Crookes Radiometer
The radiometer is made from a glass bulb from which much of the air has been removed to form a partial
vacuum. Inside the bulb, on a low friction spindle, is a rotor with several (usually four)
vertical lightweight metal vanes spaced equally around the axis. The vanes are polished or white on one side, black on the other. When exposed to
sunlight, artificial light, or infrared radiation
(even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing. Cooling the
radiometer causes rotation in the opposite direction.
The Crookes Radiometer is available for $13.00 plus $3.50 shipping.