ACTION OF LIGHT ON SELENIUM. 
349 
No. 14. — This ranks with Nos. 7 and 8. On heating the marked end there is a 
current of 50 from platinum to selenium at heated end ; and on heating unmarked end 
there is a current of 70 from selenium to platinum at the heated end. On heating the 
whole of the piece there was a current of 50 from the marked to the unmarked end. 
This piece has a resistance of 207,000 ohms for a direct current, and 195,000 ohms for 
a reverse current. 
No. 10. — The resistance of this piece is now only one ten-thousandth part of its value 
a year ago, and its place in the thermoelectric scale is completely altered. Whether 
the heat be applied at the marked or at the unmarked end, there is a current of 90 
from the selenium to platinum at the heated end. On heating the whole piece there 
is no effect. 
Experiments have been made with Nos. 21, 22, and 24 to determine whether their 
resistance is diminished on exposure to light while a battery-current is being sent 
through them. 
In all these pieces it was found that on exposing either end, or on exposing the whole 
to light the resistance was diminished ; but in the case of No. 24 the resistance was 
diminished from 60 to 50 ohms, and then increased to 54 before the light was shut off ; 
and on shutting off the light the resistance increased to 67 ohms. 
No. 8. — After the last experiments with this piece, which have been described above, 
its resistance was found to be 5290 ohms. 
The two balancing-resistances in the Wheatstone’s bridge were equal, and each 
100 ohms. On connecting the marked end to the positive pole of the battery, and 
exposing the marked end to the lime-light, the resistance was increased, the needle being 
deflected beyond the scale. On exposing the unmarked end the resistance was also 
increased, the needle being again deflected off the scale in the same direction. On 
exposing the whole piece to the lime-light, the resistance was also increased, and was 
found to be 6290 ohms on balancing with the bridge. On shutting off the light, the 
resistance at once began to diminish ; it had diminished to 5790 ohms in about 1 minute, 
and remained for some time at about 5650 ohms. 
On allowing the piece time to recover its former state, and then reversing the battery- 
current, the resistance was again increased by about 1000 ohms on exposing the whole 
piece to the action of the lime-light, and the resistance was also increased on exposing 
either end. On shutting off the light the resistance again rapidly diminished. 
This piece in its present state is different in character from every other piece with 
which we have experimented, in that its electrical resistance is increased when it is 
exposed to the action of light during the passage of an electric current through it. 
3d 
MDCCCLXXVII. 
