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. 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. with a
keeper in use A
magnet keeper 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, which produces work by being attracted by an electromagnet when the magnet is actuated. An
electron lens 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==