44
See also: Thales of Miletus 20 ■ Johannes Kepler 40–41 ■ Galileo Galilei 42–43 ■
Hans Christian Ørsted 120 ■ James Clerk Maxwell 180–85B
y the late 1500s, ships’
captains already relied on
magnetic compasses to
maintain their course across the
oceans. Yet no one knew how they
worked. Some thought the compass
needle was attracted to the North
Star, others that it was drawn to
magnetic mountains in the Arctic.
It was English physician William
Gilbert who discovered that Earth
itself is magnetic.Gilbert’s breakthrough came not
from a flash of inspiration, but from
17 years of meticulous experiment.
He learned all he could from ships’
captains and compass makers, and
then he made a model globe, or
“terrella,” out of the magnetic rock
lodestone and tested compass
needles against it. The needles
reacted around the terrella just as
ships’ compasses did on a larger
scale—showing the same patterns
of declination (pointing slightly
away from true north at the
geographic pole, which differs from
magnetic north) and inclination
(tilting down from the horizontal
toward the globe).
Gilbert concluded, rightly, that
the entire planet is a magnet and
has a core of iron. He published
his ideas in the book De Magnete
(On the Magnet) in 1600, causing
a sensation. Johannes Kepler and
Galileo, in particular, were inspired
by his suggestion that Earth is not
fixed to rotating celestial spheres,
as most people still thought, but is
made to spin by the invisible force
of its own magnetism. ■THE GLOBE
OF THE EARTH
IS A MAGNET
WILLIAM GILBERT (1544–1603)
IN CONTEXT
BRANCH
GeologyBEFORE
6th century BCE The Greek
thinker Thales of Miletus notes
magnetic rocks, or lodestones.1st century CE Chinese
diviners make primitive
compasses with iron ladles
that swivel to point south.1269 French scholar Pierre de
Maricourt sets out the basic
laws of magnetic attraction,
repulsion, and poles.AFTER
1824 French mathematician
Siméon Poisson models the
forces in a magnetic field.1940s American physicist
Walter Maurice Elsasser
attributes Earth’s magnetic
field to iron swirling in its outer
core as the planet rotates.1958 Explorer 1 space mission
shows Earth’s magnetic field
extending far out into space.Stronger reasons are obtained
from sure experiments and
demonstrated arguments
than from probable
conjectures and the opinions
of philosophical speculators.
William Gilbert