# How much force does it take to turn the vanes of a Radiometer?
**URL:** https://boards.straightdope.com/t/how-much-force-does-it-take-to-turn-the-vanes-of-a-radiometer/496515
**Category:** Factual Questions
**Created:** [May 15, 2009, 11:49am UTC](https://boards.straightdope.com/t/how-much-force-does-it-take-to-turn-the-vanes-of-a-radiometer/496515 "2009-05-15T11:49:02Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@nornagest](https://boards.straightdope.com/u/nornagest)
#### Post date: [May 15, 2009, 11:49am UTC](https://boards.straightdope.com/t/how-much-force-does-it-take-to-turn-the-vanes-of-a-radiometer/496515/1 "2009-05-15T11:49:02Z")
1 dyne? Less? More? I realize that it is air molecules in a greatly thinned out
globe being kicked away from the dark side of the vane that makes it move,
and not light preasure, otherwise I could calculate it directly. By “budging”
a vane I would define that as making the system rotate at 1 RPM.
Can anybody help me?
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### Author:  [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)
#### Post date: [May 15, 2009, 2:06pm UTC](https://boards.straightdope.com/t/how-much-force-does-it-take-to-turn-the-vanes-of-a-radiometer/496515/2 "2009-05-15T14:06:03Z")
It depends upon the mass of your vanes construction, how thick the paper is, and probably the residual gas pressure. If I assume an overall mass of 1 gram for your setup I estimate a force of something like 0.01 dyne in a quickie calculation. Curiously, I’ve never seen anyone estimate this.
By the way, the actual force responsible _isn’t_ merely gas pressure on the vanes. It turns out that the balance between incoming gas atoms and the outgoing gas atoms after collision rapidly equalize so that the net effect is zero. If your vanes were infinite in size they wouldn’t rotate. It’s the imbalance at the edges of the vanes that causes them to rotate, a surprisingly exotic effect that was, equally surprisingly, realized early on.
> **[Crookes radiometer](https://en.wikipedia.org/wiki/Crookes_radiometer)**
>
> The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.
> The reason for the rotation was a cause of much scientific debate in the ten years following the invention of the device, but in 1879 the currently accepted expl...
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [May 15, 2009, 6:00pm UTC](https://boards.straightdope.com/t/how-much-force-does-it-take-to-turn-the-vanes-of-a-radiometer/496515/3 "2009-05-15T18:00:03Z")
> [@](#):
>
> It depends upon the mass of your vanes construction, how thick the paper is, and probably the residual gas pressure.
It’ll also depend on the size of the needle-point the vane assembly is resting on, since the smaller the point, the lower the frictional torque will be.