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