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Speaker 1: Welcome to brain Stuff production of iHeart Radio. Hey brain Stuff,

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Speaker 1: Lauren bog Obam here. Today's episode is a classic from

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Speaker 1: our erstwhile host, Christian Sager. In this one, we break

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Speaker 1: down the surprising inner workings of an old conversation piece,

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Speaker 1: Crooks Radiometer, which is one of those glass thingies with

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Speaker 1: the little black and white sided spinny things inside. I'll

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Speaker 1: let Christian explain, Hey brain Stuff, it's Christian Sager. Crooks

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Speaker 1: Radiometer was one of the most sensational toys of the

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Speaker 1: eighteen seventies, but no one had any idea how it worked,

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Speaker 1: which caused much scientific debate. In a two thousand four

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Speaker 1: article in New Scientists, Andre Larazza of the Naval post

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Speaker 1: Graduate School in Monterey, California, went so far to say this,

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Speaker 1: even today, most physicists think they know how it works,

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

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Speaker 1: to explain how Crooks Radiometer works, but we're actually going

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Speaker 1: to explain what it is. Okay, you're ready, this is

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Speaker 1: what it is. It's a glass bulb with four veins

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Speaker 1: suspended inside with a good but not perfect vacuum. Each

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Speaker 1: vein is blackened on one side and silvered on the

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Speaker 1: other They are all attached to a rotor which is

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Speaker 1: balanced on a vertical support that turns with very little friction.

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Speaker 1: When you shine light on the veins, they spin. And

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Speaker 1: it started out as a toy, but it's now marketed

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Speaker 1: as like a conversation piece, and it's often called a

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Speaker 1: light mill. Maybe you've seen one. So how is this

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Speaker 1: thing invented? Well? Even Crooks, the guy who invented it,

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Speaker 1: he didn't understand how it worked when he built the

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Speaker 1: first radiometer in eighteen seventy three, and it was a

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Speaker 1: byproduct of his chemical research. Came about while he was

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Speaker 1: weighing samples of thallium in a vacuum chamber to deduce

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Speaker 1: the effect of air currents. Krook noticed when sunlight shone

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Speaker 1: on the balance, his measurements were disturbed. Warm samples appeared

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Speaker 1: to weigh less than cold ones. Investigating further, he discovered

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Speaker 1: a black surface was repelled more strongly than a silver one.

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Speaker 1: Based on these findings, he built the first radiometer, which

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Speaker 1: became a popular toy and novelty item that same year.

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Speaker 1: Krooks suggested that the mill spun because of quote the

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Speaker 1: pressure of light, and it was pushing it like a

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Speaker 1: water mill. But the light falling on the black side

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Speaker 1: should have been absorbed, while the light falling on the

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Speaker 1: silver side would be reflected, causing the radiation pressure to

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Speaker 1: push on the silver side. But Krook's radiometer was pushing

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Speaker 1: on the black side, meaning it was turning the wrong

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

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Speaker 1: here we go. The vacuum in the radiometer is important.

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Speaker 1: It has to be just right for the spin to work.

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Speaker 1: If there's no vacuum, there's too much drag for the

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

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Speaker 1: veins won't spin unless they're held in place with the

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Speaker 1: impediment of friction. But if the veins have frictionless support

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Speaker 1: from the rotor and the vacuum is good but incomplete,

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Speaker 1: then thermal transpiration takes place and it looks like the

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Speaker 1: light is pushing against the black sides, but in fact

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Speaker 1: the black side is moving away from the light. Osborne

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Speaker 1: Reynolds provided the correct solution in eighteen seventy nine. He

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Speaker 1: explained that thermal transpiration, or I like to refer to

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Speaker 1: it as thermal creep which some other people use was

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Speaker 1: the flow of gas caused by a temperature difference on

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Speaker 1: either side of the veins. If gas is originally at

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Speaker 1: the same pressure on each side, it flows from the

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Speaker 1: colder to the hotter side, resulting in higher pressure on

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Speaker 1: the hotter side. In the case of the veins, the

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Speaker 1: faster molecules from the warm side strike their edge, imparting

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Speaker 1: more force than the cold molecules and moving the vein

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Speaker 1: away from the warmer guests. Today's episode was written by

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Speaker 1: Christian and produced by Tyler Clain. Brain Stuff is production

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Speaker 1: of I Heart Radio's How Stuff Works. For more on

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Speaker 1: this and lots of other topics, visit our home planet

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