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Old 08-09-2002
fer@fer
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N hemispere binoc w compass not working in S?

Hi Bulleye,

Some principles of magnetism should be discussed to better understand what the vendor tried to explain.

The force of attraction magnetic field is greatest at the poles of the earth and least in the area between the two poles. The perfect place to be: Equator. The magnetic field is formed by lines of force flow from each of these poles, bending around and flow toward the opposite pole. Points of equal variation connected by an isogonic line, are plotted on Isogonic charts . In our navigation charts, we get only magnetic declination for the area. As we now, primarily correction for this, is the magnetic variation. In some places this variation is easterly; other places it is westerly.

The lines of force in the earth''s magnetic field pass through the center of the earth, exit at both magnetic poles, and bend around to re-enter at the opposite pole. Near the Equator, these lines become almost parallel to the surface of the earth. However, as they near the poles, they tilt toward the earth. In the immediate area of the magnetic poles they dip rather sharply into the earth. Since the poles of a compass tend to align themselves with the magnet lines of force, the magnet within the compass tends to tilt or dip toward the earth in the same manner as the lines of force.

Because the compass is suspended to swing freely, it tends to align with the earth''s magnetic lines of force. If you happen to be at North pole with a Southern calibrated compass, you''ll see that the card of the compass rose will almost point downwards. (And of course will get stuck, and will not longer free rotate as you change rumb.

This is known as the dip error: the compass card tends to align itself with the earth''s magnetic field. At or near the Equator this causes little or no problem, but as the ship nears either of the magnetic poles, the dip error (vertical dip in the needle) may become significant. In some areas, it may happen that the needle just dug itself into the bottom and wouldn''t read anything.

That is the main error for a compass in a ship. (In airplanes there are other errors in turning and acceleration, which I will skip discussing at this Honorable Sailing Forum)

How are these dip corrections managed by compass manufacturers: in compass for the Northern Hemisphere, a small counterweight is placed on the south seeking end of the compass needle, allowing the needle being parallel to the earth surface. In the Southern hemisphere the opposite applies and the counterweight is positioned on the north seeking end of the needle. That is the reason they only work in one hemisphere.

But, good compasses allow for this by allowing for a fairly large vertical play in the needle. But, the simple, flat compasses don''t work well in this situation because the tip of the needle hits the surface of the compass.

I recall one of the racing Cats racing The Race, changed compasses after entering Southern Hemisphere, since the needle plate got stucked.

Sorry so long, Regards

Fernando

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