Leon Foucault

Jean Bernard Léon Foucault (Paris, September 18, 1819-Ib., February 11, 1868) was a French physicist and astronomer. He experimentally demonstrated the terrestrial rotation in 1851 using a huge pendulum, the so-called "Foucault pendulum" (installed first at the Paris Observatory and a few weeks later at the Panthéon in Paris). Among other contributions, he measured the speed of light, took the first photographs of the Sun, discovered eddy currents, and invented the gyroscope.

Biography

The son of a publisher, Leon Foucault was educated in his early years largely in his own home (in Paris and Nantes), and later was a student at the Stanislas College in Paris. He began to study medicine, a discipline that he abandoned to turn to physics, in which he became interested in very varied fields of experimentation. First, he analyzed Louis Daguerre's experiences in photography, and for three years he was interested in Alfred Donné's work on microscopic anatomy, attending his lectures.

With Hippolyte Fizeau, he conducted a series of experiments on the intensity of sunlight, comparing it with that of the carbon arc lamp and with that of the oxy-hydrogenated lime torch flame. He also worked on the interference of infrared radiation, on the propagation of light rays and on the chromatic polarization of light.

In the 1840s he devised an electromagnetic regulator to improve the performance of arc lamps, and, in collaboration with his friend Jules Regnauld, wrote a paper on binocular vision.

In 1850 he carried out with Fizeau (first as a collaborator, and later as a rival) a series of experiments on the relative speed of light in different media, confirming that it varies inversely with the refractive index of the medium in which it propagates. He obtained his doctorate at the Faculty of Sciences in Paris, with a thesis entitled On the speed of light in air and water (presented in 1853).

His demonstration in 1851 of the diurnal movement of the Earth by the rotation of the plane of oscillation of a heavy and long pendulum freely suspended in the Panthéon in Paris, caused a sensation both in the world of the literate and in the popular world, since which was the first dynamic demonstration of the Earth's rotation. In the following years he invented (and named) the gyroscope as a conceptually simpler experimental test.

In 1855 he received the Royal Society's Copley Medal for his "remarkable experimental investigations." Earlier that same year, he was appointed physicien (physicist) at the Imperial Observatory in Paris.

In September 1855 he discovered that the force required for the rotation of a copper disk increases when it is rotated between the poles of a magnet, at the same time that the disk begins to heat up from the eddy currents induced in the metal.

Tomb of Léon Foucault in the cemetery of Montmartre (Paris)

In 1857 Foucault invented the polarizer that bears his name, and in the following years he devised a method for testing the mirrors of reflective telescopes to determine their shape. The so-called Foucault test allowed the worker to determine if the mirror is perfectly spherical, or if it is deformed. Prior to Foucault's invention, the testing of telescope mirrors was a "trial and error" process.

In 1862, Foucault determined the speed of light to be 298,000 km/s, a value whose accuracy could not be improved until 1907.

That same year, 1862, he was made a member of the Bureau des Longitudes and an officer of the Légion d'Honneur. In 1864 he was made a member of the Royal Society of London and, the following year, of the mechanical section of the institute.

In 1865 his articles were published on a modification of James Watt's regulator, on which he had been experimenting for some time with the idea of making its period of revolution constant, in a new device to regulate electric light. A year later he demonstrated how, by depositing a thin, transparent layer of silver on the outer end of a telescope objective, the sun could be observed without damaging the eyes.

Foucault died of what was probably a rapid form of multiple sclerosis on February 11, 1868 in Paris and was buried in the Cimetière de Montmartre.

Measuring the speed of light

Diagram of a variant of the Foucault experiment to measure the speed of light where a laser is used as a light source.

Continuing the experiment abandoned by François Arago in 1843 because of the blindness he suffered, Foucault was able in 1850 to carry out a series of experiments on the speed of light, perfecting the revolving mirror designed by Sir Charles Wheatstone, demonstrating Light travels faster in air than in water.

This result meant invalidating the corpuscular theory in favor of the wave theory of light, until the wave-particle duality unified these two concepts in the context of quantum physics. He confirmed that the speed of light varies inversely with the refractive index of the medium in which it propagates (see the Fizeau and Foucault Experiment).

In 1862 he obtained a measurement of the speed of light of 298,000 kilometers/second (with an accuracy of ±500 km/s), 10,000 km/s less than that previously obtained by Hippolyte Fizeau in 1849, with a difference of only 0.6% error on the currently accepted value. To do this, he used a procedure similar to the one described in the attached illustration, based on a mirror oscillating at high speed. This value endured for 45 years, until Rosa & Dorsey made more precise measurements in 1907.

Foucault's pendulum

Péndule de Foucault in the Paris Pantheon

Repetition of the original experiment by its fiftieth anniversary (1902).

His most spectacular experiment, the one that made him enormously popular, was performed on March 26, 1851 at the Panthéon in Paris, becoming an impressive demonstration for the general public. He officiated as a pendulum for a 26kg brass-clad cannonball, suspended from the vault by a 220-foot-long cable that took sixteen seconds to go back and forth each time. Attached to the bullet, at its bottom, was a small stiletto and the floor of the Pantheon was covered with sand. With each round trip, the stylus left a different mark in the sand, each one about two millimeters to the left of the last, thus demonstrating that the Earth rotated.

Some posts

Recueil des travaux scientifiques de Léon Foucault, 1878

• 1844-1845. Cours de microscopie complémentaire des études médicales. Anatomie microscopique et physiologie des fluides de l'économieJ.-B. Baillière (Paris).

• 1863. Notice sur les travaux de M. Léon Foucault, Mallet-Bachelier (Paris)

• Recueil des travaux scientifiques de Léon Foucault (in French) 1. Paris: Gauthier-Villars. 1878.
• Recueil des travaux scientifiques de Léon Foucault (in French) 2. Paris: Gauthier-Villars. 1878.

• 1913. Mesure de la vitesse de la lumière. He had a choice of surfaces. Mémoires de Léon Foucault Archived on 3 March 2016 at Wayback Machine., A. Colin (Paris), collection Les Classiques de la science, reissued in 1922.

• 2001. Recueil des travaux scientifiques de Léon Foucault, Librairie scientifique et technique (Paris). (facsim. de la ed. de Paris: Gauthier-Villars, 1878), ISBN 2-85367-214-X.

Acknowledgments

• Member of the Legion of Honor.
• Member of the Royal Society and Medalla Copley (in 1855).
• It is one of 72 scientists whose name is registered at the Eiffel Tower.
• The asteroid (5668) Foucault and the lunar crater Foucault bear this name in their honor.
• In 1988 the novel by Italian writer Umberto Eco was published Foucault pendulum, set in the Conservatoire National des Arts et Métiers of Paris, in the room where is the pendulum that gives title to the story.
• The Google searcher dedicated a doodle on September 18, 2013 on the occasion of its 194th anniversary.
• On 14 September 2015, a copy of his pendulum was installed at the top of the Paris Pantheon.