A pole piece is a structure composed of material of high

magnetic permeability In electromagnetism, permeability is the measure of magnetization produced in a material in response to an applied magnetic field. Permeability is typically represented by the (italicized) Greek letter ''μ''. It is the ratio of the magnetic ...

that serves to direct the

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

produced by a

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, steel, nickel, ...

. A pole piece attaches to and in a sense extends a pole of the magnet, hence the name. Pole pieces are used with both

permanent 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, steel, nickel, c ...

s and

electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire (likely copper) wound into a electromagnetic coil, coil. A current through the wire creates a magnetic ...

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

magnetic core A magnetic core is a piece of magnetism, magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, ele ...

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 A magnetic core is a piece of magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, ...

. While still often used with permanent magnets, soft iron suffers from eddy currents which make it less suitable for use with electromagnets, and particularly inefficient when the magnet is excited by

alternating current Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...

Shapes and forms

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

dynamic loudspeaker An electrodynamic speaker driver, often called simply a speaker driver when the type is implicit, is an individual transducer that converts an electrical audio signal to sound waves. While the term is sometimes used interchangeably with the ...

has a distinctive annular magnet and pole piece structure which serves to concentrate the magnetic flux on the

voice coil A voice coil (consisting of a former, collar, and winding) 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. Th ...

. 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 Moving or Movin' may refer to: Moving of goods * Relocation (personal), the process of leaving one dwelling and settling in another * Relocation of professional sports teams * Relocation (computer science) * Structure relocation Music Album ...

has a different and equally distinctive structure. MagnetEZ

magnet keeper A magnet keeper, also known historically as an armature, is a bar made from magnetically soft iron or steel, which is placed across the poles of a permanent magnet to help preserve the strength of the magnet by completing the magnetic circuit; ...

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 of an

electromechanical solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whos ...

, which produces work by being attracted by an electromagnet when the magnet is actuated. An

electron lens Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...

contains two specialised pole pieces used to guide the electromagnetic field lines of the wire coils of the permanent magnet, these field lines guides the spiral of electrons. In the objective electron lens an upper annular polepiece surrounds the column and a lower planar polepiece with a 'pinhole' on the optic axis is closest to (adjacent) the object (specimen)

References

Magnetic devices {{technology-stub