Crookes Radiometer

Comprising partly evacuated glass bulb approx 70mm diameter, with contained at its centre a fine pivot which supports four lightweight metal arms. One side of each vane is blackened, the reverse side is white. Infrared radiation heats the black faces of the vanes more than the white, causing a pressure difference on either side of the vanes which drives the vanes round, providing a pleasant visual demonstration of several physical effects. Discussion of the mechanism turning the vanes covers several areas of physics including: thermal radiation and absorption; thermal transfer; air pressure; and momentum of gas molecules. The unit is mounted on a round plastic base.