ABSTRACT
The International System of Units (SI) is the modern form of the metric system agreed at an international conference in 1960. It has been adopted by the International Standards Organisation (ISO) and the International Electrotechnical Commission (IEC) and its use is recommended wherever the metric system is applied. It is now being adopted throughout most of the world and is likely to remain the primary world system of units of measurement for a very long time. The indications are that SI units will supersede the units of existing metric systems and all systems based on Imperial units. SI units and the rules for their application are contained in ISO
ResolutionR1000 (1969, updated 1973) and an informatory document SI-Le Syste`me International d0 Unite´s, published by the Bureau International des Poids et Mesures (BIPM). An abridged version of the former is given in British Standards Institution (BSI) publication PD 5686 The Use of SI Units (1969, updated 1973) and BS 3763 International System (SI) Units; BSI (1964) incorporates information from the BIPM document. The adoption of SI presents less of a problem to the electron-
ics engineer and the electrical engineer than to those concerned with other engineering disciplines as all the practical electrical units were long ago incorporated in the metre-kilogram-second (MKS) unit system and these remain unaffected in SI. The SI was developed from the metric system as a fully
coherent set of units for science, technology and engineering. A coherent system has the property that corresponding equations between quantities and between numerical values have exactly the same form, because the relations between units do not involve numerical conversion factors. In constructing a coherent unit system, the starting point is the selection and definition of a minimum set of independent ‘base’ units. From these, ‘derived’ units are obtained by forming products or quotients in various combinations, again without numerical factors. Thus the base units of length (metre), time (second) and mass (kilogram) yield the SI units of velocity (metre/ second), force (kilogram-metre/second-squared) and so on. As a result there is, for any given physical quantity, only one SI unit with no alternatives and with no numerical conversion
factors. A single SI unit ( joule ¼ kilogram metre-squared/ second-squared) serves for energy of any kind, whether it be kinetic, potential, thermal, electrical, chemical . . . , thus unifying the usage in all branches of science and technology. The SI has seven base units, and two supplementary units of
angle. Certain important derived units have special names and can themselves be employed in combination to form alternative names for further derivations. Each physical quantity has a quantity-symbol (e.g. m for
mass) that represents it in equations, and a unit-symbol (e.g. kg for kilogram) to indicate its SI unit of measure.
