Re: Crooke's Radiometer

[Thomas Gleich Harrison <tomh@jove.acs.unt.edu>]

I know that as a class demo the intensity and the energy distribution
affects the speed of vane rotation. For example, you can take a Kodak
Carousel projector and either change the bulb luminence(a switch)  or back
the projector off or closer in and affect the speed of rotation.  Also,
you can put the projector downstream of a AC potentimeter and thereby
change the spectral energy distribution of the radiation i.e.  Planck
function, of the light being emitted, and it too will affect the speed
albeit you also change the bulb luminence by adjusting the bulb
temperature(Stefan-Boltzman law). So, photon flux is a key factor, i.e.
energy density, ..... and in any case, the students love it.
But to demonstrate photon momentum, at ths point I caste my ballot for the
Compton Effect !

Tom Harrison Ph.D., RSO
Engineering Physics
University of North Texas
Denton, TX


On Tue, 6 Apr 1999, J. Andrew Tompkins wrote:

> As I recall the speed of rotation of the vane assembly is somewhat
> proportional to the infrared energy deposited on the vanes, and thus could
> give a measure of the radiant energy deposited.
> 
> Andy Tompkins
> Woodstock, GA
> 
> jatalbq@mindspring.com
> 
> 
> >On Tue, 6 Apr 1999, Otto G. Raabe wrote:
> >
> >> April 6, 1999
> >> Davis, CA
> >> 
> >> Maybe it is called a radiometer because (1) it responds to electromagnetic
> >> radiation in the form of light and especially infrared radiation, and (2)
> >> it was named before the discovery of ionizing radiation.
> >
> >	-I would bet that it was meant as a demonstration of radiation
> >pressure, the fact that radiation has momentum, and sometime later
> >somebody noticed that the rotation was in the wrong direction for that
> >explanation.
> >
> >> 
> >Bernard L. Cohen
> >Physics Dept.
> >University of Pittsburgh
> >Pittsburgh, PA 15260
> >Tel: (412)624-9245
> >Fax: (412)624-9163
> >e-mail: blc+@pitt.edu
> >
> >
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