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250 change. But if the cyclic variation be carried out with very great rapidity, the phenomenon of lag comes into play. The conductivity variation then lags behind the impressed electromotive variation, and the receiver does not instantaneously recover its original resistance.

For example, in the case of the self-recovering receiver which at 0·2 volt gave a current represented by two galvanometer divisions, the resistance being equal to 50,000 ohms, the cyclic electromotive variation was quickly carried through the range from 0·2 volt to 1·2 volt and back to 0·2 volt, the immediate value of the current at this last point was not two, but six, divisions of the galvanometer. Thus the receiver, owing to lag, does not instantaneously recover its original resistance; the deflection, however, soon creeps back to two, exhibiting a complete recovery. This characteristic of self-recovery is also exhibited by rapid electromotive variation as under electric radiation (see fig. 56).

Space allows only a brief reference to the characteristic cyclic curve of negative class of substance exemplified by potassium. In this we are presented with the extraordinary phenomenon that an increase of E. M. F. is attended by a diminution of current, so that at a critical E. M. F. the current disappears altogether.

Summary

1. Under the action of electric radiation the conductivity of metallic particles exhibits variation. In the positive class, like iron, there is an increase, and in the negative, like K, a diminution, of conductivity. Each class again falls into two sub-classes, (a) sensitive substances which exhibit self-recovery, and (b) sensitive substances which do not. In the case of