Number of systems of units have been used at various times in the past days. The different systems of units are available in the different countries. The M K S and C G S systems of units have been used in earlier days. However, for the sake of uniformity of units all over the world, an international organization, the General Conference of Weight and Measures, recommended a unified systematically constituted system of units, in 1960. This system of units is called SI (System International Units) system of units. The SI system of units is divided into three categories namely:. Fundamental Units: The units which are independently chosen and not dependent on any other units are called fundamental units. These are another called base units. The seven such base units form the basis of SI system of units. These base units are metre, kilogramme, second, ampere, kelvin, mole, candela. Supplementary Units: In addition to the fundamental units, there’s two supplementary units added to the SI system of units. These units are defined as:1) Radian for the plane angles: The plane angle subtended by an arc of a circle equal in length to the radius of the circle. The radian is denoted as rad 2) Steradian for the solid angles: The solid angle subtended at the centre of a sphere by the surface whose area is equal to the square of the radius of the sphere. The steradian is denoted as sr. Derived Units: The units other than fundamental and supplementary are derived from the fundamental and supplementary units. Hence these units are called derived units. The derived units can be mainly classified as:1) Mechanical units such as mass, velocity, acceleration, force, weight, torque, work, energy, power etc 2) Electric and Magnetic units such as power, energy, ohms, farads, henries, magnetic flux in webers, tesla etc 3) Thermal units such as latent heat, specific heat capacity, sensible heat, calorific value etc. There’s number of other derived units available to specify the quantities like are, volume, density, luminous flux, luminance etc . When a particular instrument indicates a reading, to specify the reading and use it in the further calculations, it is necessary to specify type and magnitude for that reading. The magnitude is nothing except the reading obtained on the instrument. The type of the reading is nothing except the unit of the physical quantity which is measured by the instrument. Without unit, only magnitude has no physical meaning. Thus the unit can be defined as follows: The standard measure of each type of physical quantity to be measured is called unit. The physical, chemical, electrical quantity, property, process, variable or a condition to be measured is referred as measurand. The measurement involves comparison of a quantity with a standard value