Magnet keeper

A magnet keeper, also known historically as an armature, is a ferromagnetic bar made from 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; it is important for magnets which have a low magnetic coercivity, such as Alnico magnets.

Keepers also have a useful safety function as they stop external metal being attracted to the magnet.

Most magnets do not need a keeper - just those with low coercivity, meaning they are easily susceptible to stray fields.

You can think of a magnet as the sum of lots of little magnetic domains, which may only be a few microns or smaller in size. Each domain carries its own small magnetic field, which can point in any direction. When all of domain are pointing in the same direction, the fields add and you get a strong magnet. When these all point in random directions, they cancel each other and the net magnetic field is zero.

In magnets with low coercivity, the direction that the magnetic domains is pointing is easily swayed by external fields, such as the Earth's magnetic field or perhaps by the stray fields caused by flowing currents in a nearby electrical circuit. Given enough time, such magnets may find their domains randomly oriented, and hence their net magnetization greatly weakened. A keeper for low-coercivity magnets is a just a strong permanent magnet that keeps all the domains pointing the same way, and realigns those that may have gone astray.