ELECTRIC DISCHARGES IN GASES AT L0^^" PRESSURES. 
407 
and a mercury pump of small capacity. By using this reservoir and the pump of 
small capacity it ^Yas possible to diminish the pressure in the discharge tube by such 
exceedingly small amounts that it was easy to obtain a series of discharge potentials 
over the whole range of pressures investigated without the necessity of admittincr 
fresh gas to the chamber. 
In making measurements, one terminal of the battery was joined to eartli and the 
other terminal was connected througli a. resistance of xylol to one of the electrodes of 
the discharge tube. The other electrode was permanently joined to one pair of 
quadrants of a quadrant electrometer, the second pair of whicli was kept to earth. 
In determining tlie potential difterence necessary to produce discharge at a given 
piessuie, tlie electrometer electri'ide w.as tirst earthed, a^ given pr)tential applied to 
the battery electrode, and the earth connection of the electrometer electrode tlien 
removed. 
If after waiting some minutes no discharge passed, the operation Avas repeated 
with a slightly higher potential applied to the liatteiy electrode. This procedure 
was folloAA^ed until a potential sufficiently high aavus reached to lireak doAvii the gas 
and cause a discharge. The passage of the discharge could be readily noted, as it was 
accompanied by a violent deflection of tlie electrometer needle. 
The AAffil-knoA\-n phenomenon of delay in tlie jiassing of the discharge, Avhich has 
been iiiA’-estigated at length by Warbttrg,- Avas observed througliout the experiments. 
It AAuis especially mai-ked in the neighbourhood of the critical pressure, discharge 
being frequently obtained ten or ei’en fifteen minutes after the requisite Amltage had 
been applied. 
In every case, therefore, as the minimum sparking potential for any pressure was 
atjproached, a considerable time aa^s alloAved to elapse, Avitli a given ajiplied potential 
difference, before any increase Avas made. 
III. Expe)'im(n\ls in Air. 
In the experiments on atinosjilieric air the avIioIo discharge ajiparatus Avas first 
exhausted to a, very low pressure and then re-filled by fresli air, AA'hicli bubbled in 
very sloAvly, first through a Avash-bottle of sulphuric acid and then tlirough a tube 
tightly packed Avith phosjihoric jientoxide. The discharge chamber aaiis then 
exhausted to aliout 20 millims. of mercury and alloAved to staiul at tins jiressiire for a 
jieriod of from eight to tAA-elve hours. 
Dining this time the air Avas abvays in contact Avith phosjihoric jientoxide in the 
drying tube, and AV'as therefore entirely free from moisture Avhen the measurements 
were taken. 
The first measurements Avere made Avith the electrodes millims. apart, and the 
spark potentials were determined over a range of pressures extending from 
51 millims. doAvn to 'ofi millim. of mercury. The spark potentials corresjionding to 
Warburg, ‘ Ann. d. Pbys.,’ vol, 62, p, 385, 
■X- 
