348 Mr. S. Bidwell on the Electrical Resistance 



Amongst other things, the effect was tried of mixing it with 

 graphite. The sulphur was heated to a temperature a little 

 above its melting-point (115° C), an d when quite liquid a 

 small quantity of tolerably pure powdered plumbago was 

 stirred into it. The liquid mixture was then poured into 

 moulds and allowed to cool quickly. Sticks or plates were 

 thus produced of a substance which in general appearance 

 closely resembled crystalline selenium, its surface being dull 

 and slate-coloured, and its fracture metallic, not unlike that 

 of cast iron. 



A short stick of this substance was fitted with platinum 

 electrodes (platinum wires being made red hot and pressed 

 into the two ends), and was joined up in circuit with a 

 Leclanche cell and a reflecting galvanometer. The spot of 

 light was brought to the zero-point at the middle of the scale 

 by means of a magnet, and the sulphur rod exposed to the 

 radiation of a gas-flame which was held within a few inches of 

 it. The spot at once began to move, showing a considerable 

 change in the resistance of the rod. The gas-flame was extin- 

 guished; and the spot slowly returned to zero. Upon exami- 

 nation, however, it appeared that the behaviour of this sulphur 

 rod differed from that of crystalline selenium in two important 

 particulars. In the first place, the change of resistance was 

 clearly an effect not of light, but of temperature. Burning- 

 magnesium produced no greater change than the gas-flame 

 when held at the same distance. Sunlight was found to have 

 a powerful effect, which was scarcely diminished when the 

 light was caused to pass through red glass; but blue glass or 

 a cell containing water formed an effectual screen. A black 

 hot poker, or even the warmth of the finger, caused a greater 

 deflection than a strong light which was too far removed to 

 heat the rod sensibly. 



Now there can be little doubt that the decrease in the 

 resistance of selenium which occurs under the influence of 

 radiation is totally distinct from any effect produced by tem- 

 perature. It is of course true that absorption of radiations 

 is followed by a rise of temperature, particularly when such 

 radiations belong to the red and infra-red part of the spec- 

 trum ; but it is not, I think, the fact that such rise of tempe- 

 rature is in any way connected with the remarkable variation 

 of resistance under the influence of light, which, owing to 

 the invention of the photophone, has lately attracted so much 

 attention. The electrical effects of radiation are, in this case 

 at least, no more due to rise of temperature than are its 

 chemical effects. The evidence in favour of this assertion 

 seems to me to be overwhelming; and I hope to return to it 



