A bar of soft iron placed vertically in or near the housing of a ship’s compass to correct deviation caused by the local magnetic field of the ship.Permanent magnets or iron placed near the compass are used for compensation; an iron Flinder's bar is one example....
In ship's compasses, the compasses usually have to be equipped with an iron Flinder's bar and maybe quadrantial spheres; they're supposed to eliminate compass deviation caused by a ship's hull.
Origin
Late 19th century: name after Captain Matthew Flinders, Matthew.