The Sensitivity of Selenium Cells

Abstract

MR. THORNE BAKER'S observations on the enhanced light sensitivity of selenium cells actuated by alternating, instead of direct, current (NATURE, June 19, p. 858), are possibly related to some phenomena investigated recently by Mr. J. W. Avery and myself. We have found that selenium cells of a particular type (those made to the design of Prof. H. Thirring) display, when used in the ordinary way. with direct current, appreciable and persistent polarisation effects. Almost accidentally it was discovered that this polarisation disappeared as a consequence of prolonged exposure of the cell to the operation of a drying agent. By the same process the ‘dark’conductance of the cell was reduced in the proportion of about 4 to 1, while the ‘light’ conductance was scarcely altered. The dried cell had thus become much more sensitive to light. We have attributed both the polarisation observed and a large part of the “dark” conductance of the undried cell to the presence of a film of water in parallel with the selenium between the electrodes, and our observations agree quantitatively with this assumption.