In physics we deal with physical quantities which are usually quantified by measuring. The process of measuring involves assigning a number to a physical quantity by comparing it with a standard. The standard quantity is called unit and used to measure the given physical quantity. For instance when we say the length is 5 m, it means the standard used is one meter ( 1 m) and the length is 5 times of the standard length. A standard adopted as a unit should have the following features: i) It must be universally acceptable and of reasonable size. ii) Its magnitude must be definite and definable iii) Its magnitude should not change with time, temperature and pressure i.e. it should be invariable and iv) It should be reproducible.