Question

In the potentiometer circuit shown, the null point is at X. State with reason, where the balance point will be shifted when:

(a) Resistance R is increased, keeping all other parameters unchanged; 

(b) Resistance S is increased, keeping R constant.

Answer 1

Let l be the balance length of the segment AX on the potentiometer wire for given resistance R and S.

(a) If resistance R is increased, the current flow in the main circuit (or wire AB) will

decrease. From relation K = \(\frac{pI}{L}\) the potential gradient along the wire AB will decrease.

To balance the emf of the cell, the point X will shift toward the point B, i.e.,

ε = kl = k′l′

If k′ < k, so l′ > l

(b) For the given resistance R, the potential gradient along the wire remain same. Balance length ‘l’ remain constant. ε = kl

And no current flows in the resistance S. If resistance S is increased/decreased there is no change in the balance length.