Speaker 1: Welcome to brain stuff from how Stuff works. Hey, brain stuff,
Speaker 1: it's Christian saga. Crooks Radiometer was one of the most
Speaker 1: sensational toys of the eighteen seventies, but no one had
Speaker 1: any idea how it worked, which caused much scientific debate.
Speaker 1: In a two thousand four article in New Scientists, Andre
Speaker 1: Larazza of the Naval post Graduate School in Monterey, California,
Speaker 1: went so far to say this, even today, most physicists
Speaker 1: think they know how it works, while few actually do. Well. Today,
Speaker 1: we're not only going to explain how Crooks radiometer works,
Speaker 1: but we're actually going to explain what it is. Okay,
Speaker 1: you're ready. This is what it is. It's a glass
Speaker 1: bulb with four veins suspended inside with a good but
Speaker 1: not perfect vacuum. Each vein is blackened on one side
Speaker 1: and silvered on the other. They are all attached to
Speaker 1: a rotor, which is balanced on a vertical support that
Speaker 1: turns with very little friction. When you shine light on
Speaker 1: the veins, they spin. And it started out as a toy,
Speaker 1: but it's now marketed as like a conversation piece, and
Speaker 1: it's often called a light mill. Maybe you've seen one.
Speaker 1: So how was this thing invented well. Even Krooks, the
Speaker 1: guy who invented it, he didn't understand how it worked
Speaker 1: when he built the first radiometer in eighteen seventy three,
Speaker 1: and it was a byproduct of his chemical research. Came
Speaker 1: about while he was weighing samples of thallium in a
Speaker 1: vacuum chamber to reduce the effect of air currents. Krook
Speaker 1: noticed when sunlight shone on the balance, his measurements were disturbed.
Speaker 1: Warm samples appeared to weigh less than cold ones. Investigating further,
Speaker 1: he discovered a black surface was repelled more strongly than
Speaker 1: a silver one. Based on these findings, he built the
Speaker 1: first radiometer, which became a popular toy and novelty item
Speaker 1: that same year. Crooks suggested that the mill spun because
Speaker 1: of quote, the pressure of light, and it was pushing
Speaker 1: it like a water mill. But the light falling on
Speaker 1: the black sides should have been absorbed, while the light
Speaker 1: falling on the silver side would be reflected, causing the
Speaker 1: radiation pressure to push on the silver side. But Crook's
Speaker 1: radiometer was pushing on the black side, meaning it was
Speaker 1: turning the wrong way. From his explanation, so how does
Speaker 1: this thing actually work? All? Right? Here we go. The
Speaker 1: vacuum in the radiometer is important. It has to be
Speaker 1: just right for the spin to work. If there's no vacuum,
Speaker 1: there's too much drag for the veins to move. If
Speaker 1: there's a near perfect vacuum, the veins won't spin unless
Speaker 1: they're held in place with the impediment of friction. But
Speaker 1: if the veins have frictionless support from the rotor and
Speaker 1: the vacuum is good but incomplete, then thermal transpiration takes
Speaker 1: place and it looks like the light is pushing against
Speaker 1: the black sides, but in fact the black side is
Speaker 1: moving away from the light. Osborne Reynolds provided the correct
Speaker 1: solution in eighteen seventy nine. He explained that thermal transpiration,
Speaker 1: or I like to refer to it as thermal creep,
Speaker 1: which some other people use, was the flow of gas
Speaker 1: caused by a temperature difference on either side of the veins.
Speaker 1: If gas is originally at the same pressure on each side,
Speaker 1: it flows from the colder to the hotter side, resulting
Speaker 1: in higher pressure on the hotter side. In the case
Speaker 1: of the veins, the faster molecules from the warm side
Speaker 1: strike their edge, imparting more force than the cold molecules,
Speaker 1: and moving the vein away from the warmer gas. Check
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Speaker 1: dot com.