Page:Collected Physical Papers.djvu/401

Rh. This class exhibits an increase of resistance under electric radiation (p. 251).

External disturbances or stimuli impinging on a substance induce a molecular upset, the effectiveness of the stimulus depending on the rapidity of the onset of the disturbance. The molecular upset is attended by changes induced in the properties of the substance. The most sensitive means for its detection is electrical: by the method of conductivity or of electromotive variation (p. 170).

Stimulation of the substance can be produced by electric radiation, by visible light, and by mechanical vibration.

Method of conductivity variation.—This is specially well suited for studying the effect of electric radiation on discontinuous particles; for since the action of radiation is one of surface layer, the larger the area, the more pronounced is the effect, and in loose particles the effective surface is greatly enlarged. The effective total resistance of the particles, moreover, is due to the resistances of surface contacts; any change in the property of the surface layer therefore causes a great variation of the total resistance. There are two classes of substance of which the positive responds by a diminution and the negative by an increase of resistance (p. 135). The effect of radiation is annulled by mechanical vibration. The continuity of effect of electric radiation and light is seen in the galena receiver which responds to both visible and invisible lights by a diminution of resistance (p. 269).