A pole piece is a structure composed of material of high magnetic permeability that serves to direct the magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 produced by a magnet

Magnet

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...

. A pole piece attaches to and in a sense extends a pole

Magnetic pole

Magnetic pole may refer to:* One of the two ends of a magnet* The magnetic poles of astronomical bodies, a special case of magnets, two special cases of which are the Geomagnetic poles:...

 of the magnet, hence the name.

Pole pieces are used with both permanent magnets and electromagnet

Electromagnet

An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...

s. In the case of an electromagnet, the pole piece or pieces simply extend the magnetic core

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...

 and can even be regarded as part of it, particularly if they are made of the same material. In the case of a permanent magnet, the distinction between the magnet itself and the pole piece or pieces is more clear cut.

Materials

The traditional material for pole pieces was soft iron. While still often used with permanent magnets, soft iron suffers from eddy current

Eddy current

Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...

s which make it less suitable for use with electromagnets, and particularly inefficient when the magnet is excited by alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

.

Shapes and forms

Pole pieces take many shapes and forms depending on the application.

A traditional dynamic loudspeaker has a distinctive annular magnet and pole piece structure which serves to concentrate the magnetic flux on the voice coil

Voice coil

A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...

. The central, cylindrical pole piece surrounded by the voice coil is normally referred to as the pole piece. A second pole piece in turn surrounds the voice coil.

A moving coil meter has a different and equally distinctive structure.
A magnet keeper

Magnet keeper

A magnet keeper is ferromagnetic bar placed across the poles of a permanent magnet that helps preserve the strength of the magnet by completing the magnetic circuit, and are particularly useful for magnets which have a low magnetic coercivity, such as AlNiCo magnets.Keepers also have a useful...

 is a specialised pole piece used to temporarily connect the poles of a permanent magnet, to help to preserve the magnetism, and for safety in the case of large and powerful magnets.

Another specialised pole piece is the armature

Armature (electrical engineering)

In electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine–generally in a motor or generator, but it may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay....

 of an electromechanical solenoid, which produces work by being attracted by an electromagnet when the magnet is actuated.