[ 403 ] 
X. On the Laws Governirui Electric Discharr/es in Gases at Low Pressnres. 
By W. R. Care, B.A., Post-graduate Student, University of Toronto. 
Communicated by Professor J. J. Thomson, F.R.S. 
deceived, February 11,—Read, March 5, 1903. 
1. Introduction. 
The researches of recent years have conclusively settled the general connection 
between the spark potential and the jDressure of a gas. It is now well known that as 
the jjressure of a gas diminishes the difference of potential necessary to produce a 
discharge between electrodes in the gas, a fixed distance apart, also diminishes, until, 
at a critical jM'essure, the spark potential reaches a minimum value. It is further 
established that below the critical pressure the potential difference required to 
produce discharge raj^idly increases as the pressure is lowered. 
This connection between the spark potential and the corresponding pressure of 
a gas has been well illustrated in a series of curves drawn by Peace,^' who 
investigated the sj^arking j^otentials betv'een a pair of parallel plates at pressures 
ranging from one-half an atmosj^here down to a little below the critical pressure. 
Among others, Strutt! and Bouty| have carried on the investigation at pressures 
considerably below the critical point, and their results shoAv that, once the critical 
pressure has been passed, the rise in potential difference necessary to produce discharge 
is exceedingly rapid. 
The effect of varying tlie distance between the electrodes was first determined by 
Paschen,§ Avho observed the existence of a simple law connecting the pressure at 
which discharge took place with the corresponding spark potential and the distance 
Ijetween the electrodes. 
Paschen’s results showed that when a given potential difference was applied to 
two spherical electrodes, whose distance apart could be varied, the maximum 
pressure at which discharge occurred varied inversely with the distance between the 
spheres. 
The range of pressures over which he found the law to apply, while considerable, 
did not extend below 2 centims. of mercury, and his results do not in any case indicate 
that the critical pressuie had been readied. It is evident, then, that Pasohen’s 
conclusions are confined to pressures higher than the critical pressures. 
* Peace, ‘Roy. Soc. Proc.,’ vol. 52, p. 99. 
t Strutt, ‘Phil. Trans.,’ A, vol. 193, p. 377. 
t Booty, ‘Compt. Rend.,’ vol. 131 (2), p. 113. 
§ Paschex, ‘Ann, d. Phys.,’ vol. 37, p. 69. 
VOL. CCl.—A 340. 3 F 2 8.7.03 
