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Ancient Knowledge:
The ancient Greeks are the first people to be associated with knowledge about magnets. They knew about lodestones which are rare natural magnets that attract iron. The term ‘magnetism’ has, however, come from Magnesia city in Asia Minor (the ‘Turkey’ of today) where such stones were found.
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The Chinese (c. AD 1000, according to some) were the first to note that a lodestone placed on a boat floating in a bowl of water made the boat rotate to the south though the boat was not pulled bodily in the south direction. Shon Kua (AD 1030- 93) wrote about fortune-tellers who rubbed the point of a needle with a magnet to make it indicate the southern direction. The Chinese knew steel needles could be permanently magnetised. The earliest reference in Europe to magnetising iron can be traced back to AD 1200.
The Development of the Compass:
It is believed that the discovery of magnetising iron travelled from China to Europe. The sea voyages of navigators and explorers like Da Gama, Columbus and Ferdinand Magellan would not have been possible without the use of the magnetic compass. The compass was also used in small folding personal sundials that showed time when aligned northwards. By 1187, magnetic needles were mounted on pivots so that they could rotate freely towards any horizontal direction.
This was the traditional compass made using a flat steel needle balanced on a pivot. It was magnetised by striking it with a lodestone. It was, however, noted by George Hartmann in 1544 that the compass’ north-pointed end slanted down and, in order to keep the balance, its tip had to be cut off or a counterweight had to be attached to it.
The Strange feature was explained by British Robert Norman (The Newe Attractive) in the sixteenth century. He stated that north of the equator; the force on the needle was not horizontal but slanted downwards at an angle (‘dip’ or ‘inclination’). He measured the dip angle using a compass needle (‘dip circle’) which rotated freely in the north-south plane. It pointed downwards at the dip angle.
It was also discovered that even the horizontal part of the force was not northwards exactly but showed a few degrees of variance from true north (an angle now termed ‘declination’). Compass- makers had to rotate their dials to make up for the variation observed.
William Gilbert (c. 1544-1603), president of the Royal College of Physicians in Britain, discovered or confirmed to be true many findings about magnetism. He studied different types of attraction of materials, such as amber.
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The attractive force of these materials when rubbed with other substances was referred to as the ‘electrick force’. Gilbert studied its differences from magnetic attraction. Gilbert also stated, for the first time, that the Earth itself was a giant magnet in his De Magnete which is today a benchmark on the boundary between medieval scholarship and modern science of geomagnetism.
In 1634, Henry Gellibrand (1597-1636) showed that Gilbert’s prediction of ‘perpetual immutability’ was false. That is, the magnetic declination did change from place to plan systematically. He raised a crucial question on the Earth’s magnetism: he wondered how its magnetism could vary if it was permanently magnetised? The solution-was offered by Edward Halley: the Earth’s interior, consisted of concentric spherical shells and each was magnetised differently and some rotated differently from others.
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The search, however, was still on for discovering the best manner of making magnetic needles, suspending them, ensuring that they were in the true magnetic meridian and accounting for their regular diurnal variations. In 1777, a French military engineer, Charles Augustin Coulomb, prepared the instrument ‘torsion balance’ which served as model for magnetic instruments for some 200 years. His instrument allowed the measurement of more than the magnetic diurnal variation. He showed that the magnetic attraction and repulsion between magnetic poles varied inversely with the square of the distance.
Hans Christian Oersted (1777-1851) was the first to give clear-cut evidence connecting electricity and magnetism. Through his experiments that involved a magnetic compass, an electric battery and a thin metal wire, he showed that whenever the wire was connected to the battery, and a current flowed, the magnetic needle moved; when the current flow stopped, the needle came back to its original place. Andre-Marie Ampere (1777- 1836) solved the puzzle by stating that the basic ingredient of magnetism was the electric current.
An electric current circulating around a wire loop. A current flowing in a coil of a thousand turns created magnetism a thousand times stronger as the magnetic forces of all its loops added up; two coils with the same axis repelled or attracted each other on the basis of whether their flows were opposed or parallel. According to him, the magnetism of iron could be because iron atoms had small circulating current which reinforced each other.
After Ampere, a number of scientists further unravelled the mystery of magnetism and geomagnetic features. In the first half of the nineteenth century, Karl Friedrich Gauss (1777- 1855) deeply studied magnetism and found ‘Gottingen Magnetic Union’ and developed spherical harmonic analysis of the scalar magnetic potential (1836-39).
Sabine, in 1852, found that magnetic storms follow the sunspot cycle and in 1859, Richard Carrington observed white-light solar flares followed by a large magnetic storm. Examining ancient lava flows, Bernard Brunhes (1867-1910) found that their magnetisation was reversed, a feature seen as evidence of actual reversals by Motenori Matuyama (1884-1958) of Japan. But in the early 1950s, Jan Hospers who studied Icelandic basalts concluded that rocks with reversed magnetism were not self-reversed but stood as relics of epochs when our planet had reversed magnetic polarity.
Continental Drift/Plate Tectonics:
In 1918, Alfred Wegener published The Origin of the Continents and Oceans where he put forward the theory of continental drift. That continents were slabs of lighter rock floating on top of denser material below was already known; Wegener proposed that the continental slabs could move slowly.
Then came the theory of polar wandering promoted by Keith Runcorn (1922-1995) which stated that the Earth’s crust slowly slid around the interior so that a particular area would have had a different climate a long time ago. But a lot of controversies came up such as in interpreting magnetic data. Observations from different continents but from the same period did not agree as to the positions of ancient magnetic poles.
The convincing evidence to put to rest most controversies emerged from magnetic surveys. By the end of the 1950s, electromagnetic magnetometers were used by geophysicists. The instruments helped in observing magnetic anomalies that showed no consistent patterns on land but well-ordered patterns above the floor of the ocean. There were ‘stripes’ of alternating magnetisation parallel to the mid-oceanic ridges. This is evidence for continuous formation of new rock at the ridges.
With more rock forming, the other rock is pushed farther away from the ridge, in the process producing the symmetrical stripes. The materials in the stripes on one side mirror those of the stripes on the other side of a ridge.
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And there is alternation of one stripe of normal polarity and another of reversed polarity. Geologists have found that rocks found in different parts of the planet with the same ages have the same magnetic characteristics. Rocks in Alaska have magnetic materials aligned in such a way that they must once have been at or near the equator.
The orientation of magnetic materials on the east coast of South America shows similarity to those on the west coast of Africa. Explaining this phenomenon, Lawrence Morlay in 1962 as well as Fred Vine and Drummond Matthews, both independently, stated that the ocean floor spread out from the mid-ocean ridges at about an inch every year. The magma that oozed out hardened and acquired the prevailing magnetization.
It was then gradually carried away to either side of the ridge. As the main dipole revised its direction, so did the magnetic imprint. Each of the magnetic stripes consisted of magma cooled during a particular epoch having a certain magnetic polarity. So the Earth’s magnetic field was faithfully recorded over time.
The phenomenon of the new sea floor generation that came to be known as seafloor spreading has thus been supported by (i) at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest; (ii) the youngest rocks at the ridge crest always have present-day (normal) polarity; and (iii) stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth’s magnetic field has reversed many times.
The hypothesis represented a major advance in the development of the plate-tectonics theory.
If the earth’s crust was expanding along the oceanic ridges, however, it must be shrinking elsewhere. Harry Hess suggested that new oceanic crust continuously spreads away from the ridges. Millions of years later, the oceanic crust descends into the oceanic trenches.
The ocean basins are thus being continually ‘recycled’. Hess and, later, Robert Dietz pointed out that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate. All this finally led to the confirmation of the plate-tectonics theory.
Developments in the Latter Half of the 20th Century:
The era of spaceflight began with Sputnik 1 and 2 in the 1950s. This meant that experiments in the laboratories and observations in general could be corroborated by space flight discoveries. Importantly, satellites and spacecraft’s were sent into space that made amazing discoveries about the Earth’s magnetism and that of the other planets and aspects related to them. Listed below are some of the astounding findings and voyages undertaken into space for that purpose.
i. Explorers 1 and 3 discovered the inner radiation belt in 1958. In the same year, Eugene Parker predicted the presence of solar wind.
ii. Tom Gold coined the word ‘magnetosphere’ and Drake proposed that Jupiter had a radiation belt. Two years later, ideas on magnetic reconnection and plasma convection in the magnetosphere were proposed.
iii. Explorer 12 crossed the mapetosphere in 1963 and Mariner 2 mapped solar wind. In the same year, IMP 1 was launched which produced the first mapping of the magneto tail of the Earth. A year later, scientists analysed morphology of magnetic substorms.
iv. In the 1970s, lunar satellites from Apollo 15 and 16 surveyed the lunar magnetic field. OGO 7 observed the nature of coronal holes (1972). The various planets had their magnetospheres observed: Pioneer 10 was the first to cross Jupiter’s magnetosphere (December 1973), Mariner 10 observed Mercury’s magnetic field (1974) and Pioneer 11 first passed Saturn’s magnetosphere (1979).
v. The magnetospheres of Uranus and Neptune were passed by Voyager 2 in 1986 and 1989 respectively. In 1997, the Mars Global Surveyor observed Mars’ crustal magnetisation.
vi. The Oersted satellite was launched to map the Earth’s main field in 1999.