Helmholtz coil ($B_0=(4/5{)}^{3/2}{\mu }_0{N}_H{I}_H/R_H$)

A Helmholtz coil arrangement uses two identical coaxial circular windings separated by their radius, carrying equal electric currents in the same circulation sense. Their combined magnetic field is especially uniform near the midpoint, making it useful for a current-loop torque experiment. The cancellation of the axial second derivative there follows from the on-axis magnetic field of a circular current loop.