Abstract: An electromagnetic device such as a power transformer comprises at least two tubular magnetic core sections spaced apart in substantially parallel alignment. The windings of the device are substantially disposed within each tubular magnetic core. The outer surfaces of the magnetic core sections are essentially unobstructed and available for contact with heat extraction elements. The core sections may be elongated relative to their cross-sectional dimensions in order to increase the cooling surface area. The device configuration accommodates a means of providing for selected values of leakage inductance using opposing projections extending inward from the tubular side walls of the cores. The configuration also accommodates a means of actively adjusting the leakage inductance of the device.