Abstract: An oven for thermo-magnetic treatment of toroidal coils of amorphous ferro-magnetic ribbon material whereby to improve the eddy current and hysteresis losses thereof for utilization of such coils in a distribution transformer construction. The oven comprises a housing formed as an annular chamber with thermo insulating material disposed at the interior of the chamber. A wire coil is wound about the annular chamber through the inner central passage thereof whereby to generate a magnetic field about the housing to orient the ferro-magnetic domains in the amorphous ribbon material being heat treated in the chamber. At least one toroidal coil is disposed within the chamber between a pair of heating plates to which controlled heat is applied. Preferably, the chamber is evacuated or an inert gas is provided therein.