CBSE Class 12 · 2010 · Magnetic Effects of CurrentShort Answer

\textbf{Principle:} a coil carrying current in a radial magnetic field experiences a deflecting torque $\tau =NIAB$, balanced by the restoring torque $k\varphi$, giving $\varphi \propto I$. \textbf{Why not used directly:} (i) it gives full-scale deflection for a very small current (order $\mu \text{A}$), so a large current would damage it; (ii) it has appreciable resistance, so connecting it in series would change the current it is meant to measure. \textbf{Current sensitivity} $\frac{\varphi }{I}=\frac{NAB}{k}$ depends on the number of turns $N$ and the area $A$ of the coil (and on $B$ and the torsion constant $k$).