MP Board · Class 12 · Physics · Moving Charges and MagnetismExplain the working principle of a moving coil galvanometer. How is a galvanometer converted into an ammeter?

A moving coil galvanometer works on the principle that a current-carrying coil placed in a uniform magnetic field experiences a magnetic torque. When current $I$ flows through a coil of $N$ turns and area $A$ placed in a magnetic field $B$, the deflecting torque is given by $\tau = NIAB \sin\theta$. For a radial magnetic field, $\sin\theta = 1$, so $\tau = NIAB$. This torque is balanced by the restoring torque of the suspension spring, $\tau_r = k\phi$, where $k$ is the torsional constant and $\phi$ is the angular deflection. Thus, $NIAB = k\phi \implies \phi = \left(\frac{NAB}{k}\right)I$, meaning deflection is directly proportional to current. Conversion into an Ammeter: A galvanometer is converted into an ammeter by connecting a very small resistance, known as a shunt ($S$), in parallel with the galvanometer. This protects the sensitive coil from high currents and allows large currents to pass through the shunt, extending the range.