International System of Units

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International System of Units,

officially called the Système International d'Unités, or SI, system of units adopted by the 11th General Conference on Weights and Measures (1960). It is based on the metric systemmetric system,
system of weights and measures planned in France and adopted there in 1799; it has since been adopted by most of the technologically developed countries of the world.
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. The basic units of length, mass, and time are those of the mks systemmks system,
system of units of measurement based on the metric system and having the meter of length, the kilogram of mass, and the second of time as its fundamental units. Other mks units include the newton of force, the joule of work or energy, and the watt of power.
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 of metric units: the metermeter,
abbr. m, fundamental unit of length in the metric system. The meter was originally defined as 1/10,000,000 of the distance between the equator and either pole; however, the original survey was inaccurate and the meter was later defined simply as the distance between two
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, kilogramkilogram,
abbr. kg, fundamental unit of mass in the metric system, defined as the mass of the International Prototype Kilogram, a platinum-iridium cylinder kept at Sèvres, France, near Paris.
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, and secondsecond,
abbr. sec or s, fundamental unit of time in all systems of measurement. In practical terms, the second is 1/60 of a minute, 1/3,600 of an hour, or 1/86,400 of a day. Since the length of the day varies, however, the second must be defined in more precise terms.
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. Other basic units are the ampereampere
, abbr. amp or A, basic unit of electric current. It is the fundamental electrical unit used with the mks system of units of the metric system. The ampere is officially defined as the current in a pair of equally long, parallel, straight wires 1 meter apart that produces
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 of electric current, the kelvin of temperature (a degree of temperature measured on the Kelvin temperature scaleKelvin temperature scale,
a temperature scale having an absolute zero below which temperatures do not exist. Absolute zero, or 0°K;, is the temperature at which molecular energy is a minimum, and it corresponds to a temperature of −273.
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), the candela, or candlecandle,
in weights and measures, unit of luminous intensity; it is defined as 1-60 of the intensity of a blackbody, or ideal radiator, at the temperature at which platinum solidifies (2,046°K;).
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, of luminous intensity, and the molemole,
in chemistry, a quantity of particles of any type equal to Avogadro's number, or 6.02×1023 particles. One gram-molecular weight of any molecular substance contains exactly one mole of molecules.
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, used to measure the amount of a substance present. All other units are derived from these basic units.


See U.S. National Bureau of Standards, Spec. Pub. 330, International System of Units (1971).

International System of Units


(Système International d’Unitées), a system of units of physical quantities, approved by the Eleventh General Conference of Weights and Measures (1960). The abbreviated designation of the system is SI. The International System of Units was developed to replace a complex set of systems of units and individual subsidiary units that had grown out of the metric system of measures and also to simplify the use of units. The advantages of the system include universality (it embraces all branches of science and technology) and coherence (that is, the consistency of derived units, which are found from equations without proportionality constants). As a result, if the magnitudes of all quantities are expressed in SI units, calculations may be carried out without introducing into the formulas any coefficients that depend on the choice of units.

The international and Russian names and symbols for base, supplementary, and some derived units of the International System of Units are given in Table 1. Russian symbols are given in accordance with the current GOST’s (All-Union State Standards); designations envisioned in the GOST Units of Physical Quantities Draft are also given. Definitions of base and supplementary units and their quantitative relationships are given in articles on the respective units.

The first three basic units (the meter, kilogram, and second) make possible the development of coherent derived units for quantities of a mechanical nature; the rest have been added to permit the formation of derived units for quantities that cannot be reduced to mechanical quantities (the ampere for electric and magnetic quantities, the kelvin for thermal quantities, the candela for quantities of light, and the mole for quantities in physical chemistry and molecular physics). The supplementary units radian and steradian are used in forming derived units of quantities depending on a plane angle or solid angle.

The International System of Units provides special prefixes to form the names of decimal multiple or fractional units: deci- (to form units equal to 10-1 of the original unit), centi- (10-2), milH- (10-3), micro- (10-6), nano- (10-9), pico- (10-12), femto(10-15), and atto- (l0-18); and decca- (101), hecto- (102), kilo(103), mega- (106), giga- (109), and tera- (1012).


Burdun, G. D. Spravochnikpo Mezhdunarodnoisisteme edinits. Moscow 1971.
Table 1. Base and derived units of the International System of Units
OuantityName of unitinternationalRussian
* Formerly “degree Kelvin” (°K)
Base Units
Length ........metermmM
Mass ........kilogramkgmkr
Time ........secondscekc
Electric current ........ampereAaA
Thermodynamic temperature ........kelvin*KKK
Luminous intensity ........candelacdk∂
Amount of substance ........molemolmojbMOAb
Suplimentary Units
Plane angle ........radianradpa∂paд
Solid angle ........steradiansrcmepcp
Derived units
Area ........square meterm2M2M2
Volume, capacity ........cubic meterm3M3M3
Frequency ........hertzHzГuѓЦ
Velocity ........meter per secondm/sM/cekM/C
Acceleration ........meter per second squaredm/s2M/cek2M/C2
Angular velocity ........radian per secondrad/spa∂/cekpaд/C
Angular acceleration ........radian per second squaredrad/s2pa∂/cek2paд/C2
Density ........kilogram per cubic meterkg/m3κǀ/M3kГ/m3
Force ........newtonNǀǀH
Pressure, mechanical stress ........pascalPaIIo(n/M2)Лa(H/M2)
Kinematic viscosity ........square meter per secondm2/sM2/ceκM2/C
Dynamic viscosity ........pascal-secondPa·secIIu·ceκΠa·C
Work, energy, quantity of heat ........jouleJ∂Mдж
Power ........wattWBMBT
Quantity of electricity ........coulombCκКA
Voltage, electromotive force ........voltVBB
Electric field strength ........volt per meterV/mB/MB/M
Electric resistance ........ohmOMOM
Electric conductance ........SiemensScǀǀMCM
Electric capacitance ........faradFФФ
Magnetic flux ........weberWbB6
Inductance ........henryHǀǀǀГ
Magnetic flux density ........teslaTMAT
Magnetic field strength ........ampere per meterA/ma/mA/M
Magnetomotive force ........ampereAaA
Entropy ........joule per kelvinJ/KMдж/К
Specific heat ........joule per kilogram-kelvinJ/(kg·K)M/(κi·К)дж/(κГ·К)
Thermal conductivity ........watt per meter-kelvinW/(m·K)6m/(m·К)BT/(M·К)
Radiation intensity ........watt per steradianW/sr6m/cmepBT/cp
Wave number ........unit per meterm-1m-1M-1
Luminous flux ........lumenImǀmAM
Brightness ........candela per square metercd/m2κ∂/M2κд/M2
Illuminance ........luxIxǀκAκ
Le Système International d’unitées (SI), Edition du Bureau International des poids et mesures. Paris, 1970.


International System of Units

[¦in·tər¦nash·ən·əl ¦sis·təm əv ′yü·nəts]
A system of physical units in which the fundamental quantities are length, time, mass, electric current, temperature, luminous intensity, and amount of substance, and the corresponding units are the meter, second, kilogram, ampere, kelvin, candela, and mole; it has been given official status and recommended for universal use by the General Conference on Weights and Measures. Also known (in French) as Système International d'Unités. Abbreviated SI (in all languages).

International System of Units (SI)

A system of units based on the following fundamental quantities: metre, kilogram, second, ampere, kelvin, candela, and mole.
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