What is S.I unit ? || Standard unit || Explained || Priyam Kumar

The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system and is the most widely used system of measurement, with an official status in most countries.[Note 1] It comprises a coherent system of units of measurement including seven base units, which are the second (the unit of time with the symbol s), metre (length, m), kilogram (mass, kg), ampere (electric current, A), kelvin (thermodynamic temperature, K), mole (amount of substance, mol), and candela (luminous intensity, cd).[1] The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units.[Note 2] Twenty-two derived units have been provided with special names and symbols.[Note 3] The seven base units and the 22 derived units with special names and symbols may be used in combination to express other derived units,[Note 4] which are adopted to facilitate measurement of diverse quantities. Another characteristic feature of the SI system is that it provides twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units, always in powers of ten. For example, k, 'kilo', stands for a factor of 1000, so one kilometre (1 km) is 1000 m. As a consequence, the conversion of units within the SI is always through a power of ten.[Note 5] The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves.

Since 2019, the SI system of units has been defined as the system of units in which seven particular defining constants have certain exact numerical values when expressed in terms of their SI units. For example, one of these defining constants is the speed of light in vacuum c, which in the SI by definition has the exact value of c = 299792458 m/s. The other six are the hyperfine transition frequency of caesium {\displaystyle \Delta \nu _{\text{Cs}}}{\displaystyle \Delta \nu _{\text{Cs}}}, the Planck constant h, the elementary charge e, the Boltzmann constant k, the Avogadro constant NA, and the luminous efficacy Kcd. Mathematically, every SI unit is a unique[Note 6] product of powers of the defining constants divided by their exact numerical SI values.[Note 7] The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant Kcd.[1]:128–9. Prior to 2019, h, e, k, and NA were not defined a priori but were rather very precisely measured quantities. In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units. One consequence of the redefinition of the SI is that the distinction between the base units and derived units is in principle not needed, since any unit can be constructed directly from the seven defining constants. Nevertheless, the distinction is retained because 'it is useful and historically well established', and also because the ISO/IEC 80000 standard specifies base and derived quantities that necessarily have the corresponding SI units.[1]:129

The current way of defining the SI system is a result of a decades-long move towards increasingly abstract and idealized formulation in which the realizations of the units are separated conceptually from the definitions. As far as realization of units, the CIPM Consultative Committees, based on the best available science of the moment, recommend a variety of experimental methods that can be used to realize the definitions of the base units. Descriptions of what are believed to be currently best experimental realizations are provided as so-called "mises en pratique" (practical techniques).[2] The great advantage of doing it this way is that as science and technologies develop, new and superior realizations may be introduced without the need to redefine the unit.[Note 8] A disadvantage of this system is that it lacks the simplicity and conceptual clarity of e.g. using artefacts to define the units, in which case the definition and the realization are equivalent. One problem with artefacts is that they can be lost, damaged, or changed; another is that they largely cannot benefit from advancements in science and technology. The last artefact used by the SI was the International Prototype of the Kilogram, a cylinder of platinum-iridium. Concerns regarding its stability on the one hand, and progress in precise measurements of the Planck constant and the Avogadro constant on the other, led to a revision of the definition of the base units, which was put into effect on 20 May 2019[5] and resulted in the definition of the SI system as described above. The past definitions of the SI relied on various other approaches, e.g. a specific physical state such as the triple point of water for the