A tangent galvanometer shows no deflection when a current is passed through it, but when the current through it is reversed it gives a deflection of 1800, then the plane of the coil

Initially, when coil is perpendicular to magnetic meridian, the field produced by coil is in opposite direction to the Earth's magnetic field (BH).Finally , when direction of current is reversed, the field produced by coil is also reversed and adds up with Earth's magnetic field (BH) , to give deflection of 180o.