An evacuated electronic tube which converts light into a measurable electric current. Light falling on a photocathode releases electrons, which are accelerated by an electric field and attracted to the first dynode (positive electrode), where they liberate more electrons which are attracted to the second dynode, and so on. A common type of photomultiplier in astronomical use has ten dynodes, each at an increasingly positive electric potential. The flow of electrons arriving at the final anode is proportional to the amount of light falling on the photocathode. Photomultipliers are widely used for photometric measurements in astronomy, such as of variable stars.