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The "explicit-constant" formulation
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Summary
Why change the SI?
What changes are proposed?
The "explicit-constant" formulation
What is a mise en pratique?
When might the changes take place?
What is the BIPM contributing?
Discussions on the New SI in the Consultative Committees
Discussion in the scientific literature
FAQs, Frequently Asked Questions about the New SI
Key documents
Discussion Meeting
Direct access

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 "New 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

      The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
    8th edn. of the SI brochure (2006), Section 2.1.1.1

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

      The metre, symbol m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly 299 792 458 when it is expressed in the SI unit m s–1.

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 unit of electric current; its magnitude is set by fixing the numerical value of the elementary charge to be equal to exactly 1.602 17X x 10–19 when it is expressed in the SI unit s A, which is equal to C.

Further information can be found in Resolution 1 and in the Draft Chapter 2 of the 9th edition of the SI brochure.