Re: Crooke's Radiometer

[Bernard L Cohen <blc+@pitt.edu>]

	Why is it called a "radiometer" if radiation is not involved? I
apologize for the explanation I gave based on radiation being the
controlling effect. I am quite sure I have seen a radiometer in which that
was the case; perhaps it was under vacuum. I don't remember it being
called a "Crookes" radiometer; that was about 50 years ago. It would be
interesting to pump the air out of a Crookes radiometer and see the
direction of rotation change as the air is removed.

Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu


On Fri, 2 Apr 1999, Otto G. Raabe wrote:

> 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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