Magnetism and Magnetic Materials

Описание

3 1.1 A brief history of magnetism
day. Usually red, and marked with ‘North’ and ‘South’ poles, horseshoe magnets still feature in primary school science books all over the world, despite the
fact that these horseshoes have been quite obsolete for the past 50 years.
A lodestone ‘terella’ used
by Gilbert to demonstrate
how the magnetic field of
the Earth resembles that of
a magnet.
Ren´ e Descartes, ´
1596–1650.
An eighteenth century
horseshoe magnet.
The obvious resemblances between magnetism and electricity, where like or
unlike charges repel or attract, led to a search for a deeper connection between
the two cousins. Luigi Galvani’s ‘animal electricity’, stemming from his celebrated experiments on frogs and corpses, had a physical basis – nerves work
by electricity. It inspired Anton Messmer to postulate a doctrine of ‘animal
magnetism’ which was enthusiastically embraced in Parisian salons for some
years before Louis XVI was moved to appoint a Royal Commission to investigate. Chaired by Benjamin Franklin, the Commission thoroughly discredited
the phenomenon, on the basis of a series of blind tests. Their report, published
in 1784, was a landmark of scientific rationality.
It was in Denmark in 1820 that Hans-Christian Oersted eventually discovered the true connection between electricity and magnetism by accident. He
demonstrated that a current-carrying wire produced a circumferential field
capable of deflecting a compass needle. Within weeks, Andre-Marie Amp ´ ere `
and Dominique-Franc¸ois Arago in Paris wound wire into a coil and showed
that the current-carrying coil was equivalent to a magnet. The electromagnetic
revolution was launched.
The remarkable sequence of events that ensued changed the world for ever.
Michael Faraday’s intuition that the electric and magnetic forces could be conceived in terms of all-pervading fields was critical. He discovered electromagnetic induction (1821) and demonstrated the principle of the electric motor with
a steel magnet, a current-carrying wire and a dish of mercury. The discovery
of a connection between magnetism and light followed with the magneto-optic
Faraday effect (1845).
All this experimental work inspired James Clerk Maxwell’s formulation3 of a
unified theory of electricity, magnetism and light in 1864, which is summarized
in the four famous equations that bear his name:
∇ · B = 0, (1.1a)
0∇ · E = ρ, (1.1b)
(1/µ0)∇ × B = j + 0∂ E/∂t, (1.1c)
∇ × E = −∂ B/∂t. (1.1d)
These equations relate the electric and magnetic fields, E and B at a point in
free space to the distributions of electric charge and current densities, ρ and j
in surrounding space. A spectacular consequence of Maxwell’s equations is the
existence of a solution representing coupled oscillatory electric and magnetic
3 ‘From a long view of the history of mankind there can be little doubt that the most significant event
of the nineteenth century will be judged as Maxwell’s discovery of the laws of electrodynamics’
(R. Feynman The Feynman Lectures in Physics. Vol. II, Menlo Park: Addison-Wesley (1964)).

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