Kilogram

The kilogram (also kilogramme) is the base unit of mass in the metric system, formally the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering, and commerce worldwide, and is often called a kilo. The kilogram is almost exactly the mass of one litre of water. The kilogram was originally defined in 1795 as the mass of a litre of water. This was a simple definition, but hard to replicate precisely. In 1799, the Kilogramme des Archives, a platinum artefact, replaced it as the standard of mass. In 1879 a cylinder of platinum-iridium, the International Prototype of the Kilogram (IPK) became the standard of the unit of mass for the metric system, and remained so until May 20, 2019,, making the kilogram the last of the metric base units to be defined by a physical artefact. Despite best efforts to maintain it, evidence accumulated that the mass of the International Prototype Kilogram had been changing; the IPK had diverged from its replicas by approximately 50 micrograms since their manufacture late in the 19th century. This led to several competing efforts to develop measurement technology precise enough to allow replacing the kilogram artefact with a definition based directly on physical fundamental constants. This culminated in 2018 with a redefinition of the kilogram in terms of the Planck constant. As a result of this redefinition, the kilogram is now defined in terms of the second and the metre, replacing the IPK as primary standard. The new definition was approved by the General Conference on Weights and Measures (CGPM) on November 16, 2018. Physical standard masses such as the IPK and its replicas still serve as secondary standards. The kilogram is defined in terms of three fundamental physical constants: The speed of light c, a specific atomic transition frequency ΔνCs, and the Planck constant h. The formal definition is: