Orals Revision: Magnetic Compass — Parts, Coefficients A–E & Corrections

Orals revision card — MIN 653 (M) Amendment 1, competence 'Determine and allow for compass errors': knowledge of the principles of magnetic and gyro-compasses, ability to determine and allow for their errors, and knowledge of the compass correction procedure. This card covers the magnetic side.

Parts of a Magnetic Compass

• Bowl — the sealed, liquid-filled housing containing the card and pivot, dampens oscillation
• Card — the graduated ring carrying the magnets, floats/pivots freely to align with the local magnetic field
• Gimbal mounting — keeps the bowl level regardless of the vessel's roll and pitch
• Lubber line — the fixed reference mark on the bowl aligned with the ship's fore-and-aft line; the card reading against this line gives your heading
• Binnacle — the stand housing the compass, and critically, the corrector magnets and soft-iron correctors described below

Two Kinds of Compass Error

• Variation — the angle between True North and Magnetic North at your position, caused by the Earth's own magnetic field. Not a fault of your compass — it changes with geographic location and is shown on the chart's compass rose.
• Deviation — the angle between Magnetic North and Compass North, caused by the ship's own magnetism (permanent and induced) and nearby ferrous/electrical equipment. This one is your compass's fault, and it changes with the ship's heading — which is why it needs a deviation card, not a single fixed correction.

The Five Deviation Coefficients (A–E)

Deviation on any given heading can be broken down into five components, expressed as: Deviation = A + B·sinθ + C·cosθ + D·sin2θ + E·cos2θ — found in practice by swinging the ship through 8 headings and solving for each coefficient.

• Coefficient A — a constant error, the same on every heading. Usually caused by misalignment of the lubber line or the compass itself relative to the ship's fore-and-aft line — not caused by magnetism, so it isn't correctable with magnets.
• Coefficient B — varies as the sine of heading (maximum on N/S, zero on E/W). Caused by permanent magnetism and induced magnetism acting fore-and-aft. Corrected with a fore-and-aft permanent magnet.
• Coefficient C — varies as the cosine of heading (maximum on E/W, zero on N/S). Caused by permanent/induced magnetism acting athwartships. Corrected with an athwartship permanent magnet.
• Coefficient D — quadrantal deviation from symmetrically-arranged induced magnetism in soft iron, repeats twice per compass rotation. Corrected with the quadrantal spheres either side of the binnacle.
• Coefficient E — quadrantal deviation from asymmetrically-arranged soft iron. Usually small on a well-built ship — know it exists, don't over-invest revision time here versus A–D.

Correction in Practice

• Heeling error — a separate vertical-plane effect from the ship's permanent and induced vertical magnetism, corrected by the Flinders bar (a soft-iron corrector) and a heeling magnet beneath the binnacle
• Residual deviation after correction is recorded on a deviation card, kept near the compass, and used to correct any compass course to true
• MIN 653 explicitly expects you to know the compass correction procedure, not just the theory — be ready to talk through swinging the ship and applying correctors, not just recite the coefficient table

Quick Self-Check

• Which coefficient is corrected by the quadrantal spheres, and why does it repeat twice per rotation instead of once?
• Variation vs. deviation — which one is on your chart, and which one is on your deviation card?
• Why isn't Coefficient A correctable with a magnet?

Test yourself: Navigation Knowledge Checks.

Related Reading

• Boatmaster's Licence Compass Work — the foundational version of this topic at Tier 1 Level 2