The present invention relates to an apparatus and a method for generating and changing a magnetic field in a field of view (28), said magnetic field having a, in particular ball-shaped or line-shaped, first sub-zone (62) having a low magnetic field strength and a second sub-zone (64) having a higher magnetic field strength. The proposed apparatus comprises at least three pairs of first coils (136a-136d), wherein the coils are arranged along a ring around the field of view and wherein the two coils of each pair are opposingly arranged on opposite sides of the field of view, at least one pair of second coils (116) opposingly arranged on opposite sides of the field of view at the open sides of said ring, generator means (110, 130) for generating current signals for provision to said first and second coils for generating the desired magnetic fields by said first and second coils, and control means (150) for controlling said generator means.
