The Compass:
Shen Kua describes the magnetic device that changed
the world
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This is one of the earliest
magnetic compasses: A floating fish-shaped iron leaf, mentioned
in the Wu Ching Tsung Yao which was written around 1040. The
book describes how iron can be heated and quenched to produce
thermoremanent magnetisation. The first clear account of suspended
magnetic compasses in any language was written by Shen Kua
in 1088.
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| Chinese
Compass |
Portuguese
Compass |
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The
compass changed the world. It allowed Columbus' ship, the
Santa Maria, to follow a Westerly course day and night to
land on the coast of Hispaniola: The discovery of America
in 1492. Chinese oceangoing junks, led by the eunuch admiral
Shen He, plied routes to Cambodia, Ceylon, Java and discovered
Africa. But such exploration was stopped in the Ming period,
leaving the globe open for voyages of discovery by the European
powers.
Hard steel
wire for the needles, originally exported from Damascus and
Hyderabad, was later manufactured world-wide, although in
these two pictures, both drawn around the 16th century, we
can see quite a contrast in the working conditions prevalent
in China and in England!
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Shen Kua's account preceded the first comparable European text,
written in Paris by the English monk Alexander Neckham, by more
than a century. And it was not until 1269 that the first Western
account of a magnet and its properties "Epostolia de Magnete",
was written by Petrus Perigrinus in 1269. It describes numerous
experiments, especially with a spherical lodestone which he called
"terella". He found the poles of the magnet although he
ascribed this behaviour to celestial poles rather than the magnet
itself.
Two of Perigrinus' proposals -a perpetual motion machine (above
left) and a levitation device with a floating magnet- have inspired
a wealth of hare-brained schemes ever since. The perpetual motion
idea is parodied in the 1920's Berlin postcard featuring Albert
Einstein in his magnetic autocar. In the eighteenth century, Temple
Henry Croker twice presented his ideas for a perpetual motion machine
(below left) to the Royal Society. It was supposed to work in Barbados,
at the magnetic equator, where the strong horizontal terrestrial
field would flip the bar magnet M repeatedly. The US patent office
was issuing patents for magnetic perpetual motion machines as late
as the 1970's.
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Proponents of magnetic levitation came in for similar ridicule.
The picture on the left, taken from a 1727 French translation
of Gulliver's Travels, shows Gulliver waving at Laputa, the
flying magnetic island with a shiny adamantine base. Galleries
around its sloping sides were connected by stairs to the palace
at the top. Swift included a map (below) in the first edition
of 1726. Parodying the style recommended by natural philosophers,
he explained how c represents the attractive end, and d the
repelling end of the lodestone travelling above the dominions
of Balnibarbi.
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| It seemed that the last word on the subject
was given by Earnshaw in 1839. He proved a theorem which implied
that it was impossible for a magnet to achieve stable equilibrium
in all three directions in a static magnetic field created
by an arrangement of permanent magnets. Only recently has
it been understood that it is possible to levitate
a permanent magnet with the help of diamagnetic material.
On the right we see levitation of a Ni-Fe-B magnet in the
field of another magnet, stabilised using the feeble diamagnetism
of two human fingers. Diamagnetic objects like drops of water
or frogs can be levitated when the product of magnetic field
and field gradient exceeds about 14 tesla2/cm. |
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Development of the compass and the circumnavigation of the globe
led to the beginings of the modern science of magnetism.
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William Gilbert wrote De Magnete, arguably the first ever
scientific text, in 1600. The book starts with a resolve to
strip away all myth from the subject and reason from observation:
"In follies and fables do philosophers of the vulgar
sort take delight; and with such like do they cram readers
a-hungered for things obtruse, and every ignorant gaper for
nonsense. But when the nature of the lodestone shall have
been by our labours and experiments tested, then will the
hidden and recondite but real causes of this great effect
be brought forward, proven, demonstrated...and the foundations
of a grand magnetic science being laid will appear anew, so
that high intellect may no more be deluded by vain opinions"
Gilbert's observation of the magnetic dipole field of a lodestone
terella and other shapes of magnets led to the brilliant insight
"Magnus magnes ipse est globus terrestis"- the Earth
itself is a great magnet. A key idea was the Orbis Virtutis(below
left)-the sphere of influence of the magnet, which influenced
Kepler in formulating his laws of planetary motion and was
forerunner of the field concept. |
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Gilbert was a believer in an animistic philosophy, and despite
its modernity, the six books of De Magnete are cast in animistic
and anthromorphic terms.
The first person to break free from the animistic idea
of the lodestone possesing a soul was the 17th century philosopher
and physicist René Descartes, who offered the first
theory with a recognisable image of the magnetic field. He
believed in effluvia of "threaded parts", which
preferred to travel through lodestone or iron rather than
air, and propagated by passing through a network of one-way
ducts in the earth or in iron. Lodestones have the best ducts,
with fur-like cilla which must be undamaged if magnetism is
to persist.
By re-establishing confidence in the power of reason and
by postulating a dichotomy of the soul and the body, Descartes
laid the foundations for the birth of theoretical science
and opened the door to the study of nature on her own terms.
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But it was not to be all plain sailing...
  
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