CBSE Class 12 · 2017 · Delhi · Current ElectricityShort Answer

\textbf{Principle:} a coil of $N$ turns, area $A$, carrying current $I$ in a field $B$ experiences a torque $\tau =NIAB$, balanced by the suspension torque $k\theta$, so $\theta =\frac{NAB}{k}I\propto I$. (i) A radial field makes the plane of the coil always parallel to $B$, keeping the torque $=NIAB$ for all positions, so the scale is linear. (ii) The soft-iron core concentrates the flux and makes the field radial and stronger, increasing sensitivity. \textbf{Current sensitivity} $=\frac{\theta }{I}=\frac{NAB}{k}$. A galvanometer alone cannot measure current: it is uncalibrated and its fine coil would be damaged by ordinary currents, so a low shunt is added to convert it into an ammeter.