Definition
A soft iron bar placed across the poles of a permanent magnet during storage to complete the magnetic circuit and prevent the magnet from losing its strength over time.
Plain English
A small iron bar laid across the ends of a magnet when it is not in use, so the magnet stays strong and does not weaken in storage.
Context Anchor
Seen in discussions of aircraft magnetos, permanent magnets, and ignition-system maintenance.
Derivation
The word 'keeper' here means something that 'keeps' or preserves. The bar 'keeps' the magnetism intact by giving the magnetic field a continuous iron path to flow through, rather than letting it leak away through the air.
Why Pilots Care
A demagnetized magneto magnet produces weak sparks and can cause hard starting or rough engine operation.
Intuition Check
A keeper is not a person, a latch, or a storage box here. It is a piece of iron that helps a magnet keep its strength.
Example Sentence 1
Before placing the magneto rotor on the shelf, the technician fitted the magnet keeper across its poles.
Example Sentence 2
Never store a permanent magnet without its keeper or the magnet will lose strength.