**Principle:** A current carrying coil suspended in a magnetic field experiences a torque.

Let I be the current through the loop PQRS.

a, b = Sides of the rectangular loop

A = ab = Area of the loop

N = Number of turns in the loop

According to Fleming’s left hand rule, the side AB experiences a normal inward force, F1= IaB and side CD experiences a normal outward force, F2 = IaB. These two equal and opposite forces form a couple, which exerts a torque given by,

= Force × Perpendicular distance

= IaB × b sin

= IBab sin

τ = IBA sin

For N turns,

τ = NIBA sin

The current sensitivity of a moving coil galvanometer depends on

(i) number of turns in the galvanometer coil

(ii) torsional constant of the suspension fibre.