Question

State the principle of working of a moving coil galvanometer giving its labelled diagram.
Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Justify this statement.

Answer 1

Principle of moving galvanometer.

A current carrying coill when kept inside a uniform magnetic field, can experience a torque.

When current (I) is passed in the coil, torque acts on the coil given by,

$\tau =NIABsin\theta$

where q is the angle between the normal to plane of coil and the magnetic field of strength B.

N is the number of turns in a coil.

When the magnetic field is radial, as in the case of cylindrical pole pieces and soft iron core, then in every position of coll the plane of the coil, is parallel to the magnetic field lines, so that

$\theta ={90}^{\circ }$ and $sin{90}^{\circ }$ = 1

Deflecting Torque is given by,

if C is the torsional rigidity of the wire and q is the twist of suspension strip, then restoring torque = C

For equilibrium deflecting torque = restoring torque,

$i.e.,NIAB=C\theta$

$\therefore \theta =\frac{NAB}{C}l$

$i.e.,\theta \propto l$

Current sensitivity, $S-1=\left(\frac{\theta }{I}\right)=\frac{NAB}{C}$ ...(1)

Voltage sensisvicy, $S_v=(\frac{\theta }{V}=\frac{NAB}{GC}$ ....(2)

Dividing (2) by (1)

$\frac{S_v}{S_I}=\frac{1}{G}{S}_I$

Clearly, the voltage sensitivity is dependent on current sensitivity and the resistance of the galvanometer. If we increase current sensitivity and resistance G is larger, then it is not certain that voltage sensitivity will be increased. Thus, the increase of current sensitivity does not imply the increase of voltage sensitivity.