A galvanometer in which a coil is moved between the poles of a permanent magnet and its defection measures the electric current is called a “Moving Coil Galvanometer”. It is used to measure the small electric current.
The various parts of a galvanometer are:
• A U shaped permanent magnet with cylindrical concave polepieces.
• A flat coil of thin enamel insulated wire.
• A soft iron cylinder.
• A pointer.
• A scale.
When a current flow in a rectangular coil placed in a magnetic field, it experiences magnetic torque. It rotates through an angle proportional to the current flowing through it Its rotation and deflection shows a current through it.
The coil rotates until the elastic restoring torque due to the strip does not equal and cancel the deflecting magnetic torque and then it attains equilibrium and stops rotating.
Deflection Is Directry Proportional To The Current:-
When a current ‘I’ is passed through the galvanometer, the coil is deflected through an angle ‘u’ with respect to the field and the deflecting magnetic torque acting on the toil is given by the relation.
t = BINA cos u .
B = Strength of magnetic field.
I = Current in the coil.
N = Number of turns in the coil.
A = Area of the coil.
u= Angle of deflection of the coil.
The twist produces a restoring torque which is equal and opposite to the deflecting torque. The restoring torque (couple) due to twisting of suspension wire is proportional to the ~gle of deflection The restoring elastic torque obeys Hooke’s law and it is given by