History of the metre
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 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.
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 dematerialise 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°. However, dematerialising the definition of units of length by means of the pendulum would prove less reliable than artefacts.
During the mid nineteenth century, following the American Revolution and the decolonisation 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 by a new standard metre made of platinum-iridium, and 29 bars calibrated against it were distributed to different nations. This improved standardisation involved the development of specialised measuring equipment and the definition of a reproducible temperature scale.
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:
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.
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, various "universal measures" of length were proposed which would be based on reproducible natural phenomena, in particular the pendulum and the Earth.Decimals
Using a decimal scale for measurements was proposed by Simon Stevin, a Flemish mathematician in 1586.The seconds pendulum and the Earth
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. 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.In around 1602, Galileo had observed that the regular swing of the pendulum depended on its length. In 1645 Giovanni Battista Riccioli had determined the length of a pendulum whose swing is one second each way, a "seconds pendulum".
In 1671, Jean Picard proposed this length as a unit of measurement to be called the Rayon Astronomique. In 1675, Tito Livio Burattini suggested calling it metro cattolico. However in 1671–1673, astronomer Jean Richer discovered that the length of a seconds pendulum varies from place to place depending on latitude.
In the 1790s, French scientists did not want to introduce another dimension into the definition of the unit of length, which was the unit on which the metric system was based. However, it was originally also planned to dematerialise the definition of the metre by counting the number of swings of a one-metre-long pendulum during a day, in a vacuum, at sea level, at the temperature of melting ice and at a latitude of 45°.
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.
Mètre des Archives
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 and decided that the new measure should be equal to one ten-millionth of the distance from the North Pole to the Equator. This was to be measured along the meridian passing through the centre of Paris Observatory.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, provisional metal metre bars were distributed in France in 1795-1796.
File:Kilometre definition.svg|thumb|An early definition of the metre was one ten-millionth of the Earth quadrant, the distance from the North Pole to the Equator, after the arc measurement of Delambre and Méchain.
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 Mètre des Archives. Another platinum metre, calibrated against the Mètre des Archives, and twelve iron ones were made as secondary standards.
Adoption
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. 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.
In 1855, the Dufour map, 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 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 was created during the Exposition Universelle in Paris and called for the international adoption of the metric system.
In the United States, the Metric Act of 1866 allowed the use of the metre in the United States, and in 1867 the General Conference of the European Arc Measurement proposed the creation of the International Bureau of Weights and Measures.
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. 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.
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.
At the Metre Convention 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": the metre was defined as the distance between two lines marked on the bar, to make any wear at the ends irrelevant. 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.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 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 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. For example, the United States received No. 27 with a calibrated length of .
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 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 μm. At this time, it was decided that a more formal definition of the metre was required, and this was agreed at the 7th CGPM in 1927.
These support locations are at the Bessel points 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 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 × 16 mm.
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. 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, of the length smaller, 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.