Metre Convention

The Metre Convention (French: Convention du Mètre), also known as the Treaty of the Metre,[1] 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, Germany, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States of America, and Venezuela.

The Metre Convention established a permanent organizational structure for member governments to act in common accord on all matters relating to units of measurement.

In 1999, the CGPM created in the status of associate, to allow non-member states and economic entities to participate in some activities of the BIPM through their national metrology institutes (NMIs).

[4] During the eighteenth century, in order to facilitate trade, Peter the Great, Czar of Russia adopted the English system of measure.

Profusion of units of measures was a practical problem of importance before the French Revolution and its reform was one of the items on the agenda of National Assembly.

[6] In 1805, a Swiss immigrant Ferdinand Rudolph Hassler brought copies of the French metre and kilogram to the United States.

[7] In view of the doubts being cast on the reproducibility of the metre and the kilogram and the threat that a rival standard might be set up, Napoleon III invited scientists from all the world's nations to attend a conference in Paris.

The principal tasks facing the delegates at the 1875 conference was the replacement of the existing metre and kilogram artefacts that were held by the French Government and the setting up of an organization to administer the maintenance of standards around the globe.

[12] Spain notably supported France for this outcome and the first president of the International Committee for Weights and Measures, the Spanish geodesist, Carlos Ibáñez e Ibáñez de Ibero received the Grand Officer medal of the Légion d'Honneur for his diplomatic role on this issue and was awarded the Poncelet Prize for his scientific contribution to metrology.

[13] Indeed, as Carlos Ibáñez e Ibáñez de Ibero was collaborating with the French on the Paris meridian arc (West Europe-Africa Meridian-arc) remeasurement since 1853, and was president of both the Permanent Committee of the International Metre Commission since 1872 and the Permanent Commission of the International Association of Geodesy since 1874, he was to play a pivotal role in reconciling French and German interests.

Prior to the 1870 conference, French politicians had feared that the world community might reject the existing metre as it was 0.03% (300 μm) shorter than its design length, ordering a new meridional measurement.

They were eventually reassured when the German-born Swiss delegate Adolphe Hirsch said "no serious scientist would in our day and age contemplate a metre deduced from the size of the earth".

When the conference was reconvened in 1875, it was proposed that new prototype metre and kilogram standards be manufactured to reproduce the values of the existing artifacts as closely as possible.

The current definition of the metre is "the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second".

On 16 November 2018, the 26th General Conference on Weights and Measures (CGPM) voted unanimously in favour of revised definitions of some SI base units, in particular the kilogram.

The first, the CGPM provides a forum for representative of member states, the second, the CIPM is an advisory committee of metrologists of high standing.

The General Conference on Weights and Measures (Conférence générale des poids et mesures or CGPM) is the principal decision-making body put on place by the convention.

The International Committee for Weights and Measures (Comité international des poids et mesures or CIPM) is made up of eighteen (originally fourteen)[24] individuals from a member state of high scientific standing, nominated by the CGPM to advise the CGPM on administrative and technical matters.

It has custody of the now historical international prototype of the kilogram and provides metrology services for Member States and hosts formal meetings.

In particular the treaty was amended in 1921 with the result that many other international organizations have a forum within the CIPM to ensure harmonization of measurement standards across many disciplines.

In addition, what were originally conceived as standards for the purposes of trade have now been extended to cover a large number of aspects of human activity including medicine, science, engineering and technology.

In 1921 the convention was extended to permit the promotion of standards relating to any physical quantity which greatly increased the scope of the CIPM's remit and implicitly giving it freedom to exploit Giorgi's proposals.

In response to formal requests made by the International Union of Pure and Applied Physics and by the French Government to establish a practical system of units of measure, the CGPM requested the CIPM to prepare recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to the Metre Convention.

The formal definition of International System of Units (SI) along with the associated resolutions passed by the CGPM and the CIPM are published by the BIPM on the Internet and in brochure form at regular intervals.

[40] During the 1940s, the United States government recognized the benefits of its suppliers keeping quality control records in respect of manufactured goods that would provide traceability of the process.

At the 20th CGPM (1995), it was recognized that although ad-hoc recognition of instrument calibration between cooperating countries had been taking place for a hundred years, a need had arisen for a more comprehensive agreement.

The International Earth Rotation Service monitors these changes relative to the stars at regular intervals and proposes leap seconds as and when these are needed.

[49] At the 21st meeting of the CGPM (1999), national laboratories were urged to investigate ways of breaking the link between the kilogram and a specific artefact.

Independently of this drift having been identified, the Avogadro project and development of the Kibble (or watt) balance promised methods of indirectly measuring mass with a very high precision.

[51] Over the next few years the support for natural constants grew and details were clarified,[52][53][54][55][23] until in November 2018, the 26th General Conference on Weights and Measures voted unanimously in favour of revised definitions of the SI base units.