ACTION OF LIGHT ON SELENIUM. 
;29 
mated at the ends, so that the change of connexions could be rapidly and easily 
effected. 
On joining A F and DE,a simple circuit is formed through the battery and selenium ; 
the wires AF and DE are then removed, AB and CD are joined, forming a simple 
circuit through the selenium and galvanometer. 
The first experiment for polarization was performed with tube No. 14. The current 
from 20 Leclanche cells, in series, was sent through it for about a minute. The battery 
was then thrown out of circuit, and the selenium circuit completed through the galvano- 
meter, when a deflection of 27 scale-divisions was obtained, indicating a current from 
the selenium in the opposite direction to that of the original one. On short-circuiting 
the selenium by means of a bit of thick copper wire across B C, the deflection of the 
needle at once came back to zero. 
The battery-current was then sent through the selenium for about two minutes in the 
opposite direction to its previous one. On throwing the battery out of circuit and com- 
pleting the galvanometer-circuit, a deflection of 15 scale-divisions on the other side of 
zero was obtained. 
The next piece used was the W.S. plate. The current from 20 Leclanche cells was 
sent through it for two minutes, and this gave a polarization-current indicated by a 
galvanometric deflection of 47 scale-divisions to the right of zero. The selenium was 
then short-circuited, and in a few minutes all evidence of the polarization-current had 
disappeared. The battery-current was then put on for four minutes in the opposite 
direction, and this gave rise to a polarization-current represented by a deflection of 
160 scale-divisions to the left of zero. 
Another selenium plate, viz. that in tube No. 2, was then tried, and the same battery- 
current as before was sent through it for a short time. This gave rise to a distinct 
polarization-current, indicated by a deflection of 15 divisions to the left of zero. The 
same current was then sent for a considerable time through this plate in the opposite 
direction, and produced a polarization-current indicated by a deflection of 45 divisions 
to the right of zero. 
That this is not due to a current arising from thermoelectric action in consequence 
of the junctions where the current enters and leaves the selenium being unequally 
heated is evident, since a current may be obtained from the selenium a considerable 
time after the battery-current has been interrupted, so long as the selenium electrodes 
have not been connected in the interval. 
Here, then, we had distinct evidence of the existence of polarization when an elec- 
trical current was sent through the selenium ; and it became an interesting matter to 
know what was the actual intensity of the currents which we were thus observing. This 
would be at once known if we knew the value of the current in absolute measure which, 
with a known electromotive force and through a known resistance, produced the unit 
deflection on the galvanometer-scale. 
For this purpose a Daniell’s cell was taken, and the porous cell was filled with a 
MDCCCLXXVII. 
6 A 
