Effect of Light.
The molecular effect due to visible radiation may as in the previous case be detected by the method of conductivity or electromotive variation. That light does produce conductivity variation is seen in selenium. I have also succeeded in detecting the effect of light in producing variation of contact resistance. One and the same receiver responded in the same way when alternately acted on by visible and invisible (electric) radiation. It is, however, difficult to discriminate the effect of light from that due to the rise of temperature. That the effects observed were not solely due to temperature was evident from the fact that there was a tendency towards reversal, and that the same receiver which normally exhibited a diminution of resistance exhibited an increase of resistance when it underwent a molecular modification. The peculiarities of this universal radiometer was 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 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 other cases the potential is lowered by the action of light.
I now proceed to show the remarkable similarity of the curves of effect produced by electric radiation and light. For the photo-electric cell I used two silver strips fastened at the back by paraffin on 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.
The two plates being opposed, there would be no resultant effect if both were illuminated.
But if the two plates are slightly different, then the effects on the two are not the same. There will then be an electromotive variation, even when both the plates are exposed.
9. Effects of Flashes of Light.
In fig. 9, (a), is shown the effect of flashes of light of 2 seconds duration on AgBr plate. The source of light—an incandescent gas-burner—was at a distance of 12 inches. If the plates are kept in the dark for several days, the sensitiveness is then very much enhanced, and
