The plane of the coil of tangent galvanometer is kept parallel to magnetic meridian to

The plane of the coil of tangent galvanometer is kept parallel to magnetic meridian so that the angle between the magnetic field due to coil and the earth's horizontal magnetic field will be 90⁰ and tangent law can be applied. 

$\mathrm{tan\theta }=\mathrm{B}/{\mathrm{B}}_{\mathrm{H}}$ 

B - field due to coil , B_H - Horizontal component of earth's magnetic field