推 revivalworld:丘成桐也講過類似的話耶 @@ 05/22 18:38
http://physicsworld.com/cws/article/print/18485
The revolutionary thinker William Gilbert, who died 400 years ago this month,
was largely responsible for creating the science of magnetism. David Tilley
and Stephen Pumfrey believe that his achievements deserve to be much more
widely recognized
When William Gilbert of Colchester died on 30 November 1603, England lost one
of its greatest Elizabethan scientists. Three years earlier he had published
a book called De Magnete, which was nothing less than the first ever work of
experimental physics. Its fuller title, translated from the original Latin,
was On the Magnet, Magnetic Bodies and that Great Magnet the Earth. In 1651 a
collection of Gilbert's manuscripts, which had been edited by his half
brother, was posthumously published. Going under the title De Mundo Nostro
Sublunari Philosophia Nova, the book provided "a new philosophy of our
sublunary world".
Despite the revolutionary nature of both of these works, Gilbert has in
recent years sunk - quite unfairly we believe - into the footnotes of
history. So why is his revolutionary achievement in creating the science of
magnetism not better known?
A mind made for magnetism
Gilbert's story began in Colchester, Essex, where he was born in 1544. He
went up to Cambridge University at the age of 14, where he followed the
standard undergraduate course of the day. It was here that he met, and
subsequently rejected, the current scientific orthodoxy of Aristotle's
natural philosophy, Galen's medicine and Ptolemy's astronomy. The latter
placed an immovable Earth at the centre of the universe, around which the
planets and stars revolved in crystal spheres.
In contrast to the academic conservatism of Cambridge, Gilbert found in
London - where he became a physician in the early 1570s - a booming centre of
navigational expertise, technology and applied mathematics. Gilbert's
research into magnetism, as well as his medical practice, led him - unusually
for the time - to seek out navigators and skilled instrument makers,
collating their magnetic data and discoveries about lodestones and compass
needles. Also in these circles were Copernicans who fuelled Gilbert's then
radical belief that the Earth was just one planet in an infinite universe.
Gilbert used his leisure and status as a court physician to launch his attack
on traditional university science of the Earth by the publication of De
Magnete in 1600. This single volume was organized into six separate books,
each of which contained numerous chapters. The centrepiece was a hypothesis,
which he had probably formed in the early 1580s, that the Earth is a giant
magnet. Indeed, Gilbert had spent many years and much money - said to be ￡
5000 - proving this hypothesis in a new, experimental way.
These experiments mainly involved using a spherical magnetic lodestone (known
as a terella, or "little Earth") and a freely pivoting miniature compass
needle (or versorium). The London instrument maker Robert Norman had already
discovered in 1581 that a standard compass needle points at a certain dip, or
inclination, below the horizontal, in addition to pointing roughly north and
south. However, he had no idea what this dip might be elsewhere on Earth. By
studying how the dip of a versorium varies at different points around a
terella, Gilbert successfully predicted that this relationship between dip
and latitude on a terella models the dip of a compass needle around the
Earth. In book 5 of De Magnete Gilbert was therefore able to propose a law
for the dip of a compass needle at all points on the globe.
Nautical challenges
De Magnete also announced a new instrument called an inclinometer, with which
navigators could keep a rough check on their latitude in cloudy weather. The
instrument is illustrated in De Magnete, and several European sailors
reported successful sea trials, although the technique ultimately proved of
less use in practice.
A more ambitious scheme was to correlate longitude with "magnetic variation",
which is the deviation of magnetic north from true (astronomical) north.
Unfortunately, this work foundered after the discovery in 1634 (inspired,
ironically, by Gilbert's research) of the time-dependence of this magnetic
variation. This value was found to have decreased from 11º east of true
north in 1580 to 4º east by 1634 - a discovery that shocked all European
experts of the age.
Despite these subsequent setbacks, Gilbert's navigational aims were endorsed
by the mathematician Edward Wright in 1600 in his laudatory preface to De
Magnete. "In truth, in my opinion," he wrote, "there is no subject matter of
higher importance or of greater utility to the human race."
Gilbert carried out many other experiments, including the study of spherical
lodestones that were floated on water in small wooden boats. This work showed
that magnetic forces often produce circular motion, which led him to develop
a magnetic cosmology of the rotating Earth. We now believe this cosmology was
his main motivation for the study of magnetism.
Having shown that the Earth, which he called tellus or "Mother Earth", has an
immaterial magnetic force, Gilbert credited the Earth with a soul (anima) -
then a common explanation for planets and other "self-moving" entities. In
his view, the Earth's magnetic soul rotated the planet around its axis, which
was magnetically stabilized to a point near the Pole Star. Magnetism, in
other words, caused the Earth's Copernican diurnal rotation.
As Gilbert hinted in De Magnete (and expanded upon in De Mundo), he believed
that animate forces of all the celestial bodies "conspired" dynamically to
produce the regular but non-circular celestial motions. This first Copernican
physics, elaborated in the final book of De Magnete and in De Mundo, was, of
course, superseded some 80 years later by Newton's work on gravitation.
Groundbreaker
The grand aim of De Magnete - to take magnetism beyond the simple use of the
compass to find north - was not as successful as Gilbert and Wright had
hoped. His magnetic cosmology was also soon superseded. However, we do not
believe that Gilbert's contributions to navigation and to cosmology should
simply be discounted. Gilbert's magnetic Earth is the foundation of
geomagnetism. He showed experimentally that magnetism involves a force at a
distance, which encouraged other astronomers and physicists like Johann
Kepler, Robert Hooke, Christopher Wren - and possibly Newton himself - to
think of universal gravitation as analogous to magnetism.
Suppose, however, that we do discount the two grand aims of De Magnete and
also ignore the chapters on navigation, practical instruments and on
mathematical constructions, which might have been the work of his
collaborator Edward Wright. Even then, in our view, the core that remains
still stands as the first great work of experimental physics.
Consider the achievements that can undoubtedly be attributed to Gilbert at a
time when the only magnetic materials known were lodestone (magnetite), iron
and steel, and when the whole framework of modern science was still to be
invented. The most famous, and rightly so, is his demonstration in De Magnete
that the Earth is a great magnet. He showed this by means of what we would
now call model experiments, including those on dip mentioned above, in which
he showed that the behaviour of the versorium and terrella simulate that of
the compass needle and the Earth.
Meanwhile, Gilbert's qualitative discussion of magnetostatics is just about
complete and carefully based in experiment. The direction of the internal
field, the polarity of a cut magnet, as well as the processes of
magnetization and demagnetization are all explored in detail. In chapter 11
of book 5, for example, Gilbert even comes close to the idea of a magnetic
field and makes a very good shot at describing the dipolar field of the
terrella. What is lacking is any attempt at the difficult problem of
quantifying magnetic attractions beyond his comparisons of "weak" and
"strong" lodestones.
The interaction between temperature and magnetism also receives penetrating
and accurate discussion in several sections. The final main achievement is
the account of electrostatics in a chapter of book 2 entitled "Of the
attraction exerted by amber". This appears partly to distinguish electric
from magnetic effects, and establishes the very large number of "electrics".
Although Gilbert made no distinction between positive and negative charges -
this would take another 150 years - this single chapter is still enough to
have won him the title of "father of electrical science".
When considering magnetism, the physicist and polymath William Whewell wrote
in 1859: "Gilbert's work contains all the fundamental facts of the science,
so fully examined, indeed, that even at this day we have little to add to
them."
Gilbert's deployment of experiment was deliberate, considered and
groundbreaking. The first sentence of his preface starts: "In the discovery
of secret things and in the investigation of hidden causes, stronger reasons
are obtained from sure experiments and demonstrated arguments than from
probable conjectures and the opinions of philosophical speculators of the
common sort."
The organization of De Magnete also has a very modern feel. Each topic is
introduced by a careful review of previous work followed by an account of new
experiments. Gilbert, however, was very much a man of his age in his mastery
of invective. For example, when criticizing claims for magnetic
perpetual-motion machines, he writes: "May the gods damn all such sham,
pilfered, distorted works, which do but muddle the minds of students."
Fallen star
Gilbert was more celebrated in his tercentenary year than he will be today.
The great enthusiast Silvanus P Thompson was the leading light of the Gilbert
Club, which found a considerable amount of new information on Gilbert. The
members were perhaps not disinterested; the rising electric power industry
could only benefit from acclaiming the eminent Englishman Dr Gilbert as the
father of electrical science. Two translations of De Magnete appeared;
although Thompson began first, he was pre-empted by P Fleury Mottelay's 1893
edition.
Derek Price, who edited a 1958 reprint of Thompson's Gilbert Club edition,
eloquently describes Gilbert's achievements. "One might well feel that
Gilbert had invented the whole process of modern science rather than merely
discovering the basic laws of magnetism and static electricity," he writes.
"Certainly he was the first to have the tenacity to work through a whole
segment of physics methodically, appealing to experiment and reason
throughout. Gilbert's work formed the pattern for the subsequent treatment of
other parts of physics, and much later for chemistry and biological
subjects."
Gilbert's star has, however, fallen in recent decades for several reasons. It
is not so fashionable to seek heroic ancestors. Historians, meanwhile, no
longer see the scientific era of Gilbert, Kepler and Galileo as quite so
"modern" and it is now impossible to dismiss Gilbert's pre-modern hypotheses
about the Earth's and other planetary souls as irrelevant to his experiments.
But the legacy of his magnetic science is still profound. Just look at De
Magnete and judge this sharply written and profoundly innovative work for
yourself.
--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 163.25.118.134