Page:Collected Physical Papers.djvu/245

Rh long period. This view of conductivity variation, as the expression of some molecular change, will receive independent support from the experiments on a self-recovering receiver to be described later.

The conductivity changes take place not only under very rapid Hertzian oscillations, but also under much slower electromotive variations. Thus an electromotive variation, whether quick or slow, is the effective cause of induced change of conductivity. In substances which exhibit recovery from the action of radiation so protracted, as to appear practically non-recovering, the induced molecular change is attended by a more or less permanent variation of conductivity. Recovery in such cases can be hastened by molecular vibration, by mechanical tape, or by the application of gentle heat.

After a rapid cyclic variation of E. M. F. the substance is thus transformed from its primitive condition; in other words a more or less irreversible molecular change is induced at the end of the process as exhibited by a hysteresis of conductivity. If this view be correct, then we can study the molecular change step by step, by observing the conductivity variations undergone by the substance as it is carried through a complete cycle of electromotive variation. The rapidity of this process must be just sufficiently slow to allow the successive changes to be recorded. The suddenness of electric variation no doubt exerts an influence on the amount of change; but this is a question of degree only.

The investigation thus resolves itself into the determination of the variation of conductivity of the sensitive particles, as the mass is subjected (1) to a