Page:Collected Physical Papers.djvu/171

Rh obtain reversals; there is, besides, a tendency of self-recovery in the receiver. In order to expedite the reversals, the incident electric radiation is made very strong.

Before entering into the details of the results obtained, I will say a few words about the principal types likely to be met with. We may have the following:—

I. Substances in which the B state is unstable under the given conditions; the B state will therefore only persist during the action of radiation, the substance relapsing into the original condition on the cessation of radiation. Two cases are possible (i) when the substance is positive, (ii) when the substance is negative.

The latter case is exemplified by potassium. In the above curve (fig. 24), A and B represent the two molecular states. The substance being negative, A is more conducting; a represents the conductivity of the fresh specimen; the thick dots S S′. . . the individual flashes. It is seen that the effect of radiation is to produce a sudden diminution of conductivity, due to the transient formation of B variety with its diminished conductivity. The substance, electrically speaking, is highly elastic, and it exhibits automatic recovery on cessation of radiation. With the majority of substances, however, self-recovery is only possible when the narrow limit of elasticity is not exceeded.