The international prototype of the kilogram IPK is the artefact whose mass defines at present the SI unit of mass: “The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram”.
The international prototype of the kilogram was sanctioned in 1889. Its form is a cylinder with diameter and height of about 39 mm. It is made of an alloy of 90 % platinum and 10 % iridium. The IPK has been conserved at the BIPM since 1889, initially with two official copies. Over the years, one official copy was replaced and four have been added.
Access to the IPK and its official copies is under strict supervision of the International Committee for Weights and Measures (CIPM).
The unit of mass is disseminated throughout the world by comparisons with the IPK made indirectly through a hierarchical system. The first step of these comparisons is normally with a subset of the “official copies” of the IPK, followed by calibrations of additional copies known as the “national prototypes” which are the primary national standards. Historically the IPK has been compared to its official copies at intervals of about 40 years. In between, working standards of the BIPM are used to disseminate the kilogram unit to the Member States.