RE: Crooke's Radiometer

["Frame, Paul" <FRAMEP@ORAU.GOV>]

The origin of the device ultimately lies with an observation by Crookes that
the measured weights of some thallium samples depended on the pressure
inside the balance he was using(for accuracy he compared the measured
weights in air and in a vacuum) and on the temperature of the sample.  Later
(1873) he observed that a pipe would attract a weight inside an evacuated
tube if the pipe were colder and repel the weight if the pipe were hotter.
He then did the same thing with suspended pith balls. Crookes called what he
believed was a new force at work as the x-force.  Among other things, he
thought this phenomenon explained why the tail of a comet pointed away from
the sun.

He developed what we consider the radiometer (a glass bulb with the rotating
vanes painted black and white) in 1874 and called it a "Light Mill" He
believed that light would transfer momentum directly to an illuminated
object and that the phenomenon did not involve residual gas in the evacuated
bulb in which the vanes rotated.

When his explanation was proven wrong, he renamed the device the radiometer.
The latter was a poorly chosen name since the instrument measured nothing
(although as an earlier post pointed out it was a precursor to various
measuring devices). Crookes also used the terms otheroscope and elaunoscope
to describe variations on the radiometer. He actually wore one miniature
version as a scarf pin.

All of this comes from "The infancy of atomic physics:Hercules in his
cradle" (absolutely first rate IMHO)by Alex Keller, Clarendon Press, Oxford
1983.

Paul Frame
Professional Training programs
ORAU
framep@orau.gov
http://www.orau.gov/ptp/ptp.htm
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