ELECTRIC DISCHARGES IN GASES AT LOW PRESSURES. 
405 
A short discussion ol the uppcirtitiis will rcvctil oiio coiisidcrcibl© delect. The object 
of the investigations of both Paschen and Peace was to determine the electromotive 
intensity requisite to cause discharge in a gas. Idiroughout the range of pressures 
investigated by Paschen the discharge always took place along the shortest distance 
between the spherical electrodes, and the electromotive intensity requisite to break 
down the gas was therefore directly proportional to the spark potentials obtained by 
him. At points below the critical pressure, as Peace’s results indicate, discliarge 
occurs more easily ovei‘ a longer distance than over a shorter one, and if tlie values 
of the electromotive intensities necessary to break down a gas at difterent })ressnres 
lire to be compared, it Is neces.sary to know in each case not only the jiotential 
ditierence applied to the electrodes, but also the path betv'een the electrodes along 
which the initial discharge occurs. 
To insure passage of the discharge over the same length of path Ueace used plane 
parallel plates of very large diameter as electrodes, but while in this way lie obtained 
a uniform field ol considerable extent, and so was able to obtain an accurate measure 
of the electromotive intensity between the electrodes, he failed to make certain that 
the path along Avhich the gas initially broke down wars always confined to the uniform 
part of the field. As mentioned in his paper, there A\'as considerable tendency, at 
loAv pressures, to a brush discharge from tlie edges of the plates, and this indicated a 
detect in his apparatus, which apparently he did not completely eliminate. 
In the present paper an account is given of an investigation on the potentials 
necessary to produce discharge in a gas, with a form of apparatus which insured the 
passage of the discharge in a uniform electric field. 
With this apparatus the discharge potentials liave been determined, for different 
distances betAveen the electrodes, over a range extending considerably above and 
beloAv the critical pi-essure. The results of the investigation not only confirm the 
truth of the kiAv enunciated by Paschen for discharges at high pressures, but also 
demonstrate, beyond doubt, the applicability of the same laAv to the critical pressure 
and to all pressures beloAv it. 
'I'he existence of the same relation has been sought in each of the gases air, 
liydrogen, and carbon dioxide, and the result of the investigation has been the 
establishment Avith equal certainty of the same general kiAv for all pressures, viz., that 
Avith a given potential difference, the field being uniform, fhe product of the ])ressure 
at AAdiich discharge occurs and the distance lietween tlie electrodes is constant. 
II. Description of Apparatus. 
The form of the discharge chamber is shown in fig. 1. 
The electrodes consisted of two plane brass plates a, a, 3'6 centims. in diameter, 
embedded in ebonite, as shown in the figure, the outer faces of the electrodes being 
hush -with the surface of the ebonite. These pieces of ebonite which cai'ileel the 
electrodes serA'ed also to close the glass tube T, T, avIucIi thus constituted a discliarge 
