克鲁克斯辐射计的工作原理

克鲁克斯辐射计的工作原理

克鲁克斯辐射计包括一个玻璃泡，里面悬挂着四个叶片。玻璃泡内为高度真空。当您将光线照射到辐射计内的叶片时，它们就会旋转起来——在明亮的太阳照射下，叶片的转速可达每分钟数千转！
玻璃泡内的真空程度是辐射计能否正常工作的关键。如果泡内不是真空（也就是说里面充满了空气），叶片会由于阻力过大而无法转动。如果泡内接近完全真空，叶片也不会旋转（除非叶片是以无摩擦方式固定的）。如果支持叶片的装置之间没有摩擦力，而玻璃泡内又为完全真空，则从叶片的银色面弹出的光子会推动叶片转动。但这一推力仍非常微弱。
如果玻璃泡内为高度真空而非完全真空，那么在叶片的边缘会发生另一种称为热发散的效应，这种效应使我们看到的旋转就象是光在推动叶片的黑色面。叶片的黑色面会沿远离光的方向运动。
看下这篇文章嘛，或许对你有帮助
http://www.jys.edu.cn/shuzihua/20071010101350.asp

网上搜到的一段有关的文字。说是因为腔内的气体的一种叫做Knudsen Force的作用使得叶片转动的。具体的我也不太清楚。
We have all seen the Crookes Radiometer at some point. Shine a light on it and it spins. However, the most common misconception is that this is due to the radiation force of the light. If you look at how the radiometer spins, then if this were the case it is going backwards! In fact, the light only imparts motion onto the device indirectly, by heating it. It is the gas in the chamber that actually moves the vanes.
This only works, though, if the chamber is under partial vacuum. Bring the radiometer up to atmospheric pressure and nothing happens. This is because the force that moves the vanes, known as a Knudsen Force, depends on the mean free path of the gas, or how far the molecules of gas can go before hitting other molecules. At atmospheric pressure, this is clearly a shorter distance than under partial vacuum.
Even under vacuum, the force is tiny. If you look at Knudsen forces, it turns out there are some inefficient things about the design of the radiometer's vanes. First, the force only acts on the source within one mean free path of the edge. In atmosphere, this is about 70 nm. By evacuating the chamber, we increase the area involved, and thus the force. Another issue is the surface. If only 70 nm are doing anything, and the rest is dead weight, then it will take most of the force just to move the vane. However, since only the edge is involved and the rest isn't doing anything, why not make more edges? By using a holey vane rather than a solid one, we can get many times the force. Lastly, it turns out that you also want the thickness to be about the mean free path too.