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.
"The keeper is first placed on the magnet, which has an attachment whereby a transmitter can be centrally associated with it; the other terminal having three connections that can be attached to this medium. The impulse is given simultaneously to the three leads after setting the instrument to represent forty-two thousand eight hundred vibrations on the harmonic, the same on the enharmonic and on the diatonic." [The Operation of the Vibratory Circuit]
"When a magnet is brought into contact with a keeper, there is no induction of magnetism from the magnet into the keeper. The static force of the magnet remains unchanged, and the action between the two may be compared to a sympathetic outreach of a very limited range of motion." [More on Keelys Theories]
"The time approaches when electric magnetic waves will be produced with an outreach of two feet, as powerful at that distance as is now shown when the keeper is almost touching the poles. These waves will demonstrate a radiating force too stupendous for measurement with present instruments." [POLARIZATION AND DEPOLARIZATION]
"The time is approaching when electromagnetic waves with an outreach of two feet will be produced, having an energy equal to that now shown up on the magnet when it is about to kiss its keeper, and showing a radiating force too stupendous for actual measurement." [ELECTROMAGNETIC RADIATION - Snell]
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 that 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, only those with low coercivity, meaning that they are easily susceptible to stray fields.
Magnet can be considered as the sum of many little magnetic domains, which may only be a few micrometers 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, yielding 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 in which the magnetic domains are 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 just a strong permanent magnet that keeps all the domains pointing the same way and realigns those that may have gone astray. Wikipedia, Magnet Keeper