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Speaker 1: Brand with today's question, how does a Crooks radiometer work?
Speaker 1: A Crooks radiometer has four veins suspended inside a glass bulb.
Speaker 1: The veins are black on one side and silver on
Speaker 1: the other. Inside the bulb, there's a good vacuum. When
Speaker 1: you shine a bright light on the veins in a radiometer,
Speaker 1: they spin. In bright sunlight, they can spin at several
Speaker 1: thousand rotations per minute. The vacuum is important to the
Speaker 1: radiometer's success. If there's no vacuum, that is, if the
Speaker 1: bulb is full of air, the veins don't spin because
Speaker 1: there's too much drag. If there's a near perfect vacuum,
Speaker 1: the veins do not spin unless they are held in
Speaker 1: an absolutely frictionless way. If the veins have a frictionless
Speaker 1: support and the vacuum is complete, then photons bouncing off
Speaker 1: the silver side of the veins push the veins, causing
Speaker 1: them to rotate. However, this force is exceedingly small. If
Speaker 1: there's a good but incomplete vacuum, then a different effect
Speaker 1: called thermal transporation occurs along the edges of the veins.
Speaker 1: The effect looks as though the light is pushing against
Speaker 1: the black faces. The black side of the veins moves
Speaker 1: away from the light. For more on this and thousands
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