What are the disadvantages of moving coil galvanometer?

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]:

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

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 :

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?