URL: http://www.phys.canterbury.ac.nz/~phys146/foucault.html

Léon FOUCAULT (1819-1868)

( Dr. William Tobin)

Jean Bernard Léon Foucault was the French physicist who in 1851 set up an enormous pendulum in the Panthéon in Paris. The slow clockwise veering of the swing-plane of the bob demonstrated that the Earth was slowly turning anticlockwise below. In 1851 no one doubted that the Earth was spinning on its axis, but this first dynamical proof of the fact ended a quest that had begun in Galileo's time, over two centuries earlier. It established Foucault's fame then and subsequently. However, this was by no means his only significant contribution to 19th-century science. Others were:

• The invention of the gyroscope in 1852, which also demonstrated the terrestrial rotation.
• A comparison of the speed of light in air and in water in 1850. The result that light travelled faster in air than in water showed that Newton's and Descartes' corpuscular theory of light could not be correct.
• The invention of the reflecting telescope in essentially its modern form with metallized-glass mirrors figured to sub-wavelength surface accuracy (c.1857).
• The first terrestrial measurement of the speed of light in absolute units (i.e. kilometres per second). The value obtained by Foucault in 1862 is in agreement with the modern value. It was also some 3 per cent smaller than the previously-accepted value. This was in line with a prediction by Focault's boss at the Paris Observatory, Urbain Le Verrier, that the then-accepted distance to the Sun was 3 per cent too big.

The biography will aim to cater for three distinct readerships. The main text should be accessible to the general reader with some scientific background---readers for whom terms such as velocity and mass do not need definition. Appendices with fuller technical details will enlarge on Foucault's work for those with a more-advanced training in physics. Finally, a scholarly apparatus of endnotes and citations should satisfy researchers in the history of science. The expected publication date is the (northern) autumn of 2002 for the French version and a little later in English. A list of chapter titles in given below.

If you would like to receive notification once the biography has been published, please e-mail contact details to: W.Tobin@phys.canterbury.ac.nz

Some details of Foucault's work have already been published in journals. See:

• Foucault's invention of the silvered-glass reflecting telescope and the history of his 80-cm reflector at the Observatoire de Marseille. W. Tobin. Vistas in Astronomy 30, 153-184 (1987).
• Perfecting the Modern Reflector. W. Tobin. Sky & Telescope 74, 358-359 (1987).
• Toothed wheels and rotating mirrors: Parisian astronomy and mid-nineteenth century experimental measurements of the speed of light. W. Tobin. Vistas in Astronomy 36, 253-294 (1993).
• Foucault, his Pendulum and the Rotation of the Earth. W. Tobin & A.B. Pippard. Interdisciplinary Science Reviews 19, 326-337 (1994).
• Le grand art des pièges à lumière. W. Tobin & G. Chevalier. Les Cahiers de Science & Vie No. 25, 50-67 (1995).
• Foucault, son pendule, et la rotation de la Terre. W. Tobin. l'Astronomie 110, 50-60 (1996).
• Léon Foucault. W. Tobin. Scientific American 279, 52-59 or 70-77 [pagination varies with edition] (July 1998).
• Léon Foucault. W. Tobin. Pour la Science No. 251 80-87 (September 1998).