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[Phys-L] Albion Perfide - or - Crookes' Radiometer



How does a light-mill work?

From a work by Phillip Gibbs (1996), U Cal Riverside, noticed by Paolo Brenni on RETE list:
<http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html>

"Early in 1879 Reynolds submitted a paper to the Royal Society in which he considered what he called "thermal transpiration", and also discussed the theory of the radiometer. By "thermal transpiration", Reynolds meant the flow of gas through porous plates caused by a temperature difference on the two sides of the plates. If the gas is initially at the same pressure on the two sides, it flows from the colder to the hotter side, resulting in a higher pressure on the hotter side if the plates cannot move. Equilibrium is reached when the ratio of pressures on either side is the square root of the ratio of absolute temperatures. This counter intuitive result is due to tangential forces between the gas molecules and the sides of the narrow pores in the plates. ...

The vanes of a radiometer are not porous. To explain the radiometer, therefore, one must focus attention not on the faces of the vanes, but on their edges. The faster molecules from the warmer side strike the edges obliquely and impart a higher force than the colder molecules.... The net movement of the vane due to the tangential forces around the edges is away from the warmer gas and towards the cooler gas, with the gas passing around the edge in the opposite direction. The behavior is just as if there were a greater force on the blackened side of the vane (which as Maxwell showed is not the case); but the explanation must be in terms of what happens not at the faces of the vanes, but near their edges.

Maxwell refereed Reynolds' paper, and so became aware of his suggestion. Maxwell at once made a detailed mathematical analysis of the problem, and submitted his own paper, "On stresses in rarefied gases arising from inequalities of temperature", for publication in the Philosophical Transactions; it appeared in 1879, shortly before his death. The paper gave due credit to Reynolds' suggestion that the effect is at the edges of the vanes, but criticized Reynolds' mathematical treatment. Reynolds' paper had not yet appeared (it was published in 1881), and Reynolds was incensed by the fact that Maxwell's paper had not only appeared first, but had criticized his unpublished work! "

Brian Whatcott Altus OK.