Moving Coil Galvanometer - Practice Questions & MCQ

Moving coil galvanometer-

Moving coil galvanometer is an electromagnetic device which is used measure small values of current. It consists of permanent horse-shoe magnet, coil, soft iron core, pivoted spring, non-metallic frame, scale, and pointer as shown in the figure

As we have studied that the torque acts on a current carrying coil suspended in the uniform magnetic field. Due to this, the coil rotates. Hence, the deflection in the coil of a moving coil galvanometer is directly proportional to the current flowing in the coil.

In this, the coil is suspended between the pole pieces of a strong horse-shoe magnet. The magnetic field is made radial and for this the pole pieces are made cylindrical and a soft iron cylindrical core is placed within the coil without touching it. The benifit of this type of field is that the plane of the coil always remains parallel to the field. Therefore θ=90^o and the deflecting torque always has the maximum value. 

$
\tau_{\text {deflection }}=N B i A
$

Now if the coil deflects, a restoring torque is set up in the pivoted spring. If $\alpha$ is the angle of twist, the restoring torque is

$
\tau_{\text {restoring }}=C \alpha
$

where C is the torsional constant of the fibre.
When the coil is in equilibrium, then -

$
\begin{aligned}
& N B i A=C \alpha \\
& i=\frac{C}{N B A} \cdot \alpha \Rightarrow i=K \alpha
\end{aligned}
$

where $K=\frac{C}{N B A}$ is the galvanometer constant.
This linear relationship between i and $\alpha$ makes the moving coil galvanometer useful for current measurement and detection.

Here we will discuss two important terminologies -
1. Current sensitivity $\left(\mathbf{S}_{\mathrm{i}}\right)$ : The current sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit current flowing through it. So it can be written as -

$
S_i=\frac{\alpha}{i}=\frac{N B A}{C}
$

2. Voltage sensitivity $\left(\mathrm{S}_{\mathrm{v}}\right)$ : Voltage sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit voltage applied to it. So it can be written as -

$
S_V=\frac{\alpha}{V}=\frac{\alpha}{i R}=\frac{S_i}{R}=\frac{N B A}{R C}
$

(By using Ohm's law)