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    Re: Finding the magnetic pole
    From: George Huxtable
    Date: 2010 Jan 28, 00:27 -0000

    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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