314 
PROFESSOR W. Gr. ADAMS AND MR. R. E. DAT ON THE 
with binding-screws fixed on a plate of ebonite. The whole is contained in an oblong 
wooden box 30 centims. long, which is furnished with a draw-lid. Through the end 
of this box, in which there are two small holes lined with ebonite, the binding-screws 
are connected by means of insulated copper wires with the set of resistance-coils which 
are used for measuring the resistance of the selenium. 
A Wheatstone’s-bridge arrangement was formed, in which the four sides were the 
selenium, a resistance-box from 1 to 10,000 ohms, a fixed resistance of 4 ohms, and a 
coil of 2000 ohms — the battery electrodes being connected to the junction of the 
selenium and resistance-box and to the junction of the two fixed resistances, and the 
galvanometer electrodes being connected to the other two junctions. 
In the first series of experiments, the object of which was to determine the general 
character of the action, the method of procedure was as follows : — 
The box containing the selenium was placed on its side, and, close in front of it, was 
placed a black screen with a rectangular aperture, 6x3^ centims., opposite the selenium. 
On the further side of this aperture was a ledge for supporting, when necessary, coloured 
glasses and other absorbing media. Beyond this was placed the light whose effect 
was to be examined. The resistance of the selenium was then balanced as nearly as 
possible, without, however, permitting the current to remain on for any length of time, 
since its prolonged passage rapidly increases the resistance ; the lid of the box was then 
withdrawn, and the circuit was again completed, so that the exposure and the passage 
of the current went on simultaneously for, in general, about 15 seconds, after which 
the box was closed, and then the circuit interrupted. 
The resistance of this plate of selenium increases with an increase of the atmospheric 
temperature ; but in the dark, and at ordinary temperatures, its resistance is about 2^ 
megohms. 
When not exposed to the light the resistance of this bar of selenium begins to increase 
as soon as the electric circuit is completed, and continues to do so at a uniform rate, 
owing to the heating-effect of the current or to polarization. Although the heating- 
effect of the current must be small, yet the selenium is so sensitive to the slightest 
changes of temperature that this continuous increase of resistance may be due in 
part to the heating-effect of the current. On one occasion unintentionally the current 
from the battery was allowed to pass continuously for an hour and a half, and the 
resistance was found to increase continuously during the whole of that time. The rate 
of increase was the same as that observed when the current was allowed to pass only 
for a short time. 
On exposure to light while the current is passing the resistance is diminished, but when 
the light is again eclipsed the resistance of the selenium returns in a very few minutes 
nearly to its previous value. This diminution of resistance is generally manifested by 
a strong “ throw ” of the galvanometer-needle towards the side of lessening resistance, 
followed by a more steady deflection in the same direction, which gradually attains 
