Abstract: A wound core transformer is described which features low core losses by the addition of magnetic material in those sides of the wound core which are located outside of the coil. In one embodiment the wound core includes: a base-core is formed from two strips of magnetic material of generally uniform thickness and wrapped about each other in the form of a closed rectangular loop with the ends of each strip offset from each other and the ends of the adjacent strip; and a filler strip of magnetic material which is interleafed between the two strips forming the base-core with its base disposed adjacent the ends of the strips forming the base-core. An electrical conductor is disposed opposite the filler strip and wrapped about the two strips forming the base core so as to form a coil. Under this arrangement, the magnetic induction formed in that side of core containing the base portion of the filler strip is lower than that side of the core around which the coil is wrapped.