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NAU 102

Lesson 15

Magnetic Compass

Basic Principle

Small magnets are placed under a compass card.

The card is free to move and aligns with the horizontal component of the Earth’s

magnetic field.

Magnetic Compass

Early Compasses

A lodestone placed on a chip of wood, floating in a bowl of water.

Modern Compasses

Basically the same. Just more parts!

Marine Compasses

Many ships will have two compasses:

Standard CompassLocated on the centerline, away from

magnetic influences.

Directions stated as “per standard compass” (psc)

Marine Compasses

Many ships will have two compasses:

Steering Compass

Located near the helm console.

Directions stated as “per steering compass” (p stg c)

Compass Components

Bowl

Contains and supports all other compass components

Constructed of non-magnetic material

Glass cover is secured with a Bezel Ring.

Bowl & Bezel Ring

Bowl

Bezel Ring

Compass Components

Lubber’s Line

Aligned with the centerline of the vessel.

Indicates the compass heading of the vessel.

Lubber’s Line

Compass Components

Compass Card

Stays aligned with magnetic meridians.

Usually graduated in 360° and cardinal and intercardinal points.

Compass Card

Compass Components

Directive Magnets

Two or more magnets, mounted underneath the compass card.

Align the card with the magnetic meridian.

Directive Magnets

Compass Components

Pivot

Pin in bowl which supports the compass card.

Compass card rotates on a jeweled bearing.

Pivot

Compass Components

Compass Fluid

Clear liquid, resistant to freezing.

Usually glycol or petroleum based.

Reduces friction and oscillations.

Compass Components

Card Float or Air Chamber

Attached to the card.

Supports most of the weight of the card.

Compass Components

Expansion Bellows

Allow the fluid to expand and contract with temperature.

Pivot

Mounting Components

The compass is mounted on gimbals.

Double, hinged rings of non-magnetic material.

Allow the compass to remain level as the ship rolls and pitches.

Gimbals

Mounting Components

Gimbaled compass is mounted in a Binnacle.

Supports the compass.

Protects from environment and

magnetic interference

Binnacle Components

Cover (Hood)

Protects the compass from the environment.

Spider Glass

Magnifying glass to enable reading of the compass.

Binnacle Components

Light

Enables the use of the compass at night.

Correctors

Minimizes the effects of magnetic interference (deviation).

Correctors

Quadrantal Spheres

Soft Iron Spheres

Reduce deviation on intercardinal

headings

Correctors

Fore and Aft / Athwartships Magnets

Reduce deviation on cardinal headings

Magnets placed in movable trays under

the compass

Correctors

Heeling Magnet

Vertically suspended

beneath the compass.

Corrects compass errors caused by rolling and pitching.

Correctors

Compensates for induced magnetism caused by vertical

component of force.

Flinders Bar

Soft iron contained in a tube on the outside of the binnacle.

Binnacle

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