Page:Collected Physical Papers.djvu/205

Rh the case of receivers for exhibition of conductivity variation, a careful adjustment of pressure is necessary for obtaining the best result. In order that the effect observed might be purely due to electromotive variation and not to the variation of conductivity, the cell was connected with a capillary electrometer. Owing to the opacity of the electrolyte the intensity of radiation has to be strong; the radiator was therefore placed at a distance of 6 inches from the cell. The incident radiation induced a responsive variation of the electromotive force. Long continued radiation induced a reversal as observed in the case of conductivity-variation previously described.

I give below the results of three experiments with different combinations:—

I was next desirous of obtaining a continuous record of electromotive changes induced by the continued action of electric radiation. For this purpose I used a galvanometer.

A cell was made, in the way previously described, with two specimens of magnesium powder. Owing to some differences in the two portions there was an initial P. D. of 0.042 V. between the two electrodes. The E. M. F. of the cell was balanced by the potentiometer method, a sensitive galvanometer (with an interposed high resistance) being used as the detector of electromotive variation. Fig. 39 (a) shows the deviation from