Page:Collected Physical Papers.djvu/191

exhibited by various substances in regard to self-recovery is one merely of degree. I give below typical cases which exhibit the gradual transition from so-called non-recovery to complete self-recovery.

The galvanometer used is a dead-beat D'Arsonval of moderate sensitiveness. The receiver was appropriately fixed on a heavy base. This was supported on one or two sets of pneumatic tyres so as to protect the sensitive receiver from mechanical vibration.

Positive Type.—In fig. 33 (a) is shown the effect of radiation on Fe₃O₄ when cold. Only the upper portion of the curve is given; the flash of radiation produced a deflection of the galvanometer of sixty-four divisions.

Fig. 33. Different stages in the evolution of a Molecular Receiver from a non-recovering to a self-recovering condition. The substance used is Fe₃O₄, representing positive type, which exhibits a diminution of resistance under electric radiation. (a) the so-called non-recovery, really a case of very slow recovery; (b) the same slightly warmed, exhibiting a partial and arrested recovery; (c) the same with increased molecular mobility, recovery in 30″. In this and in the following, thick line represents response to radiation, thin line exhibits recovery.

It will be observed that it had recovered to the extent of three divisions in the course of 60 seconds; if the rate