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Description: | LO 68. (old departmental catalogue number in aluminium paint). Crookes's Radiometer engraved on the glass envelope MANU Unsigned. late19th early 20th century] DES after CROOKES,William,Sir. 1861 London, England. This is a form of Crookes' paddle wheel tube which was thought to demonstrate that photons have mass and hence can exert a pressure.. However it was proved by the Irish physicist George Johnstone Stoney (1826-1911) that the apparent effect is not caused by light pressure but by thermal expansion and contraction effects in the system altering the center of gravity. The apparatus consists of a hand blown spherical envelope, under vacuum, at the end of tube which is fixed in a lead weighted wooden base. A central glass cane supports four circular mica vanes, painted black on one side, which are fixed to a small brass support by fine steel wres. When the instrument is placed near to a strong light source the mica paddles rotate. The theory of Obituary notice 'Proceedings of The Royal Society', Series A, Vol. 86, 1912 Stoney is given, in a some-what crude state, in the ' Phil. Mag. ' for March and April, 1876. Stoney's view may be summarised in the statement that for a certain distance in front of the heated vane, and reaching from it to the glass envelope, when the vessel is not too large, the molecular motions of the rarefied gas are polarised by the thermal conditions, and interpenetrate one another in a degree greater than prevails elsewhere in the gas. The greater molecular interpenetration in the line between glass and vane involves nothing of the nature of a wind, but determines a greater stress in the direction of polarisation. Errors in the earlier statements of his views are corrected in his paper in the Trans. Roy. Soc. Dub.,' 1878, and republished in the ' Phil. Mag.' of December, 1878; and a mathematical expression for the Crookes stress is given, based upon an investigation of Clausius of the stress across a layer of gas conducting heat normal to a heater and cooler. FitzGerald took a part in the discussion (cf. his ' Collected Papers ') by showing that the stress parallel to the heater and cooler could not be the same as the perpendicular stress--or, in other words, a polarisation stress must exist ('Nature,' vol. 17, p. 514)---and by a mathematical discussion of the subject in the ' Trans. Roy. Soc. Dub.,' 1878. | License: | http://www.hmag.gla.ac.uk/spirit/rights/ | Publisher: | Hunterian Museum and Art Gallery, University of Glasgow | Rights holder: | Hunterian Museum and Art Gallery, University of Glasgow | Subjects: | SCIENTIFIC COLLECTION : | Source: | Hunterian Museum | Creator: | Hunterian Museum and Art Gallery, University of Glasgow | Identifier: | http://www.huntsearch.gla.ac.uk/cgi-bin/... | Language: | en-GB | Go to resource |
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