180 ROYAL SOCIETY OF CANADA 
VI. Spark POTENTIALS WITH DIFFERENT ELECTRODES. 
It has now been shown, using brass electrodes of constant size, 
that for discharges in a uniform field in any gas, the values of the 
spark potentials are determined solely by the product of the pressure 
of the gas and the distance between the electrodes. From this result 
it appeared that if the size or material of the electrodes did not affect 
the results, the spark potentials were dependent only upon the quan- 
tity of the gas per unit cross section between the electrodes. 
In order to determine this point the brass electrodes which had 
been used up to this time were replaced in turn by electrodes of iron, 
zinc and aluminium of exactly the same size. The results of the 
experiments showed that there was no variation in the different sets 
of readings and it was evident that there was not the slightest effect 
produced in any case by a change in the material of which the elec- 
trodes were made. 
In order to see if the size of the electrodes affected the values 
of the spark potentials for the different pressures, provided the dis- 
charge took place in a uniform field, a reduction was made in the 
surface of the electrodes exposed to the gas. This was done by 
replacing the ebonite rings C, C, Fig. I., which had an inner diam- 
eter of 3 cm., by others whose inner diameter was but 1 cm. By 
this device the areas of the electrodes exposed to the gas were reduced 
to about */,, of their value in the early experiments, and the condition 
that the discharge could only take place in a uniform field still held. 
Using this apparatus, with air, no difference could be observed in the 
values of the discharge potentials corresponding to the different pres- 
sures, and it was therefore certain that the value of the spark potential 
was in no way influenced by the size of the electrodes. 
It is therefore clearly established that the only factors that affect 
the spark potentials are pressure and the distance between the elec- 
trodes, and hence Paschen’s law is most accurately expressed by 
saying: “ That, with a given applied potential difference, discharge in 
a uniform field, in any gas, at pressures ‘both above and below the 
critical pressures, is dependent solely on the constancy of the quan- 
tity of matter per unit cross section between the electrodes.” 
Every assistance towards the carrying of my research to a success- 
ful issue has been given me throughout by President Loudon, and I 
gratefully accept this opportunity of thanking him. I also wish to re- 
cord my appreciation of the many kind suggestions of Professor J. C. 
McLennan, in whose laboratory the experiments were performed and 
to whom I owe much for their success. 
