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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A temperature difference in the free-molecule gas causes a thermomolecular pressure difference that drives the vanes. By using this site, you agree to the Terms of Use and Privacy Policy. The internal air molecules are heated up when they touch the black side of the vane. The effect cannot be explained in this way.

One needs to use a much better vacuum, suspend the vanes from fine fibers and coat the vanes with an inert glass to prevent out-gassing. The effect is also known as thermal creep, since it causes gases to creep along a surface that has a temperature gradient. Inresearchers at the University of Texas, Austin created a monocolored light mill which has four curved vanes; each vane forms a convex and a concave surface.

If the gas is initially at the same pressure on the two sides, it flows from the colder to the hotter side, resulting in a higher pressure on the hotter side if the plates cannot move.

Crookes radiometer – Wikipedia

The air pressure inside the bulb needs to strike a balance between too low and too crookse. Maxwell refereed Reynolds’ paper, and so became aware of his suggestion. Feldman; Shaochen Chen The correct solution to the problem was provided qualitatively by Osborne Reynolds, better remembered for the “Reynolds number”.

Inside the bulb, there is a good vacuum.


Crookes radiometer light heat thermal transpiration. Inwhile investigating infrared radiation and the croookes thallium, the eminent Victorian experimenter Sir William Crookes developed a special kind of radiometer, an instrument for measuring radiant energy of heat and light.

The wheel turns backwards because the net exchange of heat between the black sides and the environment initially cools the black sides faster than the white sides. Britannica does not currently have an article on this topic. It was clear that the black side of each vane would absorb heat from infrared radiation more than the silver side.

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. The radiometer is made from a glass bulb from which much of the air has been removed to form a partial vacuum. Particle Acceleration Takes a Leap Forward.

Reynolds wanted his protest to be published by the Royal Society, but after Maxwell’s death this was deemed inappropriate. Crookes’s Radiometer is today marketed as a conversation piece called a light-mill or solar engine.

How does a light-mill work?

This page was last edited on 10 Novemberat It consists of four vanes, each of which is blackened on one side and silvered on the other. When you shine a light on the vanes in the radiometer, they spin — in bright sunlightthey can spin at several thousand rotations per minute! July Learn how and when to remove this template message. From Wikipedia, the free encyclopedia.

No net force can be generated by normal forces on the faces of the vanes, because pressure would quickly equalise to a steady state with just a flow of heat through the gas. By “thermal transpiration”, Reynolds meant the flow of gas through porous plates caused by a temperature difference on the two sides of the plates.

Clerk 1 January Radiometers are now commonly sold worldwide as a novelty ornament; needing no batteries, but only light to get the vanes to turn.


So this explanation in terms of warm gas is wrong, but even the Encyclopaedia Britannica gives this false explanation today. In that case, the obvious explanation is that the pressure of the gas on the darker side increases with its temperature, creating a higher force on the dark side of crookws vane which thus pushes the rotor around.

A strong vacuum inside the bulb does not permit motion, because there are not enough air molecules to cause the air currents that propel the vanes and transfer heat to the outside before both sides of each vane reach thermal equilibrium by heat conduction through the vane material.

How does a light-mill work?

The faster molecules from radiometre warmer side strike the edges obliquely and impart a higher force than the colder molecules. Inside radiometre bulb, on a low friction spindle, is a rotor with several usually four vertical lightweight vanes spaced equally around the axis.

Maxwell at once made a detailed mathematical analysis of the problem, and submitted his own paper, “On stresses in rarefied gases arising from inequalities of temperature”, for publication in the Philosophical Transactions; it appeared inshortly before his death.

Equilibrium is reached when the ratio of rasiometer on either side is the square root of the ratio of absolute temperatures.

He discovered that, in fact, the warmer gas would simply expand in such a way that there would be no net force from this effect, just a steady flow of heat across the vanes.