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Concept
Kilogram
Summary
The kilogram (also kilogramme) is the base 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 kilogram is defined in terms of the second and the metre, both of which are based on fundamental physical constants. This allows a properly equipped metrology laboratory to calibrate a mass measurement instrument such as a Kibble balance as the primary standard to determine an exact kilogram mass.
The kilogram was originally defined in 1795 during the French Revolution as the mass of one litre of water. The current definition of a kilogram agrees with this original definition to within 30 parts per million. In 1799, the platinum Kilogramme des Archives replaced it as the standard of mass. In 1889, a cylinder of platinum-iridium, the International Prototype of the Kilogram (IPK), became the standard of the unit of mass for the metric system and remained so for 130 years, before the current standard was adopted in 2019.
The kilogram is defined in terms of three fundamental physical constants:
a specific atomic transition frequency ΔνCs, which defines the duration of the second,
the speed of light c, which when combined with the second, defines the length of the metre,
and the Planck constant h, which when combined with the metre and second, defines the mass of the kilogram.
The formal definition according to the General Conference on Weights and Measures (CGPM) is:
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 6.62607015e-34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
Defined in term of those units, the kg is formulated as:
kg = (299792458)^2/(6.62607015e-34)(9192631770)h ΔνCs/c^2 = 917097121160018/62154105072590475e42h ΔνCs/c^2 ≈ (1.475521399735270e40)h ΔνCs/c^2 .
Official source
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