Kilogram

The kilogram (kg) is equal to the mass of the international prototype of the kilogram.

Up to now, the unit of mass cannot be traced back to fundamental constants with sufficient accuracy. Thus, the reference object for mass measurement is still the international prototype of the kilogram. It was manufactured in 1889 of an alloy of 90 % platinum and 10 % iridium. It is kept at the Bureau Internationel des Poids et Mesures (BIPM) at Sèvres near Paris.

To distribute the mass unit, copies of the international prototype kilogram - so-called national prototypes - have been provided to the majority of member states of the Metre Convention. These are used in national metrology institutes to check the working standards against which the weights and balances used in practice can be calibrated.

The comparison of the working standards to the national prototype kilogram is carried out in Switzerland with the help of an automatic mass comparator, housed inside an air-sealed steel casing. With this comparator it is possible to compare up to six mass standards in an atmosphere which is very close to perfectly shielded from pressure and temperature variations, with a reproducibility of 0.25 µg.

To bring the mass unit into line with other base units, the worldwide attempt to find a means of referencing the mass unit to natural constants continues; e. g. via electrical quantum standards, or the manufacture of high-purity silicon mono-crystals with a known number of atoms. METAS is working actively on such a project, called the Watt Balance. The aim of this experiment is to link the mass unit to fundamental constants by an extremely precise comparison of the electrical and the mechanical power.