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History of the metre
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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 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. The mètre 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 of 1/334.[1]
Following the arc measurement of Delambre and Méchain, the historical French official standard of the metre was made available in the form of the Mètre des Archives, a platinum bar held in Paris. It was originally also planned to dematerialize the definition of the metre by counting the number of swings of a one-metre-long pendulum during a day at a latitude of 45°.[2] However, dematerializing the definition of units of length by means of the pendulum would prove less reliable than artefacts.[3][4]
During the mid nineteenth century, following the American Revolution and the decolonization of the Americas, the metre gained adoption in Americas, particularly in scientific usage, and it was officially established as an international measurement unit by the Metre Convention of 1875 at the beginning of the Second Industrial Revolution.
The Mètre des Archives and its copies such as the Committee Meter were replaced from 1889 at the initiative of the International Association of Geodesy by thirty platinum-iridium bars kept across the globe.[5] A better standardisation 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.[6]
In collaboration with the International Geodetic Association created to measure the Earth, the International Bureau of Weights and Measures became the world reference centre for the measurement of geodetic bases thanks to the discovery of invar, an alloy of nickel and iron with a coefficient of thermal expansion close to zero.[7][8]
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 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; this definition was reworded in 2019:[9]
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 c to be 299792458 when expressed in the unit m⋅s−1, where the second is defined in terms of the caesium frequency ΔνCs.
Where older traditional length measures are still used, they are now defined in terms of the metre – for example the yard has since 1959 officially been defined as exactly 0.9144 metre.[10]
Background
[edit]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, various "universal measures" of length were proposed which would be based on reproducible natural phenomena, in particular the pendulum and the Earth.
Decimals
[edit]Using a decimal scale for measurements was proposed by Simon Stevin, a Flemish mathematician in 1586.[11][12]
The seconds pendulum and the Earth
[edit]In the 18th century, the French Academy of Sciences organised work on cartography and geodesy which included measuring the size and shape of the Earth.[13] Through surveys in Ecuador and Lapland it was found that the Earth is not a perfect sphere but rather an oblate spheroid, as Newton had deduced from the variations in the seconds pendulum's length with latitude.[14]
In around 1602, Galileo had observed that the regular swing of the pendulum depended on its length.[15] In 1645 Giovanni Battista Riccioli had determined the length of a pendulum whose swing is one second each way, a "seconds pendulum".[16]
In 1671, Jean Picard proposed this length as a unit of measurement to be called the Rayon Astronomique (astronomical radius).[17][18][19] In 1675, Tito Livio Burattini suggested calling it metro cattolico (universal measure).[20] However in 1671–1673, astronomer Jean Richer discovered that the length of a seconds pendulum varies from place to place depending on latitude.[21][19]
In the 1790s, French scientists did not want to introduce another dimension (time) into the definition of the unit of length,[2] which was the unit on which the metric system (metre and kilogram) was based.[22] However, it was originally also planned to dematerialize the definition of the metre by counting the number of swings of a one-metre-long pendulum during a day (86,400 seconds), in a vacuum, at sea level, at the temperature of melting ice and at a latitude of 45°.[2]
The second was added to the system following a proposal by Carl Friedrich Gauss, in 1832, to base a system of absolute units on the three fundamental units of length, mass and time.[23]
Mètre des Archives
[edit]
In 1790, during the French Revolution, the National Convention tasked the French Academy of Sciences with reforming the units of measurement. The Academy formed a commission, which rejected using the pendulum as a unit of length[24] 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 centre of Paris Observatory.[25][26]
However, pending completion of that work, a measurement from Dunkirk on the English Channel to Collioure on the Mediterranean coast made in 1740 was used, and following legislation on 7 April 1795,[28] provisional metal metre bars were distributed in France in 1795-1796.[29]
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.[30][23] 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 Mètre des Archives.[31] Another platinum metre, calibrated against the Mètre des Archives, and twelve iron ones were made as secondary standards.[32]
Adoption
[edit]In the 19th century, measuring instruments calibrated on the metre were devised for American, Spanish and Egyptian cartography.
One of the iron metre standards was brought to the United States in 1805.[33] It became known as the Committee Meter in the United States and served as a standard of length in the United States Coast Survey until 1890.[34][33][35][36]
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.[37][38] On the sidelines of the Exposition Universelle (1855) 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.[1] A Committee for Weights and Measures and Monies (French: Comité des poids, mesures et monnaies) was created during the Exposition Universelle (1867) in Paris and called for the international adoption of the metric system.[7][1]
In the United States, the Metric Act of 1866 allowed the use of the metre in the United States,[39] and in 1867 the General Conference of the European Arc Measurement (German: Europäische Gradmessung) proposed the creation of the International Bureau of Weights and Measures.[40][41]
In 1869, the Saint Petersburg Academy of Sciences sent a report inviting his French counterpart to undertake joint action to ensure the universal use of the metric system in all scientific work.[4] The French Academy of Sciences and the Bureau des Longitudes in Paris drew the attention of the French government to this issue. The same year, Napoleon III issued invitations to join the International Metre Commission.[7]
The Commission called for the creation of a new international prototype metre which length would be as close as possible to that of the Mètre des Archives and the arrangement of a system where national standards could be compared with it.[4]
At the Metre Convention of 1875 the metre was adopted as an international scientific unit of length.
International prototype metre
[edit]
In the late nineteenth century, a new international standard metre, called a "prototype",[a] was made along with copies to serve as national standards. It was a "line standard": the metre was defined as the distance between two lines marked on the bar, to make any wear at the ends irrelevant.[22][1] When replacing British standard measurements after the burning of parliament, William Simms inaugurated the principle, which inspired Henri Tresca, of marking lines indicating the length of the unit on the neutral plane of the standard.[4]
The construction was at the limits of technology. The bars were made of a special alloy, 90% platinum and 10% iridium, 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.[10][1] The first castings proved unsatisfactory, and the job was given to the London firm of Johnson Matthey who succeeded in producing thirty bars to the required specification. One of these, No. 6, was determined to be identical in length to the mètre des Archives, 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.[42] For example, the United States received No. 27 with a calibrated length of 0.9999984 m ± 0.2 μm (1.6 μm short of the international prototype).[43][1]
As bar lengths vary with temperature, precise measurements required known and stable temperatures and could even be affected by a scientist's body heat,[44] so standard metres were provided with precise thermometers.[45]
The first (and only) follow-up comparison of the national standards with the international prototype was carried out between 1921 and 1936,[10][42] and indicated that the definition of the metre was preserved to within 0.2 μm.[46] 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.[47]
The unit of length is the metre, defined by the distance, at 0°, between the axes of the two central lines marked on the bar of platinum–iridium kept at the Bureau International des Poids et Mesures and declared Prototype of the metre by the 1st Conférence Générale des Poids et Mesures, this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.
These support locations are at the Bessel points of the prototype – the support points, separated by 0.5594 of the total length of the bar,[48] that minimise shortening of the bar due to bending under its own weight.[49] Because the prototype is a line standard, its full length is 102 cm, slightly longer than 1 metre.[50][51] Cross-sectionally, it measures 16 mm × 16 mm.[52]
The representation of the unit of length by means of the distance between two fine lines on the surface of a bar of metal at a certain temperature is never itself free from uncertainty and probable error, owing to the difficulty of knowing at any moment the precise temperature of the bar; and the transference of this unit, or a multiple of it, to a measuring bar will be affected not only with errors of observation, but with errors arising from uncertainty of temperature of both bars. If the measuring bar be not self-compensating for temperature, its expansion must be determined by very careful experiments. The thermometers required for this purpose must be very carefully studied, and their errors of division and index error determined.[53] In the 19th century, careful comparisons with several standard toises showed that the Mètre des Archives was not exactly equal to the legal metre or 443.296 lines of the toise of Peru, but, in round numbers, 1/75 000 of the length smaller,[14] or approximately 0.013 millimetres. Moreover, we now know that the metre is 0.197 millimetres shorter than it should be according to its original proposed definition, mainly due to not taking into account a vertical deflection in the southern end of the arc measurement of Delambre and Méchain.[54][55]
From standard bars to wavelength of light
[edit]Charles Sanders Peirce'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 the 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. Albert Abraham Michelson soon took up the idea and improved it.[3][56]
Interferometric options
[edit]
The first interferometric measurements carried out using the international prototype metre were those of Albert A. Michelson and Jean-René Benoît (1892–1893)[57] and of Benoît, Fabry and Perot (1906),[58] both using the red line of cadmium. These results, which gave the wavelength of the cadmium line (λ ≈ 644 nm), led to the definition of the ångström as a secondary unit of length for spectroscopic measurements, first by the International Union for Cooperation in Solar Research (1907)[59] and later by the CIPM (1927).[42][60] Michelson's work in "measuring" the prototype metre to within 1⁄10 of a wavelength (< 0.1 μm) was one of the reasons for which he was awarded the Nobel Prize in Physics in 1907.[10][42][61]
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 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 isotopes 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).[10]
Several isotopes of cadmium, krypton and mercury both fulfil the condition of zero nuclear spin and have bright lines in the visible region of the spectrum.
Krypton standard
[edit]Krypton is a gas at room temperature, allowing for easier isotopic enrichment and lower operating temperatures for the lamp (which reduces broadening of the line due to the Doppler effect), and so it was decided to select the orange line of krypton-86 (λ ≈ 606 nm) as the new wavelength standard.[10][62]
Accordingly, the 11th CGPM in 1960 agreed a new definition of the metre:[47]
The metre is the length equal to 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom.
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 of air).[10][46] 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
[edit]
The krypton-86 discharge lamp operating at the triple point of nitrogen (63.14 K, −210.01 °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, of which the first working version was constructed in the same year as the redefinition of the metre.[63] Laser light is usually highly monochromatic, and is also coherent (all the light has the same phase, unlike the light from a discharge lamp), both of which are advantageous for interferometry.[10]
The shortcomings of the krypton standard were demonstrated by the measurement of the wavelength of the light from a methane-stabilised helium–neon laser (λ ≈ 3.39 μm). 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.[b] The asymmetry also affected the precision to which the wavelengths could be measured.[64][65]
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,[further explanation needed] instead of inferring the frequency from the wavelength and the speed of light. 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 region. The frequency of the light from the methane-stabilised laser was found to be 88.376 181 627(50) THz.[64][66]
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 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.[c] This ambiguity was resolved in 1975, when the 15th CGPM approved a conventional value of the speed of light as exactly 299 792 458 m s−1.[67]
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.[68][69] That same year, the 17th CGPM adopted a definition of the metre, in terms of the 1975 conventional value for the speed of light:[70]
- The metre is the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second.
This definition was reworded in 2019:[9]
- 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 c to be 299792458 when expressed in the unit m⋅s−1, where the second is defined in terms of the caesium frequency ΔνCs.
The concept of defining a unit of length in terms of a time received some comment.[71] 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 as opposed to four parts in 109 for the metre in 1983).[60][71] 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.[71]
Summary of definitions since 1798
[edit]| Basis of definition | Date | Absolute uncertainty |
Relative uncertainty |
|---|---|---|---|
| 1⁄10,000,000 part of one half of a meridian, measurement by Delambre and Méchain | 1798 | 0.5–0.1 mm | 10−4 |
| First prototype Mètre des Archives platinum bar standard | 1799 | 0.05–0.01 mm | 10−5 |
| Platinum-iridium bar at melting point of ice (1st CGPM) | 1889 | 0.2–0.1 μm | 10−7 |
| Platinum-iridium bar at melting point of ice, atmospheric pressure, supported by two rollers (7th CGPM) | 1927 | n/a | n/a |
| 1,650,763.73 wavelengths of light from a specified transition in krypton-86 (11th CGPM) | 1960 | 0.01–0.005 μm | 10−8 |
| Length of the path travelled by light in a vacuum in 1⁄299,792,458 of a second (17th CGPM) | 1983 | 0.1 nm | 10−10 |
See also
[edit]Notes
[edit]- ^ The term "prototype" does not imply that it was the first in a series and that other standard metres would come after it: the "prototype" metre was the one that came first in the chain of comparisons, the metre to which all other standards were compared.
- ^ Taking the point of highest intensity as the reference wavelength, the methane line had a wavelength of 3.392 231 404(12) μm; taking the intensity-weighted mean point ("centre of gravity") of the krypton line as the standard, the wavelength of the methane line is 3.392 231 376(12) μm.
- ^ The measured speed of light was 299 792.4562(11) km s−1 for the "centre-of-gravity" definition and 299 792.4587(11) km s−1 for the maximum-intensity definition, with a relative uncertainty ur = 3.5×10−9.
References
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- ^ Levallois, J.J. (1991). "La méridienne de Dunkerque à Barcelone et la déterminiation du mètre (1792-1799)". Vermessung, Photogrammetrie, Kulturtechnik: VPK = Mensuration, photogrammétrie, génie rural. 89 (7): 375. doi:10.5169/seals-234595. ISSN 0252-9424.
- ^ Lenzen, Victor F. (1965). "The Contributions of Charles S. Peirce to Metrology". Proceedings of the American Philosophical Society. 109 (1): 29–46. ISSN 0003-049X. JSTOR 985776.
- ^ Michelson, A.A.; Benoît, Jean-René (1895). "Détermination expérimentale de la valeur du mètre en longueurs d'ondes lumineuses". Travaux et Mémoires du Bureau International des Poids et Mesures (in French). 11 (3): 85.
- ^ Benoît, Jean-René; Fabry, Charles; Perot, A. (1907). "Nouvelle détermination du Mètre en longueurs d'ondes lumieuses". Comptes rendus hebdomadaires des séances de l'Académie des sciences (in French). 144: 1082–1086.
- ^ "Détermination de la valeur en Ångströms de la longeur d'onde de la raie rouge du Cadmium considérée comme étalon primaire" [Determination of the value in Ångströms of the wavelength of the red line of cadmium under consideration as a primary standard]. Transactions of the International Union for Cooperation in Solar Research (in French). 2: 18–34. 21 May 1907. Bibcode:1908TIUCS...2...17.
- ^ a b Hollberg, L.; Oates, C.W.; Wilpers, G.; Hoyt, C.W.; Barber, Z.W.; Diddams, S.A.; Oskay, W.H.; Bergquist, J.C. (2005). "Optical frequency/wavelength references" (PDF). Journal of Physics B: Atomic, Molecular and Optical Physics. 38 (9): S469 – S495. Bibcode:2005JPhB...38S.469H. doi:10.1088/0953-4075/38/9/003. S2CID 53495023.
- ^ Nobel Prize in Physics 1907 – Presentation Speech, Nobel Foundation, retrieved 14 August 2010
- ^ Baird, K.M.; Howlett, L.E. (1963). "The International Length Standard". Applied Optics. 2 (5): 455–463. Bibcode:1963ApOpt...2..455B. doi:10.1364/AO.2.000455.
- ^ Maiman, T.H. (1960). "Stimulated optical radiation in ruby". Nature. 187 (4736): 493–494. Bibcode:1960Natur.187..493M. doi:10.1038/187493a0. S2CID 4224209.
- ^ a b Evenson, K.M.; Wells, J.S.; Petersen, F.R.; Danielson, B.L.; Day, G.W.; Barger, R.L.; Hall, J.L. (1972). "Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser". Physical Review Letters. 29 (19): 1346–1349. Bibcode:1972PhRvL..29.1346E. doi:10.1103/PhysRevLett.29.1346.
- ^ Barger, R.L.; Hall, J.L. (1973). "Wavelength of the 3.39-μm laser-saturated absorption line of methane". Applied Physics Letters. 22 (4): 196–199. Bibcode:1973ApPhL..22..196B. doi:10.1063/1.1654608. S2CID 1841238.
- ^ Evenson, K.M.; Day, G. W.; Wells, J.S.; Mullen, L.O. (1972). "Extension of Absolute Frequency Measurements to the cw He☒Ne Laser at 88 THz (3.39 μ)". Applied Physics Letters. 20 (3): 133–134. Bibcode:1972ApPhL..20..133E. doi:10.1063/1.1654077. S2CID 118871648.
- ^ Resolution 2 of the 15th CGPM. 15th Meeting of the General Conference on Weights and Measures. International Bureau of Weights and Measures. 1975.
- ^ Pollock, C.R.; Jennings, D.A.; Petersen, F.R.; Wells, J.S.; Drullinger, R.E.; Beaty, E.C.; Evenson, K.M. (1983). "Direct frequency measurements of transitions at 520 THz (576 nm) in iodine and 260 THz (1.15 μm) in neon". Optics Letters. 8 (3): 133–135. Bibcode:1983OptL....8..133P. doi:10.1364/OL.8.000133. PMID 19714161. S2CID 42447654.
- ^ Jennings, D.A.; Pollock, C.R.; Petersen, F.R.; Drullinger, R. E.; Evenson, K.M.; Wells, J.S.; Hall, J.L.; Layer, H.P. (1983). "Direct frequency measurement of the I2-stabilized He–Ne 473-THz (633-nm) laser". Optics Letters. 8 (3): 136–138. Bibcode:1983OptL....8..136J. doi:10.1364/OL.8.000136. PMID 19714162.
- ^ Resolution 1, 17th Meeting of the General Conference on Weights and Measures, 1983
- ^ a b c Wilkie, Tom (27 October 1983). "Time to remeasure the metre". New Scientist (27 October 1983): 258–263.
- ^ Cardarelli, François (2003). Encyclopaedia of Scientific Units, Weights and Measures. Springer-Verlag London Ltd. ISBN 978-1-4471-1122-1.
External links
[edit]- Chisholm, Hugh, ed. (1911). . Encyclopædia Britannica. Vol. 18 (11th ed.). Cambridge University Press. p. 299.
History of the metre
View on GrokipediaPre-Metric Motivations
Fragmented Length Units in History
Throughout history, length measurements were often derived from human anatomy or local customs, resulting in significant variability across regions and eras. In ancient Egypt, the royal cubit, used for monumental constructions like the pyramids, measured approximately 0.524 meters and was subdivided into 7 palms or 28 fingers, though slight variations occurred due to manufacturing tolerances in granite rods.[3] Similarly, the Roman foot (pes) was standardized at about 0.296 meters, based on the average adult male foot, and served as a basis for engineering feats such as roads and aqueducts, yet regional adaptations led to inconsistencies of up to several millimeters.[4] These units, while practical for local use, lacked universality, complicating cross-cultural exchanges in the ancient world. In medieval and early modern Europe, such fragmentation persisted and intensified, fueling practical challenges. The English yard, decreed by King Henry I in 1101 as the distance from his nose to the thumb of his outstretched arm (roughly 0.914 meters), exemplified body-based standards that varied with rulers and artisans, often causing disputes in cloth trade where a single yard's discrepancy could affect entire shipments.[5] National differences exacerbated this; the French pied (foot) measured approximately 0.325 meters, rooted in the Paris toise, while the Prussian foot (Fuss) was about 0.314 meters under the 1816 standardization.[1] These inconsistencies not only hindered accurate land surveying and construction but also sparked frequent trade conflicts, as merchants navigated mismatched standards across borders. By the 18th century, Enlightenment scholars increasingly criticized these disparate systems for impeding scientific advancement and international commerce. Jean-Charles de Borda and fellow members of the French Academy of Sciences argued that the proliferation of local units—varying significantly between nations—obstructed precise experimentation, reliable taxation, and efficient global trade, necessitating a rational, invariant alternative.[1]Enlightenment Proposals for Universal Standards
During the Age of Enlightenment, intellectuals sought rational and universal standards for measurement to overcome the fragmentation of local units derived from human anatomy or arbitrary artifacts, which hindered scientific progress and trade. A seminal proposal came from French astronomer and clergyman Gabriel Mouton in 1670, who advocated for a decimal-based system of lengths grounded in natural phenomena, specifically one minute of arc along the Earth's meridian as the base unit, subdivided decimally into smaller units like the milliare and centuria.[6] In the mid-18th century, the French Academy of Sciences engaged in discussions to establish more reliable standards, collaborating with the British Royal Society around the 1750s to explore nature-based units for length, weight, and time, aiming for universality that transcended regional variations and supported empirical science. These efforts emphasized deriving measures from invariant natural properties, such as astronomical observations or physical constants, to ensure reproducibility across borders.[4] Inspired by contemporaneous French ideas, American statesman Thomas Jefferson proposed a comprehensive decimal system for the United States in a 1790 report to Congress, recommending a base unit of length equivalent to the length of a seconds pendulum at 45° latitude to provide a globally consistent and natural standard. Jefferson's plan extended decimal divisions to weights, volumes, and currency, arguing that such a system would facilitate commerce and scientific exchange by eliminating the inconsistencies of inherited British units.[7] Key Enlightenment texts further advanced these concepts, notably the 1791 report by mathematician and philosopher Nicolas de Condorcet, then permanent secretary of the French Academy of Sciences, which called for a reformed system of weights and measures derived from immutable natural laws rather than variable human proportions like the foot or cubit, promoting decimal subdivisions for precision and universality in scientific and economic applications.[8]French Revolutionary Origins
Advocacy for Decimal Reforms
During the French Revolution, Charles Maurice de Talleyrand-Périgord, Bishop of Autun, advocated for a comprehensive reform of weights and measures to establish a universal, invariable system based on natural principles rather than arbitrary human-derived units. On March 9, 1790, Talleyrand presented a proposal to the Constituent Assembly, emphasizing the need for decimal divisions to simplify calculations and promote international trade harmony.[9] In response, the National Assembly tasked the French Academy of Sciences with developing this new framework, forming a committee in 1790 that included prominent scientists such as Jean-Charles de Borda, Joseph-Louis Lagrange, and Antoine-Laurent de Lavoisier.[1] This group, building on Enlightenment precursors like the Marquis de Condorcet, aimed to create a rational system applicable "for all times, for all peoples."[10] The push for decimal reforms was deeply intertwined with revolutionary rhetoric, portraying the new system as an embodiment of republican ideals such as equality, reason, and unity. Proponents argued that standardizing measures would eliminate regional variations that favored the privileged, ensuring "one law, one weight, and one measure" across the republic and fostering social equity by making knowledge and commerce accessible to all.[10] Talleyrand's assembly address highlighted how decimalization aligned with Enlightenment reason, freeing society from the "superstitions" of the Ancien Régime's fragmented units and promoting liberty through universal reproducibility.[9] This ideological framing positioned the reforms as a cornerstone of national unification, reflecting the Revolution's broader quest for rational governance and international harmony.[1] The decimal principle was formalized in the law of 18 Germinal, Year III (April 7, 1795), which adopted the metric system for the French Republic, mandating decimal subdivisions for all measures including length, area, volume, and weight.[11] This decree replaced the traditional toise—an inconsistent unit roughly equivalent to a man's outstretched arms—with a decimalized structure, where the old toise was eventually aligned to two meters in later adjustments to ease transition.[10] Initial efforts also extended decimals to time and angles, alongside length, with the base unit of length named the mètre, derived from the Greek word metron meaning "a measure," to signify its universal applicability.[12] The 1795 law established a provisional platinum prototype, marking the system's official implementation despite ongoing resistance.[11]Debates on Pendulum versus Meridian Definitions
In the late 1780s and early 1790s, as part of the French Revolution's push for decimal-based reforms, the French Academy of Sciences grappled with defining a new universal unit of length, the metre, leading to intense debates between two primary proposals: basing it on the length of a seconds pendulum or on a fraction of Earth's meridian arc. On October 27, 1790, an Academy committee reported in favor of a decimal system overall. A subsequent commission, formed on 16 February 1791 and chaired by Borda, focused on the length unit.[13][11] The seconds pendulum proposal, which gained traction for its conceptual simplicity and ease of replication, defined the metre as the length of a pendulum that completes a full swing (period of 2 seconds) at 45° latitude, a location chosen to balance gravitational variations. This length was calculated using the formula for a simple pendulum's period, , rearranged to , where seconds and is the local gravitational acceleration (approximately 9.806 m/s² at 45° latitude). At Paris (roughly 48.85° latitude), measurements by astronomers like Cassini yielded a length of about 0.994 m, slightly longer due to higher latitude effects on gravity, but the proposal's appeal lay in its direct tie to timekeeping without requiring extensive geodetic surveys.[14][15] Advocated prominently by Jean-Charles de Borda, a naval engineer and Academy member who had invented precise pendulum instruments, this approach emphasized practicality: the unit could be determined locally with relative ease using clocks and gravitational measurements, avoiding the logistical challenges of planetary-scale surveys. Borda, as chair of the relevant commission formed on 16 February 1791, highlighted the pendulum's "natural" basis in universal physical laws, though its dependence on local gravity made it vary slightly by location, undermining true universality.[11][15] In contrast, astronomers Jean-Baptiste Joseph Delambre and Pierre Méchain championed the meridian-based definition, proposing the metre as exactly 1/10,000,000th of the Earth's quadrant meridian—the distance from the equator to the North Pole along a great circle through Paris—for its independence from local conditions and direct connection to the planet's geometry. This method, also considered by commission members like Lagrange, Laplace, and Monge, prioritized a latitude-independent standard that reflected humanity's shared terrestrial home, arguing that the pendulum's gravitational variability (up to 0.5% across latitudes) rendered it less suitable for a global system.[14][11] Despite the pendulum's advantages in simplicity, the Academy's commission reported on 19 March 1791 in favor of the meridian definition, a decision ratified by the National Assembly on 26 March 1791, to establish a unit intrinsically linked to Earth's scale rather than time or locality. This choice underscored the era's Enlightenment ideals of rational, invariant measures, setting the stage for the subsequent geodetic expeditions while rejecting the pendulum despite its instrumental feasibility.[14][11]Creation of the Initial Prototype
The Metre des Archives Expedition
The Metre des Archives Expedition, spanning 1792 to 1799, was a pivotal geodesic survey aimed at measuring a portion of the Paris meridian to establish the metre as one ten-millionth of the distance from the North Pole to the Equator. This effort followed earlier debates favoring a meridian-based definition over alternatives like a pendulum standard, as it promised a universal, Earth-derived unit independent of local variations. Astronomers Jean-Baptiste Joseph Delambre and Pierre François-André Méchain led the project, employing astronomical triangulation with a repeating circle to determine distances between observation points along the arc. Delambre surveyed the northern segment from Dunkirk to Rodez (approximately 743 km), while Méchain handled the southern segment from Rodez to the Montjuïc Fortress near Barcelona (about 333 km), dividing the total arc of roughly 1,076 km spanning 9°40' of latitude.[16][17] The expedition faced severe challenges amid the French Revolution's political instability and ensuing wars. Delambre encountered detention by suspicious local militias near Paris in 1793, narrowly avoiding execution as counterrevolutionary suspicions mounted during the Reign of Terror. Méchain, working in Spanish territory, was imprisoned by authorities in 1793 after being caught behind enemy lines during the Franco-Spanish War, delaying his progress and forcing him to navigate hostile borders under cover. Both astronomers grappled with technical difficulties, including measurement errors from atmospheric refraction, which distorted zenith angles and baselines—particularly evident in Méchain's observations at Montjuïc compared to those from a Barcelona hotel rooftop. These obstacles extended the survey far beyond its initial timeline, with Delambre completing his portion by 1798 and Méchain returning to Paris only in 1799 after repeated attempts to verify data.[18][17][16] By combining their triangulated data with prior surveys, Delambre and Méchain extrapolated the full quarter-meridian length from the North Pole to the Equator as 5,130,740 toises. This yielded a provisional metre defined as 0.513074 toise (or approximately 3 feet and 11.296 lignes), adopted by decree on August 1, 1793, to enable immediate metric implementation while final measurements were pending. Provisional brass metre bars based on this value were distributed across France starting in 1795. However, Méchain privately discovered a discrepancy of about 0.2 mm in the metre's length during his southern observations, stemming from refraction-induced latitude errors at Barcelona; tormented by the implications for the project's precision, he concealed the issue, even adjusting data in reports to align with expectations and protect his reputation. Delambre, unaware at the time, incorporated the altered figures into the official calculations, embedding the subtle inaccuracy into the foundational metre standard.[19][16][17]Adoption and Early Implementation
In 1799, following the completion of the meridian arc measurement expedition led by Jean-Baptiste Delambre and Pierre Méchain, French authorities constructed the Mètre des Archives, a platinum bar intended as the definitive standard for the metre. This bar was crafted to an end-to-end length of exactly 443.296 lines of the Toise du Pérou, a pre-metric reference standard, which equated to approximately 0.999998 metres by modern definition. Deposited in the Archives de la République on June 22, 1799, it served as the primary artifact embodying the unit's value, derived from one ten-millionth of the Earth's quadrant along the Paris meridian.[20] The French Directory formalized the metre's adoption through the law of 10 December 1799 (19 Frimaire an VIII), which legalized the metric system and designated the Mètre des Archives as the official length standard, replacing the provisional metre established in 1795. This decree integrated the metre into a broader decimal framework for weights and measures, aiming for uniformity in scientific, commercial, and administrative applications across France. To facilitate implementation, secondary standards—copies of the platinum prototype—were produced and distributed to the nation's departments, ensuring local replication and verification of the unit. Accompanying these efforts were public education initiatives, including the installation of engraved marble metre markers in prominent urban locations to familiarize citizens with the new scale.[21][9][22] Despite these measures, the metric system's rollout encountered significant resistance, particularly from artisans and regional communities accustomed to traditional units like the pied and toise, which varied locally and complicated everyday trade and craftsmanship. The 1799 decree permitted continued use of older measures alongside the metric system, but a follow-up decree on 18 June 1801 mandated exclusive adoption, granting a grace period until that date to mitigate disruptions and allow adaptation. This two-year transition addressed practical challenges in recalibrating tools and contracts, though full acceptance remained gradual amid economic and cultural inertia.[23]International Prototype Era
Metre Convention and Bar Prototypes
The Metre Convention, signed on 20 May 1875 in Paris by representatives of seventeen nations—Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, Germany, Italy, Peru, Portugal, Russia, Spain, Sweden-Norway, Switzerland, Turkey, the United States, and Venezuela—established a permanent international framework for metric standards.[24] This treaty created the International Bureau of Weights and Measures (BIPM) in Sèvres, France, tasked with maintaining and comparing prototype standards to ensure global uniformity in measurements. The convention addressed the need for shared artifacts following the French Revolution's initial metric efforts, promoting cooperation among signatory states for the preservation and verification of length and mass units.[25] At the first General Conference on Weights and Measures (CGPM) in 1889, delegates sanctioned the International Prototype Metre (IPM), a bar crafted from a platinum-iridium alloy consisting of 90% platinum and 10% iridium for durability and resistance to corrosion.[26] Johnson Matthey & Co. in London produced 30 such metre bars, with Bar No. 6 selected as the IPM after comparisons to the earlier French Mètre des Archives prototype. The metre was defined as the length of this IPM at the temperature of melting ice (0°C), serving as the fundamental unit of length. This prototype served as the international standard until 1960.[26] To establish the IPM's dimensions precisely, comparisons involved specialized instruments such as microscopes and cathetometers, which measured the distance between the two central engraved lines on the bar's neutral surface under controlled conditions at 0°C.[27] These line standards allowed for accurate readings by focusing on the axes of the lines, minimizing errors from end wear, with tolerances set at 0.01 mm for national variants relative to the IPM.[26] The 29 remaining bars were distributed as national prototypes to signatory nations, with the United States receiving Prototype No. 27, certified as 1 m minus 1.6 μm relative to the IPM and serving as the U.S. standard from 1890.[28] Under the Metre Convention, these national prototypes required periodic verification against the IPM at the BIPM every ten years to maintain consistency and detect any dimensional changes.[29] This process ensured the metre's stability across international borders until subsequent redefinitions.[2]Challenges with Material Standards
The platinum-iridium alloy used for the international prototype metre and its national copies introduced significant practical challenges in the 19th and early 20th centuries, primarily due to the material's susceptibility to environmental influences that compromised long-term stability. Thermal expansion and contraction of the alloy required meticulous temperature corrections during measurements, as even small variations could alter the perceived length. The linear thermal expansion coefficient for the 90% platinum-10% iridium alloy was approximately 8.8 × 10^{-6} /°C at 20 °C, meaning a 1 °C change could cause a length shift of about 8.8 micrometres in a 1-metre bar, necessitating controlled conditions at 0 °C for reference but complicating practical use.[30] Wear from handling, polishing, and comparative measurements further exacerbated instability, leading to gradual erosion of the engraved lines defining the metre's endpoints. Over decades of use, this handling-induced wear resulted in length changes of a few micrometres in some prototypes, as revealed in BIPM's periodic verifications starting in the 1920s.[31] Manufacturing tolerances in casting and finishing the alloy bars also produced subtle variations among the 30 national prototype metres distributed in 1889, with differences of up to 10 micrometres that propagated uncertainties in international length standards. These discrepancies hindered consistency in scientific experiments and trade measurements, as national laboratories reported length values deviating by up to 0.001% from the international prototype kept at the BIPM.[31] To mitigate these issues, the BIPM organized international comparisons from 1921 to 1936, involving interferometric verifications that recalibrated national copies against the international prototype and exposed the limitations of material artifacts, ultimately underscoring the demand for a more invariant definition. Contributions to interferometry in the early 1900s came from scientists like Charles Fabry and Alfred Pérot.[31]Transition to Optical Standards
Interferometry Developments
The limitations of physical prototypes, such as the platinum-iridium bar's susceptibility to thermal expansion, mechanical wear, and manufacturing inconsistencies, motivated the development of more stable optical standards through interferometry in the late 19th and early 20th centuries.[2] Interferometry offered a way to define length in terms of reproducible light wavelengths, reducing reliance on material artifacts.[32] In 1893, Albert A. Michelson and J.-R. Benoît adapted the interferometer to precisely measure the international prototype metre against the wavelength of the red cadmium spectral line (from ^{114}Cd) at 643.84696 nm.[33] Their measurements determined that the metre corresponded to 1,553,164.03 wavelengths of this line in air at standard conditions, achieving an accuracy of about 0.1 micrometers or 1/10 of a wavelength.[34] This work demonstrated interferometry's potential for calibrating length standards and contributed to Michelson's 1907 Nobel Prize in Physics for precision optical instruments. Building on this approach, in 1906, J.-R. Benoît, Charles Fabry, and Alfred Pérot employed the Fabry-Pérot interferometer—a high-resolution etalon using multiple reflections between partially silvered plates—to refine the metre's calibration against the same cadmium red line.[35] Their setup produced sharp interference fringes, allowing counts of wavelength intervals across the prototype's length with improved precision, yielding 1,553,164.13 wavelengths per metre and an uncertainty of about 0.1 wavelength (relative uncertainty of approximately 6.5 × 10^{-8}).[34] This method, involving the superposition of light waves to form observable fringes, established interferometry as a foundational technique for linking material standards to spectral lines.[33] The calibration process relied on the fundamental relation where the prototype length is expressed as an integer number of wavelengths times the wavelength : or equivalently, with determined by counting interference fringes as the etalon spacing varies.[34] By the 1920s, such interferometric methods were routinely applied at the International Bureau of Weights and Measures (BIPM) for verifying metre prototypes, and in 1927 the 7th CGPM adopted the cadmium red line as a supplementary standard (1 metre = 1,553,164.13 wavelengths in air), paving the way for proposals to redefine the unit directly in terms of spectral wavelengths.[36] Leading into the mid-20th century, international committees explored various atomic spectral lines for a universal standard, initially favoring cadmium-114 and mercury-198 due to their stability and accessibility before settling on krypton-86. In 1960, the 11th General Conference on Weights and Measures (CGPM) adopted the krypton-86 orange-red line as the basis for the metre, formalizing interferometry's role in achieving sub-micrometer reproducibility.[37]Krypton-86 Wavelength Adoption
In 1960, the 11th General Conference on Weights and Measures (CGPM) adopted a new definition of the metre, marking the shift from material artifacts to an optical standard based on atomic spectroscopy. The resolution specified that the metre is the length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels and of the krypton-86 atom.[37] This definition utilized the orange-red emission line at approximately 605.78 nm, realized through interferometry with a krypton-86 discharge lamp operated under controlled conditions, including a temperature of 15 °C.[37] The adoption of the krypton-86 wavelength offered significant advantages over the platinum-iridium bar prototypes, primarily its invariance as a universal physical phenomenon independent of material properties or manufacturing variations, and its high reproducibility across laboratories equipped with spectroscopic apparatus. This change reduced the relative uncertainty in realizing the metre from about 0.2 parts per million with the International Prototype Metre to 0.01 parts per million, enhancing global consistency in length measurements.[38] Interferometry served as the enabling technology, allowing precise counting of interference fringes produced by the krypton radiation. International comparisons conducted shortly after adoption confirmed that the krypton-based metre aligned closely with the International Prototype Metre, differing by less than 0.1 μm.[39] This alignment ensured minimal disruption to existing calibrations while establishing the new standard's validity. The transition to widespread use involved laboratories upgrading to interferometric techniques, with full adoption in national metrology institutes generally achieved by 1965.[40]Modern Physical Constant Definition
Speed of Light Redefinition in 1983
In 1983, the 17th Conférence Générale des Poids et Mesures (CGPM) adopted Resolution 1, which established a new definition of the metre as the length of the path travelled by light in vacuum during a time interval of of a second, thereby fixing the speed of light in vacuum at exactly 299,792,458 m/s.[41] This replaced the 1960 definition based on the orange-red emission line of krypton-86, abrogating it to shift from a material artifact or spectral standard to a fundamental physical constant.[41] The primary rationale for this redefinition was to anchor the metre to invariants of nature, linking length directly to the second (already defined via caesium atomic transitions) and the universal constant , which ensures long-term stability independent of material degradation or measurement artifacts.[42] By defining exactly, the uncertainty in its value was reduced to zero, eliminating previous measurement errors associated with determining the speed of light and providing a more reproducible and precise basis for metrology.[40] The numerical value of was specifically chosen based on the best available measurements at the time, which aligned closely with the length scale of the prior krypton standard, thereby preserving continuity in practical realizations of the metre.[1] This redefinition enabled significant advancements in the practical realization of the metre through laser interferometry synchronized with atomic clocks, allowing for length determinations with relative uncertainties below , a substantial improvement over the uncertainty inherent in the krypton standard.[43] Such precision stems from the ability to count interference fringes produced by stabilized lasers over paths defined by time intervals from highly accurate caesium or other atomic frequency standards, fundamentally enhancing the reproducibility and universality of length measurements worldwide.Implications for Precision and Stability
The 1983 redefinition of the metre, linking it to an exact value of the speed of light in vacuum, has profoundly enhanced the reproducibility of length measurements across scientific disciplines by enabling realizations through frequency-stabilized lasers, which provide intrinsic traceability without reliance on physical artifacts. This approach achieves relative uncertainties as low as 10^{-11} or better, far surpassing previous material-based standards, and supports applications in global navigation satellite systems (GNSS) like GPS, where precise laser interferometry ensures sub-centimeter positioning accuracy over vast distances.[44] In nanotechnology, such laser realizations underpin atomic force microscopy and other interferometric tools for calibrating nanoscale dimensions with traceability to the SI metre, enabling reproducible fabrication and characterization at the nanometre scale.[45] Similarly, in particle physics, frequency-stabilized lasers facilitate high-precision alignment and displacement measurements in accelerators, such as those at CERN, where beam path lengths must be controlled to micrometre levels for collision experiments.[46] The Bureau International des Poids et Mesures (BIPM) plays a central role in disseminating this definition globally by coordinating key comparisons among national metrology institutes through the Consultative Committee for Length (CCL), ensuring consistency in metre realizations via shared calibrations of optical frequency standards.[47] These comparisons, documented in the BIPM Key Comparison Database (KCDB), involve bilateral and multilateral verifications of laser-based length standards, with results demonstrating agreement at the 10^{-9} level or better, thereby maintaining worldwide uniformity in precision measurements.[48] The BIPM also provides mise en pratique guidelines for practical implementations, including recommended radiations and uncertainty evaluations, which national laboratories use to propagate the standard to end-user applications. The 2019 revision of the International System of Units (SI) by the General Conference on Weights and Measures (CGPM) introduced minor textual adjustments to the metre's definition to align with the new framework of fixed constants, but reaffirmed the exact value of the speed of light (c = 299 792 458 m/s) without altering the metre's magnitude or realization methods. This continuity ensured no disruption to existing calibrations, as the metre remained defined by the distance light travels in vacuum during 1/299 792 458 of a second, preserving its stability and precision for ongoing scientific use.[49] Despite these advances, realizing the metre definition outside vacuum conditions presents ongoing challenges, primarily due to the refractive index of air, which modifies the effective path length of light and introduces uncertainties up to several parts in 10^7 if uncorrected. These effects are addressed through standardized corrections using the Edlén formula or its updates, which account for air's composition, temperature, pressure, and humidity to achieve vacuum-equivalent precision in ambient environments. Such corrections are essential for practical metrology in laboratories and industry, where vacuum conditions are often infeasible, and ongoing research refines dispersion models to further minimize residual errors.[50]Chronology of Definitions
Key Milestones from 1791 to Present
In 1791, the French Academy of Sciences proposed defining the metre as one ten-millionth part of the length of the Earth's meridian from the equator to the North Pole, aiming for a universal standard based on natural constants.[2] By 1799, France officially adopted the metre, with the Mètre des Archives—a platinum bar constructed from meridian measurements—deposited in the National Archives as the national prototype.[2] The Metre Convention, signed on 20 May 1875 in Paris by representatives from 17 nations, established the International Bureau of Weights and Measures (BIPM) to maintain and promote uniform metric standards globally.[24] In 1889, the 1st General Conference on Weights and Measures (CGPM) sanctioned the International Prototype Metre, a platinum-iridium bar, as the definitive standard, replacing earlier national prototypes.[2] The 7th CGPM in 1927 specified precise conditions for measuring the International Prototype Metre, including temperature and support mechanisms, to enhance accuracy and reproducibility.[2] Shifting to optical standards, the 11th CGPM in 1960 redefined the metre as exactly 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in vacuum.[51] In 1983, the 17th CGPM redefined the metre as the distance traveled by light in vacuum in 1/299,792,458 of a second, fixing the speed of light at exactly 299,792,458 m/s and linking the unit to fundamental physical constants.[52] In 2019, the 26th CGPM adopted the revised International System of Units (SI), fixing the numerical value of the speed of light c at exactly 299,792,458 m/s when expressed in units of m s⁻¹, thereby reaffirming the metre's 1983 definition with exact dependence on this invariant constant.[2]Comparative Table of Evolving Standards
The evolution of the metre's definition is summarized in the following table, which highlights key changes in its basis, formulation, achievable precision, and oversight.| Year | Definition Basis | Key Equation/Description | Precision Level (Relative Uncertainty) | Governing Body |
|---|---|---|---|---|
| 1799 | Terrestrial meridian arc | One ten-millionth of the distance along the Earth's meridian from the North Pole to the Equator, realized as the length of the platinum Mètre des Archives bar at 0 °C.[11][1] | ~2 × 10^{-4} (0.2 mm short due to measurement errors in arc survey) | French Academy of Sciences and National Archives |
| 1889 | Material artefact | Distance between two lines on the International Prototype Metre bar (90% platinum, 10% iridium) at the temperature of melting ice (0 °C).[1] | ~1 × 10^{-8} initially, degrading to ~2 × 10^{-6} by 1960 due to instability | 1st CGPM (International Metre Convention) |
| 1960 | Optical (atomic wavelength) | 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p_{10} and 5d_5 levels of the krypton-86 atom (⁸⁶Kr; λ = 605.780211 nm).[37][1] | 4 × 10^{-9} | 11th CGPM |
| 1983 | Speed of light in vacuum | The length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second (c = 299,792,458 m/s exactly).[41][1] | 0 (exact by definition) | 17th CGPM |
| 2019 | Speed of light in vacuum (reaffirmed) | Fixed numerical value of the speed of light c to be 299,792,458 m/s when expressed in metre per second (no change from 1983).[1] | 0 (exact by definition) | 26th CGPM (SI revision) |