ACTION OF LIGHT ON SELENIUM. 
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above platinum in the thermoelectric scale. Some of these pieces which gave no 
current or very little current when their junctions were exposed to the lime-light, gave 
stronger thermoelectric currents when their junctions were held between the finger and 
thumb. The pieces which stand below platinum in the thermoelectric scale are 
Mr. W. Smith’s plate, and Nos. 4, 7, 8, 10, & 14. 
On examining No. 11, the current produced on warming the marked end by the 
finger gave a deflection of 15 divisions in the direction from selenium to platinum at 
the heated end. On warming the unmarked end by the finger there was a deflection of 
110 divisions, indicating a current from selenium to platinum at the heated end. 
On heating the marked end by the hot blast, there was a deflection of 55 divisions, 
indicating a current from selenium to platinum at the heated end ; and on heating the 
unmarked end, there was a deflection of 115 divisions, indicating a current from selenium 
to platinum at the heated end. 
The unmarked end of this piece is much more sensitive than the marked end. 
On referring to the experiments with lime-light, it appears that the unmarked end 
was more sensitive to lime-light than the marked end, but that when the light was 
brought to a focus on the unmarked end, there was a current produced in a direction 
from platinum to selenium at the illuminated junction. Thus there seems in this case 
to be a difference in character between the action of the heat of the hand, or of the 
blast, and the action of the lime-light. 
This is the first instance in which we have found that the process of annealing has 
placed selenium above platinum in the thermoelectric scale. 
On referring to the paper, we see that after sending a current from a battery through 
this piece of selenium very strong polarization-currents were obtained from it, and that 
on exposure to light there was a slight increase in the intensity of the polarization- 
current. 
No. 15 was the first piece from which a current was obtained by the action of light 
alone. It shows polarization-currents very well, and in it we first found that exposure 
to the light of a candle produced a current which was opposed to and greatly over- 
balanced the polarization-current. This piece is very singular in its action when its 
junctions are heated. 
It will be convenient to call a current giving a deflection of 50 divisions of the scale 
a current of 50, and to denote platinum and selenium by P and S respectively. 
On applying heat to one of the junctions by touching it with the hand, a current was 
produced from platinum to selenium at the heated junction. On warming the other 
junction in the same way, the current was reversed, so that, in both cases, at the heated 
junction the current passed from the platinum to selenium. 
On repeating the experiment, with the marked end heated by the hand, there was at 
first a current giving a deflection of 120, or a current of 120, from P to S at the heated 
junction; this diminished to 50. When the hand was removed there was a deflection 
of 50 on the other side of zero, and the needle rested for some time at 10, showing a 
M D CCCLXX VII . 3 C 
