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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Hot air engines Electromagnetic radiation meters Radiometry External combustion engines Heat transfer Energy conversion Novelty items. Jul 19, Crookes radiometer light heat thermal transpiration. Again, these are the same thermomolecular forces responsible for Reynolds’ thermal transpiration. The details of exactly how this moves the crokkes side of the vane forward are given in the section below.

How Crookes’ Radiometer Works

Radiometers are now commonly sold worldwide as a novelty ornament; needing no batteries, but only light to get the vanes to turn. When the radiometer is heated in the absence of a light source, it turns in the forward direction i. The white or silver side of the vanes are slightly warmer than the internal air temperature but cooler than the black side, as some heat conducts through the vane from the black side.

Print “How does a Crookes’ radiometer work? Philosophical Transactions of the Royal Society of London.

How does a Crookes Radiometer work?

The black side of the vane moves away from the light. Retrieved from ” https: The internal air molecules are heated up when they touch the black side of the vane. The radiometer is made from a glass bulb from which much of the air has been removed to form a partial vacuum. Britannica does not currently have an article on this topic. The principle of this radiometer has found numerous applications in the development of sensitive measuring instruments.


They rotate with the white or silvered side advancing and the dark side receding. For similar reasons, the theory that the vanes are propelled by electrons dislodged via the photoelectric effect can also be ruled out. The Crookes radiometeralso known as a light millconsists of an airtight glass bulb, containing a partial vacuum.

Crookes radiometer – Wikipedia

Christopher Condit; Nate J. 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 rotor bears four light, horizontal arms mounted at right angles….

Clerk 1 January This can be demonstrated by cooling the radiometer, for then the rotor turns the other way. How does a Crookes’ radiometer work? In other projects Wikimedia Commons.

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. Design, Fabrication, and Mathematical Modeling”. On a last note, it is possible to measure radiation pressure using a more refined apparatus. Although it may seem like a device you generally see only in a museum, Crookes radiometers are in fact quite common and are sold across the world as novelty ornament.


The torque was greatly enhanced by the resonant coupling of the incident light to plasmonic waves in the gold structure. 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. This would cause the rarefied gas to be heated on the black side.

When this was realised, other explanations for the radiometer effect were sought and some that people came up with are still mistakenly quoted as correct. Therefore, a pressure of about 1E-2 torr is preferred in order to obtain maximum effects. Equilibrium is reached when the ratio of pressures on either side is the square root of the ratio of absolute temperatures.

Wikimedia Commons has media related to Light mills. A Crookes radiometer consists of a glass bulb from which most of the air has been removed, thereby creating a partial vacuum, and a rotor that is mounted on a vertical support inside the bulb.

It was clear that the black side of each vane would absorb heat from infrared radiation more than the silver side. Allegedly, during his experiments he observed that when light shone on the balance the weighing process was disturbed, and chose to further investigate the effect.