Units of physical quantities, particular physical quantities to which, by definition, numerical values equal to unity are assigned. Many units of physical quantities are realised by material measures used for measurements (for example, the metre, the kilogram). Historically, units of physical quantities first appeared for measuring length, area, volume, mass and time, and the sizes of the units did not agree from one country to another. As trade expanded and science and technology developed, the number of units of physical quantities grew and the need to unify them and to create systems of units was felt ever more keenly. In the 18th century the metric system of measures was proposed in France and gained international recognition. On its basis a number of metric systems of units were built, which were used in various fields of physics and technology. The further ordering of units of physical quantities is now going on, on the basis of the International System of Units (SI).
Units of physical quantities are divided into system units, that is, those belonging to some system of units, and units outside the systems (for example, the millimetre of mercury, the horsepower, the electronvolt). System units are subdivided into base units, chosen arbitrarily (the metre, the kilogram, the second and others), and derived units, formed from the equations relating physical quantities (the newton, the joule and so on). For the convenience of expressing various amounts of some quantity that are many times larger or smaller than the units of physical quantities, multiple units and submultiple units are used. In the metric systems of units, multiple and submultiple units (apart from the units of time and of angle) are formed by multiplying the system unit by 10n, where n is a positive or negative whole number. To each of these numbers there corresponds one of the decimal prefixes adopted for forming the names of multiple and submultiple units.
Units of physical quantities are divided into system units, that is, those belonging to some system of units, and units outside the systems (for example, the millimetre of mercury, the horsepower, the electronvolt). System units are subdivided into base units, chosen arbitrarily (the metre, the kilogram, the second and others), and derived units, formed from the equations relating physical quantities (the newton, the joule and so on). For the convenience of expressing various amounts of some quantity that are many times larger or smaller than the units of physical quantities, multiple units and submultiple units are used. In the metric systems of units, multiple and submultiple units (apart from the units of time and of angle) are formed by multiplying the system unit by 10n, where n is a positive or negative whole number. To each of these numbers there corresponds one of the decimal prefixes adopted for forming the names of multiple and submultiple units.