Scientists taking part in a meeting of the Bureau International des Poids et Mesures (BIPM), in Versailles, have voted to change the way the kilogram and three other base units are defined.
The unanimous decision was driven by the critical need for scientific accuracy. The international prototype kilogram (IPK), a metal cylinder locked in a vault in Paris, has defined the weight since 1889.
Over time there has been a tiny fluctuation in the weight of the IPK. For this reason the kilogram will in future be defined by Planck’s constant, which relates the amount of energy carried by a photon to the frequency of its electromagnetic wave.
The scientists decided that the units for electric current (ampere), temperature (kelvin) and amount of substance (mole) would also be linked to constants of nature: the electric charge, the Boltzmann constant and the Avogadro constant respectively.
The base units (in all there are seven) are the building blocks of the modern metric system, standardized as the International System of Units (SI). The SI references the ISO/IEC 80000 quantities and units series.