RE: Crooke's Radiometer

[Glen.Vickers@ucm.com]

I haven't followed this thread to closely, but I believe this is commonly
described in physics textbooks.  If this is the little dealy with the black
and white sided vanes that turn when exposed to light, I believe it does
indeed work on the principles of differential thermal absorption by the
different colors and increased gas pressure on the black or warmer side.  

Obviously a physics professor would make students postulate how it works
before telling them...

Sincerely,
Glen
glen.vickers@ucm.com

	-----Original Message-----
	From:	Otto G. Raabe [SMTP:ograabe@ucdavis.edu]
	Sent:	Friday, April 02, 1999 6:50 PM
	To:	Multiple recipients of list
	Subject:	Re: Crooke's Radiometer

	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
	              *****************************************************
	
************************************************************************
	The RADSAFE Frequently Asked Questions list, archives and
subscription
	information can be accessed at
http://www.ehs.uiuc.edu/~rad/radsafe.html
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html