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    Re: Historical Magnetic Variation/Declination
    From: Gary LaPook
    Date: 2004 Jun 22, 02:39 -0700
    Some compasses have a piece of wire wrapped around the south end of the needle that can be slid in towards the pivot or out to balance the dip of the magnetic field. Of course this means that the compass is designed for use in the northern hemisphere.

    All of these problems show up in aircraft compasses where aircraft accelerations cause the card to lie in a non horizontal plane which then allows it to pivot in response to the dip of the magnetic field causing, in flying parlance, turning error and acceleration error.

    Gary LaPook

    George Huxtable wrote:
    Brooke Clarke wrote-
     >Most compasses that use a pin pivot are made to
    work in some band of latitudes by adding a weight to one end of the
    pointer so it will rest about level at the center of the latitude band.
    There's a patent on the idea of having the magnetic part of the needle
    on a hinge so it actually points along the magnetic vector and the
    needle stays level, good for any latitude.
    Response from George.
    I have some doubts about how useful that would be.
    It can't really be "good for any latitude", can it? Above a magnetic pole,
    where the field points vertically, there's simply no way to extract a
    Northerly direction from a compass, whether or not the magnetic part of the
    needle is free to pivot about a horizontal axis: that magnet will simply
    point straight up and down.
    At a location away from the magnetic pole, but near it, will the turning
    moment on the needle assembly, trying to draw it into the North-South
    direction, be affected by allowing the magnet to tilt freely to align
    itself with the steep tilt of the field? My guess is that it will be
    considerably reduced, but I am not sure I have got the 3-dimensional vector
    diagram right. Confirmation or refutation would be equally welcome.
    Brooke Clarke tells us that a patent exists on the idea, but I haven't seen
    any compass on the market that works on that basis: perhaps we can draw our
    own conclusions.
    One of the virtues of the old dry-card compasses (Kelvin type) that used to
    be so common on big ships, was that it was easy to take off the glass top
    and rebalance the card by shifting a weight, as the latitude changed.
    It's certainly true that tilt of the card due to unbalance can lead to
    errors in reading it, especially if the gimballing is not perfectly free to
    move, because of parallax effects between the card and the lubber-lines.
    contact George Huxtable by email at george@huxtable.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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