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    Re: Finding the magnetic pole
    From: Greg R_
    Date: 2010 Jan 27, 18:43 -0800

    Excellent (and very informative) post - thanks, George.
    --- On Wed, 1/27/10, George Huxtable  wrote:
    > From: George Huxtable 
    > Subject: [NavList] Re: Finding the magnetic pole
    > To: NavList@fer3.com
    > Date: Wednesday, January 27, 2010, 4:27 PM
    > John Huth "Apache Runner" asked-
    > | Does anyone have information about the kind of equipment
    > used to find the
    > | North/South magnet poles?���I'm assuming
    > that there's some kind of compass
    > | that has horizontal and vertical bearings, but I don't
    > know where to find
    > | some examples of this sort of thing.
    > |
    > | I know that Amundsen mapped out the N mag
    > pole.���Could this exercise have
    > | helped him on his journey to the S. pole, given that the
    > S mag pole had 
    > only
    > | recently been found when he set out?
    > =======================
    > John is thinking of a Dip Circle or Inclinometer. This has
    > a carefully 
    > balanced compass needle, pivoted on a horizontal axis,
    > which can be 
    > carefully set by a spirit level to be precisely horizontal.
    > If the needle 
    > points exactly vertical, at all orientations of that axis
    > (two directions, 
    > at right angle, suffice) then there is no horizontal
    > component, and any 
    > magnetic field is truly vertical, which defines the
    > position as a magnetic 
    > pole. See examples at-
    > http://physics.kenyon.edu/EarlyApparatus/Electricity/Dip_Needle/Dip_Needle.html
    > James Clark Ross reached the North Magnetic pole, on a
    > sledge journey from 
    > his ship "Victory", in 1831. It was on the West coast of
    > Boothia Peninsula, 
    > in about 70�.N, 97�W. Boothia got its name after one of
    > the sponsors of the 
    > expedition, the maker of Booth's Gin!
    > Amundsen reached the same spot in 1905 (or 1904?), but
    > found that the 
    > Magnetic Pole had moved on further northward, so he didn't
    > actually reach 
    > the Magnetic Pole itself. However, this demonstrated that
    > the Magnetic Pole 
    > shifted, which was hardly in much doubt anyway. It's a
    > somewhat hollow 
    > achievement, to have reached such a transient spot as the
    > magnetic pole, but 
    > it allowed travellers to claim some sort of scientific
    > purpose.
    > Now, the North Magnetic Pole has shifted much further
    > North, into the Arctic 
    > Ocean.
    > In answer to John's second question, I doubt if that
    > knowledge of the North 
    > Magnetic Pole was of much help to an explorer of the
    > southern Arctic. There 
    > is no basic reason why the two poles should be antipodal,
    > and they are 
    > generally quite a long way from being so. Indeed, it's not
    > even necessary 
    > for there to be a single pole in each hemisphere. In theory
    > there caould be 
    > more, driven as they are by the chaotic churning of the
    > molten interior of 
    > the Earth.
    > Amundsen's southern journey took him rather a long way from
    > the South 
    > magnetic pole, which was probably a good thing, as it
    > allowed magnetic 
    > compasses to be usable over that long journey over the
    > featureless plateau.
    > I've just been reading an interesting book, "Earth's
    > Magnetism in the days 
    > of Sail", by A R T Jonkers (2003). This is about how the
    > study of old logs 
    > of wooden sailing vessels, with their observations of
    > magnetic variation (= 
    > declination to a geophysicist), has allowed changes in the
    > Earths magnetic 
    > field to be reconstructed, over the period 1600 to 1800.
    > Perhaps I'll say 
    > more about it later.
    > George.
    > contact George Huxtable, at� george@hux.me.uk
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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