SI base unit

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SI base unit

The International System of Units

International System of Units

The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten....
(SI) defines seven dimensionally independent

Dimensional analysis

Dimensional analysis is a conceptual tool often applied in physics, chemistry, and engineering to understand physical situations involving certain physical quantities....
SI base units. All other physical units can be derived from these base units: these are known as SI derived unit

SI derived unit

SI derived units are part of the SI system of measurement Units of measurements and are derived from the seven SI base units.Note that while the names of all SI units are in lowercase, the symbols of units named after people are written with an initial capital letter ....
s.

Note that while the names of all SI units are in lowercase, the symbols of units named after people are written with an initial capital letter (eg: meter, m; ampere, A). The exception is degrees Celsius, which refers to degrees on the Celsius

Celsius

Celsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death....
scale.

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style="font-size:larger;font-weight:bold;"|SI

Si, si, or SI may refer to :...
base units

Name

Symbol

Measure

Definition

Historical Origin / Justification

met

Metre

The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes....

length

Length

Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end....

17th CGPM (1983, Resolution 1, CR, 97)

Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
's equator to the North Pole measured on the circumference through Paris

Paris

Paris is the Capital of France and the country's largest city. It is situated on the river Seine, in northern France, at the heart of the ?le-de-France Regions of France ....
.

kilogram

Kilogram

The kilogram or kilogrammeThe spelling kilogram is used by the International Committee for Weights and Measures and the U.S....

mass

Mass

In physical science, mass refers to the degree of acceleration a body acquires when subject to a force: bodies with greater mass are accelerated less by the same force....

"The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram."
3rd CGPM (1901, CR, 70)

The mass of one lit

Litre

The litre or liter is a unit of volume. There are two official symbols: the Latin letter L in lower and upper case . The lower case L is often written as a cursive l to avoid confusion with the number 1 in antiqua fonts....
of water

Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
.

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Encyclopedia

The International System of Units

International System of Units

Dimensional analysis

SI derived unit

Celsius

Celsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death....
scale.

style="font-size:larger;font-weight:bold;"|SI
Si

Si, si, or SI may refer to :...
base units
Name	Symbol	Measure	Definition	Historical Origin / Justification
met Metre The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes....	m	length Length Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end....	17th CGPM (1983, Resolution 1, CR, 97)	Earth Earth Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather... 's equator to the North Pole measured on the circumference through Paris Paris Paris is the Capital of France and the country's largest city. It is situated on the river Seine, in northern France, at the heart of the ?le-de-France Regions of France .... .
kilogram Kilogram The kilogram or kilogrammeThe spelling kilogram is used by the International Committee for Weights and Measures and the U.S....	kg	mass Mass In physical science, mass refers to the degree of acceleration a body acquires when subject to a force: bodies with greater mass are accelerated less by the same force....	"The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram." 3rd CGPM (1901, CR, 70)	The mass of one lit Litre The litre or liter is a unit of volume. There are two official symbols: the Latin letter L in lower and upper case . The lower case L is often written as a cursive l to avoid confusion with the number 1 in antiqua fonts.... of water Water Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam.... . A lit is one thousandth of a cubic met.
second Second The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....	s	time Time Time is a component of the measurement used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects....	13th CGPM (1967/68, Resolution 1; CR, 103) "This definition refers to a caesium atom at rest at a temperature of 0 K." (Added by CIPM in 1997)	A second is of the day Day A day is a units of measurement of time equivalent to approximately 24 hours. It is not an International System of Units unit but it is accepted for use with SI....
ampere Ampere The ampere is the International System of Units unit of electric current. The ampere, in practice often shortened to amp, is an SI base unit, and is named after Andr?-Marie Amp?re, one of the main discoverers of electromagnetism....	A	electric current Electric current Electric current is the flow of electric charge. The electric charge may be either electrons or ions.The International System of Units unit of electric current intensity is the ampere....	9th CGPM (1948)	The original "International Ampere" was defined electrochemically (1893) as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the SI ampere, the difference is 0.015%.
kelvin Kelvin The kelvin is a Units of measurement of temperature and is one of the seven SI base units. The Kelvin scale is a Thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero ....	K	thermodynamic temperature Thermodynamic temperature Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics. Thermodynamic temperature is an ?absolute? scale because it is the measure of the fundamental property underlying temperature: its null or zero point, absolute zero, is the temperature at which the particle constitue...	13th CGPM (1967/68, Resolution 4; CR, 104) "This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: mole of ²H per mole of ¹H, mole of ¹⁷O per mole of ¹⁶O, and mole of ¹⁸O per mole of ¹⁶O." (Added by CIPM in 2005)	the Celsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is absolute zero Absolute zero Absolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible, and cannot be reached, by artificial or natural means.... ).
mole Mole (unit) The mole is a Units of measurement of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity....	mol	amount of substance Amount of substance The amount of substance, n, of a sample or system is a physical quantity which is Proportionality to the number of Elementary entity present....	"1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol”. / 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles." 14th CGPM (1971, Resolution 3; CR, 78) "In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to." (Added by CIPM in 1980)	atomic weight Atomic weight Atomic weight is a Dimensionless quantity physical quantity, the ratio of the average mass of atoms of an chemical element to 1/12 of the mass of an atom of carbon-12.... or molecular weight multiplied by the molar mass constant Molar mass constant The molar mass constant, symbol Mu, is a physical constant which relates atomic weight and molar mass. Its value is defined to be 1?10?3 kg/mol in SI units.... , 1 g/mol
candela Candela The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function ....	cd	luminous intensity Luminous intensity In Photometry , luminous intensity is a measure of the wavelength-weighted Power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye....	16th CGPM (1979, Resolution 3; CR, 100)	the candlepower Candlepower Candlepower is an obsolete scientific unit of luminous intensity based on the light emitted from a candle made to a specified formula.The candlepower as a scientific measure was replaced in 1948 by the international unit known as the candela.... , which is based on the light emitted from a candle

Proposal to modify the definitions

There have been several modifications to the definitions of the base units, and additions of base units, since the Metre Convention in 1875. Since the redefinition of the metre in 1960, the kilogram is the only unit which is directly defined in terms of a physical artifact rather than a property of nature. However, the mole, the ampere and the candela are also linked through their definitions to the mass of this platinum

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements....
–iridium

Iridium

Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is the second densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 ?C....
cylinder stored in a vault near Paris. It has long been an objective of metrology

Metrology

Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement....
to find a way to define the kilogram in terms of a fundamental constant, in the same way that the metre is now defined in terms of the speed of light

Speed of light

The speed of light in an free space is an important physical constant usually written as c, with a value of 299,792,458 metres per second....
.

The 21st General Conference on Weights and Measures

General Conference on Weights and Measures

The General Conference on Weights and Measures is the English name of the Conf?rence g?n?rale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du M?tre of 1875....
(CGPM, 1999) placed these efforts on an official footing, and recommended "that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram." Two main possibilities have attracted attention: the Planck constant

Planck constant

The Planck constant , also called Planck's constant, is a physical constant used to describe the sizes of quantum in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory....
and the Avogadro constant.

In 2005, the International Committee for Weights and Measures

International Committee for Weights and Measures

The International Committee for Weights and Measures is the English name of the Comit? international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention ....
(CIPM) approved the preparation of new definitions for the kilogram, the ampere and the kelvin, and noted the possibility of a new definition for the mole based on the Avogadro constant. The 23rd CGPM (2007) decided to postpone any legal change until the next General Conference in 2011.

SI base unit

Proposal to modify the definitions

See also

External links