(a) With the help of a diagram , explain the principle and working of a moving coil galvanometer .
(b) What is the importance of a radial magnetic field and how is it produced ?
(c) Why is it while using a moving coil galvometer as a voltmeter a high resistance in series is required whereas in an ammeter a shunt is used ?

Explanation

(a) Moving coil galvanometer : It is a device used for the detection and measurement of small electric current .
Principle : The working is based on the fact that a current carrying coil suspended in a magnetic field experiences a torque .
Construction : It consists of a coil having a large number of turns of insulated copper wire wound on a metallic frame . The coil is suspended by means of a phosphorbronze strip and is surrounded by a horse shoe magnet NS . A hair spring is attached to lower end of the coil . The other end of the spring is attached to the scale through a pointer .
Working : When current is passed , say along ABCD , the couple acts on it . Since the plane remains always parallel to the magnetic field in all position of the coil (radial field) , the force on the vertical arms always remains perpendicular to the place of the coil .

Let I = the current flowing through coil .
B = magnetic field supposed to be uniform and always parallel to the coil .
A = area of the coil
Deflecting torque acting on the coil is
`tau = NI BA sin 90^(@) = NIBA " " [ because sin 90^(@) = 1]`
Due to deflecting torque , the coil rotates and suspension wire gets twisted . A restoring torque is set up in the suspension fibre . If `phi` is angle through which the coil rotates and K is the restoring torque per unit angular twist , then restoring torque , `tau = k phi` .
In equilibrium
Deflecting torque = Restoring torque
`NIBA = k phi`
or `" " phi = ((NAB)/(k))` is the galvanometer constant
`therefore " " phi prop 1`
This provides a linear scale for the galvanometer .
(b) Radial magnetic field : It is the field in which the plane of the coil always lies in the direction of the magnetic field . A radial magnetic field is produced by(i) properly cutting the magnetic pole pieces in the shape of concave faces and (ii) using a soft iron core within the coil .
(c) A galvanometer can be converted into a voltmeter by connecting a high resistance in series with galvanometer to draw a very small current . A galvanometer can be converted into an ammeter by connecting a low resistance shunt is parallel with galvanometer to draw large value of current .