The transformer (10) comprises a core (12), a primary winding (14) and a secondary winding 16. The core comprises an elongate limb (13) having a main axis (15) and comprising a plurality of segments (12.1 to 12.n) of a magnetic material and gaps (18.1 to 18.n−1) between segments arranged in alternating relationship along the main axis (15). The main axis (15) is parallel to a direction of a magnetic field in the limb (13). Each gap has a linear segment separating extent (gj which is parallel to the main axis (15). The value of n is larger than three and the gaps are filled with an isolation medium (20).