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.
Do you want to know moreabout Earth’s Magnetism?
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.
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.
Do you want to know more about Gilbert’s Experiment?
Who is William Gilbert? Dr. William Gilbert Born: 24 May 1544 Colchester Died: 30 November 1603 (aged 59) London Occupation: Physician Known for: Studies of magnetism
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 fathers 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 honour.
In 1600 William Gilbert published a book entitled De Magnete wherein he explained the behavior of the magnetic rock. He proposed the earth itself was a huge spherical magnet with its poles situated almost at the geographic poles.
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 earth. Thus, he concluded that the earth is a MAGNET. As a magnet, the earth is surrounded by a magnetic field whose lines of force meet at the magnetic poles.
Compass and the magnet actually align themselves to the line of the magnetic force. This alignment makes the compass point north-south. It’s because the earth’s magnetic poles are not located exactly at the geographic poles. At present the magnetic north pole is located at 75°N latitude and 100°W longitude.
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.
Magnetic DeclinationThese varies at different places on Earth.
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 magnet. End that points the south is the south seeking pole, o t the S-pole.
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.
A compass constructed specifically to allow tilting is called tilting or dipping compass. It turns on a horizontal axis and on a vertical plane. The angle formed between the dipping needle and the horizontal plane is called the magnetic inclination.
Gilberts theory that the earth is a giant magnet is now accepted in modern science. But it led to another question: ―What caused the Earth’s Magnetism?‖
Do you want to know more about magnetic field?
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.
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.
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.
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.
NegativeExperiment What is Negative Experiment?
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 magnetometer.
Blackett failed, however, to prove his theory. His report entitled ―Negative Experiment‖, became widely acclaimed because of the full description of the method he used and the precise measurements he made.
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.
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 outer core.
The inner and outer core move with different speeds. It is this difference in speed that is the source of magnetism. This is known as the Dynamo Theory.
A dynamo, or generator, is a device for producing electricity by rotating an electrical conductor across magnetic lines of force. If electricity is to be produced continuously, there should be a continuous rotation of the conductor. In the earth, the energy us thought to come from convection currents set in motion by heat produced by radioactivity in the core.
The dynamo theory involves a chain of complex processes taking place in the earth’s core. 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.
1940’s, scientists became interested in paleomagnetism, or the history of the earth’s magnetism. 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 fossil magnets.
These are bits of magnetic materials (iron) in rocks which have preserved the magnetism of the earth at the time the rocks were formed. 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 strengths.
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 these poles. A third interpretation is that the magnetic poles are fixed only the upper layers of the earth are shifting about.
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.