A radiometer with aluminium vanes. An instrument designed for measuring the mechanical effect of radiant energy. Mounted in a partial vacuum, the central rotor spins when the vanes are exposed to light. Chemist William Crookes (1832-1919) initially believed the vanes with black surfaces were pushed around by pressure from light radiation. This was incorrect and other theories were put forward. It was not until 1879 that Osborne Reynolds suggested the correct explanation, warmer molecules striking the edge of the vanes obliquely with a higher force than the colder molecules.ca. 1875
