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The systems formalised the concept of a collection of related units called a coherent system of units. In a coherent system, base units combine to define derived units without extra factors. [4]: 2 For example, using metre per second is coherent in a system that uses metre for length and second for time, but kilometre per hour is not coherent.
The other units of length and mass, and all units of area, volume, and derived units such as density were derived from these two base units. Mesures usuelles ( French for customary measures ) were a system of measurement introduced as a compromise between the metric system and traditional measurements.
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).
SI derived units are units of measurement derived from the seven SI base units specified by the International System of Units (SI). They can be expressed as a product (or ratio) of one or more of the base units , possibly scaled by an appropriate power of exponentiation (see: Buckingham π theorem ).
The International System of Units is the modern metric system. It is based on the metre–kilogram–second–ampere (MKSA) system of units from early in the 20th century. [20] It also includes numerous coherent derived units for common quantities like power (watt) and irradience (lumen).
Stored charge per unit electric potential farad (F = C/V) L −2 M −1 T 4 I 2: scalar Catalytic activity concentration: Change in reaction rate due to presence of a catalyst per unit volume of the system kat⋅m −3: L −3 T −1 N: intensive Chemical potential: μ: Energy per unit change in amount of substance J/mol L 2 M T −2 N −1 ...
A standard (or etalon) is an object, system, or experiment with a defined relationship to a unit of measurement of a physical quantity. [31] Standards are the fundamental reference for a system of weights and measures by realising, preserving, or reproducing a unit against which measuring devices can be compared. [ 2 ]
The kilogram is defined by setting the Planck constant h exactly to 6.626 070 15 × 10 −34 J⋅s (J = kg⋅m 2 ⋅s −2), given the definitions of the metre and the second. [1] kg: SI: Physics: Basic: ampere: A: I: electric current: The flow of exactly 1 / 1.602 176 634 × 10 −19 times the elementary charge e per second.