It is a differential method of measurement in which a very small difference between the value of the quantity to be measured and the reference is determined by the observation of the coincidence of certain lines or signals. For example, measurement by vernier calliper micrometer.
In this method the value of the quantity to be measured is directly indicated by a deflection of a pointer on a calibrated scale.
In this method the value of the quantity to be measured is combined with a known value of the same quantity. The combination is so adjusted that the sum of these two values is equal to predetermined comparison value .. For example, determination of the volume of a solid by liquid displacement.