Explain the construction and working of …

PRINCIPLE
When a current carrying coil is placed in a uniform magnetic field, it experiences a torque.

CONSTRUCTION
It consists of a rectangular coil of an insulated copper wire wound a non conducting frame. The coil is suspended between the concave shaped poles of a permanent horse-shoe magnet by a phosphor-bronze wire. The upper end of this wire is connected to torsional head. The lower end of the coil is connected to phosphor-bronze spring. A small concave mirror M is attached to measure the deflection of the coil.
The Young's modules of phosphor bronze is high but rigidity modules is low. S, it can be twisted very easily but cannot be elongated.
A soft iron cylinder is arranged inside the rectangular frame of the coil. This cylinder increases the field intensity in between the poles. The combination of curved poles and the soft iron produces a radical magnetic field. In radical field, the plane of the coil will be always parallel to the field and experiences a maximum and constant torque. The whole arrangement is kept inside a brass case provided with a glass window. The deflection of the coil can be measured using lamp and scale arrangement.

WORKING
When the current to be measured is passed through the coil, experiences a deflecting torque. Then the coil begins to turn. As the coil turns the phosphor-bronze wire gets twisted. As a result, an oppositely directed restoring couple develops in the phosphor-wire.
Let us consider n be the number of turns of the coil, A be the area of the coil B be the intensity of magnetic induction field, and i be the current through the coil.
Now, As the field is radical, the plane of the coil is always parallel to the magnetic filed. Then the coil experiences a constant and maximum deflecting torque.
τd = niAB
If C is the restoring copper per unit twist and θ is the deflection of the coil, then restoring torque.
τr = Cθ
In equilibrium position, the deflecting torque is equal to the restoring torque.
τd = τr
niAB = Cθ
(OR)  i = C/nABθ
(OR) i = Kθ where K = C/nAB is a constant for the galvanometer.
So, i ∝ θ
Thus, the current flowing through the moving coil galvanometer is directly proportional to the deflection of the coil. It means the moving coil galvanometer has a linear scale. It is an important advantage because the instrument can be accurately calibrated.