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    Re: Historical Magnetic Variation/Declination
    From: Joel Jacobs
    Date: 2004 Jun 20, 18:42 -0400

    Bravo Zulu Bob Peterson!
    
    Thank you,
    
    Joel Jacobs
    ----- Original Message -----
    From: "RSPeterson" 
    To: 
    Sent: Sunday, June 20, 2004 4:09 PM
    Subject: Re: Historical Magnetic Variation/Declination
    
    
    > 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.
    >
    > References:
    > 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.
    > >
    > >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.
    > >================================================================
    > >
    > >
    > >
    > >
    >
    >
    > --
    > Robert S. Peterson
    > 31 N Alfred, Elgin IL  60123  USA
    > 847/697-6491
    > Compass Adjusting & Repair for Lake Michigan Navigators Since 1985
    > Physics {at} Bartlett HS
    > e-mail: rspeterson(at)wowway.com
    
    
    

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