an apparatus for shimming the magnetic field generated by a magnet arrangement of a magnetic resonance imaging 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 .