The method making a dry type, air cooled transformer having a mitered magnetic core and a compression bonded coil with air ducts formed in the end portions of the coil. The coil on each of the core legs is generally rectangular in shape and comprises a plurality of layers of wound conductor with the conductor layers in the end portions of the coil being spaced apart to form a plurality of air ducts for the passage of cooling air therethrough. The conductor layers in each of the side portions of the coil are compressed and then bonded together in their compressed state by means of a heat cured adhesive coated on opposite sides of the sheets of insulation between adjacent layers. This compression bonding of the coil sides squares up the inner and outer surfaces of the coil so as to improve the coil and core space factors thereby allowing a smaller core. Conversely, compression bonding allows higher output power ratings to be achieved by packing added conductor material through the same size core window. The air ducts in the coil are formed by inserting temporary and permanent duct spacers in the coil during winding thereof, wherein the temporary duct spacers are located at the corners of the conductor layers. Following winding, the temporary duct spacers are loosened and removed from the coil thereby leaving only the permanent duct spacers in place.