Selenium cells

The element selenium was discovered by Berzelius in 1817. It can be obtained in various forms and applied to many uses, although its peculiar properties have been generally understood for only about thirty-five years. In 1873 while experimenting with selenium as an ohmic resistance, Willoughby Smith discovered that the resistance of this metal was reduced by exposure to light. In using sticks of selenium as a resistance in an experimental telegraph circuit, he had trouble with the current varying; on investigating he found that when the cover to a box in which he had placed the selenium was closed, the current was less than with the box open. Further investigation showed that the electrical resistance of selenium varied with the amount of light falling upon it.