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The "explicit-constant" formulation

Resolution 1 adopted by the CGPM in 2011 takes note of the CIPM's intention to express the definitions of all seven base units of the SI in a uniform manner using the "explicit-constant formulation", in which "the unit is defined indirectly by specifying explicitly an exact value for a well-recognized fundamental constant".

As further explained in the Draft Chapter 2 of the 9th edition of the SI brochure, the revised SI will be scaled so that the numerical values of seven constants are fixed. Using the "explicit-constant formulation" each definition will state explicitly which numerical value it fixes.

For example, the definition of the metre might be reformulated from the existing

to a completely equivalent but "explicit-constant" form which might be

      The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m/s, where the second is defined in terms of capital DeltanuCs.

to make clear that this definition fixes the numerical value of the speed of light in vacuum, c, expressed in metres per second.

Along the same lines, the new definition for the ampere might be reworded with, for example, the following sentence:

      The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10–19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of capital DeltanuCs.

Further information can be found in Draft Resolution A (26th CGPM - November 2018) and in the Draft text for the 9th edition of the SI Brochure.


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