Metre Convention (French: Convention du Mètre), 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. The treaty set up an institute for the purpose of coordinating international metrology and for coordinating the development of the metric system. The treaty also set up associated organizations to oversee the running of the institute. Initially it was only concerned with the units of mass and length but, in 1921, at the 6th meeting of the General Conference on Weights and Measures (CGPM), it was revised and its mandate extended to cover all physical measurements. In 1960, at the 11th meetings of the CGPM, the system of units it had established was overhauled and relaunched as the "International System of Units" (SI).
The Convention created three main organizations:
Membership of the convention is restricted to countries who have diplomatic relations with France, but in 1999 the category of associate membership was introduced for those nations that wished to partake in the calibration and measurement aspects of the CIPM Mutual Recognition Agreement (CIPM-MRA) program without taking part in the activities of the BIPM.
In England in 1215, clause 25 of the Magna Carta set out the standards of measure that were to be applied throughout the realm prefixed. The wording of the clause emphasised that "There is to be a single measure ... throughout our realm". Five centuries later, when in 1707 England and Scotland were united into a single kingdom, the Scots agreed to use the same units of measure that were already established in England. During the eighteenth century, in order to facilitate trade, Peter the Great, Czar of Russia adopted the English system of measure.
Abuse of units of measures were one of the causes of the French Revolution and its reform was one of the items on the agenda of National Assembly. Talleyrand, an influential leader of the Assembly invited British and American participation in the establishment of a new system, but in the event, the Assembly went it alone and introduced the metre and the kilogram which were to form the basis of the metric system, manufacturing prototypes which, in 1799, were lodged with Archives.
Between 1850 and 1870, a number of countries adopted the metric system as their system of measure including Spain, many South American republics and many of the Italian and German states (the Netherlands had adopted the system in 1817). In 1863, the International Postal Union used grams to express permitted weights of letters.
In the 1860s, inspections of the prototype metre revealed wear and tear at the measuring faces of the bar and also that the bar was wont to flex slightly when in use. 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. In July 1870, two weeks before the conference was due to start, the Franco-Prussian War broke out. Although the delegates did meet (without a German delegation), it was agreed that the conference should be recalled once all the delegates (including the German delegation) were present.
France was defeated in the war, Napoleon went into exile and Germany and Italy, now unified nations, adopted the metric system as their national system of units, but with the prototype copy of the kilogram and metre under the control of the Third French Republic. In 1872 the new republican government reissued the invitations and in 1875 scientists from thirty European and American countries met 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 organisation to administer the maintenance of standards around the globe. The conference did not concern itself with other units of measure. The conference had undertones of Franco-German political manoeuvring, particularly since the French had been humiliated by the Prussians during the war a few years previously. Although France lost control of the metric system, they ensured that it passed to international rather than German control and that the international headquarters were based in Paris.
The conference was called to discuss the maintenance of international standards based on the existing French standards rather than using French standards which, at that time, were 70 years old and which, through wear and tear, might not be exactly the same as when they adopted in 1799.
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 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.
Although the new standard metre had the same value as the old metre, it had an "X" cross-section rather than a rectangular cross-section as this reduced the flexing when taking measurements. Moreover the new bar, rather than being exactly one metre in length was a little longer than one metre and had lines engraved on them that were exactly one metre apart. The London firm Johnson Matthey delivered 30 prototype metres and 40 prototype kilograms. At the first meeting of the CGPM in 1889 bar No. 6 and cylinder No. X were chosen by lot as the international prototypes. The remainder were either kept as BIPM working copies or distributed by lot to member states as national prototypes. The metre was retained as the international standard until 1960 when the metre was redefined in terms of the wavelength of the orange-red line of krypton-86. As of 2012 the prototype kilogram was still in use, though it is expected to be replaced by a new definition within the next few years.
The Convention created three main organizations to facilitate the standardisation of weights and measures around the world. The first, the CGPM provided a forum for representative of member states, the second, the CIPM was an advisory committee of metrologists of high standing and the third, the BIPM was an institute that provided appropriate secretarial and laboratory facilities in support of the GCPM and CIPM.
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. It is made up of delegates from member states and [non-voting] observers from associate states. The conference usually meets every four to six years to receive and discuss a report from the CIPM and to endorse new developments in the SI on the advice of the CIPM though at the 2011 meeting, it agreed to meet again in 2014 rather than 2015 to discuss the maturity of the new SI proposals. It is also responsible for new appointments to the CIPM and decides on major issues concerning the development and financing of the BIPM. It normally meets every four to six years, The body comprises delegates from all member states. 
The International Committee for Weights and Measures (Comité international des poids et mesures or CIPM) is made up of eighteen (originally fourteen) individuals from a member state of high scientific standing, nominated by the CGPM to advise the CGPM on administrative and technical matters. It is responsible for the running of ten consultative committees (CCs), each of which investigates different aspects of metrology – one CC discusses the measurement of temperature, another the measurement of mass and so on. The CIPM meets annually at Sèvres to discuss annual reports from the various CCs, to submit an annual report to the governments of member states in respect of the administration and finances of the BIPM and to advise the CGPM on technical matters as and when necessary. Each member of the CIPM is from a different member state – with France, in recognition of its work in setting up the Convention, always having one seat on the CIPM.
The International Bureau of Weights and Measures (Bureau international des poids et mesures or BIPM) is an organisation based at Sèvres, France that has custody of the International Prototype Kilogram, provides metrology services for the GCPM and CIPM, houses the secretariat for these organisations and hosts their formal meetings. It also has custody of the former International Prototype Metre which was retired in 1960. Over the years the various international prototype metres and kilograms were returned to BIPM headquarters for recalibration services.
Initially it has a staff of 9, falling to 4 once the initial batch of prototypes had been distributed; in 2012 it has a staff of over 70 people and a budget of over €10 million. The director of the BIPM is ex-officio a member of the CIPM and a member of all consultative committees.
The original treaty was written in French and the authoritative language of all official documents is French. Communication between the BIPM and member states is, in the case of France, via the French Foreign Minister and in the case of all other members, via the members' ambassador to France.
The French government offered the treaty members the Pavillon de Breteuil in Sèvres to house the BIPM. The pavillon was originally built in 1675 on the estate of the Château de Saint-Cloud and was home to, amongst others, Emperor Napoleon III. The château was all but destroyed during the Franco-Prussian War (1870–1) and the pavillon badly damaged. The pavillon has been fully restored and, as headquarters of an intergovernmental organization has extraterritorial rights (similar to those enjoyed by embassies).
The science of metrology has increased vastly since 1875. In particular the treaty was amended in 1921 with the result that many other international organisations have a forum within the CIPM to ensure harmonisation of measurement standards across many disciplines. In addition, what was originally conceived as standards for the purposes of trade has now been extended to cover a large number of aspects of human activity including medicine, science, engineering and technology.
The metre convention was originally drawn up with the main purpose of providing a standards of length and mass only. Standards relating to other quantities were under the control of other bodies – time was measured by astronomers, electrical units by a series of ad-hoc international conferences, and other physical standards and concepts were maintained or defined by international bodies such as International Congress of Applied Chemistry or the International Union of Pure and Applied Physics.
In 1901 Giorgi published a proposal for building a coherent set of units based on four base units – the metre, kilogram, second and one electrical unit (ampere, volt or ohm). 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. The 8th CGPM (1933) resolved to work with other international bodies to agree standards for electrical units that could be related back to the international prototypes. This was agreed in principle by the International Electrotechnical Commission at its congress in Brussels in 1935 subject to the choice of the fourth unit being agreed with, amongst others, the appropriate consultative committee of the CIPM.
In 1948, three years after the end of the Second World War and fifteen years after the 8th CGPM, the 9th CGPM was convened. 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. At the same time the CGPM formally adopted a recommendation for the writing and printing of unit symbols and of numbers. The recommendation also catalogued the recommended symbols for the most important MKS and CGS units of measure and for the first time the CGPM made recommendations concerning derived units.
The CIPM's draft proposal, which was an extensive revision and simplification of the metric unit definitions, symbols and terminology based on the MKS system of units, was put to the 10th CGPM in 1954. In the proposal the CIPM recommended that the ampere be the base unit from which electromechanical would be derived. After negotiations with the CIS and IUPAP, two further base units, the degree kelvin and the candela were also proposed as base units. The full system and name "Système International d'Unités" were adopted at the 11th CGPM. During the years that followed the definitions of the base units and particularly the mise en pratique to realise these definitions have been refined.
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. The eighth edition of the brochure Le Système International d'Unités – The International System of Units was published in 2006.
During the 1940s, the United States government recognised the benefits of its suppliers keeping quality control records in respect of manufactured goods that would provide traceability of the process. This process was formalised by the British Government and in 1979 as the quality control standard BS 5750. In 1987 BS 5750 was adopted by ISO as the basis for ISO 9000. ISO 9000 is a general purpose quality control standard which works in conjunction industry-specific standards: for example ISO 15195:2003 which gives the specific requirements for reference measurement laboratories in laboratory medicine.
International trade is hampered by one country not recognising the quality controls in place in other countries – often due to standards being different or being incompatible with each other. At the 20th CGPM (1995), it was recognised 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. Consequently the CIPM was mandated to investigate the setting up of a Mutual Recognition Agreement in respect of instrument calibration. Any such agreement would require the keeping of records that could demonstrate the traceability of calibrations back to the base standards. Such records would be recorded within an ISO 9000 framework. Four years later, in 1999 the text of the CIPM-MRA was agreed at the 21st CGPM.
The CIPM-MRA scheme is to catalogue the capabilities of National Measurement Institutes (NMIs) such as NIST in the United States or the National Physical Laboratory in the United Kingdom whose calibration procedures have been peer-assessed. The essential points of CIPM-MRA are:
Subsequent to launch of the CIPM MRA and in response to a European Community directive on in vitro medical devices, the Joint Committee for Traceability in Laboratory Medicine (JCTLM) was created in 2002 through a Declaration of Cooperation between the International Committee of Weights and Measures (CIPM), the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC), and the International Laboratory Accreditation Cooperation (ILAC). The joint committee provides a forum for the harmonisation of standards of the various participants.
With the advent of the atomic clock it has been possible to define and measure International Atomic Time with sufficient precision that variations in the Earth's rotation can be detected. The International Earth Rotation Service monitors these changes relative to the starts and at regular intervals and proposes leap seconds as and when these are needed. Currently there are over 200 atomic clocks in over 50 national laboratories around the world and the BIPM, in terms of the mandate given to it under the Metre Convention coordinates the various atomic clocks.
Since 1960, when the definition of the metre was linked to a particular wavelength of light rather than the international prototype metre, the only unit of measure that has been dependent on a particular artifact has been the kilogram. Over the years, small drifts which could be as high as 20×10−9 kilograms per annum in the mass of the international prototype kilogram have been detected. At the 21st meeting of the GCPM (1999), national laboratories were urged to investigate ways of breaking the link between the kilogram and a specific artifact.
Independently of this drift having been identified, the Avogadro project and development of the Watt balance promised methods of indirectly measuring mass with a very high precision. These projects provided tools that would enable alternative means of redefining the kilogram.
A report published in 2007 by the Consultative Committee for Thermometry to the CIPM noted that their current definition of temperature has proved to be unsatisfactory for temperatures below 20 K and for temperatures above 1300 K. The committee was of the view that the Boltzmann constant provided a better basis for temperature measurement than did the triple point of water, as it overcame these difficulties.
At its 23rd meeting (2007), the CGPM mandated the CIPM to investigate the use of natural constants as the basis for all units of measure rather than the artifacts that were then in use. The following year this was endorsed by the International Union of Pure and Applied Physics (IUPAP). In 2011 CIPM presented a resolution for consideration at the 24th CGPM to agree in principle the new definitions that had been proposed the previous year by the Consultative Committee for Units, but not to implement them until the details have been finalised. This resolution was accepted by the conference, and in addition the CGPM moved the date of the 25th meeting forward from 2015 to 2014 when it is hoped to implement these proposals.
The CGPM recognises two classes of membership – full membership for those states that wish to participate in the activities of the BIPM and associate membership for those countries or economies[Note 1] that only wish to participate in the MRA program. Associate members have observer status at the CGPM. Since all formal liaison between the convention organisations and national governments is handled by the member state's ambassador to France,[Note 2] it is implicit that member states must have diplomatic relations with France, though during both world wars, nations that were at war with France retained their membership of the CGPM. The opening session of each CGPM is chaired by the French foreign minister and subsequent sessions by the Président de l'Académie des Sciences de Paris.
On 20 May 1875 representatives from seventeen of countries that attended the Conference of the Metre in 1875, signed the Convention of the Metre.[Note 3] In April 1884 HJ Chaney, Warden of Standards in London unofficially contacted the BIPM inquiring whether the BIPM would calibrate some metre standards that had been manufactured in the United Kingdom. Broch, director of the BIPM replied that he was not authorised to perform any such calibrations for non-member states. On 17 September 1884, the British Government signed the convention on behalf of the United Kingdom. This number grew to 21 in 1900, 32 in 1950, and 49 in 2001. As of 21 July 2014[update], the General Conference membership was made up of 56 Member States, 41 Associate States and Economies and four international organisations as follows (with year of partnership between brackets):
|Name||Year of partnership||Notes|
|Austria||1875||Joined originally as Austria-Hungary|
|Czech Republic||1922||Joined originally as part of Czechoslovakia|
|Germany||1875||Joined originally as the German Empire|
|Ireland||1925||Joined originally as the Irish Free State|
|Norway||1875||Joined originally as part of Sweden and Norway|
|Russia||1875||Joined originally as the Russian Empire|
|Serbia||1879||Joined as the Principality of Serbia in 1879, as the Kingdom of Yugoslavia in 1929, and as the Federal Republic of Yugoslavia in 2001|
|Slovakia||1922||Joined originally as part of Czechoslovakia|
|Sweden||1875||Joined originally as part of Sweden and Norway|
|Turkey||1875||Joined originally as the Ottoman Empire|
|Country||Year of partnership|
|Bosnia and Herzegovina||2011|
The following international organisations have signed the CIPM MRA:
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