State the principle of a moving coil galvanometer.

(N/A) The principle of a moving coil galvanometer is based on the magnetic effect of electric current. When a current-carrying coil is placed in a uniform magnetic field,it experiences a magnetic torque. This torque causes the coil to rotate. The magnitude of this torque is given by $\tau = NIAB \sin \theta$,where $N$ is the number of turns,$I$ is the current,$A$ is the area of the coil,$B$ is the magnetic field strength,and $\theta$ is the angle between the normal to the coil and the magnetic field. In a radial magnetic field,$\theta = 90^\circ$,so the torque becomes $\tau = NIAB$. This torque is balanced by the restoring torque of the spring,$\tau_r = k\phi$,where $k$ is the torsional constant and $\phi$ is the angle of twist. Thus,$NIAB = k\phi$,which implies that the deflection $\phi$ is directly proportional to the current $I$ $(I \propto \phi)$.