(a) Draw a labelled diagram of a moving coil galvanometer. Describe briefly its principle and working. (b) Answer the following: (i) Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer? (ii) Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain, giving reason.
Solution
\textbf{Principle/working:} a current coil in the radial field feels torque τ=NIAB, balanced by kϕ, so ϕ=kNABI∝I. (i) The cylindrical \textbf{soft-iron core} concentrates the flux, making the field strong and radial so the plane of the coil always lies along B — this gives a linear scale and higher sensitivity. (ii) Current sensitivity =kNBA, voltage sensitivity =kRNBA. Increasing N raises the current sensitivity but also raises the coil resistance R proportionally, so the voltage sensitivity kRNBA need not increase.
