Page:Collected Physical Papers.djvu/402

382 Method of electromotive variation.—A difference of electromotive force is generated between two areas of the same substance, when one is exposed and the other shielded from electric or visible radiation. The characteristics of response to electric radiation and light under diverse conditions are found to be essentially similar (p. 187).

Rapid mechanical vibration, also induces an electromotive variation between the stimulated and unstimulated portions of the same wire (p. 195). The response to mechanical stimulation is found to be of opposite sign to that under light as demonstrated by alternately subjecting one of the two wires of a strain cell to the two modes of stimulation. It is moreover possible to exactly balance the effect of one by that of the other, a slight increase or decrease of either producing an immediate upset of the balance in one direction or its opposite (p. 206).

1. The molecular effect induced by stimulus can be detected in all cases by the methods of conductivity or of electromotive variation.

2. Substances exhibit quick recovery after moderate stimulation; the recovery becomes protracted in consequence of overstrain caused by intense stimulation.

3. Response is modified by the previous history of the substance and by changes of the environment. Slight rise of temperature and annealing ensures an increased sensitiveness and quick recovery.