Question

(a) Draw a labelled diagram of a moving coil galvanometer. Describe briefly its principle and working.

(b) Answer the following :

(i) Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer.

(ii) Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain, giving reason.

Answer 1

(a) Moving coil galvanometer : It is a device used for detecting and measuring small electric current.


Principle : The working is based upon the principle that a current carrying coil suspended in a magnetic field experiences a torque.

Construction : It consists of a coil with a large number of turns of insulated copper wire wounded on a metallic frame.

The coil is suspended by means of a phosphor bronze strip and a horse shoe magnet's poles surrounds it. The lower end of the coil is attached to a hair spring. The scale of the pointer is attached to the other end of the spring.

Working : When current is passed, a couple acts on it. Since the plane remains parallel to the magnetic field in all position of the coil, the force on the vertical arms always remains perpendicular to the plane of the coil.

Let I = the current flowing through coil,

B = magnetic field parallel to the coil, and

A = area of the coil.

Deflection acting on the coil is

$\tau =NIBAsin{90}^{\circ }=NIBA[\because sin{90}^{\circ }=1]$

Due to the deflection torque, the coil rotates and suspended wire gets twisted. The suspension fiber experiences a restoring torque. If $\Phi$ is angle through the coil rotates and k is the restoring torque per unit angular twist, then

Restoring torque, $\tau =k\Phi$

In equilibrium, Deflection torque = Restoring torque

$NIBA=k\Phi$

$\Phi =\left(\frac{NBA}{k}\right)I$

$\Rightarrow \Phi =\left(\frac{I}{G}\right).I$

$I=G\Phi$

where $G=\frac{k}{NBA}$ known as galvanometer constant.

$\therefore \Phi \alpha I$

This provides a linear scale for the galvanometer.

(b) (i) When a soft iron core is used the magnetic field lines tend to crowd through the core. It is because, soft iron core is ferromagnetic in nature. As a result, the strength of the magnetic field due to the field magnet increases, which in turn increases the sensitivity of the galvanometer.

(ii) Current sensitivity of a galvanometer is defined as the deflection produced in the galvanometer when a unit current flows through it.

$I_s=\frac{\theta }{I}=\frac{nBA}{K}...\left(i\right)$

Voltage sensitivity of a galvanometer is defined as the deflection produced in the galvanometer when a unit voltage is applied across two terminals.

$V_s=\frac{\theta }{V}=\frac{\theta }{IR}=\frac{nBA}{KR}=\frac{I_f}{R}...\left(ii\right)$

Where n = number of turns in the coil of galvanometer

B = Magnetic field around coil

A = Area of coil

K = restoring torque per unit twist

From equation (i) we can say that current sensitivity increases by increasing n, B, A and decreases by K.

From equation (ii) we can say that voltage sensitivity increases by increasing n, B, A and decreases by K,R.

In case of current sensitivity if we increase the no. of turns n its current sensitivity increases. Since resistance of galvanometer R also increase, voltage sensitivity remains same or unchaged.

Therefore increase in current sensitivity of galvanometer may not necessarily increase the voltage.