Page:Collected Physical Papers.djvu/400

380 induced molecular strain (attended by change of conductivity) to their original condition and original conductivity. In the positive class, a tap or application of heat restores the substance to original condition by an increase of resistance; in the negative class the restoration is by a diminution of resistance to the normal (p. 158).

The conductivity change is produced not only under very rapid electromotive variation by electric waves, but also by a comparatively slow electric variation. Electric conduction in metallic particles sensitive to electric radiation does not obey Ohm's Law. The conductivity is not constant and independent of E. M. F. but varies with it. The two classes of substances, positive and negative can be discriminated from each other by their characteristic curves under cyclic electromotive variation.

The characteristic curve is obtained by recording the variation of the electric current produced under increasing E. M. F. For surface contact of positive class of substance like iron, the curve is concave to the axis of current. Under cyclic E. M. F. variation, the forward and return curves do not coincide; there is a hysteresis. The greater is the range of E. M. F. the greater is the area enclosed. The residual effect can be dissipated by mechanical vibration.

In the negative class of substance like potassium, the curve is of an opposite character being convex to the axis of the current. Increase of E. M. F. produces an actual diminution of current or an increase of resis-