A bobbin is adapted to support a winding on a permeable core and has a wall that provides a confined thermally conductive channel that causes conduction of heat along a predetermined path from the core to a location outside the winding. A value of magnetizing inductance in a transformer is set by adjusting the gap until the value of magnetizing inductance has been set and attaching a segment of the bobbin to a pair of core pieces to maintain the gap. A permeable strip provides a permeable path outside of the hollow interior space and does not couple the winding, and an electrically insulating coupler is interposed between the slug and the winding to electrically insulate the winding.