A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Historical Magnetic Variation/Declination
From: Joel Jacobs
Date: 2004 Jun 20, 18:42 -0400
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 firstname.lastname@example.org, 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