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International Prototype of the Kilogram

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".

picture of the international prototype

The IPK has been conserved at the BIPM since 1889, when it was sanctioned by the 1st General Conference on Weights and Measures (CGPM). It is of cylindrical form, with diameter and height of about 39 mm, and is made of an alloy of 90 % platinum and 10 % iridium. Initially the IPK had two official copies; over the years, one official copy has been replaced and four others have been added, so that there are now six official copies.

Access to the IPK and its official copies is under the 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 of mass standards. Historically the IPK has been compared to its official copies at intervals of about 40 years, with the exception of the "extraordinary campaign" carried out in 2014, which was only 22 years after the preceding one. In the intervals in between, the working standards are used to disseminate the kilogram unit to the Member States by calibrations of their "national prototypes", which are intended to serve as national standards.

The BIPM watt balance
BIPM ensemble of mass standards
International Avogadro Project
Calibration services
International Prototype of the Kilogram
BIPM calibrations and services in mass and related quantities
Density determinations of prototypes and mass standards
Staff of the Department
Recent publications
Frequently asked questions about the International Prototype of the Kilogram
On the possible future revision of the SI
Director's Report 2014
Related articles
Unit of mass (kilogram)
The SI unit of mass (Metrologia, 2003, 40, 299-305)