# 2019 redefinition of the SI base units

In 2019 four of the seven si Base Units specified in the international System of Quantities were redefined in Terms of natural physical Constants rather than human Artifacts such as standard Kilogram. Effective 20 may 2019 the 144th Anniversary of the Metre Convention the Kilogram ampere kelvin and Mole are now defined by setting exact numerical Values when expressed in si Units for the Planck constant the elementary electric Charge the Boltzmann Constant and the. The second, Metre, and Candela had previously been redefined using physical Constants. The new Definitions were to improve the Si without changing the Value of any Units ensuring Continuity with existing Measures. In november 2018 the 26th general Conference on Weights and measures unanimously approved these Changes proposed by the international Committee for Weights and Measures earlier that Year after determining that previously agreed Conditions for the Change had been met. These Conditions were fulfilled by a Series of Experiments that measured the Constants with high Precision Relative to the old Si Definitions and were the Culmination of Decades of Research.

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