From its beginnings in the 18th century the metric system has increased in recognition and has developed towards the SI units which are now in use throughout the world for scientific, technological, and increasingly for everyday use. MKS electrical units, devised at the beginning of this century to simplify electrical calculations, have also developed, and are now incorporated in the SI units. The advent of this simple and unified system of units has stimulated a revolution in the teaching of the theory of electricity and magnetism.