<P> Derived units may be defined in terms of base units or other derived units . They are adopted to facilitate measurement in diverse fields of endeavor . 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 . The last new derived unit was defined in 1999 . </P> <P> The reliability of the SI depends not only on the precise measurement of standards for the base units in terms of various physical constants of nature, but also on precise definition of those constants . The set of underlying constants is modified as more stable constants are found, or other constants may be more precisely measured . For example, in 1983, the metre was redefined to be the distance of light propagation in vacuum in an exact fraction of a second . Thus, the speed of light is now an exactly defined constant of nature . </P> <P> The motivation for the development of the SI was the diversity of units that had sprung up within the centimetre--gram--second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them . The General Conference on Weights and Measures (French: Conférence générale des poids et mesures--CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and standardise the rules for writing and presenting measurements . The system was published in 1960 as a result of an initiative that began in 1948 . It is based on the metre--kilogram--second system of units (MKS) rather than any variant of the CGS . Since then, the SI has been adopted by all developed countries except the United States . </P> <P> The International System of Units consists of a set of base units, a set of coherent derived units, and a set of decimal - based multipliers that are used as prefixes . Coherent units are derived units that contain no numerical factor other than 1--quantities such as standard gravity and density of water are absent from their definitions . For example: 1 N = 1 kg × 1 m / s which says that one newton is the force required to accelerate a mass of one kilogram at one metre per second squared . Since the SI units of mass and acceleration are kilogram and metre per second squared respectively and F ∝ m × a, the units of force (and hence of newtons) is formed by multiplication to give 1 kg ⋅ m ⋅ s . Since the newton is part of a coherent set of units, the constant of proportionality is 1 . </P>

Why do scientists use the international system of units (si)
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