NIST's Physical Measurement Laboratory (PML) is producing a series of special reports on the worldwide consensus plan to redefine four of the seven basic units of measurement in the International System of Units in terms of invariants of nature. Explore these stories as they become available, see below.
For more than a century, the kilogram (kg) – the fundamental unit of mass in the International System of Units (SI) – has been defined as exactly equal to the mass of a small polished cylinder, cast in 1879 of platinum and iridium, which is kept in a triple-locked vault on the outskirts of Paris.
Temperature is one of the most important and frequent measurements in human life. For centuries, we have continuously improved on the systems, technologies, methods, and units used to quantify and express it. Now the next stage of that process is about to take place, when the kelvin (K) -- the SI base unit of thermodynamic temperature -- will get a radically new definition.