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    Re: Compass Adjustment - A Cautionary tale
    From: Trevor Kenchington
    Date: 2005 Jan 30, 20:45 -0400

    Thanks to George for correcting yet another of my misunderstandings.
    I have tracked down the previous discussion of this topic on Nav-L and
    it was much more recent than I had remembered. It should be in the
    archives for late June 2004. The postings by Bob Peterson on 20 & 21
    June, plus those of Henry Halboth on 22 & 23 June were particularly
    informative on the issue of compass tilt (though they were embedded in a
    thread which covered several other concerns about magnetic compasses --
    and led indirectly to my getting an old dory compass beautifully
    restored for use in my little boat!). Unfortunately, I can't glibly
    claim to have simply forgotten what Bob and Henry wrote seven months
    ago. Rather, I can now see that I didn't understand it at the time.
    Perhaps I have it right at last -- if not, hopefully somebody will
    correct me:
    The vertical component of the Earth's magnetic field produces vertical
    permanent and induced magnetism in iron and steel hulls. With the vessel
    upright, those do not affect the compass reading since they act
    vertically. However, when the vessel rolls or heels, the "vertical"
    component becomes partially horizontal and then it can affect the
    compass. In big ships, this is corrected by the combination of a
    vertical soft iron bar, the "Flinders bar", and a vertical permanent
    magnet, both being mounted in the binnacle with the permanent magnet
    directly below the compass pivot. The height of the permanent magnet is
    supposed to be adjusted as a ship moves into zones with different dip
    and its poles are reversed when the ship crosses the magnetic "equator"
    -- the line where dip is zero.
    None of that, however, relates to the tilt of the compass card.
    As to the latter: There are (at least) three compass designs. Some have
    a facility for moving a balancing weight along the needle (or across the
    card in the direction of the needles), so that the effect of gravity
    balances the tilted pull of magnetic dip and keeps the needle level.
    That used to be common in old, dry-card compasses but now is only seen
    in some specialized models (e.g. for military use).
    Low-cost, light-weight modern compasses use the same idea of a balance
    weight on one side of the needle, or card, but they have the weight
    permanently in place and thus are only really suited for use in one of
    five zones (which together include the entire globe). They can sometimes
    be used one zone away from the one for which they were balanced.
    According to Suunto, Zone 1 covers the U.S., Canada, all of Europe,
    Turkey and the rest of Asia north of a line roughly from the southern
    tip of the Caspian Sea to the southern tip of the Korean peninsular,
    including Japan. Zones 2 and 3 take in Mexico to Brazil and Peru, Africa
    generally north of the Equator, the southern parts of Asia, including
    South and Southeast Asia (but not Indonesia) and a large portion of the
    Pacific. Zone 4 has the rest of South America, the rest of Africa,
    Indonesia, Papua New Guinea and a lot of the South Atlantic and Indian
    oceans. Zone 5 has Australia, New Zealand and most of the Southern Ocean
    west of Cape Horn and east of Kerguelen. Those seem to be only
    convenient units, rather than ones rigidly defined by isoclines of
    magnetic dip.
    Other modern compasses are "global" and suited for use anywhere (except
    near the magnetic poles, of course). They avoid tilt by having the
    centre of gravity of the card set well below the pivot (and presumably
    having a heavy card). Gravity then keeps the card more or less level,
    despite the tilted pull of magnetic dip. Besides other disadvantages
    (size, weight and cost, presumably), the substantial offset between
    pivot and centre of gravity means that various accelerations acting on
    the compass can cause the card to rotate on the pivot, making it hard to
    read the compass. I assume that that is less of a problem on big ships
    than on small boats, while it is usually big ships which move across
    multiple zones and hence need "global" compasses.
    Lee wrote:
    > George, I don't understand what you are saying here, and wouldn't mind a
    > reference or source  to help develop what you are saying. My imperfect
    > understanding at the moment is that the only difference between a zoned and
    > a global needle is the bearing that the needle/card rides on. As I
    > understand it, a global needle is based upon a more intricate and expensive
    > bearing. A bearing that allows the needle to swing freely, despite the force
    > of magnetic dip upon it. Properly built, it is unclear to me that a global
    > needle will function any different to a zoned needle.
    That suggests a fourth compass design, perhaps involving a roller
    bearing around the pivot pin which would allow the card to rotate but
    not to tilt at all. Are such things made?
    Is all that about right?
    Back in June, Bob Petersen mentioned "cores" which the ocean navigator
    can change at sea to keep his compass from tilting so far that the card
    binds on the housing as he sails from one zone to another. I am not
    clear what form those take nor how they function.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@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

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