Quantities, units and some useful numerical values

2 Quantities, units and some useful numerical values 

The SI (abbreviated from the French Le Systane International d Unites), the modern metric system of measurement was developed in 1960 from the old meter-kilogram-second (mks) system, rather than the centimeter-gram-second (cgs) system, which, in turn, had a few variants. Because the SI is not static, units are created and definitions are modified through international agreement among many nations as the technology of measurement progresses, and as the precision of measurements improve. 

Long the language universally used in science, the SI has become the dominant language of international commerce and trade. The system is nearly universally employed, and most countries do not even maintain official definitions of any other units. A notable exception is the United States, which continues to use customary units in addition to SI. In the United Kingdom, conversion to metric units is government policy, but the transition is not quite complete. Those countries that still recognize non-SI units (e. g., the US and UK) have redefined their traditional non-SI units in SI units. 

It is important to distinguish between the definition of a unit and its reahzation. The definition of each base unit of the SI is carefully drawn up so that it is unique and provides a sound theoretical basis upon which the most accurate and reproducible measurements can be made. The realization of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the unit. 

Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units. Examples of such SI derived units are given in the Table 2, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted. 

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