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A committee of the International Committee for Weights and Measures (CIPM) has proposed revised formal definitions of the SI base units, which are being examined by the CIPM and which will likely〔 be adopted at the 26th General Conference on Weights and Measures (CGPM) in the fall of 2018. The metric system was originally conceived as a system of measurement that was derivable from nature. When the metric system was first introduced in France in 1799 technical problems necessitated the use of artifacts such as the prototype metre and kilogram. In 1960 the metre was redefined in terms of the wavelength of light from a specified source, making it derivable from nature, leaving the kilogram as the only unit still defined by an artifact. If the proposed redefinition is accepted, the metric system (SI) will, for the first time, be wholly derivable from nature. The proposal can be summarised as follows: ::"There will still be the same seven base units (second, metre, kilogram, ampere, kelvin, mole, and candela). Of these, the kilogram, ampere, kelvin and mole will be redefined by choosing exact numerical values for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre and candela are already defined by physical constants and it is only necessary to edit their present definitions. The new definitions will improve the SI without changing the size of any units, thus ensuring continuity with present measurements." Further details are found in the draft chapter of the ''Ninth SI Units Brochure''.〔 〕 The last major overhaul of the metric system was in 1960 when the International System of Units (SI) was formally published as a coherent set of units of measure. SI is structured around seven base units that have apparently "arbitrary" definitions and another twenty units that are derived from these base units. Although the units themselves form a coherent system, the definitions do not. The proposal before the CIPM seeks to remedy this by using the fundamental quantities of nature as the basis for deriving the base units. This will mean, amongst other things, that the prototype kilogram will cease to be used as ''the'' definitive replica of the kilogram. The second and the metre are already defined in such a manner. A number of authors have published criticisms of the revised definitions — in particular that proposal had failed to address the impact of breaking the link between the mole and the kilogram, the dalton and the unified atomic mass unit, and the Avogadro constant and Avogadro's number. ==Background== (詳細はウィキペディア（Wikipedia）』 ■ウィキペディアで「Proposed redefinition of SI base units」の詳細全文を読む スポンサード リンク
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