Abstract: The present invention refers to a magnetic core comprising flat magnetic plates superimposed and joined in order to constitute the toroidal form of the core, creating an internal space for the winding and a lateral air gap groove. There being assembled in the open space of the core or air gap, annulling it, an open hoop made of magnetic material having suitable dimensions, whose outer edge penetrates into the interior space of the winding in order thus to create a new internal air gap because the external diameter of the hoop is smaller than the internal diameter of the space available for the inductance coils. In this way a magnetic shunt is situated between the inductance coils in order to regulate their mutual inductance.