With the help of a neat and labelled diagram, explain the underlying principle and working of a moving coil galvanometer. What is the function of (i) the uniform radial field and (ii) the soft iron core in such a device?
Solution
\textbf{Principle:} a current-carrying coil in a magnetic field experiences a torque τ=NIAB, balanced by the suspension's restoring torque kϕ, giving ϕ=kNABI∝I. (i) The \textbf{uniform radial field} ensures the plane of the coil is always parallel to B, so sinθ=1 throughout and the torque stays NIAB independent of position — this makes the deflection directly proportional to current (a linear scale). (ii) The \textbf{soft-iron core} concentrates the field, making it both stronger (greater sensitivity) and radial.
