Explain the construction of moving coil galvanometer?

What is the basic principle of moving coil galvanometer?. Derive an expression for the current flowing through the galvanometer in terms of the steady angular deflection of its coil. Define voltage sensitivity of the galvanometer. What is a convenient way to increase its sensitivity?

The galvanometer deflection, when key $K_1$ is closed but $K_2$ is open, equals ${\theta }_0$ (see figure). On closing $K_2$ also and adjusting $R_2$ to $5\Omega ,$ the deflection in galvanometer becomes $\frac{{\theta }_0}{5}.$ The resistance of the galvanometer is, then, given by [Neglect the internal resistance of battery]:

The current sensitivity of a galvanometer can be increased by :
(A) decreasing the number of turns
(B) increasing the magnetic field
(C) decreasing the area of the coil
(D) decreasing the torsional constant of the spring

Choose the most appropriate answer from the options given below :

Given below are two statements:

Statement I: If the number of turns in the coil of a moving coil galvanometer is doubled then the current sensitivity becomes double.

Statement II: Increasing current sensitivity of a moving coil galvanometer by only increasing the number of turns in the coil will also increase its voltage sensitivity in the same ratio

In the light of the above statements, choose the correct answer from the options given below:

Derive the expression for the torque acting on the rectangular current carrying coil of a galvanometer. Why is the magnetic field made radial?