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The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. Ohio, The University of Akron. "the radiometer using inquiry to teach energy conversions". The University of Akron, Ohio . Retrieved 10 October 2021. The air pressure inside the bulb needs to strike a balance between too low and too high. 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. High inside pressure inhibits motion because the temperature differences are not enough to push the vanes through the higher concentration of air: there is too much air resistance for "eddy currents" to occur, and any slight air movement caused by the temperature difference is damped by the higher pressure before the currents can "wrap around" to the other side. [6] Movement with radiation [ edit ] It was invented in 1873 by the chemist Sir William Crookes as the by-product of some chemical research. 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. Investigating this effect, he created the device named after him. Calaprice, Alice; etal. (27 October 2015). An Einstein encyclopedia. Princeton University Press. p.190. ISBN 978-0691141749.

US 182172,Crookes, William,"Improvement in Apparatus For Indicating The Intensity of Radiation",published 1876-09-12

When exposed to sunlight, artificial light, or infrared radiation (even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing. Crookes, William (1 January 1874). "On Attraction and Repulsion Resulting from Radiation". Philosophical Transactions of the Royal Society of London. 164: 501–527. doi: 10.1098/rstl.1874.0015. S2CID 110306977. . Thermodynamic explanation [ edit ] A Crookes radiometer in action with the light switched on and off. (Note that the explanation given in the caption to the clip doesn't agree with the modern explanation.) Movement with absorption [ edit ] Han, Li-Hsin; Shaomin Wu; J. Christopher Condit; Nate J. Kemp; Thomas E. Milner; Marc D. Feldman; Shaochen Chen (2010). "Light-Powered Micromotor Driven by Geometry-Assisted, Asymmetric Photon-heating and Subsequent Gas Convection". Applied Physics Letters. 96 (21): 213509(1–3). Bibcode: 2010ApPhL..96u3509H. doi: 10.1063/1.3431741. Archived from the original on 22 July 2011.

Wolfe, David; Larraza, Andres (2016). Alejandro Garcia. "A Horizontal Vane Radiometer: Experiment, Theory, and Simulation". Physics of Fluids. 28 (3): 037103. arXiv: 1512.02590. Bibcode: 2016PhFl...28c7103W. doi: 10.1063/1.4943543. S2CID 119235032. The currently accepted theory was formulated by Osborne Reynolds, who theorized that thermal transpiration was the cause of the motion. [11] 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 behaves like the pores in Reynolds's plate. As gas moves from the cooler to the hotter side, the pressure on the hotter side increases. When the plate is fixed, the pressure on the hotter side increases until the ratio of pressures between the sides equals the square root of the ratio of absolute temperatures. Because the plates in a radiometer are not fixed, the pressure difference from cooler to hotter side causes the vane to move. The cooler (white) side moves forward, pushed by the higher pressure behind it. From a molecular point of view, the vane moves due to the tangential force of the rarefied gas colliding differently with the edges of the vane between the hot and cold sides. [3] 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 1879 the currently accepted explanation for the rotation was published. [3] [4] Today the device is mainly used in physics education as a demonstration of a heat engine run by light energy. Loeb, Leonard B. (1934) The Kinetic Theory of Gases (2nd Edition);McGraw-Hill Book Company; pp 353–386 Reynolds, Osborne (1 January 1879). "On certain dimensional properties of matter in the gaseous state …". Philosophical Transactions of the Royal Society of London. 170: 727–845. doi: 10.1098/rstl.1879.0078. ; Part 2.

Worrall, J. (1982). "The pressure of light: The strange case of the vacillating 'crucial experiment' ". Studies in History and Philosophy of Science. 13 (2): 133–171. Bibcode: 1982SHPSA..13..133W. doi: 10.1016/0039-3681(82)90023-1.

The prefix " radio-" in the title originates from the combining form of Latin radius, a ray: here it refers to electromagnetic radiation. A Crookes radiometer, consistent with the suffix " -meter" in its title, can provide a quantitative measurement of electromagnetic radiation intensity. This can be done, for example, by visual means (e.g., a spinning slotted disk, which functions as a simple stroboscope) without interfering with the measurement itself.Brush, S. G.; Everitt, C. W. F. (1969). "Maxwell, Osborne Reynolds, and the Radiometer". Historical Studies in the Physical Sciences. 1: 105–125. doi: 10.2307/27757296. JSTOR 27757296.