1 00:00:02,040 --> 00:00:08,240 Speaker 1: Welcome to brain stuff from how Stuff works. Hey, brain stuff, 2 00:00:08,240 --> 00:00:11,880 Speaker 1: it's Christian saga. Crooks Radiometer was one of the most 3 00:00:12,160 --> 00:00:16,439 Speaker 1: sensational toys of the eighteen seventies, but no one had 4 00:00:16,480 --> 00:00:20,079 Speaker 1: any idea how it worked, which caused much scientific debate. 5 00:00:20,400 --> 00:00:23,880 Speaker 1: In a two thousand four article in New Scientists, Andre 6 00:00:24,120 --> 00:00:28,520 Speaker 1: Larazza of the Naval post Graduate School in Monterey, California, 7 00:00:28,640 --> 00:00:33,600 Speaker 1: went so far to say this, even today, most physicists 8 00:00:33,640 --> 00:00:38,519 Speaker 1: think they know how it works, while few actually do. Well. Today, 9 00:00:38,600 --> 00:00:41,959 Speaker 1: we're not only going to explain how Crooks radiometer works, 10 00:00:42,280 --> 00:00:45,720 Speaker 1: but we're actually going to explain what it is. Okay, 11 00:00:45,880 --> 00:00:48,159 Speaker 1: you're ready. This is what it is. It's a glass 12 00:00:48,320 --> 00:00:52,800 Speaker 1: bulb with four veins suspended inside with a good but 13 00:00:53,080 --> 00:00:58,240 Speaker 1: not perfect vacuum. Each vein is blackened on one side 14 00:00:58,520 --> 00:01:01,560 Speaker 1: and silvered on the other. They are all attached to 15 00:01:01,680 --> 00:01:04,720 Speaker 1: a rotor, which is balanced on a vertical support that 16 00:01:04,880 --> 00:01:08,560 Speaker 1: turns with very little friction. When you shine light on 17 00:01:08,640 --> 00:01:12,279 Speaker 1: the veins, they spin. And it started out as a toy, 18 00:01:12,560 --> 00:01:15,600 Speaker 1: but it's now marketed as like a conversation piece, and 19 00:01:15,600 --> 00:01:19,000 Speaker 1: it's often called a light mill. Maybe you've seen one. 20 00:01:19,800 --> 00:01:23,399 Speaker 1: So how was this thing invented well. Even Krooks, the 21 00:01:23,400 --> 00:01:26,640 Speaker 1: guy who invented it, he didn't understand how it worked 22 00:01:26,640 --> 00:01:30,000 Speaker 1: when he built the first radiometer in eighteen seventy three, 23 00:01:30,280 --> 00:01:33,360 Speaker 1: and it was a byproduct of his chemical research. Came 24 00:01:33,360 --> 00:01:36,240 Speaker 1: about while he was weighing samples of thallium in a 25 00:01:36,319 --> 00:01:40,559 Speaker 1: vacuum chamber to reduce the effect of air currents. Krook 26 00:01:40,640 --> 00:01:44,880 Speaker 1: noticed when sunlight shone on the balance, his measurements were disturbed. 27 00:01:45,440 --> 00:01:50,240 Speaker 1: Warm samples appeared to weigh less than cold ones. Investigating further, 28 00:01:50,320 --> 00:01:53,800 Speaker 1: he discovered a black surface was repelled more strongly than 29 00:01:53,840 --> 00:01:56,720 Speaker 1: a silver one. Based on these findings, he built the 30 00:01:56,760 --> 00:02:01,280 Speaker 1: first radiometer, which became a popular toy and novelty item 31 00:02:01,320 --> 00:02:05,000 Speaker 1: that same year. Crooks suggested that the mill spun because 32 00:02:05,040 --> 00:02:08,359 Speaker 1: of quote, the pressure of light, and it was pushing 33 00:02:08,400 --> 00:02:11,240 Speaker 1: it like a water mill. But the light falling on 34 00:02:11,320 --> 00:02:14,120 Speaker 1: the black sides should have been absorbed, while the light 35 00:02:14,160 --> 00:02:17,680 Speaker 1: falling on the silver side would be reflected, causing the 36 00:02:17,880 --> 00:02:21,960 Speaker 1: radiation pressure to push on the silver side. But Crook's 37 00:02:22,040 --> 00:02:25,920 Speaker 1: radiometer was pushing on the black side, meaning it was 38 00:02:25,960 --> 00:02:29,200 Speaker 1: turning the wrong way. From his explanation, so how does 39 00:02:29,200 --> 00:02:32,640 Speaker 1: this thing actually work? All? Right? Here we go. The 40 00:02:32,800 --> 00:02:36,240 Speaker 1: vacuum in the radiometer is important. It has to be 41 00:02:36,840 --> 00:02:40,160 Speaker 1: just right for the spin to work. If there's no vacuum, 42 00:02:40,400 --> 00:02:43,240 Speaker 1: there's too much drag for the veins to move. If 43 00:02:43,240 --> 00:02:47,640 Speaker 1: there's a near perfect vacuum, the veins won't spin unless 44 00:02:47,680 --> 00:02:50,920 Speaker 1: they're held in place with the impediment of friction. But 45 00:02:51,600 --> 00:02:55,480 Speaker 1: if the veins have frictionless support from the rotor and 46 00:02:55,880 --> 00:03:01,320 Speaker 1: the vacuum is good but incomplete, then thermal transpiration takes 47 00:03:01,360 --> 00:03:05,119 Speaker 1: place and it looks like the light is pushing against 48 00:03:05,240 --> 00:03:08,120 Speaker 1: the black sides, but in fact the black side is 49 00:03:08,200 --> 00:03:12,920 Speaker 1: moving away from the light. Osborne Reynolds provided the correct 50 00:03:12,960 --> 00:03:17,960 Speaker 1: solution in eighteen seventy nine. He explained that thermal transpiration, 51 00:03:18,160 --> 00:03:20,680 Speaker 1: or I like to refer to it as thermal creep, 52 00:03:20,840 --> 00:03:24,160 Speaker 1: which some other people use, was the flow of gas 53 00:03:24,200 --> 00:03:27,880 Speaker 1: caused by a temperature difference on either side of the veins. 54 00:03:28,280 --> 00:03:32,160 Speaker 1: If gas is originally at the same pressure on each side, 55 00:03:32,680 --> 00:03:36,280 Speaker 1: it flows from the colder to the hotter side, resulting 56 00:03:36,560 --> 00:03:40,280 Speaker 1: in higher pressure on the hotter side. In the case 57 00:03:40,320 --> 00:03:43,920 Speaker 1: of the veins, the faster molecules from the warm side 58 00:03:44,280 --> 00:03:48,760 Speaker 1: strike their edge, imparting more force than the cold molecules, 59 00:03:48,840 --> 00:03:57,960 Speaker 1: and moving the vein away from the warmer gas. Check 60 00:03:58,000 --> 00:04:00,280 Speaker 1: out the brain Stuft channel on YouTube, and from around 61 00:04:00,280 --> 00:04:02,920 Speaker 1: this and thousands of other topics, visit how stuff works 62 00:04:03,040 --> 00:04:17,120 Speaker 1: dot com.