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 caters for three distinct readerships. The main text is accessible to the general reader with some scientific background---readers for whom terms such as velocity and mass do not need definition, but who may well have forgotten the exact definition of `sine.' Appendices provide maps, lists of Foucault's instruments in museums, and a sample of Foucault's writing. For readers with greater technical experience, a final appendix discusses the challenges involved in making a Foucault pendulum that is safe and which really does indicate the rotation of the Earth. (Many purported Foucault pendulums do not.) The book finishess with endnotes and citations to satisfy researchers in the history of science, and an index.

The French version, Léon Foucault. Le miroir et le pendule was published by EDP-Sciences in October 2002 and has won the Prix spécial du Jury of the Prix du livre de l'astronomie - Haute-Maurienne/Vanoise - Festival de l'astronomie 2003.

The English version, The Life and Science of Léon Foucault. The Man who Proved the Earth Rotates was published by Cambridge University Press in October 2003.

Chapter titles (English version)

• Web site of the French publisher (EDP-Sciences)
• Publisher's catalogue entry for Léon Foucault. Le miroir et le pendule.
• Cover of Léon Foucault. Le miroir et le pendule. (JPEG, 73 kbyte)
• Order form for Léon Foucault. Le miroir et le pendule. (PDF, 180 kbyte)
• Reviews of Léon Foucault. Le miroir et le pendule.

• Web site of the English publisher (Cambridge University Press)
• Publisher's catalogue entry for The Life and Science of Léon Foucault. The Man who Proved the Earth Rotates. (Includes downloadable version of Chapter 1.)
• Cover of The Life and Science of Léon Foucault. The Man Who Proved the Earth Rotates (JPEG, 80 kbyte)
• Order form for Léon Foucault. Le miroir et le pendule. (PDF, 43 kbyte)
• Reviews of The Life and Science of Léon Foucault. The Man Who Proved the Earth Rotates.

Articles of related interest

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). This article has been translated in foreign-language editions of Scientific American such as Pour la Science No. 251, 80-87 (septembre 1998); Swiat Nauki 9, 84-91 (Wrzesien 1998); Investigación y ciencia No. 264, 32-39 (Septiembre 1998); and Spektrum der Wissenschaft 78-86 (oktober 1999).

Some of the challenges involved in building a Foucault pendulum are outlined in Tobin & Pippard (1994) and Tobin (1996), and references therein. For a particularly nice treatment, see:

• L'expérience du pendule de Foucault au Palais de la Découverte. A. Marillier. Revue du Palais de la Découverte, 26, No. 258, 31-45 (mai 1998)

Sites of related interest

• Postcard describing exhibition (JPEG, 130 kbyte)
• On-line summary of the exhibition (under construction)

Hundreds of sites make reference to Foucault pendulums established around the world and can be found with a search engine. Notable sites include:

• Elementary introduction to the Foucault pendulum
• Foucault pendulum in the Lycée Nicolas Appert in Orvault, France.
In French. What looks like an interesting pendulum, though its suspension does not allow it to run for more than a few hours. However I would caution against certain design features. The electromagnetic drive pulls as well as pushes. A design that only pushes would be preferable, because pulling increases ellipsing. And my experience is that a Charron ring is very desirable for a 10-m length pendulum if it is to run continuously.
• An Foucault pendulum built by over-winterers at the South Pole in 2001.
A Foucault pendulum at a pole is often used in explanations of the veering law, and now three people have built one! The authors deserve full admiration for their tenacity, courage and initiative. They are to be encouraged to build a better pendulum next time they are on the ice in order to produce a result that is unquestionably a reflection of the Earth's rotation and not the numerous perturbations to which the Foucault pendulum is subject.
• Foucault's grave