Introduction

Abstract

All non-electrical physical quantities may be defined in terms of mass, length and time which are the three fundamental mechanical units. Electrical and magnetic quantities generally require four units of which three are mass, length and time and the fourth can be some electrical or magnetic quantity such as current, permeability or permittivity. It is sometimes convenient to introduce temperature as an independent unit ranking equally with length, time and mass but if it is recognized that heat is of the same nature as energy, temperature may be defined in terms of the three fundamental mechanical units. It would appear, then, that all physical quantities can be expressed in terms of four units. The measurement of mass, length and time dates back to the dawn of history, whereas electrical and magnetic phenomena were not considered quantitatively until the middle of the nineteenth century.