Loeb, Leonard B. (1934) The Kinetic Theory of Gases (2nd Edition);McGraw-Hill Book Company; pp 353–386

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Han, Li-Hsin; Shaomin Wu; J. Christopher Condit; Nate J. Kemp; Thomas E. Milner; Marc D. Feldman; Shaochen Chen (2011). "Light-Powered Micromotor: Design, Fabrication, and Mathematical Modeling". Journal of Microelectromechanical Systems. 20 (2): 487–496. doi: 10.1109/JMEMS.2011.2105249. S2CID 11055498. 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. a b c Gibbs, Philip (1996). "How does a light-mill work?". math.ucr.edu/home/baez/physics/index.html. Usenet Physics FAQ . Retrieved 8 August 2014. 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 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]

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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. Radiometers are now commonly sold worldwide as a novelty ornament; needing no batteries, but only light to get the vanes to turn. 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. 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. Another incorrect theory was that the heat on the dark side was causing the material to outgas, which pushed the radiometer around. This was later effectively disproved by both Schuster's experiments [9] (1876) and Lebedev's (1901) [8] Partially correct theory [ edit ] 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.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. a b c Kraftmakher, Yaakov (29 August 2014). Experiments and demonstrations in physics (2ed.). Singapore: World Scientific. p.179. ISBN 9789814434904. Ohio, The University of Akron. "the radiometer using inquiry to teach energy conversions". The University of Akron, Ohio . Retrieved 10 October 2021. 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. .