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PEOEESSOE W. G. ADAMS AND ME. E, E. DAY ON THE 
In these investigations we have seen : — 
(1) That a slight increase of temperature of a piece of annealed selenium is accom- 
panied by a large increase of electrical resistance. 
(2) That on increasing the strength of the current through the selenium, there was a 
diminution in its resistance, which seems to be due to a kind of polarization which is 
similar in its effects to electrolytic polarization. This polarization seems to be a perma- 
nent effect, or at least to last for a long time, so long as the ends of the selenium are 
not short-circuited. 
(3) On exposure to light while a battery-current is passing through it, the electrical 
resistance of a piece of annealed selenium is apparently diminished. 
The experiments with the Bunsen flame, with weak sources of light, and with moon- 
light, seem to show that this effect is due to the illuminating-power rather than to the 
heating effect of the source. 
(4) The apparent change in the electrical resistance is directly proportional to the 
square root of the illuminating-power of the light. 
(5) After the battery-current is disconnected from the selenium, the strength of the 
current due to polarization is increased in most cases by exposure to light. 
(6) Pieces of annealed selenium are, in general, sensitive to light ; that is to say, 
that under the action of light a kind of electromotive force is developed among the 
molecules, which, under certain conditions, can produce an electric current through the 
selenium. 
(7) This sensitiveness is different at different parts of the same piece of selenium. 
(8) In most of the pieces which we have tested, the action of light, when there is no 
battery-current passing, causes a flow of electricity from the selenium to the platinum 
at the illuminated junction. To distinguish these currents from currents arising from 
any other cause, we have called them photoelectric currents. 
APPENDIX. 
Eeceived May 17, 1877. 
The place of Selenium in the Thermoelectric Scale. 
In the above paper it has been assumed as a fact established by the researches of the 
late Dr. Matthiessen (Proc. B. S. vol. ix. p. 99, 1857, and Phil. Trans. 1858) that 
platinum stands above selenium in the thermoelectric scale, and therefore that the 
direction of a thermoelectric current would be from platinum to selenium at the heated 
joint. It has been suggested that, considering the changes produced in selenium by the 
process of annealing, it would be well to try the thermoelectric properties of the several 
pieces used in the above experiments. 
As many of these pieces are only 3 or 4 millims. in length, and as the ends of the 
