International System of Units revised in historic vote On November 16, delegates from the member nations of the International Bureau of Weights and Measures (Bureau Internaltional des Poids et Mesures: BIPM) voted to revise the definitions of some of the seven fundamental units of the SI, the International System of Units (Système International d'Unites).

The biggest one was its revision of the kilogram. It is no longer given by a platinum-iridium cylinder in a suburb of Paris, but by Planck's constant being officially fixed.

It thus goes the way of the meter, once officially the length of a platinum-iridium bar in that same suburb of Paris, but for the last quarter-century by the speed of light in a vacuum officially being fixed.

The elementary charge will also be fixed, for making precision measurements of voltage and electrical resistance with the Josephson effect and the quantum Hall effect.

Also fixed are Boltzmann's constant, giving temperature in terms of energy, and Avogadro's number or constant, giving the atomic mass unit or dalton in terms of the kilogram.

A consequence is that all of the SI's units now depend on one physical reference, the one in the definition of the second, with all the rest being handled by very successful theories: relativity, quantum mechanics, thermodynamics, and electromagnetism. Something like how energy is handled, as a result of recognition that different kinds of energy are interconvertible.