ACTION OF LIGHT ON SELENIUM. 
333 
to zero and then moved up to the 150th division to the right of zero. On screening 
off the light the needle came back at once to zero, and on removing the screen it at 
once returned to the 150th division. The light was then held on either side of the 
plate, but this made no difference in the character of the action. 
On screening off the light the deflection was at once reduced to its original value,, 
and similarly the effect on exposing the selenium to light was almost instantaneous. 
Here, then, seemed to be a case in which light actually produced an electromotive 
force within the selenium which was, in this case, opposed to and could overbalance 
the electromotive force due to the polarization. 
Currents produced in Selenium by the action of Light. 
The question of course at once presented itself as to whether it would be possible to 
start a current in the selenium merely by the action of light. 
Accordingly, the next morning the same tube, No. 15, was placed on the commutator, 
and its electrodes were connected through the galvanometer. While unexposed there 
was no action whatever. On exposing the selenium to the light of a candle at a distance 
of about an inch from it, there was at once a deflection of 150 scale-divisions. On 
screening off the light the deflection came back at once to zero. 
Hence it was clear that a current could be started in the selenium by the action of 
light alone. 
We next made use of the original W.S. plate. The box containing it was closed, 
and the electrodes were placed in the cups A and D of the commutator. On completing 
the galvanometer-circuit there was no motion whatever of the needle. A lighted candle 
was then placed opposite the centre of the box, and on opening the lid the needle at 
once swung up and settled, so that the deflection was equal to 17 divisions of the scale. 
On closing the box the deflection was at once reduced to zero. 
A small battery-current indicated by a reduced deflection of 35xl0 2 was then sent 
through this plate for about half a minute, and produced a distinct polarization-current 
represented by a deflection of 2 divisions to the left of zero. The lid of the box was 
then opened, and the selenium exposed to the light of the candle. The deflection began 
immediately to diminish, went down to zero, and then moved up to and remained at 
the 10th division on the other side. On closing the box the deflection was at once 
reduced to zero. 
The effect, on this plate, of exposure to the light of the candle at different distances 
en tried, with the following results : — 
I. 
II. 
Distance of 
Steady 
Distance of 
Steady 
candle in inches. 
deflection. 
candle in inches. 
deflection. 
3 . . . . 
. 10 
6 . . . . 
. 3‘5 
4 . . . . 
6 
5 ... . 
. 4-5 
5 ... . 
4 
4 ... . 
. 6 
6 ... . 
2 
3 ... . 
. 8 
