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 (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. 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-19th 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:

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.

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 (astronomical radius). In 1675, Tito Livio Burattini suggested calling it metro cattolico (universal measure). 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 (time) into the definition of the unit of length, which was the unit on which the metric system (metre and kilogram) 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 (86,400 seconds), 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 (a quadrant of the Earth's circumference). 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.

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.

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