"Crookes Radiometer" Designed by Sir William Crookes.
A new explanation, by Hon. Professor Leonard Shaw.
'Radiometer' glass bulbs can often be seen in shop windows where the rapid movement of the vanes inside them attracts the viewers attention. The purpose of the 'Radiometer' was to observe any force of light..
Inside the bulbs, are four vanes, free to rotate, mounted on a needle point. Each vane is black on one side to capture light with silver behind to reflect it. The glass bulb is evacuated and sealed in a similar way to electric light bulbs.
One of my 'Radiometers' without the seal, was placed in my vacuum equipment for observation and the light of a lamp was directed upon it. In room air there was no movement of the vanes, but in a moderate vacuum, the electro magnetic radiation from the lamp was captured by the black vanes, and reflected from the silver reverse. As the black vanes became more warm, they quickly rotated away from the lamp due to the increased Brownian motion of adjacent air molecules. There was a continuous heat difference due to the light as the black vanes continuously captured the electro magnetic radiation of direct light from the lamp and the silver vanes reflected this. When a good hard vacuum was obtained, probably better than that available when the 'Radiometer' was designed many years ago, there was no rotation.
Summary
Observation was made during evacuation of the Radiometer from room air to a hard vacuum. Fast rotation of the vanes depended on the degree of vacuum. No force from the electro magnetic radiation provided by the direct lamp light, was observed when the vanes were in a hard vacuum.
The original Radiometer can be seen, at The Royal Institution, 21 Albermarle Street, London, W15 4BS where all are welcome.
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