Question

In moving coil galvanometer, a strong horseshoe magnet of concave shaped pole pieces is used?
(A) Increases space for rotation of coil
(B) Reduce weight of galvanometer
(C) Protect magnetic field which is parallel to plane of coil at any position
(D) Make magnetic induction weak at the centre

Answer 1

Hint : To answer this question, we need to use the formula for the torque experienced by a current carrying coil placed in a magnetic field. From there we have to obtain the condition for the radial magnetic field and then we can get the final answer.

Formula used: The formula which has been used to solve this question is given by
$\Rightarrow \tau = NIAB\sin {{\theta }}$, here $\tau$ is the torque experienced by a coil of area $A$, having $N$ turns, and carrying a current of $I$, which is placed in a magnetic field of $B$, and ${{\theta }}$ is the angle between the magnetic field and the area vector of the coil.

Complete step by step answer
We know that a moving coil galvanometer consists of a coil placed inside a magnetic field. When a current passes through the coil, it experiences a torque due to which it rotates and hence shows the deflection. The magnitude of this torque is given by
$\Rightarrow \tau = NIAB\sin {{\theta }}$.............................(1)
Now, the coil is attached to a spring, in order to stop it to a particular deflection. If the torsional constant of the spring is $k$ and the deflection at the equilibrium is ${{\varphi }}$, then we have
$\Rightarrow \tau = k{{\varphi }}$.............................(2)
Equating (1) and (2) we have
$\Rightarrow k{{\varphi = }}NIAB\sin {{\theta }}$
$\Rightarrow {{\varphi = }}\dfrac{{NAB\sin {{\theta }}}}{k}I$
Now the galvanometer is used in measuring various circuit parameters such as current, potential difference etc. For that, the deflection has to be made proportional to the current in the coil, which in turn is proportional to the circuit parameter being measured. As we can clearly see that there is only one variable in the above relation, that is the angle between the area vector of the coil and the magnetic field, ${{\theta }}$. For making it constant, we make ${{\theta }} = {90^ \circ }$ by making the magnetic field parallel to the coil, i.e. making it radial. For this purpose, we take the horse shoe magnet of concave shape.
Hence, the correct answer is option C.

Note
The moving coil galvanometer, which has been discussed in this question, is used for many purposes in an electric circuit. For example, by connecting a low resistance shunt wire parallel to it, the galvanometer works as an ammeter which can measure the current flowing in a circuit. Also, it can work as a voltmeter to measure the voltage between two points in a circuit by connecting a high resistance in series.