A circular coil of N turns is placed vertically with its plane in east-west direction. A magnetic compass needle is placed at the centre of the coil. Will the magnetic needle show any deflection when current is passed through the coil? How will the deflection change as we turn the coil at 90° about a vertical axis keeping the magnetic needle fixed? What would happen to the deflection if the coil is further rotated through 90°?

When a circular coil of N tums is placed vertically in east-west direction and a current is passed through it, the magnetic field at the centre of coil points horizontally along north-south direction. The magnetic compass needle placed there was already pointing in north-south direction’ Hence there is no deflection shown by the magnetic needle’ When the coil is turned through 90° about a vertical axis, the coil is now in north-south direction and magnetic field due to current is along east-west direction horizontally. As a result the magnetic needle shows deflection. When the coil is further turned through 90° it again lies in east-west direction vertically and magnetic field due to current flow in the coil is along north-south horizontally. Hence, the magnetic needle remains in its equilibrium position and shows no deflection.