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The SI system after 1983, but before the 2019 revision: Dependence of base unit definitions on other base units (for example, the metre is defined as the distance travelled by light in a specific fraction of a second), with the constants of nature and artefacts used to define them (such as the mass of the IPK for the kilogram).
Giorgi later identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units (for temperature, amount of substance, and luminous intensity) were added later. [1] The early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass.
When that multiplier is one, the unit is called a coherent derived unit. For example, the coherent derived SI unit of velocity is the metre per second, with the symbol m/s. [1]: 139 The base and coherent derived units of the SI together form a coherent system of units (the set of coherent SI units). A useful property of a coherent system is ...
A number of different units (some only of historical interest) are shown and expressed in terms of the corresponding SI unit. Conversions between units in the metric system are defined by their prefixes (for example, 1 kilogram = 1000 grams, 1 milligram = 0.001 grams) and are thus not listed in this article.
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Unit prefixes are common base-10 or base-2 powers multiples and submultiples of units. While a base unit is one that has been explicitly so designated, [ 2 ] a derived unit is unit for a derived quantity , involving the combination of quantities with different units; [ 1 ] several SI derived units are specially named.
The first group of metric units are those that are at present defined as units within the International System of Units (SI). In its most restrictive interpretation, this is what may be meant when the term metric unit is used. The unit one (1) is the unit of a quantity of dimension one. It is the neutral element of any system of units. [2]
Prefix Base 10 Decimal Adoption [nb 1]Name Symbol quetta: Q: 10 30: 1 000 000 000 000 000 000 000 000 000 000: 2022 [1]: ronna: R: 10 27: 1 000 000 000 000 000 000 000 000 000: yotta: Y: 10 24: 1 000 000 000 000 000 000 000 000 ...