# 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 system**metric 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.

**.....**Click the link for more information. . The basic units of length, mass, and time are those of the mks system

**mks 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.

**.....**Click the link for more information. of metric units: the meter

**meter,**

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

**.....**Click the link for more information. , kilogram

**kilogram,**

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.

**.....**Click the link for more information. , and second

**second,**

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.

**.....**Click the link for more information. . Other basic units are the ampere

**ampere**

, 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

**.....**Click the link for more information. of electric current, the kelvin of temperature (a degree of temperature measured on the Kelvin temperature scale

**Kelvin 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.

**.....**Click the link for more information. ), the candela, or candle

**candle,**

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;).

**.....**Click the link for more information. , of luminous intensity, and the mole

**mole,**

in chemistry, a quantity of particles of any type equal to Avogadro's number, or 6.02×10

^{23}particles. One gram-molecular weight of any molecular substance contains exactly one mole of molecules.

**.....**Click the link for more information. , used to measure the amount of a substance present. All other units are derived from these basic units.

### Bibliography

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- (10^{1}), hecto- (10^{2}), kilo(10^{3}), mega- (10^{6}), giga- (10^{9}), and tera- (10^{12}).

### REFERENCES

Burdun, G. D.*Spravochnikpo Mezhdunarodnoisisteme edinits*. Moscow 1971.

Table 1. Base and derived units of the International System of Units | ||||
---|---|---|---|---|

Designation | ||||

Ouantity | Name of unit | international | Russian | |

Current | Planned | |||

* Formerly “degree Kelvin” (°K) | ||||

Base Units | ||||

Length ........ | meter | m | m | M |

Mass ........ | kilogram | kg | m | kr |

Time ........ | second | s | cek | c |

Electric current ........ | ampere | A | a | A |

Thermodynamic temperature ........ | kelvin* | K | K | K |

Luminous intensity ........ | candela | cd | k∂ | kд |

Amount of substance ........ | mole | mol | mojb | MOAb |

Suplimentary Units | ||||

Plane angle ........ | radian | rad | pa∂ | paд |

Solid angle ........ | steradian | sr | cmep | cp |

Derived units | ||||

Area ........ | square meter | m^{2} | M^{2} | M^{2} |

Volume, capacity ........ | cubic meter | m^{3} | M^{3} | M^{3} |

Frequency ........ | hertz | Hz | Гu | ѓЦ |

Velocity ........ | meter per second | m/s | M/cek | M/C |

Acceleration ........ | meter per second squared | m/s^{2} | M/cek^{2} | M/C^{2} |

Angular velocity ........ | radian per second | rad/s | pa∂/cek | paд/C |

Angular acceleration ........ | radian per second squared | rad/s^{2} | pa∂/cek^{2} | paд/C^{2} |

Density ........ | kilogram per cubic meter | kg/m^{3} | κǀ/M^{3} | kГ/m^{3} |

Force ........ | newton | N | ǀǀ | H |

Pressure, mechanical stress ........ | pascal | Pa | IIo(n/M^{2}) | Лa(H/M^{2}) |

Kinematic viscosity ........ | square meter per second | m^{2}/s | M^{2}/ceκ | M^{2}/C |

Dynamic viscosity ........ | pascal-second | Pa·sec | IIu·ceκ | Πa·C |

Work, energy, quantity of heat ........ | joule | J | ∂M | дж |

Power ........ | watt | W | BM | BT |

Quantity of electricity ........ | coulomb | C | κ | КA |

Voltage, electromotive force ........ | volt | V | B | B |

Electric field strength ........ | volt per meter | V/m | B/M | B/M |

Electric resistance ........ | ohm | Ω | OM | OM |

Electric conductance ........ | Siemens | S | cǀǀM | CM |

Electric capacitance ........ | farad | F | Ф | Ф |

Magnetic flux ........ | weber | Wb | Bõ | B6 |

Inductance ........ | henry | H | ǀǀǀ | Г |

Magnetic flux density ........ | tesla | T | MA | T |

Magnetic field strength ........ | ampere per meter | A/m | a/m | A/M |

Magnetomotive force ........ | ampere | A | a | A |

Entropy ........ | joule per kelvin | J/K | ∂M/К | дж/К |

Specific heat ........ | joule per kilogram-kelvin | J/(kg·K) | ∂M/(κi·К) | дж/(κГ·К) |

Thermal conductivity ........ | watt per meter-kelvin | W/(m·K) | 6m/(m·К) | BT/(M·К) |

Radiation intensity ........ | watt per steradian | W/sr | 6m/cmep | BT/cp |

Wave number ........ | unit per meter | m^{-1} | m^{-1} | M^{-1} |

Luminous flux ........ | lumen | Im | ǀm | AM |

Brightness ........ | candela per square meter | cd/m^{2} | κ∂/M^{2} | κд/M^{2} |

Illuminance ........ | lux | Ix | ǀκ | Aκ |

*Le Système International d’unitées (SI), Edition du Bureau International des poids et mesures*. Paris, 1970.

K. P. SHIROKOV