338 
PROFESSOR W. Gf. ADAMS AND MR. R. E. DAY ON THE 
with this piece. When the light was thrown quite on the end the resulting current 
was much weaker, being only 10, but in the same direction as before, through the 
selenium. In all these experiments we found that on shutting off the light the deflection 
of the galvanometer-needle came back at once to zero. 
Hence the exposure of any part of this piece to light causes a current in it from the 
marked to the unmarked end. 
To these currents, which are due to the action of light, and which seem to differ in 
character from thermoelectric currents, it will be convenient to give the name of photo- 
electric currents. 
Is it 'possible to balance a weak battery-current by the action of Light 1 
Knowing now that light can produce an electric current in a piece of selenium when 
no other current is passing or has ever passed through it, and that the apparent resistance 
of a plate of selenium is diminished when the selenium is exposed to the action of light, 
it is an interesting matter to examine what will be the effect on the apparent strength 
M of a current which was passing through a piece of selenium in the dark when a beam 
of light was allowed to fall upon it. 
Mr. W. Smith’s plate, enclosed in its box, was therefore put in circuit with the galva- 
nometer, and the light of burning magnesium ribbon was found, on opening the box, 
to produce a deflection of 90, without any shunt. The lid of the box was then closed, 
and the current from one Leclanche’s cell was sent through the plate, and while the 
current was passing, the lid of the box was opened and the selenium was exposed to 
the light of a gas-burner at a distance of 6^ inches. The 10th shunt being in the galva- 
nometer, the following were the observed results : — 
Deflection due to current of 1 Leclanchc’s cell 156 to right. 
ditto ditto and gas-flame 208 „ 
The poles of the selenium plate were then reversed in the mercury-cups without 
altering the actual position of the plate, and we then found that 
Deflection due to current of 1 Leclanche’s cell entering at the opposite end .... 188 to right, 
ditto ditto and gas-light 240 „ 
The battery-current was then turned off, and the light of the gas-flame at the same 
distance as before produced a deflection of 12 divisions to the left (without any shunt). 
These experiments were important, inasmuch as they lead us to notice the following 
points : — 
(1) That the same light at the same distance produced in both cases the same incre- 
ment of current, viz. 52 with the 10th shunt to the galvanometer. 
(2) That in both cases the action of the light increased the strength of the current 
flowing through the selenium. 
These experiments with the gas-flame and the current from one Leclanche’s cell 
were repeated several times, and gave substantially the same results as before. 
