[Ben Krasnow] Shows Us How A Crookes Radiometer Works

[Ben Krasnow] is tackling the curious Crookes Radiometer on his Applied Science YouTube channel. The Crookes Radiometer, a staple of museum gift shops everywhere, is a rather simple device. A rotor with black and white vanes rotates on the head of a needle. The entire assembly is inside a glass envelope. The area inside the glass is not at a hard vacuum, nor is it filled with some strange gas. The radiometer only works when there is a partial vacuum inside.

The radiometer’s method of operation was long misunderstood. Sir William Crookes and James Clerk Maxwell both believed that the vanes moved due to the pressure of the photons hitting the vanes. If that were true though, the radiometer would spin in the opposite direction it normally does when held near a light source.  It was eventually discovered that the system is a thermodynamic one. [Ben] proves this by cooling down the radiometer’s glass with a can of freeze spray.  The radiometer immediately begins spinning backwards, with no light source present.

From there [Ben] mounts the rotor of a radiometer inside his vacuum chamber, which many will recognize as the chamber from his DIY electron microscope. As expected, the vanes don’t spin at a hard vacuum. In fact, [Ben] find the vanes spin fastest when the pressure is about 7 mTorr.

21 thoughts on “[Ben Krasnow] Shows Us How A Crookes Radiometer Works

  1. I remember one of these sitting on the windowsill of my second grade classroom. I spent many hours staring at it and trying to work out how it worked instead of paying attention to the teacher.

    It’s funny how something so simple can leave an impression that has lasted well over 30 years and counting.

    1. I too would ignore class and get lost and mesmerized by it. In my case though, I was at the gift shop in the Science Museum in Boston instead of being in school. Hehe

      As for his observation, I’ve always thought that there was something wrong with many of the explanations on how it worked. Nice to know it’s a heat engine (a useless one but one none the less), that actually explains why I used to be able to hear some of them spinning while they was in their more or less opaque box.

      1. I always wanted to use a radiometer as a light intensity sensor. Correlating RPM with IR wouldn’t be hard. I couldn’t figure out how to mount an optical interrupter reliably, though. Now I would just use a reflection based optical sensor.

  2. I was stricken by them on display for 40 years in the window of a local electrical shop on main street.
    A place where you could take a fan with a bad cord and have it fixed.
    Light blubs (which they tested), sockets and plugs in bins (no plastic pacs) and radiometers,

  3. I had a big fight with a teacher over this. He showed us a radiometer and we watched it spin. Then we read the explanation of how it worked in the science book. I was confused as the device was turning the other way. I raised my hand. Big mistake. Wound up in the principal’s office (yet again).

    So NOBODY in generations besides a 5th grader who never had his homework done correctly ever noticed the thing really turns the other way?

    I keep one on my window sill and much enjoy it.

  4. I always find it amazing when science articles use strange units (i.e., from regional measurement systems that are not part of the SI system).

    *Please*, dear HaD staff, do use SI units whenever possible, especially in science articles.

    BTW: “About 7mTorr” is about 1 Pa.

          1. Always refreshing to see someone admit they were mistaken.Usually they go away in a huff without comment. I’m also trying to make more boxes inside boxes inside…

  5. “Sir William Crookes and James Clerk Maxwell both believed that the vanes moved due to the pressure of the photons hitting the vanes.” [citation needed] Newtons’s corpuscular theory of light was out of fashion when Maxwell was active. After all the equations Maxwell is most famous for describe e/m fields and resulting waves. Plank long after introduced the concept of quantization of energy, the beginning of Quantum Mechanics.

    “If that were true though, the radiometer would spin in the opposite direction it normally does when held near a light source.” It’s hard to believe that Maxwell, one of the greatest theoretical physicist, one of those truly awe-inspiring geniuses, would overlook the colour of the vanes.

    1. Solar sails work (the most successful so far is IKAROS which you can Google on). But the Crookes radiometer works on a different principle which gives much more thrust.

      By using a hard enough vacuum and a bright enough light, you could make the radiometer spin the other way using the solar sail effect, but it is not easy.

      If interplanetary space were filled with 7 mTorr gas, then you could use the Crookes radiometer effect for space propulsion and get much more thrust than solar sails do.

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