External core lamps are basically fluorescent lamps with magnetic cores wrapped around a part of the discharge tube. The core is usually made of ferrite, a ceramic material containing iron oxide and other metals. In external core lamps, high-frequency energy from a special power supply called an electronic ballast passes through wires that are wrapped in a coil around a toroidal ferrite core placed around the outside of a portion of the glass tube. This creates a high-frequency magnetic field within the ferrite core. Since the magnetic permeability of the ferrite is hundreds or thousands of times higher than that of the surrounding air or glass, and the ferrite core provides a closed path for the magnetic field, the ferrite core contains virtually all of the magnetic field.