Next message: Trevor J. Kenchington: "Re: Historical Magnetic Variation/Declination"
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 , 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 @ Bartlett HS
> e-mail: rspeterson(at)wowway.com
