New Definitions of the Kilogram and the Mole: Paradigm Shift to the Definitions Based on Physical Constants

Abstract

The kilogram is the unit of mass and was defined in 1889 by the international prototype of the kilogram. The mole is the unit of amount of substance and was defined in 1960 by the number of atoms in 0.012 kg of ¹²C. These definitions were revised in May 2019. The new definitions of the kilogram and the mole are based on the Planck constant h and the Avogadro constant N_A, respectively. The values of h and N_A used in the new definitions were determined by summarizing measurement results of the two physical constants by several national metrology institutes around the world. In this review, the history of the two units and measurement technologies used to derive the new definitions are described. The effect of the revision on the development of new measurement technologies is also introduced.