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    Compasses and zones. was [NAV-L] Compass Adjustment - A Cautionary tale
    From: George Huxtable
    Date: 2005 Feb 1, 21:32 +0000

    I wrote, on Jan 30-
    >For mariners' compasses that were not liquid-damped, it was usually a
    >simple matter to >lift the glass lid and shift a little sliding weight to
    >rebalance the card as the >latitude changed. For liquid-damped compasses,
    >where that was impossible, the suspension >was designed to make them less
    >liable to tilt as the vertical component of the field >changed with
    >magnetic latitude, by placing the centre of gravity of the card further
    >>below the pivot.
    and continued-
    >. Unfortunately, this
    > renders the compass more susceptible to the sideways accelerations of a
    > vessel in a seaway. Also, the geometry was altered so that the card could
    > tilt further before it fouled anything. In that way it was possible to make
    > a compass that would work anywhere in the world, but it's a matter of
    > compromise.
    > Presumably, a compass that's designed to operate in a restricted zone of
    > magnetic latitudes would be a better performer, in terms of motion
    > stability, than a "global" compass.
    to that last part, Lee Martin responded-
    "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."
    I'm not sure what type of bearing Lee has in mind here. There's more too it
    than just the design of the needle-bearing, that has forced compasses to be
    divided into relevant zones. It' related to the shape of the housing, and
    the introduction of the "hockey-puck" design, which for all its other
    important advantages, only allows the card headroom for a very limited
    amount of tilt, before touching its plastic housing.
    Yes, you can design a traditional needle-bearing to allow for quite a lot
    of tilt. Lee asks for references, and I can offer none on the more modern
    designs, but on my shelves I have "The Ship's Compass", (1952), by Grant
    and Klinkert. Figure 46 shows a very narrow coned pencil-point, of
    osmium-iridium alloy, at a guess subtending 20 degrees, with a sapphire
    riding on it, with a hollow-cone of about 90 degrees, so that would allow a
    card tilt of 35 degrees or so; far more tilt than the modern hockey-puck
    housing could accommodate.
    For further references, for Lee, about the points I have made, Grant &
    Klinkert, describing the dry-card compass on page 76, say
    " ... with the exception of a region near to the magnetic equator the
    earth's force is directed at an angle to the sensible horizon.. This qould
    give the card a propensity for "dipping" in any latidues other than where
    the dip is zero. This is prevented by securing the centre of gravity of the
    card and magnet system well below the centre of suspension..."
    and on page 81, about the liquid-damped card-
    "The necessity of having the centre of gravity of the card as a whole below
    the point of suspension in order to prevent "dipping" unfortunately is the
    cause of acceleration errors which are due to the movement of the ship."
    Lee writes, in another posting-
    "Certainly my impression is that other approaches to a global needle have
    relied upon a
    needle slung low below the pivot point, and riding on a superior bearing."
    That's fair enough, but to it I would add a shape for the containing
    housing that allows for considerable tilt. And for those compromises, a
    price has to be paid in terms of performance.
    Trevor Kenchington has added-
    >>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?
    I suggest that would be a VERY bad design! The compass finds the diection
    of magnetic North from the horizontal component of the magnetic field. If
    the card is constrained to pivot about an axis, as Trevor suggests, it
    becomes VERY important to ensure that that axis is at all times exactly
    vertical. Otherwise, the compass will be finding the component of the field
    about some different plane than the horizontal, and because dip angles can
    be so great, it would be VERY susceptible to any such error. It would
    require a precision and stability of gimballing that would be impossible to
    meet, I suggest.
    Now, Greg Trevillian has introduced us to another clever design. He writes-
    >Suunto is making several handheld compasses with a "global" needle.
    >Evidently, they've come up with a design where the magnet and the needle
    >are separate, so that the magnet can tilt while the needle lies flat.
    I think I see what Greg is getting at here, though I'm a bit puzzled whan
    he says "the magnet can tilt while the needle lies flat". What is the
    "needle" he refers to? If Greg had written "the magnet can tilt while the
    card lies flat", that would have been clear.
    And indeed, I think that such a design would work, though I can see one
    snag, to be dealt with later.
    Imagine  a compass card, carrying a magnet at its centre, the magnet being
    significantly shorter than the diameter of the card, the whole assembly
    resting on a conical pivot in the familiar way. The magnet isn't firmly
    fixed to the card, but instead, it can pivot about a horizontal axis, like
    a see-saw, between two horizonally-spaced bearings attached to the card
    near its centre, in the E-W direction. Thouse bearings could be similar,
    perhaps, to the balance-wheel bearings of a watch. The magnet itself would
    be balanced about that axis, so that in the absence of any magnetic field
    it would stay in any position, and in a magnetic field it would be free to
    align itself, in dip and in direction, with the field.
    Because of the freedom of those see-saw bearings, there's no way that the
    magnet can apply torque to tilt the card, so the card would stay
    horizontal, at any latitude. The compass housing would need a spherical
    bulge near its centre, to allow room for the magnet to tilt, but outside
    the radius of the magnet it could become a rather flat pillbox, allowing
    easy close access to read the card, which would remain at a constant level
    whatever the latitude.
    Have I understood how the thing operates?
    The snag? Well, everything about a compass gets more difficult at high
    latitudes, near the magnetic poles, where the dip is greatest. In a normal
    compass, in which the magnet stays roughly horizontal (except for a bit of
    card tilt), the torque aligning the compass with magnetic North varies
    roughly as cosine latitude, so at 80 degrees the compass becomes very
    sluggish. For a normal compass, with its magnet fixed roughly horizontal,
    tha magnetic torque would be down by a factor of about 6 compared with the
    "no dip" condition.
    But now consider a "global" compass with a pivoting magnet, as described
    above. At latitude 80 degrees, that magnet will have tilted to 80 degrees
    down from the horizontal, so the distance of each pole from the vertical
    card axis will be reduced, by a factor of cos 80. This means that its
    effective "magnetic moment", which measures its ability to generate torque
    from field, will also be lessened by a factor of cos 80, compared to a
    compass in which the card stays roughly horizontal. That's ANOTHER factor
    of 6!
    So the magnetic torque in the card, already down by that factor of cos 80,
    is reduced by yet another cos 80 by allowing the magnet to tilt up and
    down. This would render the compass even more sluggish at high latitudes,
    the turning force on it down by about 36 compared with the "no-dip"
    situation. It would be 6 times worse than the corresponding zoned compass.
    Perhaps I have been over-pessimistic. Perhaps the see-saw magnet suspension
    can be rebalanced somewhat, to ensure that the magnet doesn't tilt all the
    way that the field tilts.
    Purchasing such a "global" compass, based on that principle, might well
    tempt someone who is venturing into such high magnetic latitudes. And my
    concerns, above, are no more than speculation based on much guesswork.
    Still, it would seem prudent for anyone planning to use such a compass in
    high latitudes to ask some searching questions first.
    Finally, I think it was Trevor who referred to the carriage of magnetic
    compasses on aircraft as a thing of the past.
    I remember reading, can't recall where, some time in the last 15 years or
    so, that it was still a requrement for modern civil aircraft to carry some
    sort of magnetic compass, to offer some sort of guidance to the pilot if
    everything else failed, and that every 747 had such a compass tucked away
    at the corner of the windscreen. Can anyone confirm (or refute) this? Would
    a modern pilot have any idea how to apply magnetic variation?
    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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