Page:On the Continuity of Effect of Light and Electric Radiation on Matter.djvu/16

1901.] or an electrometer. In the carrying out of this method there are, however, many practical difficulties. First of all, in making a voltaic combination, some kind of electrolyte is necessary, but unfortunately all electrolytes are opaque to electric radiation. This difficulty could, however, be obviated to some extent by taking an electrolyte which is almost a non-conductor (e.g., amylic alcohol) so as to be partially transparent to electric waves. But the second difficulty is far more serious. Owing to the diffuse action of the comparatively long electric waves it is impossible to shield one plate while exposing the other. If both the plates are equally acted on, there would then be no electromotive variation. From these considerations any attempt to detect the effect of electric radiation by electromotive variation appeared to be hopeless. It was only after the conclusion of another line of investigation on the electromotive variation produced by mechanical stimulus that I learnt that the effects of the same stimulus on two pieces of the same metal, forming a voltaic element, are different if the molecular conditions of the two are not originally the same. Under such a condition a P.D. exists between the two, and stimulation of both causes a variation of the existing electromotive force.

From the similarities of the effects of radiation and mechanical strain (see the following paper) I was convinced that with radiation, too, I would get unequal effects on the two plates having a slight initial electromotive difference. The effect of radiation would then be to produce a variation of the original electromotive force. And if the effects are at all parallel to those observed in the conductivity variation method (as diminution or increase of resistance) we may likewise expect to find a diminution or increase of electromotive force.

In carrying out experiments to verify the above suppositions, I soon found my anticipations to be fully justified. I at first made a cell by taking two varieties of silver. A piece of cotton wool moistened with amylic alcohol was placed in a glass tube. Ag and Ag′ were placed on opposite sides of the moistened cotton; this formed a voltaic element. Two electrodes compressed the powder, till a current was observed to flow. The amylic alcohol acted as the electrolyte. Very careful adjustment of pressure is necessary, as in the case of receivers for exhibition of conductivity variation. 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. On allowing electric radiation to act on the cell, there was produced a variation of electromotive force; continued radiation even produced a reversal. The electric radiator—a rod used in the previous experiments—was also used in this case: owing to the opacity of the electrolyte the intensity of radiation has to be strong. The radiator was placed at a distance of 6 inches from the cell.