Crookes radiometer Crookes paddlewheel Chinatown radiometers Commentary

William Crookes fluid dynamics

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.