Moving Charges and Magnetism - Moving Coil Galvanometer

A Moving Coil Galvanometer (MCG) is an instrument used for detecting and measuring small electric currents.

The principle of an MCG is that a current-carrying coil placed in a uniform magnetic field experiences a torque, which causes it to rotate.

In a radial magnetic field, the plane of the coil is always parallel to the magnetic field, ensuring that the angle between the area vector and magnetic field $\theta = 90^\circ$ at all times. This makes the deflecting torque $\tau = NIAB$ (independent of the angle of rotation).

Equilibrium condition: The deflecting torque $\tau_d = NIAB$ is balanced by the restoring torque $\tau_r = k\phi$ provided by the spring, where $k$ is the torsional constant and $\phi$ is the angular deflection.

Current Sensitivity ($S_i$): It is defined as the deflection per unit current, given by $\frac{\phi}{I}$. Increasing $N$, $A$, or $B$, or decreasing $k$, increases the sensitivity.

Voltage Sensitivity ($S_v$): It is defined as the deflection per unit voltage, given by $\frac{\phi}{V}$. Note that increasing the number of turns $N$ does not necessarily increase $S_v$ if the resistance $R$ also increases proportionally.

Conversion to Ammeter: A galvanometer is converted into an ammeter by connecting a low resistance called a 'shunt' ($S$) in parallel with the galvanometer coil.

Conversion to Voltmeter: A galvanometer is converted into a voltmeter by connecting a high resistance ($R$) in series with the galvanometer coil.