An electrophotographic printing apparatus includes a laser or comparable image-forming apparatus which produces an image on a rotating drum having a metal core. Electrostatic images formed by the laser or the like are toned by a toning mechanism that includes a rapidly rotating magnetic core. To prevent the rapidly rotating core from producing pole transitions causing a flutter in the movement of the drum, the core has an arrangement of magnetic poles which alternate circumferentially and vary along the length of the core so that the drum is subjected to pole transistions which vary in phase along the length of the drum.