Question

To study the earth magnetic field using a tangent galvanometer

Answer 1

Galvanometer(tangent) was made in the year 1837 by Claude Pouillet who afterwards employed this sensitive galvanometer to check Ohm's law.  

Galvanometer is first set up on level surface and coil aligned with magnetic north-south direction.  

Current to be measured is now sent through coil and generates magnetic field which is perpendicular to plane of coil and directly proportional to current.  

Magnitude of magnetic field produced by coil is B and magnitude of horizontal component the earth's magnetic field is B'.  

Compass needle sets itself along vector sum of B and B' after rotating through an angle Ø from its original setting.  

So its used for measuring earth's magnetic field.