Abstract: To permit adjustment of the air gap of an inductance unit suitable as a ballast for fluorescent lamps particularly in form of a leakage transformer or as a choke, for multi-voltage connection, an outer, essentially U-shaped core structure (1, 100), and having an integral connecting cross element (1, 101a), and two parallel leg elements (1b, 101b), has at least one longitudinally slidable cross element (2, 2', 2"; 3, 3'), the cross element being formed with a central longitudinal core element (14, 15; 14', 14") extending parallel to the leg elements, and short of the adjacent cross element, and longitudinally slidable within the leg elements so that the air gap between the longitudinal element and the adjacent cross element, and forming a magnetic shunt, can be adjusted after manufacture of the attached laminate to compensate for manufacturing tolerances and provide an inductance element having design impedance regardless of variation in the size of the longitudinal core element.