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During the French Revolution, the traditional units of measure were to be replaced by consistent measures based on natural phenomena. As a base unit of length, scientists had favoured the
seconds pendulum A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 0.5 Hz. Principles A pendulum is a weight suspended from a pivot so tha ...
(a pendulum with a half-period of one second) one century earlier, but this was rejected as it had been discovered that this length varied from place to place with local gravity. A new unit of length, the ''
metre The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...
'' was introduced – defined as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an
Earth flattening An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the figure of the Earth, Earth's form, used as a frame of reference, reference frame for computations in geodesy, astronomy, and the geosciences. Various different ell ...
of . Following the
arc measurement of Delambre and Méchain The arc measurement of Delambre and Méchain was a geodetic survey carried out by Jean-Baptiste Delambre and Pierre Méchain in 1792–1798 to measure an arc section of the Paris meridian between Dunkirk and Barcelona. This arc measurement ser ...
, the historical French official standard of the metre was made available in the form of the , a platinum bar held in Paris. During the mid nineteenth century, following the
American Revolution The American Revolution (1765–1783) was a colonial rebellion and war of independence in which the Thirteen Colonies broke from British America, British rule to form the United States of America. The revolution culminated in the American ...
and independence of Latin America, the metre gained adoption in
Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...
, particularly in scientific usage, and it was officially established as an international measurement unit by the
Metre Convention The Metre Convention (), also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations: Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, German Empire, Ge ...
of 1875 at the beginning of the
Second Industrial Revolution The Second Industrial Revolution, also known as the Technological Revolution, was a phase of rapid Discovery (observation), scientific discovery, standardisation, mass production and industrialisation from the late 19th century into the early ...
. The and its copies such as the
Committee Meter Metrication is the process of introducing the International System of Units, also known as SI units or the metric system, to replace a jurisdiction's traditional measuring units. U.S. customary units have been defined in terms of metric unit ...
were replaced from 1889 at the initiative of the International Geodetic Association by thirty platinum-iridium bars kept across the globe. A better
standardisation Standardization (American English) or standardisation (British English) is the process of implementing and developing technical standards based on the consensus of different parties that include firms, users, interest groups, standards organiza ...
of the new prototypes of the metre and their comparison with each other and with the historical standard involved the development of specialised measuring equipment and the definition of a reproducible temperature scale. In collaboration with the International Geodetic Association created to measure the Earth, the
International Bureau of Weights and Measures The International Bureau of Weights and Measures (, BIPM) is an List of intergovernmental organizations, intergovernmental organisation, through which its 64 member-states act on measurement standards in areas including chemistry, ionising radi ...
became the world reference center for the measurement of geodetic bases thanks to the discovery of
invar Invar, also known generically as FeNi36 (64FeNi in the US), is a nickel–iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). The name ''Invar'' comes from the word ''invariable'', referring to its relative lac ...
, an alloy of nickel and iron with a coefficient of
thermal expansion Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...
close to zero. Progress in science finally allowed the definition of the metre to be dematerialised; thus in 1960 a new definition based on a specific number of wavelengths of light from a specific transition in
krypton-86 There are 34 known isotopes of krypton (36Kr) with atomic mass numbers from 67 to 103. Naturally occurring krypton is made of five stable isotopes and one () which is slightly radioactive with an extremely long half-life, plus traces of radioiso ...
allowed the standard to be universally available by measurement. In 1983 this was updated to a length defined in terms of the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...
; this definition was reworded in 2019: Where older traditional length measures are still used, they are now defined in terms of the metre – for example the
yard The yard (symbol: yd) is an English units, English unit of length in both the British imperial units, imperial and US United States customary units, customary systems of measurement equalling 3 foot (unit), feet or 36 inches. Sinc ...
has since 1959 officially been defined as exactly 0.9144 metre.


Background

Historically, units of measurement varied greatly, even when called by the same name. Some kingdoms and other polities standardised some measurements, but in others, such as France before the French Revolution, units could still vary from place to place. During the
Scientific Revolution The Scientific Revolution was a series of events that marked the emergence of History of science, modern science during the early modern period, when developments in History of mathematics#Mathematics during the Scientific Revolution, mathemati ...
, various "universal measures" of length were proposed which would be based on reproducible natural phenomena, in particular the pendulum and the Earth.


The seconds pendulum

In around 1602,
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 â€“ 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
observed that the regular swing of the
pendulum A pendulum is a device made of a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate i ...
depended on its length. In 1645
Giovanni Battista Riccioli Giovanni Battista Riccioli (17 April 1598 – 25 June 1671) was an Italian astronomer and a Catholic priest in the Jesuit order. He is known, among other things, for his experiments with pendulums and with falling bodies, for his discussion of ...
determined the length of a pendulum whose swing is one second each way, a "
seconds pendulum A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 0.5 Hz. Principles A pendulum is a weight suspended from a pivot so tha ...
". In 1671,
Jean Picard Jean Picard (21 July 1620 – 12 July 1682) was a French astronomer and priest born in La Flèche, where he studied at the Jesuit Collège Royal Henry-Le-Grand. He is principally notable for his accurate measure of the size of the Earth, ...
proposed this length as a unit of measurement to be called the ''Rayon Astronomique'' (astronomical radius). In 1675, Tito Livio Burattini suggested calling it ' (universal measure). However in 1671–1673, astronomer
Jean Richer Jean Richer (1630–1696) was a French astronomer and assistant (''élève astronome'') at the French Academy of Sciences, under the direction of Giovanni Domenico Cassini. Between 1671 and 1673 he performed experiments and carried out celestial ...
discovered that the length of a seconds pendulum also varies from place to place by as much as 0.28%.


The Earth

In the 18th century, the
French Academy of Sciences The French Academy of Sciences (, ) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific method, scientific research. It was at the forefron ...
organised work on
cartography Cartography (; from , 'papyrus, sheet of paper, map'; and , 'write') is the study and practice of making and using maps. Combining science, aesthetics and technique, cartography builds on the premise that reality (or an imagined reality) can ...
and
geodesy Geodesy or geodetics is the science of measuring and representing the Figure of the Earth, geometry, Gravity of Earth, gravity, and Earth's rotation, spatial orientation of the Earth in Relative change, temporally varying Three-dimensional spac ...
which included measuring the size and shape of the Earth. Through surveys in
Ecuador Ecuador, officially the Republic of Ecuador, is a country in northwestern South America, bordered by Colombia on the north, Peru on the east and south, and the Pacific Ocean on the west. It also includes the Galápagos Province which contain ...
and Lapland it was found that the earth is not a perfect sphere but rather an
oblate spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has circu ...
.


Decimals

Using a decimal scale for measurements was proposed by
Simon Stevin Simon Stevin (; 1548–1620), sometimes called Stevinus, was a County_of_Flanders, Flemish mathematician, scientist and music theorist. He made various contributions in many areas of science and engineering, both theoretical and practical. He a ...
, a Flemish mathematician in 1586.


Mètre des Archives

In 1790, during the French Revolution, the National Convention tasked the
French Academy of Sciences The French Academy of Sciences (, ) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific method, scientific research. It was at the forefron ...
with reforming the units of measurement. The Academy formed a commission, which rejected using the pendulum as a unit of length and decided that the new measure should be equal to one ten-millionth of the distance from the North Pole to the Equator (a quadrant of the Earth's circumference). This was to be measured along the meridian passing through the
Panthéon The Panthéon (, ), is a monument in the 5th arrondissement of Paris, France. It stands in the Latin Quarter, Paris, Latin Quarter (Quartier latin), atop the , in the centre of the , which was named after it. The edifice was built between 1758 ...
in Paris. However, pending completion of that work, a measurement from
Dunkirk Dunkirk ( ; ; ; Picard language, Picard: ''Dunkèke''; ; or ) is a major port city in the Departments of France, department of Nord (French department), Nord in northern France. It lies from the Belgium, Belgian border. It has the third-larg ...
to Collioure made in 1740 was used, and following legislation on 7 April 1795, provisional metal metre bars were distributed in France in 1795-1796. In 1799, the measurement of part of the meridian, from Dunkirk to Barcelona, was completed and a correction for the Earth's non-spherical shape calculated from that and another survey. A metre bar was accordingly made of platinum and designated by law as the primary standard metre. This was kept in the National Archives and known as the . Another platinum metre, calibrated against the , and twelve iron ones were made as secondary standards.


Adoption

One of the iron metre standards was brought to the United States in 1805. It became known as the Committee Meter in the United States and served as a standard of length in the
United States Coast Survey United may refer to: Places * United, Pennsylvania, an unincorporated community * United, West Virginia, an unincorporated community Arts and entertainment Films * ''United'' (2003 film), a Norwegian film * ''United'' (2011 film), a BBC Two f ...
until 1890. In 1855, the Dufour map (French: ''Carte Dufour''), the first topographic map of Switzerland for which the metre was adopted as the unit of length, won the gold medal at the Exposition Universelle. On the sidelines of the
Exposition Universelle (1855) The of 1855 (), better known in English as the 1855 Paris Exposition, was a world's fair held on the Champs-Élysées in Paris, France, from 15 May to 15 November 1855. Its full official title was the . It was the first of ten major expositions ...
and the second Congress of Statistics held in Paris, an association with a view to obtaining a uniform decimal system of measures, weights and currencies was created in 1855. A Committee for Weights and Measures and Monies (French: ''Comité des poids, mesures et monnaies'') was created during the
Exposition Universelle (1867) The of 1867 (), better known in English as the 1867 Paris Exposition, was a world's fair held in Paris, Second French Empire, France, from 1 April to 3 November 1867. It was the List of world expositions, second of ten major expositions held i ...
in Paris and called for the international adoption of the metric system. In the United States, the
Metric Act of 1866 The Metric Act of 1866, also known as the Kasson Act, is a piece of United States legislation that legally protected use of the metric system in commerce from lawsuit, and provided an official conversion table from United States customary units ...
allowed the use of the metre in the United States, and in 1867 the General Conference of the European Arc Measurement (German: ''Europäische Gradmessung'') established the
International Bureau of Weights and Measures The International Bureau of Weights and Measures (, BIPM) is an List of intergovernmental organizations, intergovernmental organisation, through which its 64 member-states act on measurement standards in areas including chemistry, ionising radi ...
. At the
Metre Convention The Metre Convention (), also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations: Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, German Empire, Ge ...
of 1875 the metre was adopted as an international scientific unit of length.


International prototype metre

In the late nineteenth century, a new international standard metre, called a "prototype", was made along with copies to serve as national standards. It was a "line standard", with the metre was defined as the distance between two lines marked on the bar, to make any wear at the ends irrelevant. The construction was at the limits of technology. The bars were made of a special alloy, 90% 
platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
and 10% 
iridium Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group, is considered the second-densest naturally occurring metal (after osmium) with a density ...
, significantly harder than pure platinum, and have a special X-shaped cross section (a " Tresca section", named after French engineer Henri Tresca) to minimise the effects of torsional strain during length comparisons. The first castings proved unsatisfactory, and the job was given to the London firm of
Johnson Matthey Johnson Matthey plc is a British multinational speciality chemicals and sustainable technologies company headquartered in London, England. It is listed on the London Stock Exchange and is a constituent of the FTSE 250 Index. History Early year ...
who succeeded in producing thirty bars to the required specification. One of these, No. 6, was determined to be identical in length to the ', and was designated the international prototype metre at the first meeting of the CGPM in 1889. The other bars, duly calibrated against the international prototype, were distributed to the signatory nations of the Metre Convention for use as national standards. For example, the United States received No. 27 with a calibrated length of (1.6 Î¼m short of the international prototype). As bar lengths vary with temperature, precise measurements required known and stable temperatures and could even be affected by a scientist's body heat, so standard metres were provided with precise thermometers. The first (and only) follow-up comparison of the national standards with the international prototype was carried out between 1921 and 1936, and indicated that the definition of the metre was preserved to within 0.2 Î¼m. At this time, it was decided that a more formal definition of the metre was required (the 1889 decision had said merely that the "prototype, at the temperature of melting ice, shall henceforth represent the metric unit of length"), and this was agreed at the 7th CGPM in 1927. These support locations are at the
Bessel points Airy points (after George Biddell Airy) are used for precision measurement (metrology) to support a length standard in such a way as to minimise bending or wiktionary:Drop, drop of a horizontally supported Beam (structure), beam. Choice of support ...
of the prototypethe support points, separated by 0.5594 of the total length of the bar, that minimise shortening of the bar due to
bending In applied mechanics, bending (also known as flexure) characterizes the behavior of a slender structural element subjected to an external Structural load, load applied perpendicularly to a longitudinal axis of the element. The structural eleme ...
under its own weight. Because the prototype is a line standard, its full length is 102 cm, slightly longer than 1 metre. Cross-sectionally, it measures 16 mm Ã— 16 mm.


From standard bars to wavelength of light

Charles Sanders Peirce Charles Sanders Peirce ( ; September 10, 1839 – April 19, 1914) was an American scientist, mathematician, logician, and philosopher who is sometimes known as "the father of pragmatism". According to philosopher Paul Weiss (philosopher), Paul ...
's work promoted the advent of American science at the forefront of global metrology. Alongside his intercomparisons of artifacts of the metre and contributions to gravimetry through improvement of reversible pendulum, Peirce was the first to tie experimentally the metre to the wave length of a spectral line. According to him the standard length might be compared with that of a wave of light identified by a line in the
solar spectrum Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically perc ...
.
Albert Abraham Michelson Albert Abraham Michelson ( ; December 19, 1852 – May 9, 1931) was an American physicist known for his work on measuring the speed of light and especially for the Michelson–Morley experiment. In 1907, he received the Nobel Prize in Physics, ...
soon took up the idea and improved it.


Interferometric options

The first
interferometric Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...
measurements carried out using the international prototype metre were those of
Albert A. Michelson Albert Abraham Michelson ( ; December 19, 1852 – May 9, 1931) was an American physicist known for his work on measuring the speed of light and especially for the Michelson–Morley experiment. In 1907, he received the Nobel Prize in Physics, ...
and Jean-René Benoît (1892–1893) and of Benoît, Fabry and Perot (1906), both using the red line of
cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...
. These results, which gave the
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
of the cadmium line (''λ'' â‰ˆ 644 nm), led to the definition of the
ångström The angstrom (; ) is a unit of length equal to m; that is, one ten- billionth of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. The unit is named after the Swedish physicist Anders Jonas Ångström (1814� ...
as a secondary unit of length for spectroscopic measurements, first by the International Union for Cooperation in Solar Research (1907) and later by the
CIPM The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre ...
(1927). Michelson's work in "measuring" the prototype metre to within of a wavelength ( Î¼m) was one of the reasons for which he was awarded the
Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...
in 1907. By the 1950s, interferometry had become the method of choice for precise measurements of length, but there remained a practical problem imposed by the system of units used. The natural unit for expressing a length measured by interferometry was the ångström, but this result then had to be converted into metres using an experimental conversion factor – the wavelength of light used, but measured in metres rather than in ångströms. This added an additional
measurement uncertainty In metrology, measurement uncertainty is the expression of the statistical dispersion of the values attributed to a quantity measured on an interval or ratio scale. All measurements are subject to uncertainty and a measurement result is complet ...
to any length result in metres, over and above the uncertainty of the actual interferometric measurement. The solution was to define the metre in the same manner as the angstrom had been defined in 1907, that is in terms of the best interferometric wavelength available. Advances in both experimental technique and theory showed that the cadmium line was actually a cluster of closely separated lines, and that this was due to the presence of different
isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
s in natural cadmium (eight in total). To get the most precisely defined line, it was necessary to use a monoisotopic source and this source should contain an isotope with even numbers of protons and neutrons (so as to have zero
nuclear spin Nuclear may refer to: Physics Relating to the nucleus of the atom: * Nuclear engineering * Nuclear physics * Nuclear power * Nuclear reactor * Nuclear weapon * Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics * Nuclear space * ...
). Several isotopes of
cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...
,
krypton Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...
and mercury both fulfil the condition of zero nuclear spin and have bright lines in the visible region of the spectrum.


Krypton standard

Krypton is a gas at room temperature, allowing for easier
isotopic enrichment Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
and lower operating temperatures for the lamp (which reduces broadening of the line due to the
Doppler effect The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described ...
), and so it was decided to select the orange line of
krypton-86 There are 34 known isotopes of krypton (36Kr) with atomic mass numbers from 67 to 103. Naturally occurring krypton is made of five stable isotopes and one () which is slightly radioactive with an extremely long half-life, plus traces of radioiso ...
(''λ'' â‰ˆ 606 nm) as the new wavelength standard. Accordingly, the 11th 
CGPM The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre ...
in 1960 agreed a new definition of the metre: The measurement of the wavelength of the krypton line was ''not'' made directly against the international prototype metre; instead, the ratio of the wavelength of the krypton line to that of the cadmium line was determined in vacuum. This was then compared to the 1906 Fabry–Perot determination of the wavelength of the cadmium line in air (with a correction for the
refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
of air). In this way, the new definition of the metre was traceable to both the old prototype metre and the old definition of the angstrom.


Speed of light standard

The krypton-86 discharge lamp operating at the
triple point In thermodynamics, the triple point of a substance is the temperature and pressure at which the three Phase (matter), phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at ...
of
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
(63.14 K, −210.01 Â°C) was the state-of-the-art light source for interferometry in 1960, but it was soon to be superseded by a new invention: the
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
, of which the first working version was constructed in the same year as the redefinition of the metre. Laser light is usually highly monochromatic, and is also coherent (all the light has the same
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...
, unlike the light from a discharge lamp), both of which are advantageous for interferometry. The shortcomings of the krypton standard were demonstrated by the measurement of the wavelength of the light from a
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
-stabilised
helium–neon laser A helium–neon laser or He–Ne laser is a type of gas laser whose high energetic gain medium consists of a mixture of helium and neon (ratio between 5:1 and 10:1) at a total pressure of approximately 1 Torr (133.322 Pa) inside a small electr ...
(''λ'' â‰ˆ 3.39 Î¼m). The krypton line was found to be asymmetrical, so different wavelengths could be found for the laser light depending on which point on the krypton line was taken for reference. The asymmetry also affected the precision to which the wavelengths could be measured. Developments in electronics also made it possible for the first time to measure the frequency of light in or near the visible region of the spectrum, instead of inferring the frequency from the wavelength and the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...
. Although visible and infrared frequencies were still too high to be directly measured, it was possible to construct a "chain" of laser frequencies that, by suitable multiplication, differ from each other by only a directly measurable frequency in the
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
region. The frequency of the light from the methane-stabilised laser was found to be  THz. Independent measurements of frequency and wavelength are, in effect, a measurement of the speed of light (''c'' = ''fλ''), and the results from the methane-stabilised laser gave the value for the speed of light with an
uncertainty Uncertainty or incertitude refers to situations involving imperfect or unknown information. It applies to predictions of future events, to physical measurements that are already made, or to the unknown, and is particularly relevant for decision ...
almost 100 times lower than previous measurements in the microwave region. Or, somewhat inconveniently, the results gave ''two'' values for the speed of light, depending on which point on the krypton line was chosen to define the metre. This ambiguity was resolved in 1975, when the 15th 
CGPM The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre ...
approved a conventional value of the speed of light as exactly . Nevertheless, the infrared light from a methane-stabilised laser was inconvenient for use in practical interferometry. It was not until 1983 that the chain of frequency measurements reached the 633 nm line of the helium–neon laser, stabilised using molecular
iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...
. That same year, the 17th CGPM adopted a definition of the metre, in terms of the 1975 conventional value for the speed of light: :The metre is the length of the path travelled by light in vacuum during a time interval of of a second. This definition was reworded in 2019: :The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum to be when expressed in the unit mâ‹…s, where the second is defined in terms of the caesium frequency . The concept of defining a unit of length in terms of a time received some comment. In both cases, the practical issue is that time can be measured more accurately than length (one part in 1013 for a second using a
caesium clock The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Loui ...
as opposed to four parts in 109 for the metre in 1983). The definition in terms of the speed of light also means that the metre can be realised using any light source of known frequency, rather than defining a "preferred" source in advance. Given that there are more than 22,000 lines in the visible spectrum of iodine, any of which could be potentially used to stabilise a laser source, the advantages of flexibility are obvious.


Summary of definitions since 1798


See also

* Hebdomometre *
Length measurement Length measurement, distance measurement, or range measurement (ranging) all refer to the many ways in which length, distance, or range can be measured. The most commonly used approaches are the rulers, followed by transit-time methods and the ...
* History of geodesy#Prime_meridian_and_standard_of_length * * Paris meridian#History


Notes


References


External links

* {{cite EB1911, wstitle=Metric System , volume= 18 , page= 299 *
Metre The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...