Léon Foucault
French physicist who proved Earth's rotation with a pendulum.
Léon Foucault was a French physicist born in Paris on 18 September 1819, the son of a publisher. Educated chiefly at home, he initially studied medicine but abandoned it due to a blood phobia, turning instead to physics. His early career involved improving Louis Daguerre’s photographic processes and serving as an experimental assistant to Alfred Donné in lectures on microscopic anatomy. With Hippolyte Fizeau, he investigated the intensity of sunlight compared to carbon arc lamps and oxyhydrogen blowpipe flames, as well as the interference of infrared radiation and chromatic polarization. In 1849, he demonstrated that absorption and emission lines at the same wavelength originate from the same material, with temperature accounting for the difference. In 1850, using a rotating mirror, he measured the speed of light, showing it travels more slowly through water than air, which contradicted Newton’s corpuscular theory. His most famous achievement came in 1851, when he suspended a long, heavy pendulum from the dome of the Panthéon in Paris, demonstrating the rotation of the Earth’s axis. The experiment became a public sensation, and similar pendulums were installed across Europe and America. The following year, he used and named the gyroscope as a simpler proof of Earth’s rotation. In 1855, he discovered eddy currents (Foucault currents) by observing that a copper disc rotating between magnetic poles required more force and became heated. He also invented a polarizer, developed the Foucault knife-edge test for shaping telescope mirrors, and in 1862 measured the speed of light at 298,000 km/s, within 0.6% of the modern value. He was made a member of the Bureau des Longitudes, an officer of the Legion of Honour, and a member of the Royal Society. Near his death, he returned to Roman Catholicism. He died on 11 February 1868 in Paris, likely from multiple sclerosis, and was buried in Montmartre Cemetery.
- field
- Physics
- nationality
- French
- known_for
- Foucault pendulum
Verified Timeline
Lore & Background
Foucault's first public exhibition of his pendulum took place in February 1851 in the Meridian of the Paris Observatory. A few weeks later, he made his most famous pendulum at the Panthéon, Paris. Because the latitude of its location was 48°52'N, the plane of the pendulum's swing made a full circle in approximately 31.8 hours, rotating clockwise about 11.3° per hour. The proper period of the pendulum was about 16.5 seconds, and Foucault reported observing 2.3 mm of deflection on the edge of the pendulum every oscillation. He explained his results in an 1851 paper entitled 'Physical demonstration of the Earth's rotational movement by means of the pendulum,' published in the Comptes rendus de l'Académie des Sciences. The original bob was moved in 1855 to the Conservatoire des Arts et Métiers in Paris. On April 6, 2010, the cable suspending the bob in the Musée des Arts et Métiers snapped, causing irreparable damage to the pendulum bob and marble flooring. An exact copy of the original pendulum has been operating under the dome of the Panthéon since 1995.
Reader's Guide
Léon Foucault's significance lies in his experimental demonstrations that reshaped fundamental physics. His Foucault pendulum provided the first public, visible proof of Earth's rotation, a milestone in understanding diurnal motion. The effect is greatest at the poles and diminishes with lower latitude until it no longer exists at Earth's equator. At the North Pole, the plane of oscillation remains fixed relative to the distant masses of the universe while Earth rotates underneath it, taking one sidereal day to complete a rotation. At other latitudes, the plane of oscillation precesses relative to Earth, with angular speed proportional to the sine of the latitude. Foucault also performed a gyroscope experiment in 1852 to further demonstrate Earth's rotation. Notably, veering of a pendulum was observed as early as 1661 by Vincenzo Viviani, a student of Galileo, but there is no evidence he connected the effect with Earth's rotation.
Did You Know?
- Foucault was inspired by observing a thin flexible rod on the axis of a lathe, which vibrated in the same plane despite the rotation of the supporting frame.
- The original Foucault pendulum bob weighed 28 kilograms and was suspended by a 67-metre-long wire from the dome of the Panthéon in Paris.
- At the North Pole, the plane of oscillation of a Foucault pendulum remains fixed relative to the distant masses of the universe while Earth rotates underneath it, taking one sidereal day to complete a rotation.
- On April 6, 2010, the cable suspending the original pendulum bob in the Musée des Arts et Métiers snapped, causing irreparable damage to the bob and marble flooring.
- Veering of a pendulum was observed in 1661 by Vincenzo Viviani, a student of Galileo, but he regarded it as a nuisance rather than connecting it with Earth's rotation.
Frequently Asked Questions
What is the Foucault pendulum?
It is a long, freely swinging pendulum whose plane of oscillation appears to drift slowly over time because the planet turns beneath it. Foucault designed it so that anyone watching could see Earth's rotation without any telescopes or complex calculations.
What other contributions did Foucault make to physics?
He carried out some of the earliest laboratory measurements of the speed of light, identified the swirling currents later called eddy currents, and is credited with coining the name "gyroscope."
Why is Léon Foucault considered important in the history of science?
His pendulum turned an abstract astronomical fact into something a layperson could watch in real time, and his work on light speed and electromagnetism helped shape experimental physics in the mid-nineteenth century.
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