Deviation card difference............?

The difference is that In the south, the magnetic field comes up out of the earth, and in the north it goes back down into the earth. As the compass needle is integral with the card of a marine compass, the upwards or downwards magnetic force affects the needle and causes the compass card to tilt. The closer to the poles, the stronger the upwards or downwards force and the greater the tilt. To counter this, the card has a small counter-weight attached to enable it to sit level.

A boat compass specifically designed for Northern Hemisphere will have a weight positioned to counter the downward magnetic force. When this compass is brought to the Southern Hemisphere, the combination of the weight and the upwards magnetic force will create an exagerated tilt on the card. Obviously, the same thing will happen to a Southern Hemisphere compass when it goes to the north.

(FYI - The Magnetic North Pole is over 1,000 miles from the Geographic North Pole and this distance is currently increasing by about 40 miles a year).

http://www.compassadjustment.com/

John

Deviation Card

I don't actually know, but I suspect it has more to do with the geographical alignment of the rolling mill used to produce the hull plates, seeing as rivetting is not commonly used anymore.

Capt. John refers to a different issue, of dip angle - the fact that a compass needle points downwards at higher latitudes. I had not heard of balancing a compass card to adjust for that, and it sounds odd to me. The dip angle is a continuous function from negative in the southern ocean to zero at the equator to positive in the arctic, so unless you are operating in a small geographic region, there's no one single compensation factor.