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    Re: Historical Magnetic Variation/Declination
    From: Bob Peterson
    Date: 2004 Jun 20, 15:09 -0500

    I tend to sit in the background lurking to the informative, educated
    discussions, but it's time to contribute as an experienced compass
    adjuster/repair station for Lake Michigan.
    George pretty well has it in order.  Without a sufficient magnetic
    moment there will be nothing to "drive" the compass magnets onto their
    correct magnetic alignment though that driving moment can be extremely
    small (I fear someone will ask me to quantify).   The problem is more
    the issue with the mechanical design of the compass.  The compass
    manufacturers do balance the compass card (and thus the magnets) for a
    particular dip angle or zone.  Usually the compass will function into
    the next adjoining zone though the card will start to show a "tilt".
    The world traveling vessel which traverses many (all?) zones hits a
    dilemma.  What to do when the compass binds up because the card is now
    extremely tilted?  I only know of one compass built to function in many
    (not sure all) zones that being a Sestrel (from England).  They do this
    by increasing the distance between the pivot point and the
    Center-of-Gravity of the card assembly.  This minimizes the dip angle
    effect, however, not without a price (of course).   These compasses are
    now susceptible to dynamic effects as the vessel rolls, pitches, yaws.
    The card can chase all over the place and not seem to stabilize on a
    steady heading.  This can drive the helmsman crazy as they "chase" the
    card.  Not good.  So its a matter of striking a balance (so to speak) in
    design between card tilt and steady readings.
    The situation worsens for compasses living aboard steel vessels.  The
    proximity of the soft iron, of course, changes the "local" dip angle
    because the compass magnet is drawn down or repelled up by the iron and
    this changes depending on the heading.   Some headings the compass will
    appear to be "stuck" and other headings will act skidderish as though
    balancing a pencil on its point.  It will not want to "stay put".  It is
    an absolute must, first step, to correct the vertical field on a steel
    vessel and return the field to the correct dip angle for which the
    compass was balanced.  This can only be done with a vertical field
    instrument or dip needle indicator (sometimes called a heeling error
    indicator).   Not to do so guarantees an unsuccessful compass adjustment
    on a steel vessel.  At least in my experience.
    Back to that world traveler:  what are they to do?  My recommendation is
    to purchase additional "cores" for their binnacle compass.  Then as they
    change zones and the card tilt bottoms out and the card binds, change
    out the "core" to a new zone.  In theory, the compass correction should
    not change, in practice, it does.  So best to check it and build a new
    deviation card.   In my book, data always outweighs theory.
    I have rebalanced cards using a home-built Helmholtz coil large enough
    to receive the card assembly (card, magnets, pivot, and jewel) then
    modifying the local vertical field with the coil.  With the vector sum
    of my invariant horizontal field, I can reproduce any dip angle
    world-wide (note: only the angle, not the strength).   I then place
    small (as in really small) weights to balance the card for that "new"
    location.   The rebalanced card is worthless in my local dip angle (72
    degrees), but will right itself when it arrives at the new zone.
    Last point:  This new card is only good and usable if it is inside of a
    compass which means sealed inside of its bowl.  This is almost beyond
    the doing for all modern compasses while underway.  George is right: in
    the old days when they used dry-card compasses, they could rebalance the
    card on the fly, but not today.  This trick requires means and parts to
    treat the compass oil to remove the entrained gases from the oil
    solution.  So, once again the voyager is back to leaving home with
    several compass cores.   If there is another approach, I would love to
    hear it.  Any thoughts?
    BTW,  all of the above applies equally to electronic, flux-gate type
    compasses.  If you don't fix the vertical field (either electronically
    or magnetically), then don't expect an accurate adjustment.
    1) Kielhorn, L. V., "A Treatise on Compass Compensation," D. Van
    Nostrand Co, 1942.
    2) Defense Mapping Agency, "Handbook of Magnetic Compass Adjustment,"
    4th ed, HO Pub No 226, 1980.
    3) Harris, M., "The Compass Book," Paradise Cay Publications, 1998.
    4) Hine, A., "Magnetic Compasses and Magnetometers," Univ of Toronto
    Press, 1968.
    Thanks to all.  -- Bob Peterson
    George Huxtable wrote:
    >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.
    Robert S. Peterson
    31 N Alfred, Elgin IL  60123  USA
    Compass Adjusting & Repair for Lake Michigan Navigators Since 1985
    Physics @ Bartlett HS
    e-mail: rspeterson(at)wowway.com

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