<P> The use of a magnetic core can increase the strength of magnetic field in an electromagnetic coil by a factor of several hundred times what it would be without the core . However, magnetic cores have side effects which must be taken into account . In alternating current (AC) devices they cause energy losses, called core losses, due to hysteresis and eddy currents in applications such as transformers and inductors . "Soft" magnetic materials with low coercivity and hysteresis, such as silicon steel, or ferrite, are usually used in cores . </P> <P> An electric current through a wire wound into a coil creates a magnetic field through the center of the coil, due to Ampere's circuital law . Coils are widely used in electronic components such as electromagnets, inductors, transformers, electric motors and generators . A coil without a magnetic core is called an "air core" coil . Adding a piece of ferromagnetic or ferrimagnetic material in the center of the coil can increase the magnetic field by hundreds or thousands of times; this is called a magnetic core . The field of the wire penetrates the core material, magnetizing it, so that the strong magnetic field of the core adds to the field created by the wire . The amount that the magnetic field is increased by the core depends on the magnetic permeability of the core material . Because side effects such as eddy currents and hysteresis can cause frequency - dependent energy losses, different core materials are used for coils used at different frequencies . </P> <P> In some cases the losses are undesirable and with very strong fields saturation can be a problem, and an' air core' is used . A former may still be used; a piece of material, such as plastic or a composite, that may not have any significant magnetic permeability but which simply holds the coils of wires in place . </P> <P> "Soft" (annealed) iron is used in magnetic assemblies, direct current (DC) electromagnets and in some electric motors; and it can create a concentrated field that is as much as 50,000 times more intense than an air core . </P>

Core of a transformer is made up of