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Convention of the Metre

The Convention of the Metre is a diplomatic treaty between fifty-one nations which gives authority to the Conférence Générale des Poids et Mesures (CGPM), the Comité International des Poids et Mesures (CIPM) and the Bureau International des Poids et Mesures (BIPM) to act in matters of world metrology.


This particularly concerns the demand for measurement standards of ever increasing accuracy, range and diversity, and the need to demonstrate equivalence between national measurement standards.

The Convention was signed in Paris in 1875 by representatives of seventeen nations.

Conférence Générale des Poids et MesuresMeets every four years and consists of delegates

of all member states.

Bureau International des Poids et MesuresInternational centre for metrology. Laboratoires and offices at Sevres with an international staff of about

seventy.

Comité International des Poids et MesuresConsists of 18 individuals elected by CGPM.

It is charged with the supervision of BIPM and the affairs of the Convention of the Metre.

Pavillon de Breteuil


The CIPM has set up 10 Consultative Committees, which bring together the world's experts in their specified fields as advisers on scientific and technical matters. The chairman of each Committee is designated by, and is normally a member of, the CIPM. The members of the Committees are metrology laboratories and specialized institutes agreed by the CIPM, which send delegates of their choice.


• Consultative Committee for Electricity and Magnetism (CCEM), new name given in 1997 to the Consultative Committee for Electricity (CCE), set up in 1927.

• Consultative Committee for Units (CCU), set up in 1964 (this committee replaced the "Commission for the System of Units" set up by the CIPM in 1954).

Renowned metrological institutions

NIST (formerly NBS) – National Institute of Standards and Technology, Gaithersburg and Boulder, USA

NRC – National Research Council, Ottawa, Canada

ETL – Electrotechnical Laboratory, Tsukuba, Japan

NML – National Measurement Laboratory, Lindfield, Australia

Renowned metrological institutions

NPL – National Physical Laboratory, Teddington, United Kingdom

PTB – Physikalisch–Technische Bundesanstalt, Braunschweig and Berlin, Germany

International System of Units (SI)

The 11th CGPM (1960) adopted the name Système International d'Unités (International System of Units, abbreviation SI), for the recommended practical system of units of measurement.

SI units are divided into 2 classes:base units (7) and derived units.

SI base units

Unit of length (metre, m):

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.

Unit of mass (kilogram, kg):

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

International prototyp of the kilogram

SI base units

Unit of time (second, s):

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

SI base units

Unit of electric current (ampere, A):

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length.

SI base units

Unit of thermodynamic temperature

(kelvin, K):

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

SI base unitsUnit of amount of substance (mole, mol):The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.

When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

SI base units

Unit of luminous intensity (candela, cd):

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

SI derived units

Derived units are expressed algebraically in terms of base units by means of mathematical symbols of multiplication and division.

Examples of SI derived units expressed in terms of base units

Quantity Symbol

volume m3

velocity m/smass density kg/m3 current density A/m2

magnetic field strength A/mluminance cd/m2

Examples of SI derived units with special names

Quantity Name Symbol

plane angle radian radsolid angle steradian srfrequency hertz Hzforce newton Npressure pascal Paenergy, work joule Jquantity of heat joule Jpower watt W



electric charge coulomb Celectric potential volt Velectric resistance ohm Ωelectric conductance siemens Selectric capacitance farad Finductance henry H



magnetic flux weber Wbmagnetic flux density tesla Tluminous flux lumen lmilluminance lux lxactivity becquerel Bqabsorbed dose gray Gy

dose equivalent sievert Sv

Decimal multiples and sub-multiples of SI units

SI prefixes

Factor Prefix Symbol1024 yotta Y1021 zetta Z 1018 exa E 1015 peta P 1012 tera T 109 giga G 106 mega M


SI prefixes

Factor Prefix Symbol103 kilo k 102 hecto h 101 deca da10-1 deci d 10-2 centi c 10-3 milli m


SI prefixes

Factor Prefix Symbol 10-6 micro µ 10-9 nano n 10-12 pico p 10-15 femto f 10-18 atto a 10-21 zepto z 10-24 yocto y

Measurement standards

Material measures, measuring instruments, reference materials or measuring systems intended to define, realize, conserve or reproduce a unit or one or more values of a quantity to serve as a reference.


International standard is a standard recognized by an international agreement to serve internationally as the basis for assigning values to other standards of the quantity concerned.

National standardis a standard recognized by a national decision to serve, in a country, as the basis for assigning values to other standards of the quantity concerned.


Primary standard is a standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity.

Secondary standardis a standard whose value is assigned by comparison with a primary standard of the same quantity.


Reference standard is a standard, generaly having the highest metrological quality available at a given location or in a given organization, from which measurements made there are derived.

Working standardis a standard that is used routinely to calibrate or check material measures, measuring instruments or reference materials.


Transfer standard is a standard used as an intermediary to compare standards.

Travelling standardis a standard, sometimes of special construction, intended for transport between different locations.

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