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, i.e., at right angles to the horizontal component of earth's magnetic field H. Under the influence of two crossed magnetic fields B and H, the magnetic needle of galvanometer undergoes a deflection θ which is given by the tangent law. Using Tangent law, we can find a relation

I α tanθ

which clearly indicates that tangent galvanometer is an instrument used for detection of electric current in a circuit.