Magnetic compass

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Image
The magnetic compass is fitted on the upper bridge exactly center line of the ship   It is referred to as standard compass     1. D ry C ard    2. W et C ard   A compass point north because all magnets have two poles north pole, south pole.   The north pole of the one magnet attracted towards the south pole of another magnet     DRY CARD COMPASS   A light aluminum ring about 254mm in diameter is attached to a brass hub by silk cord which is threaded in and out through holes in the ring.   This tight silk cord is act like bicycle wheels.   There is no other connection or support between the ring and the hub.   Silk cord is used because it doesn’t shrink or stretch due to moisture or change in atmospheric temperature.     Card support: -     The bottom of the hub has small cap fi tt ed with a sapphire bearing surface.   This rests on a pivot tipped with iridium which is hard nonmagnetic metal....
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TRUE COURSE

Is the angle at the ship between true north and ships heading.


TRUE BEARING

The true bearing of an object is the angle at the observer between the true north indicated by the meridian and the line joining the observer and the object.


COMPASS BEARING

Is the angle at the ship between the compass north and the line of sight of the object from the ship.


MAGNETIC BEARING

Magnetic bearing of a terrestrial object is the angle at the ship between magnetic north and the line of sight of the object from the ship.

CAUSE OF MAGNETIC VARIATION:-


The geographic poles and the magnetic poles are at different positions on the earth.At any given position on the earth,the angle in degrees between the true north and the magnetic north is called the variation at the place.


■The magnetic poles shift erratically about 8Nautical miles a year.This cause a slight change in the value of variation each year.

DEVIATION

Is the angle at the compass between the magnetic north and the compass north.

CN-COMPASS NORTH

MN-MAGNETIC NORTH


●Deviation of the compass is caused due to the magnetic properties of the ship.

●If we are using a wooden boat with no iron in it.Then the compass will not show any deviation.

●Nowadays magnetic compasses are used to check gyro compasses.


COMPASS ERROR

Is the angular difference at the compass between true north and compass north expressed in degrees.


■Compass error depends on two things

1.deviation

2.variation



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