1901.] Effect of Light and Electric Radiation on Matter. 171 



Effect of Light. 



The molecular effect due to visible radiation may as in the pre- 

 vious 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 pro- 

 ducing 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 electro- 

 motive 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 



