Outline of the metric system

The following outline is provided as an overview of and topical guide to the metric system – various loosely related systems of measurement that trace their origin to the decimal system of measurement introduced in France during the French Revolution.

"The metric system is for all people for all time." (Condorcet 1791) Four objects used in making measurements in everyday situations that have metric calibrations are shown: a tape measure calibrated in centimetres, a thermometer calibrated in degrees Celsius, a kilogram mass, and an electrical multimeter which measures volts, amps and ohms.

Nature of the metric system

The metric system can be described as all of the following:

  • System – set of interacting or interdependent components forming an integrated whole.
    • System of measurement – set of units which can be used to specify anything which can be measured. Historically, systems of measurement were initially defined and regulated to support trade and internal commerce. Units were arbitrarily defined by fiat (see statutory law) by the ruling entities and were not necessarily well inter-related or self-consistent. When later analyzed and scientifically, some quantities were designated as base units, meaning all other needed units of measure could be derived from them.

Essence of the metric system

Underlying philosophy

Discussions of the underlying philosophy of the metric system (and other systems of measure) include:

Metric units of measure

Articles that exist for many units of measure that are related to various flavours of the metric system are catalogued below.

Unit nameQuantitySI unitcgs unitOther metricNon-metric
abampere electric currentDerived metric unit with a special name
abcoulomb electric chargeDerived metric unit with a special name
abhenry inductanceDerived metric unit with a special name
abohm electrical resistanceDerived metric unit with a special name
abvolt potential differenceDerived metric unit with a special name
ampere electric currentBase unit
ampere-meter magnetic pole strengthDerived coherent SI unit without a special name
apostilb luminanceDerived metric unit with a special name
astronomical unit lengthNon-SI unit permitted for use with SI units
dalton massNon-SI unit permitted for use with SI units
barye pressureDerived metric unit with a special name
becquerel radioactive activityDerived SI unit with a special name
bril luminanceNo specified relationship
candela luminous intensityBase unit
candela per square metre luminanceDerived coherent SI unit without a special name
centimetre lengthMultiple or submultiple of an SI base unitBase unit
coulomb electric chargeDerived SI unit with a special name
cubic centimetre volumeDerived coherent SI unit without a special nameDerived coherent metric unit without a special name
cubic metre volumeDerived coherent SI unit without a special nameDerived coherent metric unit without a special name
cubic metre per second volumetric flow rateDerived coherent SI unit without a special nameDerived coherent metric unit without a special name
curie radioactive activityDerived metric unit with a special name
day timeNon-SI unit permitted for use with SI units
decibel levelNon-SI unit permitted for use with SI units
degree Celsius temperatureDerived SI unit with a special name
degree of arc angleNon-SI unit permitted for use with SI units
dyne forceDerived metric unit with a special name
electronvolt energyNon-SI unit permitted for use with SI units
erg energyDerived metric unit with a special name
farad capacitanceDerived SI unit with a special name
gal accelerationDerived SI unit with a special name
gauss magnetic flux densityDerived metric unit with a special name
gram massMultiple or submultiple of an SI base unitBase unit
grave massBase unit
gray absorbed doseDerived SI unit with a special name
hectare areaNon-SI unit permitted for use with SI unitsMultiple or submultiple of a base unit
henry inductanceDerived SI unit with a special name
hertz frequencyDerived SI unit with a special nameDerived metric unit with a special name
hour timeNon-SI unit permitted for use with SI unitsNo specified relationship
joule energyDerived SI unit with a special name
joule per mole energy per amount of substanceDerived coherent SI unit without a special name
joule-second angular momentumDerived coherent SI unit without a special name
katal catalytic activityDerived SI unit with a special name
kelvin temperatureBase unitBase unit
kilogram massBase unitMultiple or submultiple of a base unit
kilogram per cubic metre densityDerived coherent SI unit without a special nameDerived coherent metric unit without a special name
kilometre per hour velocityNo specified relationship
litre volumeNon-SI unit permitted for use with SI units
lumen luminous fluxDerived SI unit with a special name
lumen second luminous energyDerived coherent SI unit without a special name
lux illuminanceDerived SI unit with a special name
lux second luminous exposureDerived coherent SI unit without a special name
maxwell magnetic fluxDerived metric unit with a special name
metre per second squared accelerationDerived coherent SI unit without a special nameDerived coherent metric unit without a special name
metre squared per second angular momentumDerived coherent SI unit without a special nameDerived coherent metric unit without a special name
metre lengthBase unitMultiple or submultiple of a base unit
microgram massMultiple or submultiple of an SI base unitMultiple or submultiple of a base unit
minute timeNon-SI unit permitted for use with SI unitsNo specified relationship
minute of arc angleNon-SI unit permitted for use with SI units
mole amount of substanceBase unit
neper levelNon-SI unit permitted for use with SI units
newton forceDerived SI unit with a special name
newton-metre torqueDerived SI metric unit without a special name
newton-second impulse or momentumDerived coherent SI unit without a special name
oersted magnetic field strengthDerived SI unit with a special name
ohm electric resistanceDerived SI unit with a special name
pascal pressureDerived SI unit with a special name
phot illuminanceDerived coherent metric unit without a special name
poise dynamic viscosityDerived metric unit with a special name
radian angleDerived coherent SI unit with a special name
radian per second angular frequencyDerived coherent SI unit without a special name
rayleigh photon fluxNo specified relationship
roentgen kerma of X-rays and gamma raysDerived coherent metric unit without a special name
roentgen equivalent man radiation dose equivalentDerived coherent metric unit without a special name
second timeBase unitBase unit
siemens electric conductanceDerived SI unit with a special name
sievert radiation dose equivalentDerived SI unit with a special name
skot luminanceNo specified relationship
square kilometre areaDerived SI unit without a special nameDerived coherent metric unit without a special name
square metre areaDerived coherent SI unit without a special nameDerived coherent metric unit without a special name
statcoulomb electric chargeDerived metric unit with a special name
statvolt potential differenceDerived metric unit with a special name
steradian solid angleDerived coherent SI unit with a special name
stilb luminanceDerived coherent metric unit without a special name
stokes kinematic viscosityDerived metric unit with a special name
tesla magnetic field strengthDerived SI unit with a special name
tonne massNon-SI unit permitted for use with SI units
torr pressureNo specified relationship
volt potential differenceDerived SI unit with a special name
watt powerDerived SI unit with a special name
watt-second energyDerived coherent SI unit without a special name
weber magnetic fluxDerived SI unit with a special name

History of the metric system

History of the metric system – the metric system developed from a decimal system of measurement adopted by France after the French Revolution.

Chronological history of the metric system

Principal dates in the development of the metric system include:[1]

  • 1792 – Initiation of a decimal system of measurement by the French Revolutionary Government
  • 1799 – The Metre des archives and kilogram des archives become the standards for the metric system.
  • 1861 – Concept of unit coherence introduced by Maxwell – the base units were the centimetre, gram and second.
  • 1875 – Under the Convention of the metre, a new body the General Conference on Weights and Measures (CGPM) was set up and given responsibility for the new international prototype of the kilogram and international prototype of the metre which replaced the old French copies as the definitive prototypes of the metre and the kilogram.
  • 1881 – International Electric Congress agrees on standards for electrical units (formalised in 1893)
  • 1921 – Convention of the metre extended to cover all physical units of measure
  • 1960 – The CGPM published the metric system under the name "International System of Units" (SI) a coherent system of units based on the kilogram, metre, second, ampere and kelvin.

History of metrication

History of metrication – metrication is the process by which legacy, national-specific systems of measurement were replaced by the metric system.

Historical metric system variants

Four variants of the metric system that predate the introduction of SI (1960) are described in varying levels of detail:

Between 1812 and 1839 France used a quasi-metric system:

History of metric units

Politics of the metric system

Prior to 1875 the metric system was controlled by the French Government. In that year, seventeen nations signed the Metre Convention and the management and administration of the system passed into international control.

  • Metre Convention describes the 1875 treaty and its development to the modern day. Three organisations, the CGPM, CIPM and BIPM were set up under the convention.
  • General Conference on Weights and Measures (Conférence générale des poids et mesures or CGPM) – a meeting every four to six years of delegates from all member states.
  • The International Committee for Weights and Measures (Comité international des poids et mesures or CIPM) – an advisory body to the CGPM consisting of prominent metrologists.
  • The International Bureau of Weights and Measures (Bureau international des poids et mesures or BIPM) – an organisation based at Sèvres, France that has custody of the international prototype of the kilogram, provides metrology services for the CGPM and CIPM, houses the secretariat for these organisations and hosts their formal meetings.

Both the European Union and the International Organization for Standardization have issued directives/recommendations to harmonise the use of units of measure. These documents endorse the use of SI for most purposes.

Future of the metric system
  • 2019 redefinition of SI base units – changes in the definitions of the International System of Units, which are based on defined values of physical constants, in use since May 2019.

Metrication groups and authorities

Metric system publications

Persons influential in the metric system

See also

References
  1. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), pp. 108–110, ISBN 92-822-2213-6, archived (PDF) from the original on 2017-08-14
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