Fig. 97 (a)
A, B are the two faces of a brominated sheet of silver. One face, say A, is acted on by light. The current of response is from B to A, across the plate.
Typical experiment on the electrical effect induced by light.—This subject of the production of an electrical current by the stimulus of light would appear at first sight very complex. But we shall be able to advance naturally to a clear understanding of its most complicated phenomena if we go through a preliminary consideration of an ideally simple case. We have seen, in our experiments on the mechanical stimulation of, for example, tin, that a difference of electric potential was induced between the more stimulated and less stimulated parts of the same rod, and that an action current could thus be obtained, on making suitable electrolytic connections. Whether the more excited was zincoid or cuproid depended on the substance and its molecular condition.
Let us now imagine the metal rod flattened into a plate, and one face stimulated by light, while the other is protected. Would there be a difference of potential induced between the two faces of this same sheet of metal?
Let two blocks of paraffin be taken and a large hole drilled through both. Next, place a sheet of metal between the blocks, and pour melted paraffin round the edge to seal up the junction, the two open ends being also closed by panes of glass. We shall have then two compartments separated by the sheet of metal, and these compartments may be filled with water through the small apertures at the top ([fig. 97], a).
Fig. 97 (b).—Record of Responses obtained from the Above Cell
Ten seconds’ exposure to light followed by fifty seconds’ recovery in the dark. Thick lines represent action in light, dotted lines represent recovery.