Search results
Learn about the seven SI base units of measurement for time, length, mass, electric current, temperature, amount of substance and luminous intensity. Find out how they are defined by constants of nature and how they changed in 2019.
12 kwi 2010 · NIST provides values and a searchable bibliography for the fundamental physical constants. Definitions of all seven (7) SI base units are expressed using an explicit-constant formulation and experimentally realized using a specific mises en pratique (practical technique).
29 maj 2019 · As you can see above, the units in the revised SI are based completely on seven unchanging quantities or “universal constants,” including the speed of light, the amount of electric charge in an electron, and the Planck constant. Learn more about each of these “invariants of nature” and how they come into play in the revised SI.
The SI comprises a coherent system of units of measurement starting with seven base units, which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity).
The recommended practical system of units of measurement is the International System of Units (Système International d'Unités), with the international abbreviation SI. From 20 May 2019 all SI units are defined in terms of constants that describe the natural world.
Learn how the SI, the metric system, was transformed in 2019 by basing four of its seven base units on constants of nature. Find out what the revised SI means for science and trade, and how NIST contributed to the change.
Learn about the seven base units of the International System of Units (SI), which are time, length, mass, electric current, thermodynamic temperature, amount of substance and luminous intensity. Find out how they are defined and symbolized, and how they are related to other SI units.
