International Prototype of the Kilogram (IPK)

A kilogram (symbol kg) is the fundamental unit of mass. It was originally defined as approximately equal to the mass of 1 liter of water. According to the International System of Units (SI), the kilogram was historically defined by the mass of the international prototype of the kilogram, which is kept at the International Bureau of Weights and Measures in Sèvres, France. The international prototype of the kilogram is a cylindrical object with a height and diameter of approximately 39 mm, made from an alloy consisting of 90% platinum and 10% iridium. Copies of this prototype were created with the highest possible precision and distributed to various countries for use in measurement standards. The material used for the prototype was selected based on several criteria, including corrosion resistance, high density (to minimize the effects of buoyancy when measured in air), good electrical conductivity (to prevent static electricity), low magnetic susceptibility (diamagnetism, to limit magnetic interference), thermal stability, and hardness (for resistance to abrasion). **Note**: The international prototype is no longer the basis for the definition of the kilogram. In 2019, representatives from over 50 countries unanimously agreed to redefine the kilogram using fundamental constants of nature, such as the speed of light, the duration of time, and Planck's constant — a value that relates the energy of a photon to its wavelength. Planck's constant has an approximate value of 6.626 x 10⁻³⁴ joule-seconds.