NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Historical Magnetic Variation/Declination
From: Gary LaPook
Date: 2004 Jun 22, 02:39 -0700
From: Gary LaPook
Date: 2004 Jun 22, 02:39 -0700
Some compasses have a piece of wire wrapped around the south end of the needle
that can be slid in towards the pivot or out to balance the dip of the magnetic
field. Of course this means that the compass is designed for use in the northern
hemisphere.
All of these problems show up in aircraft compasses where aircraft accelerations cause the card to lie in a non horizontal plane which then allows it to pivot in response to the dip of the magnetic field causing, in flying parlance, turning error and acceleration error.
Gary LaPook
George Huxtable wrote:
All of these problems show up in aircraft compasses where aircraft accelerations cause the card to lie in a non horizontal plane which then allows it to pivot in response to the dip of the magnetic field causing, in flying parlance, turning error and acceleration error.
Gary LaPook
George Huxtable wrote:
Brooke Clarke wrote- >Most compasses that use a pin pivot are made towork in some band of latitudes by adding a weight to one end of the pointer so it will rest about level at the center of the latitude band. There's a patent on the idea of having the magnetic part of the needle on a hinge so it actually points along the magnetic vector and the needle stays level, good for any latitude.Response from George. I have some doubts about how useful that would be. It can't really be "good for any latitude", can it? Above a magnetic pole, where the field points vertically, there's simply no way to extract a Northerly direction from a compass, whether or not the magnetic part of the needle is free to pivot about a horizontal axis: that magnet will simply point straight up and down. 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@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. ================================================================