RE: Crooke's Radiometer

[Glen.Vickers@ucm.com]

I can see how it is called a radiometer.  It does detect the presence of an
external field.  It's just that the movement is based more on thermal energy
rather than momentum and particle interaction.  Look at the fiber in a
pencil dosimeter, the primary movement is due to the static charge in the
pencil, the radiation is just the cause of the change in static charge.  I
think they're somewhat analogous.

glen

glen.vickers@ucm.com

	-----Original Message-----
	From:	Bernard L Cohen [SMTP:blc+@pitt.edu]
	Sent:	Tuesday, April 06, 1999 9:29 AM
	To:	Multiple recipients of list
	Subject:	Re: Crooke's Radiometer

		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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