NULL METHOD OF MEASUREMENT

A null method of measurement is a simple, accurate and widely used method which depends on an instrument reading being adjusted to read zero current only. The method assumes:

(i) If there is any deflection at all, then some current is flowing;
(ii) If there is no deflection, then no current flows (i.e. a null condition).
Hence it is unnecessary for a meter sensing current flow to be calibrated when used in this way. A sensitive milli-ammeter or micro-ammeter with center zero position setting is called a galvanometer. Examples where the method is used are in the Wheatstone bridge, in the DC potentiometer and with AC bridges.