Léon Foucault
Encyclopedia
Jean Bernard Léon Foucault (ʒɑ̃ bɛʁnaʁ leɔ̃ fuko) (18 September 1819 – 11 February 1868) was a French
physicist
best known for the invention of the Foucault pendulum
, a device demonstrating the effect of the Earth's rotation. He also made an early measurement of the speed of light
, discovered eddy current
s, and although he didn't invent it, is credited with naming the gyroscope
.
, where he was born on September 18, 1819. After an education received chiefly at home, he studied medicine, which he abandoned in favour of physics
due to a fear of blood. He first directed his attention to the improvement of L. J. M. Daguerre's photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic anatomy
.
With A. H. L. Fizeau
he carried out a series of investigations on the intensity of the light
of the sun
, as compared with that of carbon
in the arc lamp
, and of lime
in the flame of the oxyhydrogen blowpipe; on the interference of infrared radiation, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light.
In 1851, he provided the first experimental demonstration of the rotation of the Earth on its axis (see diurnal motion
). He achieved this by showing the rotation of the plane of oscillation of a long and heavy pendulum suspended from the roof of the Panthéon in Paris
. The experiment caused a sensation in both the learned and popular worlds, and "Foucault pendulum
s" were suspended in major cities across Europe and America and attracted crowds. In the following year he used (and named) the gyroscope
as a conceptually simpler experimental proof. In 1855, he received the Copley Medal
of the Royal Society
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 required for the rotation of a copper
disc becomes greater when it is made to rotate with its rim between the poles of a magnet
, the disc at the same time becoming heated by the eddy current
or "Foucault currents" induced in the metal.
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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. The test focuses light point source at the center of curvature 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
(about 185,000 mi.
/s) —10,000 km/s less than that obtained by previous experimenters and only 0.6% off the currently accepted value.
and an officer of the Légion d'Honneur
. In 1864 he was made a member of the Royal Society
of London, and the next year a member of the mechanical section of the Institute. In 1865 his papers on a modification of Watt
's governor appeared, upon which he had for some time been experimenting with a view to making its period of revolution constant, and on a new apparatus for regulating the electric light; and in the year (Compt. Rend. lxiii.) he showed how, by the deposition of a transparently thin film of silver
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.
Foucault died of what was probably a rapidly developing case of multiple sclerosis
on February 11, 1868 in Paris and was buried in the Cimetière de Montmartre.
The asteroid 5668 Foucault
is named for him. His name is one of the 72 names inscribed on the Eiffel Tower.
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
physicist
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
best known for the invention of the Foucault pendulum
Foucault pendulum
The Foucault pendulum , or Foucault's pendulum, named after the French physicist Léon Foucault, is a simple device conceived as an experiment to demonstrate the rotation of the Earth. While it had long been known that the Earth rotated, the introduction of the Foucault pendulum in 1851 was the...
, a device demonstrating the effect of the Earth's rotation. He also made an early measurement of the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, discovered eddy current
Eddy current
Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
s, and although he didn't invent it, is credited with naming the gyroscope
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
.
Early years
Foucault was the son of publisher in ParisParis
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
, where he was born on September 18, 1819. After an education received chiefly at home, he studied medicine, which he abandoned in favour of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
due to a fear of blood. He first directed his attention to the improvement of L. J. M. Daguerre's photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
.
With A. H. L. Fizeau
Hippolyte Fizeau
Armand Hippolyte Louis Fizeau was a French physicist.-Biography:Fizeau was born in Paris. His earliest work was concerned with improvements in photographic processes. Following suggestions by François Arago, Léon Foucault and Fizeau collaborated in a series of investigations on the interference of...
he carried out a series of investigations on the intensity of the light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
of the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, as compared with that of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
in the arc lamp
Arc lamp
"Arc lamp" or "arc light" is the general term for a class of lamps that produce light by an electric arc . The lamp consists of two electrodes, first made from carbon but typically made today of tungsten, which are separated by a gas...
, and of lime
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
in the flame of the oxyhydrogen blowpipe; on the interference of infrared radiation, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light.
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 corpuscle theory of light when it showed that light travels more slowly through water than through air.In 1851, he provided the first experimental demonstration of the rotation of the Earth on its axis (see diurnal motion
Diurnal motion
Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for...
). He achieved this by showing the rotation of the plane of oscillation of a long and heavy pendulum suspended from the roof of the Panthéon in Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
. The experiment caused a sensation in both the learned and popular worlds, and "Foucault pendulum
Foucault pendulum
The Foucault pendulum , or Foucault's pendulum, named after the French physicist Léon Foucault, is a simple device conceived as an experiment to demonstrate the rotation of the Earth. While it had long been known that the Earth rotated, the introduction of the Foucault pendulum in 1851 was the...
s" were suspended in major cities across Europe and America and attracted crowds. In the following year he used (and named) the gyroscope
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
as a conceptually simpler experimental proof. In 1855, he received the Copley Medal
Copley Medal
The Copley Medal is an award given by the Royal Society of London for "outstanding achievements in research in any branch of science, and alternates between the physical sciences and the biological sciences"...
of the Royal Society
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
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 required for the rotation of a copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
disc becomes greater when it is made to rotate with its rim between the poles of a magnet
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, and attracts or repels other magnets.A permanent magnet is an object...
, the disc at the same time becoming heated by the eddy current
Eddy current
Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
or "Foucault currents" induced in the metal.
Foucault knife-edge test
The Foucault knife-edge test was described in 1858 by French physicist Léon Foucault to measure conic shapes of optical mirrors, with error margins measurable in fractions of wavelengths of light . It is commonly used by amateur telescope makers for figuring small astronomical mirrors...
" allows the worker to tell if the mirror is perfectly spherical or has non-spherical deviation in its figure
Figuring
Figuring is the process of final polishing of an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.-Types of figuring:...
. 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. The test focuses light point source at the center of curvature 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
Charles Wheatstone
Sir Charles Wheatstone FRS , was an English scientist and inventor of many scientific breakthroughs of the Victorian era, including the English concertina, the stereoscope , and the Playfair cipher...
’s revolving mirror he, in 1862, determined the speed of light to be 298,000 km
Kilometre
The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...
/s
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
(about 185,000 mi.
Mile
A mile is a unit of length, most commonly 5,280 feet . The mile of 5,280 feet is sometimes called the statute mile or land mile to distinguish it from the nautical mile...
/s) —10,000 km/s less than that obtained by previous experimenters and only 0.6% off the currently accepted value.
Later years
In that year, he was made a member of the Bureau des LongitudesBureau des Longitudes
The Bureau des Longitudes is a French scientific institution, founded by decree of 25 June 1795 and charged with the improvement of nautical navigation, standardisation of time-keeping, geodesy and astronomical observation. During the 19th century, it was responsible for synchronizing clocks...
and an officer of the Légion d'Honneur
Légion d'honneur
The Legion of Honour, or in full the National Order of the Legion of Honour is a French order established by Napoleon Bonaparte, First Consul of the Consulat which succeeded to the First Republic, on 19 May 1802...
. In 1864 he was made a member of the Royal Society
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
of London, and the next year a member of the mechanical section of the Institute. In 1865 his papers on a modification of Watt
James Watt
James Watt, FRS, FRSE was a Scottish inventor and mechanical engineer whose improvements to the Newcomen steam engine were fundamental to the changes brought by the Industrial Revolution in both his native Great Britain and the rest of the world.While working as an instrument maker at the...
's governor appeared, upon which he had for some time been experimenting with a view to making its period of revolution constant, and on a new apparatus for regulating the electric light; and in the year (Compt. Rend. lxiii.) he showed how, by the deposition of a transparently thin film of silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
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.
Death and afterwards
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...
on February 11, 1868 in Paris and was buried in the Cimetière de Montmartre.
The asteroid 5668 Foucault
5668 Foucault
5668 Foucault is a main-belt asteroid discovered on March 22, 1984 by Antonín Mrkos at Klet. It is named for the French physicist Léon Foucault.-External links:*...
is named for him. His name is one of the 72 names inscribed on the Eiffel Tower.
Further reading
- Amir D. Aczel, Pendulum: Léon Foucault and the Triumph of Science, Washington Square Press, 2003, ISBN 0-7434-6478-8
- Umberto Eco, Foucault's Pendulum (trans. William Weaver). Secker & Warburg, 1989.
- William Tobin, Perfecting the Modern Reflector. Sky & Telescope, October 1987.
- William Tobin, Léon Foucault. Scientific American, July 1998.
- William Tobin, The Life and Science of Léon Foucault: The Man who Proved the Earth Rotates. Cambridge University Press, 2003. ISBN 0-521-80855-3