An initially unmagnetized cylindrical ring of ferromagnetic material has internal and external radii of $a$ and $b$, respectively, while its extension along the axis is much larger. The body is winded uniformly with a current-carrying wire according to the figure on the left, making it magnetized. We may assume that the local residual magnetization is proportional to the local magnetic field due to the current.
After this, a section of the ring, under an angle $\alpha$ as measured from the symmetry axis, is removed without modifying the local magnetization. The resulting system, shown in the figure on the right, may be considered as an approximate model of a horseshoe magnet if $\alpha\approx\pi$.