Marine Glossary

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International System of Units

A modern form of the metric system adopted in 1960 by the General Conference of Weights and Measures. The units of the International system of Units (abbreviated SI) are divided into three classes. The main class of SI units are the base units or the seven well defined units which by convention are regarded as dimensionally independent: the meter (length), the kilogram (mass), the second (time), the ampere (current), the Kelvin (temperature), the mole (amount of a substance), and the candela (luminous intensity).

The second class of SI units are the derived units, i.e., the units that can be formed by combining base units according to the algebraic relations linking the corresponding quantities. Several of these algebraic expressions in terms of base units can be replaced by special names and symbols which can themselves be used to form other derived units. Examples of derived units expressed algebraically in terms of base units by means of the mathematical symbols of multiplication and division are: force of 1 newton = 1 kilogram-meter per second squared; frequency of 1 hertz = 1 cycle per second; pressure of 1 Pascal = 1 newton per meter squared; energy of 1 joule = 1 newton meter; power of 1 watt = 1 joule per second.