WEBVTT - BrainStuff Classics: How Does Crooke's Radiometer Work?

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<v Speaker 1>Welcome to brain Stuff production of iHeart Radio. Hey brain Stuff,

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<v Speaker 1>Lauren bog Obam here. Today's episode is a classic from

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<v Speaker 1>our erstwhile host, Christian Sager. In this one, we break

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<v Speaker 1>down the surprising inner workings of an old conversation piece,

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<v Speaker 1>Crooks Radiometer, which is one of those glass thingies with

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<v Speaker 1>the little black and white sided spinny things inside. I'll

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<v Speaker 1>let Christian explain, Hey brain Stuff, it's Christian Sager. Crooks

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<v Speaker 1>Radiometer was one of the most sensational toys of the

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<v Speaker 1>eighteen seventies, but no one had any idea how it worked,

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<v Speaker 1>which caused much scientific debate. In a two thousand four

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<v Speaker 1>article in New Scientists, Andre Larazza of the Naval post

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<v Speaker 1>Graduate School in Monterey, California, went so far to say this,

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<v Speaker 1>even today, most physicists think they know how it works,

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<v Speaker 1>while few actually do. Well. Today, we're not only going

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<v Speaker 1>to explain how Crooks Radiometer works, but we're actually going

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<v Speaker 1>to explain what it is. Okay, you're ready, this is

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<v Speaker 1>what it is. It's a glass bulb with four veins

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<v Speaker 1>suspended inside with a good but not perfect vacuum. Each

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<v Speaker 1>vein is blackened on one side and silvered on the

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<v Speaker 1>other They are all attached to a rotor which is

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<v Speaker 1>balanced on a vertical support that turns with very little friction.

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<v Speaker 1>When you shine light on the veins, they spin. And

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<v Speaker 1>it started out as a toy, but it's now marketed

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<v Speaker 1>as like a conversation piece, and it's often called a

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<v Speaker 1>light mill. Maybe you've seen one. So how is this

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<v Speaker 1>thing invented? Well? Even Crooks, the guy who invented it,

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<v Speaker 1>he didn't understand how it worked when he built the

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<v Speaker 1>first radiometer in eighteen seventy three, and it was a

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<v Speaker 1>byproduct of his chemical research. Came about while he was

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<v Speaker 1>weighing samples of thallium in a vacuum chamber to deduce

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<v Speaker 1>the effect of air currents. Krook noticed when sunlight shone

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<v Speaker 1>on the balance, his measurements were disturbed. Warm samples appeared

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<v Speaker 1>to weigh less than cold ones. Investigating further, he discovered

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<v Speaker 1>a black surface was repelled more strongly than a silver one.

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<v Speaker 1>Based on these findings, he built the first radiometer, which

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<v Speaker 1>became a popular toy and novelty item that same year.

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<v Speaker 1>Krooks suggested that the mill spun because of quote the

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<v Speaker 1>pressure of light, and it was pushing it like a

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<v Speaker 1>water mill. But the light falling on the black side

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<v Speaker 1>should have been absorbed, while the light falling on the

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<v Speaker 1>silver side would be reflected, causing the radiation pressure to

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<v Speaker 1>push on the silver side. But Krook's radiometer was pushing

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<v Speaker 1>on the black side, meaning it was turning the wrong

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<v Speaker 1>way from his explanation, So how does this thing actually work? All? Right,

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<v Speaker 1>here we go. The vacuum in the radiometer is important.

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<v Speaker 1>It has to be just right for the spin to work.

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<v Speaker 1>If there's no vacuum, there's too much drag for the

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<v Speaker 1>veins to move. If there's a near perfect vacuum, the

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<v Speaker 1>veins won't spin unless they're held in place with the

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<v Speaker 1>impediment of friction. But if the veins have frictionless support

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<v Speaker 1>from the rotor and the vacuum is good but incomplete,

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<v Speaker 1>then thermal transpiration takes place and it looks like the

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<v Speaker 1>light is pushing against the black sides, but in fact

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<v Speaker 1>the black side is moving away from the light. Osborne

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<v Speaker 1>Reynolds provided the correct solution in eighteen seventy nine. He

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<v Speaker 1>explained that thermal transpiration, or I like to refer to

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<v Speaker 1>it as thermal creep which some other people use was

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<v Speaker 1>the flow of gas caused by a temperature difference on

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<v Speaker 1>either side of the veins. If gas is originally at

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<v Speaker 1>the same pressure on each side, it flows from the

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<v Speaker 1>colder to the hotter side, resulting in higher pressure on

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<v Speaker 1>the hotter side. In the case of the veins, the

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<v Speaker 1>faster molecules from the warm side strike their edge, imparting

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<v Speaker 1>more force than the cold molecules and moving the vein

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<v Speaker 1>away from the warmer guests. Today's episode was written by

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<v Speaker 1>Christian and produced by Tyler Clain. Brain Stuff is production

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<v Speaker 1>of I Heart Radio's How Stuff Works. For more on

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<v Speaker 1>this and lots of other topics, visit our home planet

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