kilogramprototype

The kilogram prototype, officially known as the International Prototype of the Kilogram (IPK), was the physical artifact that defined the kilogram from 1889 until 2019. It was a cylinder made of a platinum-iridium alloy, standing 39.17 millimeters tall and having a diameter of 39.17 millimeters. This specific alloy was chosen for its exceptional stability and resistance to corrosion.

The IPK was housed in a controlled environment at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, to minimize any potential changes in its mass. Around the world, national metrology institutes maintained copies of the IPK, known as "national prototypes," which were periodically compared to the IPK to ensure consistency in mass measurements.

Despite its meticulous care, it was observed that the masses of the national prototypes had drifted slightly over time, and it was even suspected that the IPK itself might have undergone subtle changes. This led to a global effort to redefine the kilogram based on fundamental physical constants rather than a physical artifact.

In May 2019, the redefinition of the SI base units was implemented, and the kilogram is now defined by the Planck constant, a fundamental constant of nature. While the IPK is no longer the primary standard, it remains a historically significant artifact and is still preserved at the BIPM.