The seven SI base units

Symbol | Name | Base quantity |
---|---|---|

s | second | time |

m | metre | length |

kg | kilogram | mass |

A | ampere | electric current |

K | kelvin | thermodynamic temperature |

mol | mole | amount of substance |

cd | candela | luminous intensity |

The **SI base units** are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre (sometimes spelled meter) for length or distance, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. The SI base units are a fundamental part of modern metrology, and thus part of the foundation of modern science and technology.

The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology. ^{[ citation needed ]}

The names and symbols of SI base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the *metre* has the symbol m, but the * kelvin * has symbol K, because it is named after Lord Kelvin and the * ampere * with symbol A is named after André-Marie Ampère.

A number of other units, such as the litre, astronomical unit, and electronvolt, are not formally part of the SI, but are accepted for use with SI.

On 20 May 2019, as the final act of the 2019 redefinition of the SI base units, the BIPM officially introduced the following new definitions, replacing the preceding definitions of the SI base units.

Name | Symbol | Measure | Post-2019 formal definition^{ [1] } | Historical origin / justification | Dimension symbol |
---|---|---|---|---|---|

second | s | time | "The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ∆ν_{Cs}, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s^{−1}."^{ [1] } | The day is divided into 24 hours, each hour divided into 60 minutes, each minute divided into 60 seconds. A second is 1 / (24 × 60 × 60) of the day. Historically, a day was defined as the mean solar day; i.e., the average time between two successive occurrences of local apparent solar noon. | T |

metre | m | length | "The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m s^{−1}, where the second is defined in terms of ∆ν_{Cs}."^{ [1] } | 1 / 10000000 of the distance from the Earth's equator to the North Pole measured on the meridian arc through Paris. | L |

kilogram | kg | mass | "The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10^{−34} when expressed in the unit J s, which is equal to kg m^{2} s^{−1}, where the metre and the second are defined in terms of c and ∆ν_{Cs}."^{ [1] } | The mass of one litre of water at the temperature of melting ice. A litre is one thousandth of a cubic metre. | M |

ampere | A | electric current | "The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634×10^{−19} when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆ν_{Cs}."^{ [1] } | The original "International Ampere" was defined electrochemically 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%. However, the most recent pre-2019 definition was: "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10^{−7} newtons per metre of length." This had the effect of defining the vacuum permeability to be | I |

kelvin | K | thermodynamic temperature | "The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10^{−23} when expressed in the unit J K^{−1}, which is equal to kg m^{2} s^{−2} K^{−1}, where the kilogram, metre and second are defined in terms of h, c and ∆ν_{Cs}."^{ [1] } | The Celsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is absolute zero). | Θ |

mole | mol | amount of substance | "The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, N_{A}, when expressed in the unit mol^{−1} and is called the Avogadro number. The amount of substance, symbol | Atomic weight or molecular weight divided by the molar mass constant, 1 g/mol. | N |

candela | cd | luminous intensity | "The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10^{12} Hz, K_{cd}, to be 683 when expressed in the unit lm W^{−1}, which is equal to cd sr W^{−1}, or cd sr kg^{−1} m^{−2} s^{3}, where the kilogram, metre and second are defined in terms of h, c and ∆ν_{Cs}."^{ [1] } | The candlepower, which is based on the light emitted from a burning candle of standard properties. | J |

New definitions of the base units were approved on 16 November 2018, and took effect 20 May 2019. The definitions of the base units have been modified several times since the Metre Convention in 1875, and new additions of base units have occurred. Since the redefinition of the metre in 1960, the kilogram had been the only base unit still defined directly in terms of a physical artefact, rather than a property of nature. This led to a number of the other SI base units being defined indirectly in terms of the mass of the same artefact; the mole, the ampere, and the candela were linked through their definitions to the mass of the International Prototype of the Kilogram, a roughly golfball-sized platinum–iridium cylinder stored in a vault near Paris.

It has long been an objective in metrology 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. The 21st General Conference on Weights and Measures (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 possibilities attracted particular attention: the Planck constant and the Avogadro constant.

In 2005, the International Committee for Weights and Measures (CIPM) approved preparation of new definitions for the kilogram, the ampere, and the kelvin and it noted the possibility of a new definition of the mole based on the Avogadro constant.^{ [2] } The 23rd CGPM (2007) decided to postpone any formal change until the next General Conference in 2011.^{ [3] }^{[ needs update ]}

In a note to the CIPM in October 2009,^{ [4] } Ian Mills, the President of the CIPM *Consultative Committee – Units* (CCU) catalogued the uncertainties of the fundamental constants of physics according to the current definitions and their values under the proposed new definition. He urged the CIPM to accept the proposed changes in the definition of the *kilogram*, *ampere*, *kelvin*, and *mole* so that they are referenced to the values of the fundamental constants, namely the Planck constant (*h*), the electron charge (*e*), the Boltzmann constant (*k*), and the Avogadro constant (*N*_{A}).^{ [5] } This approach was approved in 2018, only after measurements of these constants were achieved with sufficient accuracy.

The **ampere** (, ; symbol: **A**), often shortened to **amp**, is the unit of electric current in the International System of Units (SI). One ampere is equal to 6.241509074×10^{18} electrons worth of charge moving past a point in a second. It is named after French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism along with Danish physicist Hans Christian Ørsted.

The **General Conference on Weights and Measures** is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre Convention through which member states act together on matters related to measurement science and measurement standards. The CGPM is made up of delegates of the governments of the member states and observers from the Associates of the CGPM. Under its authority, the **International Committee for Weights and Measures** executes an exclusive direction and supervision of the BIPM.

The **kilogram** is the unit of mass in the International System of Units (SI), having the unit symbol **kg**. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a **kilo** colloquially. It means 'one thousand grams'.

The **Metre Convention**, also known as the **Treaty of the Metre**, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations. The treaty created the International Bureau of Weights and Measures (BIPM), an intergovernmental organization under the authority of the General Conference on Weights and Measures (CGPM) and the supervision of the International Committee for Weights and Measures (CIPM), that coordinates international metrology and the development of the metric system.

The **International System of Units,** known by the international abbreviation **SI** in all languages and sometimes pleonastically as the **SI system**, is the modern form of the metric system and based on the metre as the unit of length and either the kilogram as the unit of mass or the kilogram-force as the unit of force.</ref> and the world's most widely used system of measurement. Established and maintained by the General Conference on Weights and Measures (CGPM), it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

The **mole**, symbol **mol**, is the unit of amount of substance in the International System of Units (SI). The quantity **amount of substance** is a measure of how many **elementary entities** of a given substance are in an object or sample. The mole is defined as containing exactly 6.02214076×10^{23} elementary entities. Depending on what the substance is, an elementary entity may be an atom, a molecule, an ion, an ion pair, or a subatomic particle such as an electron. For example, 10 moles of water (a chemical compound) and 10 moles of mercury (a chemical element), contain equal amounts of substance and the mercury contains exactly one atom for each molecule of the water, despite the two having different volumes and different masses.

The **caesium standard** is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory.

The **Avogadro constant**, commonly denoted **N _{A}** or

The **dalton** or **unified atomic mass unit** is a unit of mass widely used in physics and chemistry. It is defined as 1⁄12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. The **atomic mass constant**, denoted *m*_{u}, is defined identically, giving *m*_{u} = *m*(^{12}C)/12 = 1 Da.

The **metric system** is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the International System of Units (SI) in the mid-20th century, under the oversight of an international standards body. Adopting the metric system is known as *metrication*.

A **base unit** is a unit adopted for measurement of a *base quantity*. A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others. The SI base units, or *Systeme International d'unites*, consists of the metre, kilogram, second, ampere, Kelvin, mole and candela.

**Metrology** is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960.

The **molar mass constant**, usually denoted by *M*_{u}, is a physical constant defined as one twelfth of the molar mass of carbon-12: *M*_{u} = *M*(^{12}C)/12. The molar mass of any element or compound is its relative atomic mass multiplied by the molar mass constant.

A **conventional electrical unit** is a unit of measurement in the field of electricity which is based on the so-called "conventional values" of the Josephson constant, the von Klitzing constant agreed by the International Committee for Weights and Measures (CIPM) in 1988, as well as Δ*ν*_{Cs} used to define the second. These units are very similar in scale to their corresponding SI units, but are not identical because of the different values used for the constants. They are distinguished from the corresponding SI units by setting the symbol in italic typeface and adding a subscript "90" – e.g., the conventional volt has the symbol *V*_{90} – as they came into international use on 1 January 1990.

The **kelvin**, symbol **K**, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907). The **Kelvin scale** is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its null (zero) point.

In metrology, a **standard** is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared. Historical standards for length, volume, and mass were defined by many different authorities, which resulted in confusion and inaccuracy of measurements. Modern measurements are defined in relationship to internationally standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass, electrical potential, and other physical quantities.

In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram. Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela had previously been redefined using physical constants. The four new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met. These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.

The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.

The following outline is provided as an overview of and topical guide to the **metric system** – various loosely related systems of measurement that trace their origin to the decimal system of measurement introduced in France during the French Revolution.

The scientific community examined several approaches to redefining the kilogram before deciding on a redefinition of the SI base units in November 2018. Each approach had advantages and disadvantages.

- 1 2 3 4 5 6 7 8 "The International System of Units (SI), 9th Edition" (PDF). Bureau International des Poids et Mesures. 2019.
- ↑ 94th Meeting of the International Committee for Weights and Measures (2005). Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants Archived 7 August 2011 at the Wayback Machine
- ↑ 23rd General Conference on Weights and Measures (2007). Resolution 12: On the possible redefinition of certain base units of the International System of Units (SI).
- ↑ Ian Mills, President of the CCU (October 2009). "Thoughts about the timing of the change from the Current SI to the New SI" (PDF). CIPM. Retrieved 23 February 2010.
- ↑ Ian Mills (29 September 2010). "Draft Chapter 2 for SI Brochure, following redefinitions of the base units" (PDF). CCU. Retrieved 1 January 2011.

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