Question

plz explain the concept moving coil of galvanometer....


according to 12th level

Answer 1

Heya........✋
Answer is in attachment.......
hope it helps:)

Answer 2

Hi,

Here is your answer,

A moving coil galvanometer was first devised by Kelvin and later by D'Arsonval. It is used to detect ans measure small electric currents of the order 10⁻⁹ A.

PRINCIPLE :

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

CONSTRUCTION :

It consist of a rectangular coil of an insulated copper wire wound on 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 the wire is connected to a phosphor bronze spring. A small concave mirror M is attached to the phosphor bronze wire to measure the deflection of the coil.
    
         The Young's modules of phosphor bronze is high but rigidity modules is low. So, 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, he coil experiences a deflecting torque τd. Then the coil begins to run. As the coil turns the phosphor bronze wire gets twisted. As a result, an oppositely directed restoring couple develops in the phosphor-bronze wire.

            Let n be the number of turns in 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.

     As the field is radical, the plane of the coil is always parallel to the magnetic field. Then the coil experiences a constant and maximum deflecting torque given b:-
 
             τd = niAB 

Now, if C is restoring couple 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 = πr

               niAB = Cθ

(OR)           i = C/nAB θ

Or i = Kθ where K = C/nAB is a constant for the galvanometer 

So, i α 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. 

MEASUREMENTS OF DEFLECTION :

The deflection of the coil is usually measured using lamp and scale arrangements . An electric lamp is fixed to a vertical stand. A horizontal scale is arranged to the same stand such that the reflected image of light from the mirror M falls on the scale.

      When the coil deflects through an angle θ the reflected ray will be deflected 2θ. Corresponding to this, let us consider x be the displacement of the image on the scale. If d is the distance between the scale and the mirror. 

               x = d(2θ)   ( ∴ arc length = radius × angle )

            Or θ = x/2d in radius

Thus knowing θ and galvanometer constant K, the current passing through the coil can be measured.

MERITS OF MOVING COIL GALVANOMETER

1) The deflection of the coil is not affected by the presence of magnetic materials in its surrounding.

2) Without sacrificing the accuracy, sensitivity of the instrument can be improved.

DEMERITS

1) It is not a direct reading instrument.

2) The coil may get damaged if large currents are passed through it. (Using shunt the galvanometer can be protected )


Hope it helps you !