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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Crookes’s Radiometer is today marketed as a conversation piece called a light-mill or solar engine. Jul 19, When the radiometer is heated in the absence of a light source, it turns in the forward direction i. The force responsible for the rotation of the spindle has been subject to debate for many years, and although two components have been identified it is still unclear which one is stronger. The paper gave due credit to Reynolds’ suggestion that the effect is at the edges of the vanes, but criticised Reynolds’ mathematical raadiometer.

How does a Crookes Radiometer work?

At these very high vacuums the effect of photon radiation pressure on the vanes can be observed in very sensitive apparatus see Nichols radiometer but radilmeter is insufficient to cause rotation. There will be a flow of heat from the hot end to the cold end, but the force on both ends will be the same because the pressures at the radioemter are equal. Julian—Gregorian uncertainty Articles needing additional references from July All articles needing additional references Use dmy dates from July All articles with unsourced statements Articles with unsourced statements from November Articles with unsourced statements from February Articles with unsourced statements from December Commons category link is on Wikidata.


It was invented in by the chemist Sir William Crookes as the by-product of some chemical research. If the gas behaves according to the ideal gas laws with isotropic pressure, it will settle into a steady state with a temperature gradient along the tube. On a last note, it is possible to measure radiation pressure using a more refined apparatus.

How Crookes’ Radiometer Works

So this explanation in terms of warm gas is wrong, but even the Encyclopaedia Britannica gives this false explanation today. A Crookes’ radiometer has four vanes suspended inside a glass bulb. Archived from the original on 22 July These are attached to the arms of a rotor which is balanced on a vertical support in such a way that it can turn with very little friction.

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 exposed to light. How it works 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. To understand why these common explanations are wrong, think first of a simpler setup in which a tube of gas is kept hot at one end and cool at the other.

Crookes radiometer – Wikipedia

When you shine a light on the vanes in the radiometer, they spin — in bright sunlightthey can spin at several thousand rotations per minute! Heat will also make Crookes’ radiometer spin, as infrared and near-infrared radiation is being generated by the surrounding bodies.

The vacuum is important to the radiometer’s success. radiometfr

The reason for the rotation was a cause of much scientific debate in the ten years following the invention of the device, [1] [2] but in the currently accepted explanation for the rotation was published.


The Crookes radiometeralso known as a light millconsists of an airtight glass bulb, containing a partial vacuum.

Upon reaching equilibrium, typically after a minute or two, reverse rotation ceases. Radiomeher radiometer is a little more complex, but the same idea should apply. One last incorrect explanation sometimes given is that the heating sets up convection currents with a horizontal component that turns the vanes.

To rotate, a light mill does not have to be coated with different colors across each vane. The effect cannot be explained in this way.

This heat loss through the glass keeps the internal bulb temperature steady with the result that the two sides of the vanes develop a temperature difference. Any suggested mechanism that yields a stronger force on the hot end with no tangential forces along the length of the tube cannot be correct, since otherwise there would be a net force on the tube with no opposite reaction.

How does a Crookes’ radiometer work?

Inside are a set of vanes which are mounted on a spindle. Alternatively, if the bulb is too much or completely vacuumed, the thermal transpiration, the effect behind the functioning of the device, will produce too little or no force at all and the fins will not spin.

Print “How does a Crookes’ radiometer work? The pressure will be the same throughout, since otherwise net forces will disturb the gas. The density will vary inversely with temperature along the tube.

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