Page:Collected Physical Papers.djvu/206

186 the balanced position by radiation which nearly reduced the potential difference to half its original value. A second experiment [see (b)] gave almost identical results.

It is thus seen that the response-curve by electromotive variation under electric radiation is similar to that obtained by the method of conductivity variation.

It has been shown that there is a recovery when the range of electric elasticity of the substance is not narrow, or when the strain is not too great; that on subjecting the substance to the continued action of radiation there is a limiting effect; that too long continued action tends to produce an electric reversal; that too feeble an intensity may give rise to a response of opposite sign to that of the normal. We shall next investigate whether visible radiation produces similar results.

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