Abstract: A plurality of relatively high magnetic permeability strips are interleaved to form layers of a closed loop differential transformer core. Although the material of each layer by itself may have a relatively square hysteresis loop, the core formed of a plurality of interleaved layers of such material has a substantially more rounded hysteresis loop. Such a core can advantageously be used in a ground fault interruption circuit in which power leads pass through a central opening in the core to form a primary winding and a secondary winding wound on the core may have a capacitor connected in circuit therewith to complete a resonant circuit.