France – 31 May 1975 - First Day Cover
100th Anniversary of the Convention du Mètre and
the establishment of the Bureau International des Poids et Mesures (BIPM).
The stamp and cachet list the names of the seven SI Base Units.
The stamp shows:
At the left-side: signatures and the seal of the first Convention of the definition of the meter on 20 May 1875;
At the right-side: the Krypton-86 (Kr 86) atom, with a reminder of the international system of measurements (SI) based on the MKSA (meter-kg-second-ampere) system (with 7 base units); and
At the lower-side: the multiplier 1,650,765.73, which was used for the new definition of the meter on 14 October 1960 (at the 11th CGPM), as follows: The meter is defined in terms of 1,650,765.73 wavelengths of Krypton-86 atom in a vacuum. Note that the definition of the meter was revised in 1983 and was approved as the distance travelled by light through a vacuum in 1/299792458 second.