## Introduction

The seven SI base units:

Name | Symbol (单位符号) |
Measure (量) |
Definition (定义) |
Dimension symbol (量纲符号) |
---|---|---|---|---|

meter (米) | m | Length (长度) | “The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.” |
L |

kilogram (千克) | kg | Mass (质量) | “The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.” |
M |

second (秒) | s | Time (时间) | “The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” |
T |

ampere (安［培］) | A | Electric current (电流) | “The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10^-7 newton per meter of length.” |
I |

kelvin (开［尔文］) | K | Thermodynamic Temperature (绝对温度) | “The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.” |
Θ |

mole (摩［尔］) | mol | Amount of substance (物质的量) | “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. |
N |

candela (坎［德拉］) | cd | Luminous intensity (发光强度) | “The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.” |
J |

SI stands for the International System of Units, also known as the metric system.

## Formal Definition

The units are important not only in science, but also in our daily lives. The different definitions could cause many troubles. As the world developed, the units were unified and the definitions were always being updated to be more precise.

The current definition of the SI base units that we are using was approved in 2005.

## Constants

Four of the seven SI constants are also updated to have exact values. These seven constants are:

Speed of light in vacuum(真空中光速): c = 299792458 m⋅s^{−1}

Hyperfine transition frequency of cesium-133(铯-133原子的基态超精细分裂频率): ∆ν_{Cs} = Δν(^{133}Cs)_{hfs} = 9192631770 s^{−1}

luminous efficacy of monochromatic radiation of frequency 540 × 10^{12} hertz(频率为540 × 10^{12}赫兹单色辐射的光视效能): K_{cd} = 683 cd⋅sr⋅s^{3}⋅kg^{−1}⋅m^{−2}

Planck constant(普朗克常数): h = 6.62607015×10^{−34} kg⋅m^{2}⋅s^{−1}

Elementary charge(元电荷): e = 1.602176634×10^{−19} A⋅s

Boltzmann constant(玻尔兹曼常数): k = 1.380649×10^{−23} kg⋅m^{2}⋅K^{−1}⋅s^{−2}

Avogadro constant(阿伏伽德罗常数): N_{A} = 6.02214076×10^{23} mol^{−1}

These constants are important to the new definition of SI base units.

## Redefinition (2019)

New definitions of the base units were approved on 16 November 2018, in Versailles, France, to take effect 20 May 2019.

The detailed definition and explanation could be found in the links in the References part.

## References

- NIST - Current Definitions of the SI base units
- NIST - SI Redefinition
- 国际单位制的重大变革，人类文明的关键节点
- Wikipedia: SI base unit
- BIPM - measurement units
- DRAFT 9th edition of the SI Brochure by BIPM
- NIST - Meet the Constants

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