A magnetic compass needle is placed on the plane of paper near A as shown in figure. In which plane should a straight current-carrying conductor be placed, so that it passes through A and there is no change in the deflection of the compass ? Under what condition is the deflection maximum and why ?

Explanation

The deflection of the compass needle is maximum when magnetic field and current are perpendicular and minimum when they are mutually parallel.
If the straight current-carrying conductor is placed on the plane of paper so that it passes through A and is parallel to the vertical axis of the compass needle, the direction of current and magnetic field is parallel . So, no change occurs in the deflection of the compass needle.
If the conductor passes through A and is held perpendicular to the plane of paper , the deflection of the needle is maximum.