If the key in the arrangement (Figure 13.1) is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are

When the above circuit is open and the magnetic field lines are drawn over the horizontal plane ABCD, then the magnetic field lines will be in the form of concentric circles with centre at the axis of the conductor because since the circuit is open, there will be no flow of current and therefore there will be no magnetic field due to the conductor. So, at point O. only Earth’s magnetic field will be present so the magnetic field lines will be in the form of concentric circles