1876.] Action of Light on Tellurium and Selenium. 163 



II. " On the Action of Light on Tellurium and Selenium." By 

 Prof. W. G. Adams, E.R.S. Received November 10, 1875. 



Two platinum wires were attached to the ends of a small bar of 

 tellurium, about 1 inch in length, by heating the wires to a bright red 

 heat and bringing them suddenly in contact, one with each end of the 

 bar. The platinum wires melted a small portion of the tellurium, and 

 became imbedded in it. The resistance of the bar and wires was about 

 half an ohm. The tellurium was placed in a box, and its resistance 

 balanced ; then it was exposed to the light of a paraffin-lamp, just as in 

 the experiments with selenium. At first the light seemed to have no 

 effect ; but in consequence of the heat from the lamp, the resistance of 

 the tellurium was increased. 



On placing a rectangular vessel of water between the lamp and the 

 tellurium and then exposing as before, there was no change of resistance 

 in the tellurium, showing that if there was any diminution in the resist- 

 ance due to the action of light it was entirely balanced by the increase in 

 the resistance due to heating by the current and by the residual portion 

 of the radiant heat. 



On replacing the rectangular vessel of water by a beaker of water, so 

 as to focus the light of the lamp on the tellurium, there was found on 

 exposure to be a diminution in the resistance of the tellurium, which gra- 

 dually increased until the galvanometer-needle was deflected through 30 

 divisions of the scale. The needle was then brought back to zero by 

 altering one of the slide-resistances in the circuit. The agreement 

 between the amounts of the change of resistance in several successive 

 experiments, with intervals between them, was very close. Thus, in 

 three experiments on October 8th, 



The change in the first experiment was 124 millims. of wire, 

 „ „ second „ „ 120 „ „ 



third „ „ 122 „ 



showing a diminution in the resistance of the tellurium amounting to 

 about one thousandth part of its whole resistance on exposure to the 

 light of the paraffin-lamp. 



On repeating the experiments with the tellurium which had not been 

 exposed to light for seven clays, the tellurium was found to be much 

 more sensitive. When exposed at a distance of about half a metre, as 

 before, to the paraffin-lamp the needle was gradually deflected through 

 30 divisions of the scale, showing that the resistance of the tellurium 

 was now diminished as much without interposing the beaker of water as 

 it had previously been when the beaker was interposed. On making no 

 change whatever, except placing the beaker of water between the lamp 

 and the tellurium, so as to focus the light on the tellurium, the deflection 

 of the needle gradually increased to 80 and then more slowly to 100 divi- 



