Crooke's Radiometer

The Crooke's radiometer, also known as a light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. Today the device is mainly used in physics education as a demonstration of a heat engine run by light energy.