The definition of the kilogram, SI base unit of mass, is as follows:
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ΔνCs.
The kilogram, a unit defined and realised in the past through the international prototype, is today related to the Planck constant. In this way is it possible to extend the mass scale with accuracy which could not be achieved with the international prototype. The realisation of the unit of mass requires two complementary primary experiments. The first experiment, the Kibble electrodynamic balance, allows to measure, through the virtual comparison of electrical and mechanical powers, the ratio between the reference mass and the Planck constant. The second experiment uses the counting of atoms in a spherical macroscopic volume and the knowledge of the mass of an atom. With the redefinition, each experiment capable to relate a reference mass to the Planck constant can be considered a primary method (at the level of uncertainty achieved by the experiment) for the realization of the unit of mass.
For practical reasons it was decided that initially the dissemination of the kilogram is to be coordinated internationally based on the so-called "consensus value". Currently, according to this decision, the dissemination is with the platinum-iridium national prototypes, for Italy it is the copy No. 62 of the International Prototype of the Kilogram.
Today at INRIM the realization of an electrostatic balance able to measure small masses through the Planck constant is in progress.
