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Trevor Kenchington's contributions are nearly always "spot-on", and that
applies to most of what he says about compass tilt: with one exception.

He wrote-

>For binnacle compasses in ships (but not
>the "binnacle mount" compasses sold for yachts), there is a vertical
>magnet within the binnacle, below the compass, which can be adjusted in
>height to suit different amounts of magnetic dip and can be reversed in
>direction when crossing the magnetic "equator".

Which there is; that's quite correct. But that is NOT for correcting the
tilt of the compass card. It's for correcting the heeling error, caused by
vertical magnetisation of the steel of the vessel, which tries to pull the
compass card away from true North as the vessel heels.

Just like all the other components of a ship's binnacle, it's there, not to
correct anything in the compass itself, but to correct the space that the
compass is placed in, so that the magnetic field at that point is, as
nearly as possible, the same as it would have been if the vessel wasn't
there (or as if she was built of wood)

If the correction has been made properly, by means of a vertical magnet
below the compass, and an unmagnetised vertical iron bar (the Flinders
bar), usually placed just in front of the compass, the vertical component
(and the tilt) should be the same as for a land-traveller at the same
magnetic latitude.

Now for dealing with the tilt of the card. For mariners' compasses that
were not liquid-damped, it was usually a simple matter to lift the glass
lid and shift a little sliding weight to rebalance the card as the latitude
changed. For liquid-damped compasses, where that was impossible, the
suspension was designed to make them less liable to tilt as the vertical
component of the field changed with magnetic latitude, by placing the
centre of gravity of the card further below the pivot. Unfortunately, this
renders the compass more susceptible to the sideways accelerations of a
vessel in a seaway. Also, the geometry was altered so that the card could
tilt further before it fouled anything. In that way it was possible to make
a compass that would work anywhere in the world, but it's a matter of
compromise.

Presumably, a compass that's designed to operate in a restricted zone of
magnetic latitudes would be a better performer, in terms of motion
stability, than a "global" compass.

Going back to heeling error: this became important in the mid 19th century,
when sailing vessels started to be built in iron (later steel), and found
that their compass deviation would change as they made a tack. It's
important to get it right, even for power vessels; otherwise, when the
ship's rolling becomes severe, the compass card swings wildly. Though it
was easy to correct at a particular magnetic latitude, which would remain
good for voyages around that latitude, it needed rechecking at different
latitudes before a proper correction could be made.

Trevor added-

>I am not aware of any markings on compasses to show how they have been
>weighted to balance magnetic dip in one region or another. (In these
>days of Internet commerce, compass manufacturers really should do more
>than simply send compasses to wholesalers with the needle or card
>balanced for the wholesaler's location. However, I have not heard of any
>greater care being taken.)

and I agree completely with those sentiments.

George.

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01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
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