The modernized metric system, what most people mean by “the metric system,” 1960–present. The official symbol of the system, which is the term most often used to refer to it, is “SI,” from the French “Système International d'Unités.”
It consists of 7 base units and a number of derived units, some with special names.
To any of these units may be attached one of 16 decimal multiplier or submultiplier prefixes, such as kilo-, milli- and so on.
| Unit | Symbol | Year adopted by CGPM |
Measured Property |
|---|---|---|---|
| meter | m | 1889, 1927, 1960, 1983 |
length |
| kilogram | kg | 1889, 1960 | mass |
| second | s | 1960, 1967 | time |
| ampere | A | 1948 | electric current |
| kelvin | K | 1967 | thermodynamic temperature |
| mole | mol | 1971 | amount of substance |
| candela | cd | 1948, 1967, 1979 |
luminous intensity |
A class of units in SI that are completely defined in terms of the base units, but which have been given their own names. Examples include the
| Unit | Symbol | Year adopted by CGPM |
Properties measured |
|---|---|---|---|
| becquerel | Bq | 1975 | activity |
| coulomb | C | 1960 | electric charge, quantity of electricity |
| degree Celsius | °C | Celsius temperature | |
| farad | F | 1960 | capacitance |
| gray | Gy | 1975 | absorbed dose, specific energy imparted, kerma, absorbed dose index |
| henry | H | inductance | |
| hertz | Hz | 1960 | frequency |
| joule | J | 1960 | energy, work, quantity of heat |
| katal | kat | 1999 | catalytic activity |
| lumen | lm | 1960 | luminous flux |
| lux | lx | 1960 | illuminance |
| newton | N | 1960 | force |
| ohm | Ω | 1960 | electric resistance |
| pascal | Pa | 1971 | pressure, stress |
| siemens | S | 1971 | electric conductance |
| sievert | Sv | 1980 | dose equivalent, dose equivalent index |
| tesla | T | 1960 | magnetic flux density |
| volt | V | 1960 | electric potential, potential difference, electromotive force |
| watt | W | 1960 | power, radiant flux |
| weber | Wb | 1960 | magnetic flux |
This class of units (but not the radian and steradian themselves!) was abolished in 1995.
| Prefix | Sym- bol |
Year adopted by CGPM |
Meaning (using American names of numbers) |
Example |
|---|---|---|---|---|
| yotta- | Y | 1990 | 1024 = 1 000 000 000 000 000 000 000 000 = 1 septillion | |
| zetta- | Z | 1990 | 1021 = 1 000 000 000 000 000 000 000 = 1 sextillion | |
| exa- | E | 1975 | 1018 = 1 000 000 000 000 000 000 = 1 quintillion | |
| peta- | P | 1975 | 1015 = 1 000 000 000 000 000 = 1 quadrillion | |
| tera- | T | 1960 | 1012 = 1 000 000 000 000 = 1 trillion | |
| giga- | G | 1948, 1960 |
109 = 1 000 000 000 = 1 billion | gigajoule |
| mega- | M | 1960 | 106 = 1 000 000 = 1 million | megawatt |
| kilo- | k | 1960 | 103 = 1 000 = 1 thousand | kilowatt |
| hecto- | h | 1960 | 102 = 100 = 1 hundred | hectare |
| deka-* | da | 1960 | 101 = 10 = ten | |
| 100 = 1 = one | ||||
| deci- | d | 1960 | 10-1 = .1 = a tenth of a | decibel |
| centi- | c | 1960 | 10-2 = .01 = a hundredth of a | centimeter |
| milli- | m | 1960 | 10-3 = .001 = a thousandth of a | milliliter |
| micro- | μ | 1960 | 10-6 = .000 001 = a millionth of a | microfarad |
| nano- | n | 1960 | 10-9 = .000 000 001 = a billionth of a | nanometer |
| pico- | p | 1960 | 10-12 = .000 000 000 001 = a trillionth of a | picofarad |
| femto- | f | 1964 | 10-15 = .000 000 000 000 001 = a quadrillionth of a | femtosecond |
| atto- | a | 1964 | 10-18 = .000 000 000 000 000 001 = a quintillionth of a | |
| zepto- | z | 1990 | 10-21 = .000 000 000 000 000 000 001 = a sextillionth of a | |
| yocto- | y | 1990 | 10−24 = .000 000 000 000 000 000 000 001 = a septillionth of a |
* The spelling “deca-” is often used (but not in the United States).
In combining the prefix with the unit name, the prefixes' final vowel is retained, except in the case of “hectare”. The American National Standard* also calls for the spellings “megohm” and “kilohm”, but the spelling “megaohm” is often encountered.
* American National Standard for Metric Practice.
ANSI/IEEE Standard 268-1992.
New York: IEEE, Oct. 1992.
A number of bogus prefixes have been described, mostly on the Internet. These hoaxes are being tracked by Gérard Michon.
See metric system for the development and spread of the basic units prior to the Conference on the Meter in 1879.
The Conference on the Meter led to the formation of permanent international bodies (the CGPM, BIPM, and CIPM) with a commitment to the meter and kilogram, and a mandate to standardize and improve the world's weights and measures. In the early years, the organizations' attention was focused mainly on units, rather than systems of units.
In the late 19th and early 20th century, most scientists used centimeter-gram-second systems, which proliferated because there were various ways of handling electromagnetic quantities. Alongside these systems were others, also ostensibly “metric,” which defined other electric and magnetic units for practical use. A subject which involved both electric and magnetic quantities required treatment in two and possibly three different systems of units. Furthermore, as new sciences developed they tended to extemporize new units, for example, the study of radioactivity led to nuclear physics and to curies, rads, rems, barns, and other units, all “metric.”
In 1901, the Italian physicist G. Giorgi proposed a meter-kilogram-second-electrical unit system, presenting it to the International Electrotechnical Commission (IEC) in 1904. Giorgi originally suggested that the electric unit be a unit of resistance, but later that was replaced by a unit of current, the ampere. The great advantage of Giorgi's proposal was that it used familiar units of mass, length, and time and, with rationalized units (and the right choice of a value for the permeability of free space), it preserved the sizes of the practical electric units, even though they were defined in absolute rather than material terms. It was an absolute practical system. In 1935, the IEC passed a resolution adopting the Giorgi system (but without deciding whether the units should be rationalized or not) and recommending it be named after him. But it was too late; almost everywhere it was referred to as the MKSA system.
Also in 1935, the Commission on Symbols, Units and Nomenclature of the International Union of Pure and Applied Physics (IUPAP) recommended basing a system on the meter and kilogram, and proposed the name newton for the unit of force based on the kilogram, second and meter. The IEC confirmed their 1935 decision in 1938 (still without deciding the rationalization issue), and the IUPAP repeated their recommendation in 1948.
In 1948 the Ninth CGPM, prodded by the French government and noting that the IUPAP had asked the CIPM “to adopt for international use a practical international system of units” and had recommended “the MKS system and one electric unit of the absolute practical system,” formally requested the CIPM “to make recommendations on the establishment of a practical system of units of measurement suitable for adoption by all signatories” (Resolution 6).
In July, 1950, the IEC's Technical Committee No. 24 on Electrical and Magnetic Magnitudes and Units chose the ampere to be the fourth, electric, unit and finally recommended rationalized units.
The 10th CGPM (1954) adopted as base units the meter, kilogram, second, ampere, degree Kelvin, and candela, thus adopting the Giorgi system. The remaining base unit, the mole, was added by the 14th CGPM in 1971 (Resolution 3).
The 11th CGPM (1960) named the new system “International System of Units;” adopted the “international abbreviation” SI and the prefixes from tera- through pico-; and added two supplementary units and 27 derived units, 13 of which had special names. So 1960 may be taken as the birth year of SI. Since then, one base unit, 11 derived units (including five with special names) and eight prefixes have been added; the second, meter and candela have been redefined; the liter and micron have been discarded, and the “degree Kelvin” changed to “Kelvin.”
The 21st CGPM (1999) adopted the “katal.”
Bureau International des Poids et Mesures.
The International System of Units (SI). 7th edition.
Organisation Intergouvernementale de la Convention du Mètre, 1998.
The authoritative (though the French version is more authoritative), comprehensive description of SI. Available on the Web as a pdf file at www.bipm.org/utils/en/pdf/si-brochure.pdf
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Last revised: 3 February 2005.