fUN gadget

Does this help:

Dunno what it's called, but the gadget I've never been able to figure out is the one that has a rotating vane in a sealed glass container shaped like an electric light bulb. Powered by light. Mike.

Well I read the therory and built one when I was 17 in metalwork at school.

1975!

Still seems like magic!

Can you still get them? Its the "pressure" of light. Absorbed in the black and reflected from the white. But photons have no mass...

Crookes radiometer

Yep.

Umm no, that's not how they work (sadly) although light pressure is a real phenomenon. The vanes on a Crookes Radiometer are too small to get much of a push from light pressure and the pressure would be greater on the white patch than on the black. So if light pressure were the cause the rotation would have the white patch on the trailing edge. As it is, they are the other way around, the "push" comes from the black side.

The vanes are in a vacuum, but the vacuum isn't perfect, just good at about

10^-2 torr. Hence there are some gas molecules in there, just not many. The vaccuum is good enough to reduce air friction on the vanes significantly. When exposed to light, the dark side of the vane absorbs photons and warms up slightly. Any air molecule hitting that side of the vane departs with slightly more energy than it had. Any air molecule hitting the white side of the card leaves with about the same (or slightly less) energy than it had. This isn't enough to cause an imbalance in force big enough to make the vanes turn, but at the edges of the vane there are effects where the molecules of air move from the cold side to the hot side past the edge of the vane. The sum of these forces causes an imbalance in force on the two sides of the vane and it rotates.

Confirmation of this effect can be shown by evacuating more air from the bulb. By the time you get to 10^-6 torr the vanes stop moving.

But they do have a lot of energy.

Well whatever magic it uses I just ordered one.

"Burgerman" wrote in message news:mtwih.8812$ snipped-for-privacy@newsfe3-win.ntli.net...

[edit] Explanations for the force on the vanes Over the years, there have been many attempts to explain how a Crookes radiometer works: 1.. Crookes incorrectly suggested that the force was due to the pressure of light. This theory was originally supported by James Clerk Maxwell who had predicted this force. This explanation is still often seen in leaflets packaged with the device. The first experiment to disprove this theory was done by Arthur Schuster in 1876, who observed that there was a force on the glass bulb of the Crookes radiometer that was in the opposite direction to the rotation of the vanes. This showed that the force turning the vanes was generated inside the radiometer. If light pressure was the cause of the rotation, then the better the vacuum in the bulb, the less air resistance to movement, and the faster the vanes should spin. In 1901, with a better vacuum pump, Pyotr Lebedev showed that in fact, the radiometer only works when there is low pressure gas in the bulb, and the vanes stay motionless in a hard vacuum. Finally, if light pressure were the motive force, the radiometer would spin in the opposite direction as the photons on the shiny side being reflected would deposit more momentum than on the black side where the photons are absorbed. The actual pressure exerted by light is far too small to move these vanes but can be measured with devices such as the Nichols radiometer. 2.. Another incorrect theory was that the heat on the dark side was causing the material to outgas, which pushed the radiometer around. This was effectively disproved by both Schuster's and Lebedev's experiments. 3.. A partial explanation is that gas molecules hitting the warmer side of the vane will pick up some of the heat i.e. will bounce off the vane with increased speed. Giving the molecule this extra boost effectively means that a minute pressure is exerted on the vane. The imbalance of this effect between the warmer black side and the cooler silver side means the net pressure on the vane is equivalent to a push on the black side, and as a result the vanes spin round with the black side trailing. The problem with this idea is that the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the force on the vane should be exactly the same - the greater temperature causes a decrease in local density which results in the same force on both sides. Years after this explanation was dismissed, Albert Einstein showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there. The force predicted by Einstein would be enough to move the vanes, but not fast enough. 4.. The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds, but first published by James Clerk Maxwell in the last paper before his death in 1879. Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behave like the pores in Reynolds's plate. On average, the gas molecules move from the cold side toward the hot side whenever the pressure ratio is less than the square root of the (absolute) temperature ratio. The pressure difference causes the vane to move cold (white) side forward. Both Einstein's and Reynolds's forces appear to cause a Crookes radiometer to rotate, although it still isn't clear which one is stronger.

See also: photophoresis.

And yes I know its html but I dont care...

Well it was when I posted it!!!

"Burgerman" wrote in news:Ztwih.18481$ snipped-for-privacy@newsfe6-win.ntli.net:

It's still magic to me - just like that stirling engine thing. Wish I could get one. "OI !!! SANTA !!!" :-)

Yes, that's what I meant by:

"at the edges of the vane there are effects where the molecules of air move from the cold side to the hot side past the edge of the vane. The sum of these forces causes an imbalance in force on the two sides of the vane and it rotates."

Nice Wiki cut and paste BTW.

No it was on the link on the site that I bought it from.

If you don't mind me asking. How much and where? All the UK suppliers seem very expensive, considering they can be baught for around $10 in the states. Mike.

Just won one on eBay. £10.06 inc airmail delivery from Australia. Mike..