Jean Bernard Léon Foucault (, ; ; 18 September 1819 – 11 February 1868) was a French

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

best known for his demonstration of the

Foucault pendulum The Foucault pendulum or Foucault's pendulum is a simple device named after French physicist Léon Foucault, conceived as an experiment to demonstrate the Earth's rotation. A long and heavy pendulum suspended from the high roof above a circular ...

, a device demonstrating the effect of

Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star Po ...

. He also made an early measurement of the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...

, discovered eddy currents, and is credited with naming the

gyroscope A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rot ...

Early years

The son of a publisher, Foucault was born in

Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. Si ...

on 18 September 1819. After an education received chiefly at home, he studied medicine, which he abandoned in favour of

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...

due to a blood phobia. He first directed his attention to the improvement of

Louis Daguerre Louis-Jacques-Mandé Daguerre ( , ; 18 November 1787 – 10 July 1851) was a French artist and photographer, recognized for his invention of the eponymous daguerreotype process of photography. He became known as one of the fathers of photog ...

's photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic

anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having i ...

. With Hippolyte Fizeau he carried out a series of investigations on the intensity of the

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...

of the sun, as compared with that of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

in the

arc lamp An arc lamp or arc light is a lamp that produces light by an electric arc (also called a voltaic arc). The carbon arc light, which consists of an arc between carbon electrodes in air, invented by Humphry Davy in the first decade of the 1800s, ...

, and of lime in the flame of the oxyhydrogen blowpipe; on the interference of

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light. In 1849, Foucault experimentally demonstrated that absorption and emission lines appearing at the same wavelength are both due to the same material, with the difference between the two originating from the temperature of the light source.

Middle years

In 1850, he did an experiment using the Fizeau–Foucault apparatus to measure the speed of light; it came to be known as the Foucault–Fizeau experiment, and was viewed as "driving the last nail in the coffin" of Newton's corpuscular theory of light when it showed that light travels more slowly through water than through air. In 1851, he provided an experimental demonstration of the rotation of the Earth on its axis ( diurnal motion). This experimental setup had been used by Vincenzo Viviani but became well known to the public by Foucault's work. Foucault achieved the demonstration by showing the rotation of the plane of oscillation of a long and heavy pendulum suspended from the roof of the Panthéon, Paris. The experiment caused a sensation in both the learned and popular worlds, and "''

s''" were suspended in major cities across Europe and America and attracted crowds. In the following year he used (and named) the

as a conceptually simpler experimental proof. In 1855, he received the

Copley Medal The Copley Medal is an award given by the Royal Society, for "outstanding achievements in research in any branch of science". It alternates between the physical sciences or mathematics and the biological sciences. Given every year, the medal is t ...

of the

Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...

for his 'very remarkable experimental researches'. Earlier in the same year he was made ''physicien'' (physicist) at the imperial observatory at Paris. In September 1855 he discovered that the

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...

required for the rotation of a

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...

disc becomes greater when it is made to rotate with its rim between the poles of a

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...

, the disc at the same time becoming heated by the eddy current or "Foucault currents" induced in the metal. In 1857 Foucault invented the polarizer which bears his name, and in the succeeding year devised a method of testing the mirror of a reflecting telescope to determine its shape. The so-called " Foucault knife-edge test" allows the worker to tell if the mirror is perfectly spherical or has non-spherical deviation in its figure. Prior to Foucault's publication of his findings, the testing of reflecting telescope mirrors was a "hit or miss" proposition. Foucault's knife edge test determines the shape of a mirror by finding the focal lengths of its areas, commonly called zones and measured from the mirror center. In the test, light from a point source is focused onto the center of curvature of the mirror and reflected back to a knife edge. The test enables the tester to quantify the conic section of the mirror, thereby allowing the tester to validate the actual shape of the mirror, which is necessary to obtain optimal performance of the optical system. The Foucault test is in use to this date, most notably by amateur and smaller commercial telescope makers as it is inexpensive and uses simple, easily made equipment. With Charles Wheatstone’s revolving mirror he, in 1862, determined the speed of light to be 298,000 km/ s – 10,000 km/s less than that obtained by previous experimenters and only 0.6% in error of the currently accepted value.

Later years

In 1862 Foucault was made a member of the

Bureau des Longitudes Bureau ( ) may refer to: Agencies and organizations * Government agency *Public administration * News bureau, an office for gathering or distributing news, generally for a given geographical location * Bureau (European Parliament), the administ ...

and an officer of the

Legion of Honour The National Order of the Legion of Honour (french: Ordre national de la Légion d'honneur), formerly the Royal Order of the Legion of Honour ('), is the highest French order of merit, both military and civil. Established in 1802 by Napoleo ...

. He became a member of the

of London in 1864, and member of the mechanical section of the Institute a year later. In 1865 he published his papers on a modification of

James Watt James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was ...

's centrifugal governor; he had for some time been experimenting with a view to making its period of revolution constant and developing a new apparatus for regulating the electric light. Foucault showed how, by the deposition of a transparently thin film of

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye. His chief scientific papers are to be found in the ''Comptes Rendus'', 1847–1869. Near his death he returned to Roman Catholicism that he previously abandoned.

Death and afterwards

Foucault died of what was probably a rapidly developing case of

multiple sclerosis Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This ...

on 11 February 1868 in Paris and was buried in the Montmartre Cemetery. The asteroid 5668 Foucault was named for him. His is one of the 72 names inscribed on the Eiffel Tower.

Bibliography

Collected Works: * ''Recueil des travaux scientifiques de Léon Foucault'
Volume OneVolume Two
1878. (Université de Strasbourg) o
Internet Archive
(US)
Donné & Foucault ''Atlas'' of medical micrographs
1845

Early years

Middle years

Later years

Death and afterwards

Bibliography

See also

References

Further reading