Explanation

(b) When a current is passed through the galvanometer coil, then a magnetic field B is produced at right angles to the plane of the coil, ie, at right angles to the horizontal component of earth’s magnetic field $B_{H}.$ Under the influence of two crossed magnetic fields B and $B_{H},$ the magnetic needle of galvanometer undergoes a deflection $\theta$ which is given by the tangent law. Using tangent law, we can find a relation
$I \propto \tan\theta$
Which clearly indicates that tangent galvanometer is an instrument used for detection of electric current in a circuit.