An apparatus for shimming the magnetic field generated by a magnet arrangement of a magnetic resonance imaging (MRI) system, has a number of shim devices of substantially similar cross-sectional dimensions, at least some of the shim devices exhibiting differing ferromagnetic characteristics. A structure body has an elongate, tubular channel cross-sectionally dimensioned to serially receive the shim devices in a predetermined sequence to provide a required distribution of the ferromagnetic characteristics in relation to said magnetic field, the channel being of length sufficient to accommodate the shim devices serially in said predetermined sequence. The shim devices are presented at the entrance of the channel in the sequence, and are forced into the channel in the sequence by pressurized fluid directed toward the entrance.