The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:
The SI base quantities form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology. However, in a given realization of these units they may well be interdependent, i.e. defined in terms of each other.
The names of all SI units are written in lowercase characters (e.g., the metre has the symbol m), except that the symbols of units named after persons are written with an initial capital letter (e.g., the ampere has the uppercase symbol A).
Many other units, such as the litre, are formally not part of the SI, but are accepted for use with SI.
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–iridium cylinder stored in a vault near Paris. It has long been an objective of metrology 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.
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 main possibilities have attracted attention: the Planck constant and the Avogadro constant.
In 2005, the International Committee for Weights and Measures (CIPM) approved the preparation of new definitions for the kilogram, the ampere, and the kelvin and it 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.
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