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Crookes radiometer: gas: Free-molecule gas: A radiometer is a four-vaned mill that depends essentially on free-molecule effects. A temperature difference in the . Crookes’s Radiometer is today marketed as a conversation piece called a light- mill or solar engine. It consists of four vanes, each of which is blackened on one. The Crookes radiometer is a light mill consisting of a set of fins placed on a spindle that rotates inside a partially vacuumed glass bulb when.

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Maxwell refereed Reynolds’ paper, and so became aware of his suggestion. The light mill is uniformly coated by gold nanocrystalswhich are a strong light absorber. At rough vacuum, this asymmetric heating effect generates a net gas movement across each vane, from the concave side to the convex side, as shown by the researchers’ Direct Simulation Monte Carlo DSMC modeling.

A gold light mill, only nanometers in diameter, was built and illuminated by laser light that had been tuned. It was invented in by the chemist Sir William Crookes as the by-product of some chemical research.

The two sides of each vane must be thermally insulated to some degree so that the polished or white side does not immediately reach the temperature of the black side. Woodruff, The Physics Teacher 6— In that case the mill is turning the wrong way. This can be demonstrated by cooling the radiometer, for then the rotor turns the other way.

This counterintuitive result is due to tangential forces between the gas molecules and the sides of the narrow pores in the plates. Click to load comments.

How does a Crookes’ radiometer work?

Actually, such an effect does exist; but it is not the real explanation. The strange case of the vacillating ‘crucial experiment ‘ “, Studies in History and Philosophy of ScienceElsevierdoi: The rotation crooies is directly related with the intensity of the electromagnetic radiation, while the rotation direction depends on the temperature of the environment in which the device is placed.


Light falling on the black side should be absorbed, while light falling on the silver side of the vanes should be reflected. A variation on this theme is that the motion of the hot molecules on the black side of the vane provide the push. Maxwell analysed this theory carefully — presumably being wary about making a second mistake.

They come in various forms, such as the one pictured, and are often used in science museums to illustrate ” radiation pressure ” — a scientific principle that they do not in fact demonstrate. It seems that Crookfs was delighted to see a demonstration of the effect of radiation pressure as predicted by his theory of electromagnetism.

Crookes was knighted in In the course of very accurate quantitative chemical work, he was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the balance.

High inside pressure inhibits motion because the temperature differences are not enough to push the vanes through the higher concentration of air: If there is no vacuum that is, if the bulb is full of airthe vanes do not spin because there is too much drag. The rotor bears four light, horizontal arms mounted at right angles…. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.

Crookes radiometer

This would cause the rarefied gas to be heated on the black side. Inside the bulb, on a low friction spindle, is a rotor with several usually four vertical lightweight vanes spaced equally around the axis. As radiant energy comes from a particular light source, it warms the vanes on the black side at the same time causing the air molecules in the partial vacuum to heat up, to a temperature slightly higher than that of the medium.


But there is a problem with this explanation. The effect is also known as thermal creep, since it causes gases to creep along a surface that has a temperature gradient. Crookes radiometer light heat thermal transpiration.

This contrasts with sunlight, with which forward rotation can be maintained all day. Unsourced material may be challenged and removed. A temperature difference in the free-molecule gas causes a thermomolecular pressure difference that drives the vanes.

If there is a good but incomplete vacuum, then a different effect called thermal transpiration occurs along the edges of the vanes, as described on this page. The Crookes radiometeralso known as a light millconsists of an airtight glass bulb, containing a partial vacuum.

When exposed to sunlightartificial light, or infrared radiation even the heat of a croojes nearby can be enoughthe vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing.

Later work eventually led to the invention of the radiometer bearing his name.

How Crookes’ Radiometer Works

To explain the radiometer, therefore, one must focus attention not on the faces of the vanes, but on their edges. Archived from the original on 22 July The effect cannot be explained in raciometer way. Inwhile investigating infrared radiation and the element thallium, the eminent Victorian experimenter Sir William Crookes developed a special kind of radiometer, an instrument for measuring radiant energy of heat and light.

This liquid, which lacks all viscosity, will climb the sides of its container towards a warmer region. The density will vary inversely with temperature along the tube.

Again, these are the same thermomolecular forces responsible for Reynolds’ thermal transpiration.