SecrTioN III., 1902 Piety TRANS. R.. EXC 
XX.—On the Potential Difference required to produce Electric 
Discharge in Gases at low pressures—An Extension of 
Paschen’s Law. 
By W. R. Carr, B.A. 
(Communicated by President Loudon.) 
(Read May 27, 1902.) 
J. INTRODUCTION. 
The researches of recent years have conclusively settled the 
general connection between the spark potential and the pres- 
sure of a gas. It is now well known that as the pressure 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 pressure the spark potential reaches a 
minimum value. It is further established that below the critical 
pressure the potential difference required to produce discharge rapidly 
increases as the pressure is lowered. 
This connection between the spark potential and the correspond- 
ing pressure of a gas has been well illustrated in a series of curves 
drawn by Peace! who investigated the sparking potentials between 
a pair of parallel plates at pressures ranging from one-half an atmo- 
sphere down to a little below the critical pressure. 
Among others, Strutt ? and Bouty * have carried on the investiga- 
tion at pressures considerably below the critical point and the results 
show that, once the critical pressure has been passed, the rise in 
potential difference necessary to produce discharge is exceedingly 
rapid. 
The effect of varying the distance between the electrodes was 
first determined by Paschen,* who observed the existence of a simple 
law connecting the pressure at which discharge took ‘place with the 
corresponding spark potential and the distance between 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. 
1 Peace, Proc. Roy. Soc., Vol. 52, p. 99. 
2 Strutt, Se hilsirans MOI 1935 sp. S77. 
* Bouty, Comp. Rend., Vol. 131 (2), p. 443. 
“ Paschen, Ann. d. Phys., Vol. 87, p. 69. 
