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# NIST Guide to the SI, Appendix A: Definitions of the SI Base Units

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NOTE:  The BIPM SI Brochure (9th Edition) was published on May 20, 2019. The NIST SP 330 2019 version has been published and reflects the changes incorporated in the BIPM SI Brochure (9th Edition). This publication, Special Publication 811, has not yet been updated to reflect the changes in the SI that came into effect on May 20, 2019 (World Metrology Day).

A.1 Introduction

The following definitions of the SI base units are taken from Refs. [1, 2]; it should be noted that SI derived units are uniquely defined only in terms of SI base units; for example, 1 V = 1 m2 · kg · s-3 · A-1.

A.2 Meter (17th CGPM, 1983)

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458of a second.

A.3 Kilogram (3d CGPM, 1901)

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

A.4 Second (13th CGPM, 1967)

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.

A.5 Ampere (9th CGPM, 1948)

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placedmeter apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per meter of length.

A.6 Kelvin (13th CGPM, 1967)

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

A.7 Mole (14th CGPM, 1971)

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.

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.

In the definition of the mole, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to.

Note: This definition specifies at the same time the nature of the quantity whose unit is the mole.

A.8 Candela (16th CGPM, 1979)

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian.

Created January 28, 2016, Updated March 1, 2023