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Inventory no. 24393 - Former Display Label


Unsigned. Height 250mm.
Mahogany base (202mm x 150mm), small rotating mirror between steel pillars, brass inlet tube with stopcock and turbine. This piece of apparatus was the central component in the optical arrangement used by Jean Léon Foucault in 1850 to determine that light travels faster in air than in water, and in 1862 the velocity of light in air. His calculations were based on the observed angular displacement of a narrow beam of light reflected from the mirror rotating at 800 revolutions per second.
Foucault collaborated with Armand-Hippolyte-Louis Fizeau (1819-96) from 1844 to 1850 when, after a dispute, they continued their experiments on the velocity of light separately. Both tried the rotating mirror technique first used by Wheatstone in 1834 in an unsuccessful attempt to measure the speed of electricity. François Arago in 1838 was the first to attempt the determination of the velocity of light by this method, and his rather complicated apparatus, rotating at 9000 rps, was made by Louis François Clément Breguet (1804-83) in 1843. Foucault decided to construct a sophisticated instrument based on Wheatstone's original design, and this was made by the Parisian instrument maker Paul Gustave Froment (1815-65). There were minor variations between Foucault's 1850 and 1862 experimental arrangements. In the earlier version great care was taken to ensure the uniform rotation of the mirror: lubricating oil was supplied to the upper and lower pivots at a constant pressure and the pressure of the steam supplied to the turbine was also carefully regulated. The uniformity of speed of the mirror was determined by comparing the note produced by the turbine with that of a tuning fork. In the 1862 version, the pivot bearings were simplified, the steam was replaced by a blast of air, and the constancy of the mirror was determined stroboscopically by means of a toothed wheel rotating at the same speed.
Fizeau in 1849 calculated the speed of light to be 185, 420 miles per second. He used a fixed mirror and a rapidly rotating toothed wheel cutting off the reflected beam. Foucault's subsequent determination of 185, 157 miles per second was slightly lower; the modern figure is 186, 300 per second (299,800 km/sec). More accurate measurements were made by A. A. Michelson during 1880-1932. He is best known for his part in the Michelson-Morley experiment of 1887 which implied that the speed of light was constant; a conclusion that ultimately led to the theory of relativity.
Foucault bequeathed his rotating mirror apparatus to the Observatoire de Paris. The pivot arrangement of that model is the same as the 1862 experiment although its screw-threaded inlet tube is reminiscent of the 1850 version. The specimen exhibited in this case has the oil-lubricating pivot arrangement of 1850 but the ribbed inlet tube of the 1862 model. Perhaps Foucault had two models made and the inlet tubes were exchanged at a later date. This piece of apparatus, although described in comtemporary text books, has not been found in instrument makers' trade catalogues and therefore, does not appear to have been marketed. Froment also made Fizeau's apparatus and Foucault's gyroscope of 1852.

Formerly in the Conrad Cooke Collection.

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