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More about Magnetic Deviation.

I've just taken a look at the wikipedia entry on magnetic deviation, which
has pointed me to a 2-part piece in the series "Dead Reckonings" by Ron
Doefler. This is superb! If you want to see a really good account of how
compass correction was worked out mathematically, you couldn't ask for more.
It's written by a mathematician, who takes an interest in defunct aspects of
maths, of which this is certainly one. And with a really good set of
references. Doefler has done serious homework here.
You will find it at-

http://myreckonings.com/wordpress/2009/04/18/magnetic-deviation-comprehension-compensation-and-computation-part-i/

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And for anyone interested in the book by William Thomson (later, Lord
Kelvin) "Popular Lectures and Addresses", vol 3, (Navigational affairs),
these volumes have recently become available as cheap reprints in
print-on-demand form, by Kessinger.

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Finally, there's something I missed out of my earlier posting on this topic
[11064]. I addressed the difficulties of correction of a steel vessel
entirely in terms of the problem as it would have faced William Thomson. The
only sensor available to him was a compass needle, which provided a single
parameter; the magnetic heading. In terms of the two horizontal components
of the field, ahead and athwartships, what that gave him was the tan of
their ratio.

However, a fluxgate (or equivalent) compess provides those two components
individually, and I'm sure that if those two components had been available
to Thomson, he would have been able to make good use of them. It's knowing
those two components, individually, that allows a fluxgate compass to
self-compensate at all. Whether that knowledge allows it to do so precisely,
within the extreme environment of a steel hull, I really don't know, but I
seriously doubt it.

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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