The work done in rotating a magnet of magnetic moment M by an angle of 90º from the external magnetic field direction is ’n’ times the corresponding work done to turn it through an angle of 60º. Where ’n’ gives by :- 

The circular loop of a wire and a long straight wire carries currents $l_c$ and $l_e$, respectively, as shown in the figure. Assuming that these are placed in the same plane. The magnetic fields will be zero at the centre of the loop when the separation $H$ is:-