BIPMi

BIPMi, or the BIPM International Prototype of the Meter, is a physical artifact that serves as the international standard for the meter, the base unit of length in the International System of Units (SI). It is maintained by the Bureau International des Poids et Mesures (BIPM) in Sèvres, France. The BIPMi is a platinum-iridium alloy bar that is 1 meter long at the temperature of melting ice. It was adopted as the standard for the meter in 1889, replacing the previous standard, the meter bar of the Archives in Paris.

The BIPMi is used to calibrate national prototypes of the meter, which are then used to calibrate working standards. This ensures consistency in measurements across different countries and laboratories. The BIPMi is periodically compared with other national prototypes to ensure its accuracy and stability.

In 2019, the BIPM announced plans to redefine the meter in terms of the speed of light, a fundamental constant of nature. This change would make the definition of the meter more precise and stable, as it would no longer depend on a physical artifact. The redefinition is expected to come into effect in 2023.