Page:Collected Physical Papers.djvu/399

Rh

In regard to the variation of resistance induced by electric waves, it is found that substances could be divided into two classes positive and negative. The positive represented by Iron, Magnesium, and others, characteristically exhibits a diminution of resistance; the negative (Potassium, Arsenic, etc.) exhibits on the other hand, an increase of resistance. An intermediate class is found in Silver which under different molecular modifications, exhibits either a diminution or an increase of resistance. Some of these substances, moreover, exhibit an automatic recovery on the cessation of electric radiation (p. 125). The increase of resistance induced by electric radiation and automatic recovery prove that the theory of 'coherence' is inadequate in explanation of the phenomenon. The induced variation of resistance positive or negative, increases with the intensity of radiation. Under continued radiation a maximum effect is produced; there is now a balance between conductivity distortion and force of restoration, recovery becoming complete on the cessation of radiation. These and other facts prove that radiation induces a molecular change of an allotropic character, and that the variation of conductivity is an expression of the induced molecular change (p. 161).

When the range of electric elasticity is not narrow, the substance exhibits an automatic recovery. In other substances the recovery is more or less prolonged. Even in such cases the recovery is hastened by molecular vibration produced by a mechanical tap, or by heat. These restore the sensitive substances from the state of