2. Now we shall try to explain some observable
events on the earth’s surface from an
understanding of the earth’s interior.
We would like to emphasize, however that
scientists did not necessarily arrive at these
explanations in the same manner.
In fact, they used the knowledge gained from
studies of these phenomena to build a model
of the earth’s structure.
4. As early as 2600 B.C., that Chinese knew
that a certain kind of rock, when shaped into
a rod and allowed to turned freely, pointed
in a north-south direction.
Ancient Greeks discovered a certain mineral
they named magnetic could attract iron.
5. 1100 A.D. the freely turning magnet had
become a standard for navigation.
1400 A.D. sailors knew that the needle-shaped
magnet, now called a compass, did not
always point exactly to the north pole.
It sometimes pointed a little to the east
or a little to west of north.
7. Who is William Gilbert?
Dr. William Gilbert
Born: 24 May 1544
Known for: Studies of
8. William Gilbert, also known as Gilberd, (24 May 1544 –
30 November 1603) was an English physician, physicist
and natural philosopher. He was an early Copernican,
and passionately rejected both the prevailing
Aristotelian philosophy and the Scholastic method of
university teaching. He is remembered today largely for
his book De Magnete (1600), and is credited as one of
the originators of the term electricity. He is regarded by
some as the father of electrical engineering or
electricity and magnetism. While today he is
generally referred to as William Gilbert, he also went
under the name of William Gilberd. The latter was used
in his and his father's epitaph, the records of the town
of Colchester, and in the Biographical Memoir in De
Magnete, as well as in the name of The Gilberd School
in Colchester, named after Gilbert.
A unit of magnetomotive force, also known as
magnetic potential, was named the gilbert in his
9. In 1600 William Gilbert published a book
entitled De Magnete wherein he
explained the behavior of the magnetic
He proposed the earth itself was a huge
spherical magnet with its poles situated
almost at the geographic poles.
10. Gilbert discovered that when a compass is
held over different points of a spherical
magnet, it behaves in about the same way as
it does when it held at different parts of the
Thus, he concluded that the earth is a
As a magnet, the earth is surrounded by a
magnetic field whose lines of force meet at the
12. Compass and the magnet actually align
themselves to the line of the magnetic
This alignment makes the compass point
It’s because the earth’s magnetic poles are
not located exactly at the geographic
At present the magnetic north pole is
located at 75°N latitude and 100°W
13. Magnetic south pole is at 66°S latitude
and 140°E longitude.
Magnetic north pole is only about 1900
km away from the geographic north
pole — a distance slightly less than 0.05%
of the earth’s circumference.
The angle by which a compass needle
turns slightly away from the geographic
north pole, whether east or west of it, is
called the magnetic declination.
15. End of the compass or magnet that
points north is called the north seeking
pole or N-pole.
Opposite poles attracts.
Therefore, the north seeking pole must
be the south pole of the compass or
End that points the south is the south
seeking pole, o t the S-pole.
16. South seeking pole is actually the north pole
of the compass or magnet.
Thus, when a compass is placed near a
magnet, the north-seeking pole of the
compass is attracted to the south-seeking
pole, or the S-pole of the magnet.
Horizontal compass is free to turn around a
vertical axis and a horizontal plane.
However, if a compass were free to turn
around in all directions it should not only
point north-south but also tilt to one side so
along as it is not at the magnetic equator.
17. A compass constructed specifically to
allow tilting is called tilting or dipping
It turns on a horizontal axis and on a
The angle formed between the dipping
needle and the horizontal plane is called
the magnetic inclination.
22. A magnetic field is a mathematical
description of the magnetic influence of
electric currents and magnetic materials.
The magnetic field at any given point is
specified by both a direction and a
magnitude (or strength); as such it is a
vector field.[nb 1] The magnetic field is most
commonly defined in terms of the Lorentz
force it exerts on moving electric charges.
There are two separate but closely related
fields to which the name 'magnetic field'
can refer: a magnetic B field and a
magnetic H field.
23. Magnetic fields are produced by moving
electric charges and the intrinsic magnetic
moments of elementary particles associated
with a fundamental quantum property, their
spin. In special relativity, electric and magnetic
fields are two interrelated aspects of a single
object, called the electromagnetic field tensor;
the aspect of the electromagnetic field that is
seen as a magnetic field is dependent on the
reference frame of the observer. In quantum
physics, the electromagnetic field is quantized
and electromagnetic interactions result from
the exchange of photons.
24. Magnetic fields have had many uses in
ancient and modern society. The Earth
produces its own magnetic field, which is
important in navigation. Rotating magnetic
fields are utilized in both electric motors and
generators. Magnetic forces give
information about the charge carriers in a
material through the Hall effect. The
interaction of magnetic fields in electric
devices such as transformers is studied in
the discipline of magnetic circuits.
26. The simplest theory on the origin of the
earth’s magnetism is that the center of
the earth is one huge permanent
magnet made of iron and nickel.
Earth’s core consists mainly of these two
mainly materials, these theory seems
acceptable at first.
Laboratory studies show that iron loses its
magnetism at 770°C and nickel at 358°C.
28. Another hypothesis was proposed by
P.M.S. Blackett, an English physicist.
He claimed that the magnetism of the
sun, the stars and the earth was caused
by their rotation.
He set out to prove his theory by
developing a highly sensitive instrument
for measuring magnetism, the
29. Blackett failed, however, to prove his
His report entitled ―Negative
Experiment‖, became widely acclaimed
because of the full description of the
method he used and the precise
measurements he made.
30. Although he did not succeed in proving
his theory, the experiment was not a
complete failure because of the
magnetometer that he developed.
This instrument later proved to be a most
useful tool when Blackett began to study
the magnetism of rocks.
32. Walter Elsasser and Edward Bullard
developed a hypothesis that presented
the earth as an electromagnet rather
than a permanent magnet.
They attributed the earth’s magnetism to
electric current produced in the earth’s
interior, which in turn is continually
produced by the movement of the liquid
33. The inner and outer core move with
It is this difference in speed that is the
source of magnetism.
This is known as the Dynamo Theory.
35. A dynamo, or generator, is a device for
producing electricity by rotating an
electrical conductor across magnetic lines
If electricity is to be produced continuously,
there should be a continuous rotation of the
In the earth, the energy us thought to come
from convection currents set in motion by
heat produced by radioactivity in the core.
36. The dynamo theory involves a chain of
complex processes taking place in the
Conditions necessary if the earth is to work
like a dynamo:
the core of the earth must consist of a material
that conducts electricity as well as metal does;
the material must be in liquid form;
The conducting liquid must flow in some way,
the stirring processes providing the energy
needed to sustain the field.
37. 1940’s, scientists became interested in
paleomagnetism, or the history of the
Development of instruments for
measuring magnetism in rocks together
with the refinements made in the
techniques for establishing rock ages
greatly aided scientists in their study of
38. These are bits of magnetic materials (iron) in
rocks which have preserved the magnetism
of the earth at the time the rocks were
Many rocks contain iron-bearing minerals
which are naturally magnetic.
Studies of rocks in one place but which
were formed at different geologic times
reveal varying magnetic directions and
39. One interpretation states the earth is
fixed and it is the magnetic poles which
are shifting in position.
Another interpretation is that the
magnetic poles are fixed and it is the
whole earth that moves with respect to
A third interpretation is that the
magnetic poles are fixed only the upper
layers of the earth are shifting about.
40. Fossil rocks also distinguish between the
north and the south pole.
There is a strong evidence that the poles
of the earth’s field have reversed many
times in the past.
The dynamo theory presents one
possible explanation: the direction of
the magnetic field is influenced by the
direction of the movement of molten
iron in the core.