Crookes Single Bulb Radiometer on Stand, c. 1890

Crookes radiometer, also known as a light mill, consists of four mica vanes suspended inside an airtight glass bulb. A vacuum is formed inside the bulb which allows the vanes, which are each blackened on one side, to rotate when a light is shone onto them. A more intense light exposure will cause a faster rotation. The glass bulb stands on a turned blackened wooden base, which is weighted down by a lead ring.