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    Re: Re Compass Adjustment - A Cautionary tale
    From: Trevor Kenchington
    Date: 2005 Jan 30, 00:27 -0400

    Kieran,
    
    This subject has come up at least once before on the list. I think I
    remember us passing over it twice in the time I have been a member,
    during the first of which I described my experience with a UK-purchased
    "hockey-puck" bearing compass in the time that I lived in Tasmania. In
    short: I experienced exactly the same as you, except for the poles and
    hemispheres being reversed.
    
    There seems to be no solution, with low-cost hand-held compasses, except
    to buy one for each hemisphere. 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".
    
    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.)
    
    I am also not clear about over how wide an area compasses can be used
    without dip becoming a problem. In southern Australia, you have a
    southward dip of nearly 70 degrees. The UK and the inhabitable parts of
    Canada have a northerly dip of about the same amount. Clearly, the
    140-degree difference is too much. You should, however, be fine down to
    60 South, since the dip in that zone is never as much as 85 degrees.
    Your compass should also be OK anywhere in the southern temperate
    latitudes except for the eastern Pacific, western South Atlantic and
    South America, since the dip across that broad swath is always more than
    55 degrees -- assuming, of course, that your instrument is optimized for
    Sydney, Melbourne or Adelaide, rather than being on the fringe of usable
    there.
    
    If you were to go to the Antarctic continent due south of Australia, you
    would find that the magnetic dip approached 90 degrees. At that point,
    your problem isn't a compass needle which sticks on its housing. Your
    problem is that a magnetic compass provides no useful information on
    horizontal directions. Get a gyro instead!
    
    
    Trevor Kenchington
    
    
    
    You asked:
    
    > I have a couple of questions:
    >
    >    1. Is there any way you can tell from looking at a compass what its
    >       effective area of operation is ie are there any codes on compasses?
    >    2. If my compass works at mid latitudes in the southern hemisphere
    >       would it also work at extreme southern latitudes eg Antarctica
    >    3. Is there any way a compass which works fine in the S hemisphere
    >       can be adjusted so that it works fine up north
    >    4. Is the usual practice if you live down here in the south and
    >       wanted to ski, say Alaska or Northern Canada that you would have
    >       to buy a compass from a retailer at the place where you were
    >       undertaking the journey. That compass of course would then be
    >       useless when you returned home.
    
    
    --
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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