Explain the construction and principle of a moving coil galvanometer.

(N/A) Principle: $A$ current-carrying coil placed in a magnetic field experiences a magnetic torque that tends to rotate the coil,producing an angular deflection proportional to the current.
Construction: The moving coil galvanometer consists of a coil with many turns,free to rotate about a fixed axis in a uniform radial magnetic field.
$A$ cylindrical soft iron core is placed inside the coil,which not only makes the magnetic field radial but also increases the strength of the magnetic field.
The magnetic field is uniform between the two poles,ensuring that the angle between the area vector of the coil and the magnetic field $\overrightarrow{B}$ is always $90^{\circ}$ (i.e.,$\theta = 90^{\circ}$).
Therefore,the magnetic torque on the loop is $\tau = NIAB \sin(90^{\circ}) = NIAB$,which is the maximum torque.
$A$ spring $S_{p}$ provides a restoring counter-torque $K\phi$ that balances the magnetic torque $NIAB$,resulting in a steady angular deflection $\phi$.
The deflection is indicated on a scale by a pointer attached to the spring,which measures the value of the electric current.