Illustration of Crookes radiometer

1873 Crookes radiometer

The book of science

Tom Sharp

William Crookes fluid dynamics Illustration of Crookes radiometer

Crookes radiometer

Weighing chemical samples on a delicate scale in a partially evacuated chamber, William Crookes noticed that sunlight seemed to disturb the balance. So he built a device to demonstrate the effect, a small set of vanes mounted on a spindle in a glass bulb with a partial vacuum. The vanes of the spindle are painted black on one side and silver on the other, and rotate when exposed to light.

Crookes paddlewheel

William Crookes put a paddlewheel in one of his Crookes tubes a test to determine whether cathode rays were particles or waves thinking if the rays were particles then their momentum would turn the paddlewheel. The rays turned the paddlewheel but not because they were particles. The rotation effect of both radiometer and paddlewheel disappeared in a full vacuum. Only later was the effect explained as turbulence caused by differential heating.

Chinatown radiometers

Radiometers and drinking birds in the windows of shops along Grant Avenue attracted my attention. I had spending money and could explain it later. I looked closely, but never bought any. The pressure of sunlight was only one possibility.

The correct explanation for Crookes’ rotating vanes is thermal transpiration, as first proposed by Osborne Reynolds, but more on fluid dynamics later.

See also in The book of science:

Readings in wikipedia: