A new measure: the reform of the International System of Units

The metric system began with the French revolution, and now we are experiencing the greatest revolution in measurement since the French revolution. The definitions of the kilogram, ampere, kelvin, and mole are all changed, being based on chosen and fixed values for Planck’s constant, the electron charge, Boltzmann’s constant, and Avogadro’s number. I will explain how this is possible, and why it was necessary?