Question

Explain, using a labelled diagram, the principle and working of a moving coil galvanometer. What is the function of (i) uniform radial magnetic field, (ii) soft iron core ? Define the terms (i) current sensitivity and (ii) voltage sensitivity of a galvanometer. Why does increasing the current sensitivity not necessarily increase voltage sensitivity ?

Answer 1

The basic principle of a moving coil galvanometer is that when a current carrying coil is placed in a magnetic field, it experiences a torque. When a current I is passed through the coil, the torque experienced is given by $\tau =NIABsin\theta$
Where N = No. of turns of the coil,
A = Area of the coil
B = Magnetic field and
$\theta$ = Angle between normal of coil and magnetic field



(i) The uniform radial magnetic field is used to make the scale linear.
(ii) The soft iron core increases the strength of the magnetic field.
The current sensitivity is defined as the deflection produced in the galvanometer, while passing a current of 1 ampere.

Thus, current sensitivity $\left(\frac{\alpha }{I}\right)=\frac{NBA}{K}$.

The voltage sensitivity is defined as the deflection produced in the galvanometer when a potential difference of 1V is applied to the coil.

Thus, voltage sensitivity $\left(\frac{\alpha }{V}\right)=\frac{NBA}{KR}$.
Where R is the resistance.
Increasing the current sensitivity does not necessarily increase the voltage sensitivity as there is an increase in the resistance as well.