Photocells are used in television camera to reproduce sound recorded on films, in counting devices, in burglar and fire alarms, to measure the temperature of stars, to study the spectrum of heavy bodies, to operate street light, to compare the illuminating powers of two sources, in photometers, for locating minor flaws in metallic sheets, to determine the opacity of solids and liquids, to control the temperature in chemical reactions and to determine the Planck's constant.
