Re: Crooke's Radiometer

["Otto G. Raabe" <ograabe@ucdavis.edu>]

April 2, 1999
Davis, CA

The vane radiometer inside a partially evacuated glass bulb works on the
phenomenon of "thermal creep" as described by Maxwell (J.C. Maxwell, "On
stress in rarified gases arising from inequalities of temperature,"
Transactions Royal Society (London) 170:231-245, 1879). Radiation momentum
is not involved since it is too small.

The forces that arise on the vanes of the radiometer come from the
so-called "thermal creep" of gas molecules that occurs over the surface of
an unequally heated body. The black sides of the vanes absorb more heat
energy from the light than do the white (or silver) sides, so the black
sides are slightly warmer than the white sides. Gas molecules near a
surface tend to approach its temperature. The gas molecules diffuse
preferentially in the direction of increasing temperature (from near the
white side to near the black side of the vanes). This "thermal creep"
causes a slight increase in pressure on the warmer (black) sides of the
vanes. If this force overcomes the static friction at the pivot and the air
resistance, the vanes turn in the direction of faced by the white sides.

For this to work, there must be some gas in the bulb or there would be no
thermal creep. Clearly, there is an optimum gas concentration. If it is too
low, the pressure caused by the creep would be too small. If it is too
high, the gas inertial resistance would tend to prevent the vanes from
readily turning. The gas viscosity affects the magnitude of the therml
creep, but is not a major variable because it is nearly independent of gas
pressure.

Otto
		*****************************************************
		Prof. Otto G. Raabe, Ph.D., CHP
              Institute of Toxicology & Environmental Health (ITEH)
		   (Street address: Building 3792, Old Davis Road)
		University of California, Davis, CA 95616
		Phone: 530-752-7754  FAX: 530-758-6140
		E-mail ograabe@ucdavis.edu
              *****************************************************
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