WEBVTT - How Does Crooke's Radiometer Work?

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<v Speaker 1>Welcome to brain stuff from how Stuff works. Hey, brain stuff,

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<v Speaker 1>it's Christian saga. Crooks Radiometer was one of the most

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<v Speaker 1>sensational toys of the eighteen seventies, but no one had

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<v Speaker 1>any idea how it worked, which caused much scientific debate.

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<v Speaker 1>In a two thousand four article in New Scientists, Andre

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<v Speaker 1>Larazza of the Naval post Graduate School in Monterey, California,

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<v Speaker 1>went so far to say this, even today, most physicists

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<v Speaker 1>think they know how it works, while few actually do. Well. Today,

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<v Speaker 1>we're not only going to explain how Crooks radiometer works,

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<v Speaker 1>but we're actually going to explain what it is. Okay,

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<v Speaker 1>you're ready. This is what it is. It's a glass

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<v Speaker 1>bulb with four veins suspended inside with a good but

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<v Speaker 1>not perfect vacuum. Each vein is blackened on one side

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<v Speaker 1>and silvered on the other. They are all attached to

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<v Speaker 1>a rotor, which is balanced on a vertical support that

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<v Speaker 1>turns with very little friction. When you shine light on

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<v Speaker 1>the veins, they spin. And it started out as a toy,

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<v Speaker 1>but it's now marketed as like a conversation piece, and

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<v Speaker 1>it's often called a light mill. Maybe you've seen one.

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<v Speaker 1>So how was this thing invented well. Even Krooks, the

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<v Speaker 1>guy who invented it, he didn't understand how it worked

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<v Speaker 1>when he built the first radiometer in eighteen seventy three,

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<v Speaker 1>and it was a byproduct of his chemical research. Came

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<v Speaker 1>about while he was weighing samples of thallium in a

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<v Speaker 1>vacuum chamber to reduce the effect of air currents. Krook

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<v Speaker 1>noticed when sunlight shone on the balance, his measurements were disturbed.

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<v Speaker 1>Warm samples appeared to weigh less than cold ones. Investigating further,

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<v Speaker 1>he discovered a black surface was repelled more strongly than

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<v Speaker 1>a silver one. Based on these findings, he built the

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<v Speaker 1>first radiometer, which became a popular toy and novelty item

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<v Speaker 1>that same year. Crooks suggested that the mill spun because

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<v Speaker 1>of quote, the pressure of light, and it was pushing

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<v Speaker 1>it like a water mill. But the light falling on

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<v Speaker 1>the black sides should have been absorbed, while the light

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<v Speaker 1>falling on the silver side would be reflected, causing the

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<v Speaker 1>radiation pressure to push on the silver side. But Crook's

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<v Speaker 1>radiometer was pushing on the black side, meaning it was

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<v Speaker 1>turning the wrong way. From his explanation, so how does

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<v Speaker 1>this thing actually work? All? Right? Here we go. The

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<v Speaker 1>vacuum in the radiometer is important. It has to be

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<v Speaker 1>just right for the spin to work. If there's no vacuum,

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<v Speaker 1>there's too much drag for the veins to move. If

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<v Speaker 1>there's a near perfect vacuum, the veins won't spin unless

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<v Speaker 1>they're held in place with the impediment of friction. But

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<v Speaker 1>if the veins have frictionless support from the rotor and

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<v Speaker 1>the vacuum is good but incomplete, then thermal transpiration takes

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<v Speaker 1>place and it looks like the light is pushing against

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<v Speaker 1>the black sides, but in fact the black side is

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<v Speaker 1>moving away from the light. Osborne Reynolds provided the correct

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<v Speaker 1>solution in eighteen seventy nine. He explained that thermal transpiration,

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<v Speaker 1>or I like to refer to it as thermal creep,

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<v Speaker 1>which some other people use, was the flow of gas

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<v Speaker 1>caused by a temperature difference on either side of the veins.

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<v Speaker 1>If gas is originally at the same pressure on each side,

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<v Speaker 1>it flows from the colder to the hotter side, resulting

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<v Speaker 1>in higher pressure on the hotter side. In the case

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<v Speaker 1>of the veins, the faster molecules from the warm side

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<v Speaker 1>strike their edge, imparting more force than the cold molecules,

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<v Speaker 1>and moving the vein away from the warmer gas. Check

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<v Speaker 1>out the brain Stuft channel on YouTube, and from around

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<v Speaker 1>this and thousands of other topics, visit how stuff works

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<v Speaker 1>dot com.