The Flinders bar is a soft-iron bar mounted on the binnacle to counter vertical induced magnetism from structures such as the ship’s funnel and masts. When the vessel heels, its induced field can cancel the error produced elsewhere in the hull.
Compass correction is never permanent. Magnetic cargoes, maintenance, welding, and hammering can all change the vessel’s magnetic profile, so compass adjusters periodically recheck the system and navigators keep a record of remaining errors.
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