Magnetic core

A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields
in electrical,electromechanical and magnetic devices such as electromagnets, transformers, electric motors,
inductors and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds
such as ferrites. The high permeability, relative to the surrounding air, causes the magnetic field lines to be
concentrated in the core material. The magnetic field is often created by a coil of wire around the core that
carries a current. The presence of the core can increase the magnetic field of a coil by a factor of several thousand
over what it would be without the core.

The use of a magnetic core can enormously concentrate the strength and increase the effect of magnetic fields produced
by electric currents and permanent magnets. The properties of a device will depend crucially on the following factors:

• the geometry of the magnetic core.
• the amount of air gap in the magnetic circuit.
• the properties of the core material (especially permeability and hysteresis).
• the operating temperature of the core.
• whether the core is laminated to reduce eddy currents.               

Air core

A coil not containing a magnetic core is called an air core coil. This includes coils wound on a plastic or ceramic 

form in addition to those made of stiff wire that are self-supporting and have air inside them. Air core coils generally 

have a much lower inductance than similarly sized ferromagnetic core coils, but are used in radio frequency circuits

 to prevent energy losses called core losses that occur in magnetic cores. The absence of normal core losses permits

 a higher Q factor, so air core coils are used in high frequency resonant circuits, such as up to a few megahertz. 

However, losses such as proximity effect and dielectric losses are still present.

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  Pot core

  Usually ferrite or similar. This is used for inductors and transformers. The shape of a pot core is round with an 
  internal hollow that almost completely encloses the coil. Usually a pot core is made in two halves which fit together 
  around a coil former (bobbin). This design of core has a shielding effect, preventing radiation and reducing electromagnetic interference.
  
  
  *From Wikipedia, the free encyclopedia