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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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