From Francis Fahy
In his article on Crookes’s radiometer, Marcus Chown mentions the observation by Arthur Schuster that if the instrument’s bulb is suspended by a single fine thread it rotates in the opposite direction to the vanes, but he offers no explanation of the phenomenon (21 August, p 48).
As Chown indicates, the light itself does not transfer momentum to the vanes. So if the vanes are subject to a torque caused solely by the gas, then the gas must acquire an equal and opposite momentum in order to conserve the angular momentum of the vane-gas system. There must therefore be contra-rotation of the mass of gas.
If we now extend our system to include the bulb, its contra-rotation would seem to indicate that the movement of the gas applies a torque to the bulb, presumably via wall shear stress at the surface of the bulb associated with a boundary layer. Any frictional torque on the bulb induced by vane rotation would tend to induce the bulb to rotate in the same direction as the vane.
Stockbridge, Hampshire, UK
