NavList:

   

Reply

Excellent (and very informative) post - thanks, George.

--
GregR


--- 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.
> 
> 
> 
> 
> 
> 

   

Reply