444 
DR. W. MANSERGH VARLEY OA" THE PHOTO-ELECTRIC 
and, as tested with a standard cell, gave a deflection of about 70 divisions for a 
volt on a scale 3 metres away. 
In taking an observation, the electrometer key was first raised so as to insulate 
the one pair of electrometer cpiadrants and the zero position of the filament image 
on tlie scale noted. The key in the primary circuit of the induction coil was now 
closed and the spai’k allowed to run for usually 10 seconds, when the primary 
circuit was again broken, and the deflection of the electrometer needle noted. The 
spark was run and the observations taken at regular intervals as far as possible. 
Variation of the Photo-Electric Current with the Pressure of the Gras. 
The following series of experiments on the variation of the photo-electric current 
from an illuminated zinc surface with the jjressure of tlie surrounding gas, were 
carried out in hydrogen, so as to avoid errors arising from tlie tiring of the zinc 
surface which occurs in air at ordinary pressures when it is illuminated by ultra-violet 
light. In hydrogen, the zinc sliowed no signs of this fatigue eftect under the 
influence of the light, and after taking a complete series of observations it was 
always possible to repeat any one of them and obtain the same values for the 
currents as before; the same holds true in air also at pressures below about 
one-tenth of an atmosphere. 
The results obtained for a whole series of pressures, ranging from 760 millims. 
down to O'OOS millim., are given in Table I. The zinc electrode was situated 
3'5 mdlims. behind the gauze electrode, and a diflerence of potential of 1 volt between 
the electrometer cpiadrant-pairs corresponded to a deflection of 60’6 divisions on the 
scale. The currents are measured in electrometer scale divisions ])er 10 seconds. 
The total capacity of tlie leak apparatus, electrometer, and connecting wires was 
about 00 centims., so that a deflection of 60 divisions per 10 seconds corresponds to a 
current of 10~^° ampere (very nearly). 
The intensity of the idtra-violet light, as measured by the second leak apparatus, 
showed no a])preciahle change throughout the course of the observations, the 
deflections obtained for 10 seconds’ illumination being, at the beginning, 212, 208 
divisions; at intervals during the experiments, 204, 209, 212, 215 divisions; and at 
the end, 214 and 209 divisions. As a further precaution, the first set of observations 
taken (tliat at 760 millims.) was I'epeated at the close, and almost identical values for 
the currents obtained, as the nundiers given show. 
