horizontal component of earth's magnetic field at a place is uniform and its direction is south to north.A high current through horizontal power line flows at this place from west to east. consider 2 points a and b at equal distances from the wire respectively above and below it giving the reason explain where is the field more a or b. why?
So as you can see in the diagram at point "a" the magnetic field due to coil is opposite to the horizontal present magnetic field.
so the net magnetic field is less there.
but at point b the magnetic field adds up.