Understanding your magnetic compass

 As a seafarer

The magnetic compass has been the main directional tool in the seafarer’s toolbox

to try and determine true north and then use that information for steering a course or take a bearing to fix position. The problem is that it has errors[1].

Let us take a closer look at a  ships magnetic compass. Today it is likely to be found on the monkey island as a backup to satnav systems but is still a requirement on ships.

The compass card is a magnet overlaid with a card holding the points of the compass. The card has a pivot and is mounted in a bowl filled with alcohol and water to dampen the movement of the card. The bowl is then mounted in a  set of gimbals so that the compass remain as horizontal as possible with  the ships motion and the complete magnetic compass is installed in a wooden stand called a binnacle.

The binnacle provides housing also for important tools to minimise compass errors.

[2]

The first challenge is that the magnetic compass north seeking pole points toward the earth’s magnetic north pole and that is not the same as the true north pole and worse still it is on the move as the image here shows! That means there is a changing error between the true north pole and the magnetic north pole as indicated the magnetic compass. The earth’s magnetic field influences the ships magnetic compass in all places on the planet.

This error is called “variation” and needs to be included in corrections to the

magnetic compass heading. 

That information can be found on the chart currently been used. In the centre of the compass rose is a second rose showing the magnetic headings for that region and stating the current variation as several degrees east or west of the true north. In this example magnetic north is to the left or west of true north.  It is also includes a date with information on the annual change in variation for that region. Hence the navigator can accurately retrieve the current variation for his ship’s magnetic compass.

A more complex challenge is based around the fact that steel ship is also magnetised, and this affects the position of the magnetic compass north seeking pointer.

It is complex because there are two distinct effects, one by the permanent magnetism in the ship whose effect does not change with course or behaviour of the ship and one that is induced by the earth’s magnetic field that changes the polarity and strength of the ship’s magnetism with course , rolling and pitching of  the vessel.

This is where the binnacle housing containing the compass plays a role in minimising the deviation to the compass needle from the ship’s magnetism.

To minimise the effect of the permanent ship’s magnetism magnets are placed both fore and aft and athwartship in the binnacle and to cope with rolling behaviour there is a magnet placed in a swinging bucket immediately below the compass in the binnacle.

[3]But what about the induced magnetism of the ship whose strength and polarity changes with ships course? Here we have Kelvins balls, those soft iron spheres each side of the compass and for the vertical component we have a vertical brass tube in front of the binnacle that contains a soft iron cylinder which with its own induced magnetism counteracts the deviation of the needle.[4]

All these adjustments are carried out by a compass adjuster by “swinging the ship” to determine the deviation of the compass. This produces a deviation curve which the navigator can use to allow for the deviation of the compass as well as the variation from the earth’s magnetic field.

As the variation and deviation are measured in degrees west or east of the compass needle, a mnemonic is a simple way to remember how to make correction to the magnetic heading.

“These rules are often combined with the mnemonic "West is best, East is least"; that is to say, add W declinations when going from True bearings to Magnetic bearings, and subtract E ones”.[5]

 

References

‘Binnacle’. In Wikipedia, 2 November 2024. https://en.wikipedia.org/w/index.php?title=Binnacle&oldid=1254943793.

Cohen, Ofer. ‘Earth’s Magnetic Field Protects Life on Earth from Radiation, but It Can Move, and the Magnetic Poles Can Even Flip’. The Conversation, 27 November 2023. http://theconversation.com/earths-magnetic-field-protects-life-on-earth-from-radiation-but-it-can-move-and-the-magnetic-poles-can-even-flip-216231.

‘Magnetic Declination’. In Wikipedia, 25 September 2024. https://en.wikipedia.org/w/index.php?title=Magnetic_declination&oldid=1247777714.

Nguyen, Van Suong. ‘Calculation of the Deviation Coefficients for Marine Magnetic Compass’. Journal of International Maritime Safety, Environmental Affairs, and Shipping 2, no. 2 (8 February 2019): 112–15. https://doi.org/10.1080/25725084.2019.1569336.

Why Do Ships Have Two Balls?, 2023. https://www.youtube.com/watch?v=ckhPRie1iKk.

 

 

 

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[1] ‘Binnacle’.

[2] Cohen, ‘Earth’s Magnetic Field Protects Life on Earth from Radiation, but It Can Move, and the Magnetic Poles Can Even Flip’.

[3] Nguyen, ‘Calculation of the Deviation Coefficients for Marine Magnetic Compass’.

[4] Why Do Ships Have Two Balls?

[5] ‘Magnetic Declination’.