potential over a wide range of pressure was surprising, 
although it was probably illusory, and caused by the gradual 
contamination of the gas. It is rather remarkable, however, that 
another sample of exactly the same gas admitted immediately 
afterwards directly from the charcoal at what was assumed to be 
approximately the critical pressure for minimum spark potential, 
gave minimum values of 206 and 228. 
Pressure mm. and ~,th mm. 
Fig. 2. The above curves show the relation between spark potential and pressure 
in helium for plane electrodes 15 mm. and 30 mm. apart, the upper curve being 
for the latter. The curves on the right correspond to the others, but the 
horizontal scale is ten times as great to show the shape at low pressures. 
Fig. 2 shows two typical curves for helium. It may be 
mentioned here that the electrodes in the present experiments 
were placed at a considerable distance, in order that the pheno- 
menon under consideration might be studied through a wide 
range without unduly increasing the pressure. Paschen’s law 
states that the spark potential depends only upon the mass of gas 
between unit area of the electrodes, and consequently, if the 
electrodes are 15 mm. apart, it will be the same as it would be nm 
a tube with electrodes 1 mm. apart and containing gas at 15 times 
‘the pressure. When using a tube of small dimensions the volume 
of the dead space becomes relatively large, and the total quantity 
