Radiometer by Objet de Curiosite

The Crookes radiometer is a device invented in 1873 by the English chemist William Crookes. It consists of four arms each supporting a plate at their ends, painted white on one side and black on the other. The four arms supporting the plates are suspended on a needle and supported by a glass shaft to reduce friction as much as possible. The plates rotate when exposed to light, the more intense the incident light, the faster the rotation. This provides a quantitative measure of the intensity of electromagnetic radiation.