THE MOVING COIL GALVANOMETER

 Galvanometer is an electromechanical instrument which is used for the detection of electric currents     through electric circuits. Being a sensitive instrument, Galvanometer can not be used for the     measurement of heavy currents. However we can measure very small currents by using galvanometer but     the primary purpose of galvanometer is the detection of electric current not the measurement of current

WORKING PRINCIPLE
A current carrying conductor placed in a magnetic field experiences a torque

Construction

It consists of N turns of coil PQRS wound over a non-magnetic metallic frame usually rectangular or circular. The coil is suspended from a torsion head H by means of phosphor bronze strip in a magnetic pole by pieces N and S. A mirror M is attached to the phosphor bronze strip. The other end of the coil is connected to a hairspring, which in turn is connected to an external terminal.

The concave or cylindrical pole pieces ensure that the plane of coil is parallel to B thus (cosq = 1 and sina = 1) in the expression t = NIBA cosq and t = NIBA sina .

The whole arrangement is enclosed in non-magnetic case, which is provided with leveling screws. The torsion head is connected to terminal T₁. The galvanometer can be connected to the circuit through terminals T₁ and T₂.

WORKING

When a current is passed through the coil in the direction PQRS, then the coil experiences a torque
t = NIBA sina
Since the magnified field is radial the plane of the coil is parallel to the magnetic field such that a = 90^o and hence t = NIBA.
The coil rotates and the phosphor bronze strip gets twisted. As a result a restoring torque comes into play trying to restore the coil back to original position.
If f be the twist produced in the strip and C be the restoring torque per unit twist then the restoring torque = Cf.
In equilibrium,

where G is the galvanometer constant.

This shows that galvanometer has a linear scale and detects the presence of current

Note:

Current sensitivity of galvanometer is the deflection produced for a unit current flowing through it. i.e.,

A sensitive galvanometer should have a long deflection for small current through it.
Voltage sensitivity is the deflection produced for a unit voltage applied across the two terminals of the galvanometer

TORQUE ON A CURRENT CARRYING LOOP IN UNIFORM MAGNETIC FIELD

Consider a rectangular coil with sides of length a and b placed in a magnetic field of flux density B and free to rotate about an axis perpendicular to the paper, as shown in Figure . A current of I Amps flows in the coil.

The field exerts a force on the sides b given by

Force (F) on side length b = BNIb

where N is the number of turns on the coil.
If the perpendicular to the coil is at an angle θ to the field direction, then the torque exerted on the coil is Fd where d = a sin q.
Therefore the torque C is given by:

Torque (C) on the coil = Fa sinθ = BNIba sin θ   or:
Torque (C) on the coil = BANI sinθ

where A = ab, the area of one face of the coil.



The maximum torque occurs when the plane of the coil is lying along the field lines (θ = 90^o and sin q = 1). At this point, shown in Figure 2(a),

Maximum torque (C_o) = BANI

The minimum value of the torque is zero, when θ = 0.


BEST OF LUCK GUYS......

VIKAS BHATI