Photoconductivity

Author: J.B. Hoag

The resistance offered by a suitably prepared thin layer of selenium to the passage of an electric current from a battery changes according to the amount of light falling upon this material. The resistance differs greatly according to the method of manufacture. In some cases it decreases from 10 to 20 million ohms to around 1 million ohms when exposed to light. Selenium cells are sluggish in their response to changes of light intensity. After the light is first turned on the resistance decreases sharply. After this it continues to decrease slowly for several minutes or even hours, depending upon the previous history of the cell. When the light is turned off the normal dark resistance is not reached until after an appreciable time interval.