maximum effect was reached, there was a distinct trend towards reversal.
The positive class of substances, the normal response of which is positive or diminution of resistance under moderate stimulation, gives negative response (increase of resistance) under sub-minimal stimulus. This is specially noticeable in a receiver rendered inert by prolonged rest. Annealing makes the response normal, as also previous stimulation. Strong and prolonged stimulation tends to reverse the normal negative response into positive. All these peculiarities of response under electric radiation will also be found characteristically exhibited by other receivers responding to the stimulus of light or of mechanical vibration.
7. Physical Nature of the Change
From the fact that the conductivity variation above described takes place In platinum and other noble metals, and the further fact that the action goes on even
