The International system of units, SI, has just been changed once again. The new definitions, which concern the kilogram, ampere, kelvin, and mole, are to come in force on May, 20. Similarly to the definition of the metre in terms of the speed of light, those four units are now defined in the terms of exact adopted values of fundamental constants, such as the elementary charge, Planck, Boltzmann, and Avogadro constants.
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