Relevant documents and M-notices: MGN 610, SOLAS Chapter V,
An in-depth understanding of the magnetic compass is specified by the OOW and management level syllabus. This includes an understanding of their limitations and sources of error. This article will deal with the knowledge required at operational level; a future article will cover the knowledge required at management level. An excellent starting point is to read MGN 610 Annex D - Operation, Testing and Maintenance of Magnetic Compasses.
There are two types of magnetic compass - dry card (oldschool, rarely seen) and wet card. In a wet card compass the card is made from mica or plastic and does not have any magnetic properties of its own; it is the magnets below the card that cause it to turn. Modern units use ring magnets, typically 7-8 of them.
The card sits on and pivots around a needle with an iridium tip; the synthetic sapphire rotational point is the only part of the card that should show wear and tear with time. This is why spare bowls should be stored upside-down; otherwise, they will fail at the same rate (and time) as the bowl that is in use. Spares should be stored away from the bridge structure.
The liquid in a wet card compass bowl is a 2:1 mix of ethyl alcohol to water. This has a damping action on the movement of the card, adds buoyancy and prevents the card from freezing in cold temperatures.
Who must carry a magnetic compass?
SOLAS Chapter V Regulation 19:
All ships, irrespective of size shall have…a properly adjusted standard magnetic compass, or other means, independent of any power supply to determine the ship's heading and display the reading at the main steering position
Vessels over 150GT and passenger vessels irrespective of size must carry a spare; normally this means a spare compass bowl.

The binnacle
The binnacle is a non-ferrous structure, usually made from GRP or wood, within which the compass bowl is housed. This is generally located above the bridge and on the centre-line of the vessel, though it may be off-centre (for example on aircraft carriers). On the rim of the brass of the bowl is a line leading dead ahead - this is the lubber line and indicates the vessels heading.
On the outside of the binnacle are two metal spheres, one on the port side, one on the stardboard side. These are Kelvin’s spheres, or quadrantal correctors. These correct for fore and aft and athwartships soft iron. On the upper image above, note that the Kelvin’s spheres are painted red and green and sit on a track, so that they can be moved closer to or further from the compass.
Also on the outside of the binnacle is a tube which contains the Flinders bar. This is mounted on the opposite side of the binnacle to the funnel. The vertical tubes (on the right of the binnacle in the upper image, behind the binnacle in the lower one) house the Flinders bars.
Inside the binnacle, on a chain below the compass bowl, is a small basket containing needle magnets - the vertical free instrument. These correct for heeling error.
Towards the bottom of the binnacle there are slots for various correcting magnets. These correct for fore and aft hard iron and for athwartships hard iron.
Note the small hatches on the bodies of both binnacles; these allow access to the corrector magnets.
Soft iron and hard iron correctors - how to remember them
There is an obvious issue with using a magnetic compass on a modern ship; ships are made of metal and have magnetic properties of their own, which will interfere with the accurate working of the magnetic compass. The solution to this is to correct for the effects of the ship using iron placed near the magnetic compass as part of the binnacle.
A good way to remembe whether a corrector is for hard or soft iron; if it’s on the outside of the binnacle (Kelvins spheres, Flinders bar) it’s for soft iron. If it’s on the inside of the binnacle, it’s for hard iron. ‘Soft on the outside, hard on the inside’ - think of a Kinder egg.