Solar Engine spin or "Radiometer”

The Crookes radiometer consists of a glass bulb from which most of the air has been removed, thus creating a partial vacuum, and a rotor mounted on a vertical support inside the bulb. The rotor carries four light, horizontal arms mounted at right angles to each other on a central axis; the rotor is free to rotate in the horizontal plane. At the outer end of each arm is mounted a vertical metal vane. Each vane has one side polished and the other blackened; the units are arranged so that the polished side of one faces the blackened side of the other. When radiant energy strikes the polished surfaces, most of it is reflected away, but when it strikes the blackened surface, most of it is absorbed, which raises the temperature of those surfaces. The air near a blackened surface is thus heated and exerts pressure on the blackened surface, causing the rotor to rotate.