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    Re: Re Compass Adjustment - A Cautionary tale
    From: George Huxtable
    Date: 2005 Jan 30, 11:48 +0000

    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.
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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