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.