## Metric system |

The **metric system** is an internationally recognised

In its modern form, it consists of a set of * metre* for length,

The metric system was designed to have properties that make it easy to use and widely applicable, including units based on the natural world, decimal ratios, prefixes for multiples and sub-multiples, and a structure of base and derived units. It is also a *rationalisation* that eliminates certain

The units of the metric system, originally taken from observable features of nature, are now defined by phenomena such as the microwave frequency of a caesium

While there are numerous named derived units of the metric system, such as the

Units of the

The metric system is also extensible, and new derived units are defined as needed in fields such as radiology and chemistry. The most recent derived unit, the

- units
- realisation of units
- properties as a system
- international system of units
- conversion, calculation and symbol confusion incidents
- conversion table
- see also
- notes
- references
- external links

This section does not . (September 2019) ( |

The modern metric system consists of four electromechanical base units representing seven fundamental dimensions of measure: length, mass, time, electromagnetism, thermodynamic temperature, luminous intensity, and quantity of substance. The units are:

metre for lengthkilogram for masssecond for timeampere for electromagnetismkelvin for temperaturecandela for luminous intensitymole for quantity

Together they are sufficient for measuring any known quantity,^{[1]} without reference to further quantities or phenomena.

The

There are currently 22 derived units with special names in the metric system, these are defined in terms of the base units or other named derived units.

Eight of these units are electromagnetic quantities:

volt , a unit of electrical potentialohm , a unit of electrical resistancetesla , a unit of magnetic flux densityweber , a unit of magnetic fluxfarad , a unit of electrical capacitancehenry , a unit of electrical inductancesiemens , a unit of electrical conductance (the inverse ofohm )coulomb , a unit of electrical charge

Four of these units are mechanical quantities:

watt , a unit of mechanical or electrical powernewton , a unit of mechanical forcejoule , a unit of mechanical, electrical or thermodynamic energypascal , a unit of pressure

Five units represent measures of electromagnetic radiation and radioactivity:

becquerel , a unit of radioactive decaysievert , a unit of absorbed ionising radiationgray , a unit of ionising radiationlux , a unit of luminous fluxlumen , a unit of luminous intensity

Two units are measures of circular arcs and spherical surfaces:

Three units are miscellaneous:

degree Celsius , a unit of thermodynamic temperaturekatal , a unit of catalytic activity (enzymatic)hertz , a unit of cycles persecond (inverse of second)

Although SI, as published by the CGPM, should, in theory, meet all the requirements of commerce, science, and technology, certain customary units of measure have acquired established positions within the world community. In order that such units are used consistently around the world, the CGPM catalogued such units in Tables 6 to 9 of the SI brochure. These categories are:^{[2]}

**Non-SI units accepted for use with the International System of Units (Table 6)**. This list includes thehour andminute , the angular measures (Degree ,Minute and second of arc ), and the historic [non-coherent] metric units, thelitre ,tonne , andhectare (originally agreed by the CGPM in 1879)**Non-SI units whose values in SI units must be obtained experimentally (Table 7)**. This list includes various units of measure used in atomic and nuclear physics and in astronomy such as thedalton , theelectron mass , theelectron volt , theastronomical unit , thesolar mass , and a number of other units of measure that are well-established, but dependent on experimentally-determined physical quantities.**Other non-SI units (Table 8)**. This list catalogues a number of units of measure that have been used internationally in certain well-defined spheres including thebar for pressure, theångström foratomic physics , thenautical mile and theknot innavigation .**Non-SI units associated with the CGS and the CGS-Gaussian system of units (Table 9)**. This table catalogues a number of units of measure based on the CGS system and dating from the nineteenth century. They appear frequently in the literature, but their continued use is discouraged by the CGPM.

The SI symbols for the metric units are intended to be identical, regardless of the language used^{[3]} but unit names are * chilometro* (Italian),

Variations are also found with the spelling of unit names in countries using the same language, including differences in *meter* and *liter* are used in the United States whereas *metre* and *litre* are used in other English-speaking countries. In addition, the official US spelling for the rarely used *deka*. In American English the term *metric ton* is the normal usage whereas in other varieties of English *tonne* is common. *Gram* is also sometimes spelled *gramme* in English-speaking countries other than the United States, though this older usage is declining.^{[6]}

In SI, the unit of power is the "*one* joule per second".^{[7]} In the ^{[8]} Similarly, neither the US gallon nor the imperial gallon is *one* cubic foot or *one* cubic yard— the US gallon is 231 cubic inches and the imperial gallon is 277.42 cubic inches.^{[9]}

The concept of coherence was only introduced into the metric system in the third quarter of the 19th century;^{[10]} in its original form the metric system was non-coherent—in particular the ^{3} and the ^{2}. However the units of mass and length were related to each other through the physical properties of water, the gram having been designed as being the mass of one cubic centimetre of water at its freezing point.^{[11]}