By John Papiewski – Updated Mar 24, 2022
A permanent magnet is a piece of iron or a similar ferromagnetic metal that retains its own magnetic field for years under ideal conditions. However, everyday events such as accidental drops, mechanical impacts, or exposure to high temperatures can gradually erode that field. A magnet keeper—a piece of ferromagnetic material that fits snugly over a magnet’s poles—serves as a protective bridge, helping to preserve the magnet’s strength during extended storage.
All permanent magnets exhibit ferromagnetism, a property in which the magnetic domains within the material align to produce a strong, stable field. In contrast, metals like copper and aluminum are paramagnetic; they exhibit only weak attraction to magnets and never develop a permanent field. A keeper is made from a ferromagnetic material that itself remains unmagnetized, acting as a neutral bridge between the poles.
Within any ferromagnetic body, microscopic magnetic domains generate tiny fields. When these domains are aligned, they reinforce each other, creating a large, coherent magnetic field around the entire object. External shocks or heat can randomize domain orientation, weakening the field. Over time, even in a stable environment, the field can decay. By placing a keeper over the poles, the magnetic circuit remains closed, stabilizing domain alignment and prolonging the magnet’s useful life.
Permanent magnets come in bars, horseshoes, rings, and flat strips. Each magnet has a single north and a single south pole, positioned at opposite ends of the magnetic field. The classic horseshoe shape places the two poles close together, making it an ideal candidate for a keeper that bridges the gap between them. In other shapes, a keeper can be positioned to cover both poles, effectively completing the magnetic loop.
A magnetic field remains strongest when the entire magnetic circuit is made of ferromagnetic material. A horseshoe magnet typically has an air gap between its poles; a keeper fills that gap, turning the circuit into a continuous iron loop. A plain bar magnet, left unprotected, will lose strength over several months. By arranging two bar magnets with opposite poles touching, you form a temporary magnetic bridge that preserves both magnets’ fields.
