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X. On the Photo-Electric Dischareje from Metallic Surfaces in Diferent 
Gases. 
By W. Manseegh Varley, M.Sc. {Viet.), Ph.D. {Strashurej), B.A. {Cantab.); late 
1851 Exhibition Research Scholar, Emmanuel College, Cambridge; Assistant 
Professor of Physics and Electrical Engineering, Heriot-Watt College, 
Edinburgh. 
Communicated by Professor J. J. Thomson, F.R.S. 
Eeceived April 24,—Read Itlay 14, 1903. 
Ihe experiments descril^ed in tins paper were designed with a view to studying as 
systematically as possible the dej^endence of the magnitude of the photo-electric 
current from a metal surface illuminated by ultra-violet light—or Hallwachs effect, 
as it is called, alter the name of its discoverer, by continental physicists—on the 
pressure and nature of tlie gas in which the illuminated surface is enclosed. 
T ery few exjDeiiments appear to have been made on the comparison of photo-electric 
currents in different gases, and the experiments hitheito caiiied out on the photo¬ 
electric effect in gases at pressures other than atmosphei'ic have been made principally 
to test special points in theory; such as those made by Townsend (‘ Phil. Mag.,’ 
6th series, vol. 3, p. 557, 1902) in support of the theory (T the genesis of ions by 
collision. 
The researches of Stoletow and of von Schwejdler are, however, exceptions, 
though their results do not enable comparisons of the photo-electric currents at 
different pressures to be made, excejDt under certain very special conditions. The 
deductions drawn by iStoletow from his observations—which are summarized in the 
‘ Comptes liendus,’ vol. 108, p. 1241, 1889— will be dealt with more fully later, but 
it will Ije advantageous to refer at the present stage to von Sgiiweidler’s experiments 
(‘Sitz.-Ber. d. k. Akad. Wien,’ Abth. Ila, vol. 107, p. 881, 1898, and vol. 108, 
p. 273, 1899). 
E. VON bcHWEiDLER examined more especially the effect of varying the potential 
difference between tlie electrodes on the photo-electric current, giving curves 
connecting these quantities at a fev\^ different pressures between 1 and 760 millims. 
mercury. Ihe curves at the diffei'ent pressures were independently obtained, and no 
data are given which enable a compaiison of the curves with one another to be made. 
They show, however, that at each pressure examined the current increases rapidly 
with the 2 >otential difference up to a certain value, after which the increase is mucli 
nioi'e gradual until another critical value is reached, when a further increase in the 
VOL. CCII.—A 355. 
11.12.0.3 
