The Crookes radiometer, also known as the 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. The reason for the rotation has historically been a cause of much scientific debate.
It was invented in 1873 by the chemist Sir William Crookes as the by-product of some chemical research. In the course of very accurate quantitative chemical work, he was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the balance.
Investigating this photoelectric effect, he created the device named after him. It is still manufactured and sold as a novelty item.
Read more about Crookes Radiometer: General Description, Prefix radio- and Suffix -meter, Explanations For The Force On The Vanes, All-black Light Mill, Nanoscale Light Mill