Tangent galvanometer measures

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 $H$. Under the influence of two crossed magnetic fields $B$ and $H$, the magnetic needle of galvanometerundergoes 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. Note A tangent galvanometer is most accurate when its deflection is $45^{\circ}$.