Denny wrote:
On the other hand, the absence of porous plates in operating
radiometers tends to cast some doubt on your claim that the plates
must be porous.

Not "my claim," my report. The claim they must be porous arrives
through the math necessary to balance the kinetic forces.

Again, which radiometer? If you are arguing a "perfect" vacuum, then

like a free lunch, I would agree there's no such thing. The Crookes
radiometer requires a partial atmosphere to work, other radiometers
work quite fine with much less.

Depends entirely on what one intends to measure.


Despite the photon torpedoes fired at me, I have not seen a convincing
physics experiment that deflates my previous arguement...

Where the F=MA arguement fails in a radiometer is that the photons
impact both sides of the paddles leaving a zero net force for
rotation...

The fact that a Crookes Radiometer requires an atmosphere is proof of
its mode of operation. The fact that it has to be a partial vacuum
further proves how it operates (more air density means too much air
drag to allow rotation by the weak local differential pressure across
the paddle)...

Those who reject local differential pressure changes due to local
heating by claiming the pressure in the bulb is static ignore the
factor of time in molecular exchange of thermal energy gains...
Carrying their argument to the logical end means sun heating cannot
cause the winds to ever blow across the ground because the net air
pressure of Terra is static...

denny
It's 10PM somewhere, have you hugged your radio today?

Hi Denny -

The thing that seems to have certain people confused here is the fact
that, with regard to radiometers, there are two different effects at
work. Radiation pressure is in fact quite measurable, but is an
orders of magnitude smaller force than the thermal/molecular effect
that toy store radiometers demonstrate.

Radiation pressure is such a small effect that it cannot be observed
unless the vessel is first evacuated to an ultra high vacuum.
Ordinarily, one would use a torsion or micro balance to measure this
effect. But in a radiometer type arrangement, the vanes rotate in a
direction away from the more reflective side because the change in
momentum is twice as high for a reflected photon as it is for an
absorbed photon.

But in the case of the thermal, partially evacuated (toy store)
radiometer, the black side of the vanes absorbs more thermal energy
and is therefore hotter than the white side which absorbs less and
reflects more energy. Gas molecules which encounter the vanes in a
partially evacuated radiometer are ejected more energetically from the
hotter side than from the cooler side thus creating a net force in the
direction away from the black (less reflective) side. This is as you
described.

73, ac6xg