Page:Collected Physical Papers.djvu/207

Rh have also succeeded in detecting the effect of light in producing variation of contact resistance in a galena receiver. One and the same receiver responded in the same way when alternately acted on by visible and invisible (electric) radiation. The peculiarities of this universal radiometer were in every way similar to those of detectors for electric radiation.

It is, however, more satisfactory to study the effect of light in producing electromotive variation. Becquerel, Minchin, and others have shown that light produces an electromotive variation in a photo-electric cell. Like electric radiation, the effect of light is not confined to any particular metal or groups of metals, but all metals exhibit an electromotive variation under its action. Two opposite effects are likewise shown; in some cases the potential is raised, in others, the potential is lowered by the action of light.

I now proceed to show the remarkable similarity of the curves of response produced under electric radiation and under light. For the photo-electric cell I used two silver strips fastened by solid paraffin on two sides of a glass plate. The front surfaces were exposed to bromine vapour. The two strips formed the two plates of the photo-electric cell, the electrolyte being common tap-water.

If the two strips are exactly similar, then there is no P. D. between them, and the effect of light on either of the strips is the same. When both the plates are illuminated, there is then no resultant effect. But if the two plates are slightly different, then the effects on the two are not the same. An electromotive variation is induced even when both the plates are exposed to light.