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Quote
Question (from "New Scientist")
In the novel Moby Dick, the wooden whaling ship meets a typhoon south-east
of Japan and is subjected to thunder, lightning and displays of St Elmo's
fire. Subsequently, the magnetism of the ship's compass needle is discovered
to be reversed. The author, Herman Melville, maintains that such compass
reversals "have in more than one case occurred to ships in violent storms",
and sometimes when the rigging has been struck by lightning the magnetism in
a compass needle may be totally lost. Is this fact or fiction, and if it is
true how does it occur?
Alan Sloan, Buxton, Derbyshire, UK
Answers
Herman Melville's assertion is entirely plausible. Lightning involves very
high currents with high associated magnetic fields. They remagnetise exposed
outcrops of high-coercivity (high resistance to the effect of an applied
magnetic field) rocks with ease. Currents exceeding 10,000 amperes have been
deduced from rock magnetisations. The associated magnetic fields could
easily demagnetise or reversely magnetise a compass needle.
Alan Reid, Leeds, UK
Unquote
Redundancy is always a good thing...
Cheers
Bent Tolstrup
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