Draw a labelled diagram of a moving coil galvanometer, Explain the principle on which it works. Deduce an expression for the torque acting on a rectangular current carrying loop kept in uniform magnetic field. Write two factors on which the current sensitivity of a moving coil galvanometer depend.

Moving coil galavanometer is an instrument used for the detection and measurement of small currents. 

 

Principle : The working of moving coil galvanometer is that when a current carrying coil is placed in a varying magnetic field, it experiences torque. 

Torque acting on a rectangular current carrying coil: 

Let, the coil PQRS be freely suspended in a magnetic field.
Let, 
l = length of PQ or RS of the coil,
b = breadth of QR or SP of the coil,
n= number of turns in the coil, 
B= strength of magnetic field in which the coil is suspended, 
I = current passing through the coil in the direction PQRS.
Then,
Area of the coil, A = l$\times$ b 

At any instant, $\mathrm{\theta }$ is the angle which the normal drawn on the plane of the coil makes with the direction of magnetic field. 

The rectangular current carrying coil, when placed in a magnetic field experiences a torque whose magnitude is given by, 
                        $\tau = nIBA sin\theta$

Current sensitivity of a moving coil galvanometer depends on : 
i) Number of turns of the coil
ii) Magnetic field applied.