Question

In a potentiometer why e.m.f of primary circuit must be more than the e.m.f of secondary circuit to find the unknown e.m.f ( i.e secondary circuit)?

Answer 1

Because the max. potential difference across the resistance wire of potentiometer is the emf of driver cell. Thus emfs to be measured will be balanced in between the extreme points of the resistance wire of potentiometer only if their emf is less than the emf of driver cell. Eg. If we try to measure the length of a line segment, for accuracy, the ruler/scale we use must be longer than the length of line segment. Same way, resistance wire of potentiometer acts a scale to measure potential drop. Thus potential drop across that wire and hence emf of driver cell must be more than the emf to be measured.