Date: Fri, 28 Oct 2005 12:10:09 -0400 (EDT)

Author: Richard Berg

Subject: Re: radiometer


Ladies and Gentlemen,

As I recall, an old (B&W, 23 min, PSSC?) film called PRESSURE OF LIGHT
showed this effect. At VERY low pressures they were able to remove
enough friction so that a beam of light could make the radiometer go
around with the WHITE side trailing (opposite from the normal radiometer

Such low pressure is not the norm for a standard radiometer, and it is
very difficult to pump one out to the requisite level. However, if you
could reduce the pressure and the friction sufficiently, it should rotate
as expected from the momentum and collisions of photons.


On Fri, 28 Oct 2005, Jerry Hester wrote:

> What if one took one of the levitating magnets between carbon. Glue small
> wings on the magnet. Place the apparatus in a Bell jar?
> You should then have a small mass, low inertia, and essentially zero
> friction.
> You would probably still need a powerful light source, but ......
> Jerry H.
> At 04:45 PM 10/26/2005 -0400, you wrote:
>> I was thinking about breaking the vacuum and placing the whole thing in a
>> bell jar.
>> Sam
>> wrote:
>>> Sam: I think you need more momentum when it is evacuated. After all,
>>> since it is heat that propels the "wrong" direction motion, half as much
>>> heat (other side only) is not equivalent to reversing direction.
>>> In the "momentum of electron paddle wheel" demo I used a 10 KV power
>>> supply to "arc" into the wheel to get it rolling. Even at that potential,
>>> it was sluggish. My apparatus was an old Leybold device with a lot of
>>> friction, I have not seen a pumpable version of the radiometer. I would
>>> expect it to be moderately dangerous due to the thinness of the glass
>>> envelope. Karl
>>> Quoting sampere :
>>>> Anybody evacuate an electrometer and shine light on it to show the
>>>> momentum of light? Is this possible with these simple radiometers, or do
>>>> you need more elaborate apparatus?
>>>> Thanks,
>>>> Sam

Dr. Richard E. Berg, Professor of the Practice
Director, Physics Lecture-Demonstration Facility
U.S. mail address:
Department of Physics
University of Maryland
College Park, MD 20742-4111
Phone: (301) 405-5994
FAX: (301) 314-9525
From Fri Oct 28 13:04:37 2005