TheInternational System of Units is the standard modern form of themetric system. The name of this system can be shortened orabbreviated toSI, from theFrench nameSystèmeInternational d'unités.
The International System of Units is asystem of measurement based on 7 base units: themetre (length),kilogram (mass),second (time),ampere (electric current),kelvin (temperature),mole (quantity), andcandela (brightness). These base units can be used in combination with each other. This createsSI derived units, which can be used to describe other quantities, such asvolume,energy,pressure, andvelocity.
The system is used almost globally. OnlyMyanmar,Liberia, and theUnited States do not use SI as their official system of measurement.[1] In these countries, though, SI is commonly used in science and medicine.
Themetric system was created inFrance after theFrench Revolution in1789. The original system only had two standard units, the kilogram and the metre. The metric system became popular amongst scientists.
In the1860s,James Clerk Maxwell andWilliam Thomson (later known as Lord Kelvin) suggested a system with three base units– length, mass, and time. Other units would be derived from those three base units. Later, this suggestion would be used to create thecentimetre-gram-second system of units (CGS), which used thecentimetre as the base unit for length, thegram as the base unit for mass, and the second as the base unit for time. It also added thedyne as the base unit forforce and theerg as the base unit for energy.
As scientists studiedelectricity andmagnetism, they realized other base units were needed to describe these subjects. By the middle of the20th century, many different versions of the metric system were being used. This was very confusing.
In1954, the 9thGeneral Conference on Weights and Measures (CGPM) created the first version of the International System of Units. The six base units that they used were the metre, kilogram, second, ampere, Kelvin, and candela.[2] The seventh base unit, the mole, was added in1971.[3]
SI is now used almost everywhere in the world, except in theUnited States,Liberia andMyanmar, where the olderimperial units are still widely used. Other countries, most of them historically related to theBritish Empire, are slowly replacing the oldimperial system with the metric system or using both systems at the same time.
The SIbase units aremeasurements used by scientists and other people around the world. All the other units can be written by combining these seven base units in different ways. These other units are called "derived units".
Current (1967): The duration of9192631770 periods of the radiation corresponding to the transition between the twohyperfine levels of theground state of thecaesium-133 atom.
Current (2019): The kilogram is defined by setting thePlanck constanth exactly to6.62607015×10−34J⋅s (J = kg⋅m2⋅s−2), given the definitions of the metre and the second.[8] Then the formula would be 1 kg =h⁄6.62607015 × 10-34⋅m2⋅s−1
Prior (1881): A tenth of the electromagnetic CGS unit of current. The [CGS] electromagnetic unit of current is that current, flowing in an arc 1cm long of a circle 1cm in radius, that creates a field of oneoersted at the centre.[9]IEC
Interim (1946): The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1m apart in vacuum, would produce between these conductors aforce equal to2×10−7newtons per metre of length.
Current (2019): The flow of1⁄1.602176634×10−19 times theelementary chargee per second.
Current (2019): The kelvin is defined by setting the fixed numerical value of theBoltzmann constantk to1.380649×10−23J⋅K−1, (J = kg⋅m2⋅s−2), given the definition of the kilogram, the metre and the second.
Prior (1900): A stoichiometric quantity which is the equivalent mass in grams ofAvogadro's number of molecules of a substance.ICAW
Interim (1967): The amount of substance of a system which contains as many elementary entities[n 4] as there are atoms in 0.012 kilogram ofcarbon-12.
Current (2019): The amount of substance of exactly6.02214076×1023 elementary entities. This number is the fixed numerical value of theAvogadro constant,NA, when expressed in the unit mol−1 and is called the Avogadro number.
Prior (1946): The value of the new candle (early name for the candela) is such that the brightness of the full radiator at the temperature of solidification ofplatinum is 60 new candles per square centimetre.
Current (1979): The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency5.4×1014 hertz and that has a radiant intensity in that direction of watt persteradian.
Note: both old and new definitions are approximately the luminous intensity of awhale blubber candle burning modestly bright, in the late 19th century called a "candlepower" or a "candle".
Notes
↑Interim definitions are given here only when there has been asignificant difference in the definition.
↑Despite the prefix "kilo-", the kilogram is the coherent base unit of mass, and is used in the definitions of derived units. Nonetheless, prefixes for the unit of mass are determined as if the gram were the base unit.
↑In 1954 the unit of thermodynamic temperature was known as the "degree Kelvin" (symbol °K; "Kelvin" spelt with an upper-case "K"). It was renamed the "kelvin" (symbol "K"; "kelvin" spelt with a lower-case "k") in 1967.
↑When the mole is used, the elementary entities must be specified and may beatoms,molecules,ions,electrons, other particles, or specified groups of such particles.
ThePrior definitions of the various base units in the above table were made by the following authorities:
Derived units are created by combining the base units. The base units can be divided, multiplied, or raised topowers. Some derived units have special names. Usually these were created to make calculations simpler.
Very large or very small measurements can be written usingprefixes. Prefixes are added to the beginning of the unit to make a new unit. For example, the prefixkilo- means "1000" times the original unit and the prefixmilli- means "0.001" times the original unit. So onekilometre is 1000metres and onemilligram is a 1000th of agram.
