Keepers are used to reduce self-demagnetization of magnets. When a magnet is not in use, it can gradually lose its magnetic strength due to various factors, including temperature changes, vibrations, and exposure to other magnetic fields. This process is called self-demagnetization.
A keeper is a piece of ferromagnetic material that is placed across the poles of a magnet to complete its magnetic circuit. This prevents the magnetic field from leaking out and helps maintain the magnet's strength over time. By providing a closed path for the magnetic flux, the keeper ensures that the magnetic energy of the magnet is conserved and not dissipated.
Therefore, keepers are useful for storing magnets for prolonged periods to prevent self-demagnetization. They do not cancel the effect of the earth's magnetic field or protect the magnet from stray electric fields, nor do they increase the strength of the magnets.