Crookes Radiometer: A Comedy of Errors

Invented by Sir William Crookes in 1873, this device is also known as a light mill. It is still a popular novelty item. A sealed glass bulb, evacuated to about 1 Pa (~ torr), encloses a set of (usually four) vanes, black on one side, white or silver on the other, that can rotate on a low-friction spindle. When exposed to light or heat, the dark sides rotate away from the source. Conversely, if a block of ice is placed nearby, the vanes rotate in the opposite direction. The mechanism of this phenomenon was a source of scientific controversy for over half a century.

The original explanation was radiation pressure, which had recently been proposed by Maxwell. If this were true, however, the vanes would rotate in the opposite direction, since photons are absorbed by the black surfaces but reflected from the white or silver sides, thus transferring twice the momentum. A successor theory proposed that the residual traces of gas would absorb more heat in the vicinity of the black surfaces, and therefore exert slightly higher pressure. But this is invalid since the temperature rise would also produce a decrease in gas density. A number of other defective explanations, which we will not enumerate here, have been popular through the years.

The accepted mechanism, first proposed by Osborne Reynolds, focuses on the flow of gas around the edges of the vanes, from the warmer black sides to the cooler white sides. The slight tangential viscous friction then causes the vanes to move in the direction black to white.

If a cooler object, such as a block of ice, is brought near the radiometer, the motion of the gas molecules will be reversed and rotation will occur in the opposite sense.

A molecular dynamics simulation of the phenomenon is given in Z. Lu, "Experimental and Numerical Study of the Optimal Operation Pressure within Crookes Radiometer," Journal of Vacuum Science & Technology A, 23(6), 2005 pp. 1531–1534.