The system of units used throughout this book is known as SI, an abbreviation for Système International d'Unités. It is a comprehensive, logical system, designed for use in all branches of science and technology. It was formally approved in 1960 by the General Conference of Weights and Measures, the international organization responsible for maintaining standards of measurement. Apart from its intrinsic merits, it has the great advantage that one system covers all situations – theoretical and practical.

A full account of SI will be found in a publication of the National Physical Laboratory (Bell 1993). The following are the essential features of the system.

(1) SI is a metric system. There are seven base units (see next section), the metre and kilogram replacing the centimetre and gram of the old c.g.s. system.

(2) The derived units are directly related to the base units. For example, the unit of acceleration is 1 m s-2. The unit of force is the newton, which is the force required to give a body of mass 1 kg an acceleration of 1 m s-2. The unit of energy is the joule, which is the work done when a force of 1 N moves a body a distance of 1 m.

The use of auxiliary units is discouraged in SI. Thus the unit of pressure, the pascal, is 1 N m-2; the atmosphere and the torr are not used. Similarly the calorie is not used; all forms of energy are measured in joules.