In those early days I found the notion of polarity reversals— that the magnetic field could, and had, turned right upside down many times—quite incredible. Like a doubting Thomas, I had to put my fingers into the holes left in the rock once my samples had been drilled out, then go through the necessary magnetic measurements to be convinced.
There was now no turning back. I went to conferences and met the pioneers of the subject: Runcorn, Creer, Lowes, Tarling and Gubbins. I moved to a post-doctoral fellowship in Canada and met more: Irving, Cox, Dalrymple, McDougall, and eventually Roberts and Glatzmaier. I am proud to count them among my colleagues and friends, and it is my privilege to take you on a journey to meet them, and many of the earlier explorers of geomagnetism.
Have you ever wondered at the sheer uniqueness of Earth, the amazing coincidences of physics and chemistry that enabled life to flicker into existence here, to take hold and flourish on a tiny speck of dust, bound in orbit around a little ball of fire, floating through the vastness of the universe?
Earth is just the right distance from the sun to carry water as solid, liquid and vapor: glaciers, oceans and clouds. If it were closer to the sun and hotter, ice caps and glaciers would be unknown. If it were much further from the sun, it would be frigid and inhospitable. Our oxygen-rich, life-sustaining atmosphere is retained by a delicate balance of gravity and temperature: a moderate greenhouse effect ensures that most of us enjoy a cozy environment for most of the year, while a tiny amount of ozone high in the stratosphere protects us from hazardous ultraviolet rays beating in from the sun.
The atmosphere is, however, perilously thin. You could compare it to the thickness of paper covering a school globe. And interplanetary space is anything but empty—it streams with charged particles and radiation that are harmful to human health. The sun throws out protons and electrons in all directions at speeds of hundreds of kilometers per second, while even more energetic particles bombard the solar system from outer space. This solar wind and these cosmic rays would make Earth quite uninhabitable were it not for the fact that we sit in the middle of an enormous magnetic shield that arrests and diverts the onslaught way above our heads.
Most of the solar wind and cosmic ray particles are safely deflected by Earth’s magnetosphere and continue uninterrupted on their race into outer space. Only the most energetic penetrate the shield and these become trapped, forced into spiral paths around Earth’s magnetic field lines, bouncing back and forth from pole to pole in the so-called van Allen radiation belts. Occasionally an especially energetic burst gives rise, at high latitudes, to a shimmering show of lights—the aurora borealis or aurora australis.
The sun itself is strongly magnetic, and so are the four giant outer planets, Jupiter, Saturn, Neptune and Uranus, but amazingly Earth is the only one of the inner planets to have a strong magnetic field. Why should it be different from its neighbors, Venus, Mars and Mercury? Along with the rest of the solar system, all four are thought to have formed around the same time, 4.5 billion years ago, and all have similar interiors: a dense metallic core, a less dense mantle and a rigid, rocky crust. So why is only Earth magnetic?
For millennia, magnetism has commanded a magical sort of curiosity. The Greeks were mystified by the attractive properties of lodestone, magnetized rock. The first Chinese compasses were astrological instruments, used to divine the ways of the winds and waters. Even after the mariner’s compass had become an essential tool of navigation, magnetism retained its fascination and became a challenge to explorers and scientists alike. No wonder medieval scholars placed the source of the compass needle’s “virtue” in the heavens.
The history of geomagnetism is strewn with famous people and their colorful stories. It was the ever-practical William Gilbert who reasoned that Earth itself was magnetic. Between spotting comets and charting the stars, Edmond Halley was the first to plot Earth’s magnetic field accurately, while Karl Friedrich Gauss lent his mathematical genius to analyzing and understanding it. The crucial connection between electricity and magnetism was first discovered and then explored by several great nineteenth-century physicists, including Hans Ørsted, André Marie Ampère, Michael Faraday and James Clerk Maxwell. Together with a greater understanding of the Earth’s interior that came with the birth of geology and then geophysics, this eventually focused interest on planetary dynamos. Meanwhile, the geomagnetic field became much more amazing with the realization that it had flipped polarity, and not just once but many many times throughout geological history.
By 1950 the quest to understand Earth’s magnetism was focused on a hydromagnetic dynamo, but the mathematical calculations were intractable. Not until the advent of the supercomputer could the equations be finally solved, and scientists present the world with evidence of the gigantic powerhouse heaving in the core of our planet.
However, to begin at the beginning we must travel back two and a half millennia, to ancient Greece.
The Mystery of Magnetism
Just as there are in the heavens two points more noteworthy than all the others … so also in this stone … there are two points, one north and the other south.
—PETRUS PEREGRINUS, 1269
Old Magnes had come this way so many times before that his feet knew every one of the black rocks over which he was clambering. Day after day, year after year, for most of his life he had trudged up this hillside to tend his small flock of sheep. Never before, though, had his iron-studded boots stuck to the rocks the way they were doing now. The only way he could get his foot free was with a mighty kick—and then, with his next step, his boot would be sucked down again.
His staff also seemed to have taken on a life of its own. Each time he planted it on a rock to steady himself he had to tug hard to lift it again. What was going on? Last night there had been terrifing thunder and lightning, followed by torrential rain—thank the gods he had come down to his shelter and not spent the night on the mountain—but the ground was now dry again and everything looked normal.
This legend of the Greek shepherd Magnes is thought to date back to around 900 BC, but it was recorded almost a millennium later by the Roman scholar and writer Pliny the Elder. Pliny was fascinated by the world around him, and before being killed by poisonous gases from Mount Vesuvius during the AD 79 eruption that destroyed Pompeii, he spent much of his life recording his observations of nature in a multi-volume encyclopedia, Naturalis Historia. Pliny’s story of Magnes, although no doubt embellished through centuries of retelling, provides two important clues to understanding Earth’s magnetic properties: an electrical storm took place and rocks became magnetized.
Magnes was apparently climbing on Mount Ida—the same Mount Ida from which Zeus is said to have watched the sacking of Troy—in the northwest of what is now Turkey. This is not far from the region the ancient Greeks called Magnesia after their homeland in mainland Greece. Still today, Magnesia is well known for its deposits of lodestone, a rock that is rich in magnetite, an oxide of iron. Normally a lump of magnetite-bearing rock is unremarkable. However, if the rock is struck by lightning it becomes strongly magnetized. A bolt of lightning may pass an electric current of up to a million amps into the ground—not for long, but long enough for rocks within a short distance to become magnetized intensely and stably.
For centuries magnetism was thought to be unique to lodestone. What was it about lodestone, and only lodestone, the ancients wondered, that gave it this magical property? The earliest ideas on the nature and origin of magnetism are usually attributed to a Greek philosopher, Thales (c. 624–546 BC), who lived in Miletus, a busy trading city not far from Mount Ida. Together with his well-known contemporary Pythagoras, Thales is credited with having laid the foundations of not just philosophy but also physics and mathematics. None of his original writings seem to have survived, but Aristotle reported:
Thales … held soul to be a motive force … he said that the magnet has a soul because it moves the iron.
Thales of Miletus. A Greek philosopher who lived
around the sixth century BC, Thales was puzzled by the way lodestones could attract each other and pieces of iron across empty space, and decided that, like humans, they must have souls.
The Greeks recognized that lodestones did not attract only other lodestones: they also attracted pieces of metallic iron. And they had observed that a piece of iron in contact with a magnet became magnetized itself, and so was able to attract another piece of iron— a process now known as induction.
Further, a lodestone did not need to be in physical contact with another lodestone or a piece of iron in order to attract it. This “action-at-a-distance” effect, where a force acted across empty space in which no intermediary medium existed, seemed impossible to explain in material terms, so Thales reasoned that an animistic explanation was called for. Living bodies moved, and instilled motion in other material objects. Living bodies had souls. Therefore, in order to move a piece of iron the magnet, too, must possess a soul.
Thales was also familiar with another action-at-a-distance effect, namely that when a piece of amber was rubbed with fur it could attract scraps of chaff and other light particles. (This is the same “electrostatic” effect that makes our hair crackle and stand on end after brushing it on a dry day.) However, whereas rubbed amber attracted scraps of all kinds of materials, lodestone attracted only other lodestones or iron.
These action-at-a-distance effects—which, as well as magnetic and electrostatic forces, also include gravity—would challenge not just Thales. Down the ages, scientists, philosophers, teachers and students would struggle to understand them, and create many and varied explanations.
Later Greek philosophers opted for an “atomistic” view of matter. This bore little resemblance to modern atomic theory, other than the idea that matter was made up of innumerable tiny particles. In the fifth century BC, Diogenes of Apollonia maintained that a lodestone or magnet “fed” on atoms of iron. Another school of thought believed that a magnet emitted particles, and that these particles cleared the space between it and a piece of iron, thus drawing them together.
This last idea led, over ensuing centuries, to a whole host of “effluvia” theories involving invisible emissions from magnetic materials, and finally, in the nineteenth century, to the notion of the magnetic field. At this early stage, though, few theories addressed, let alone answered, the obvious question—why was magnetism confined to lodestone and iron?
Early Greek science was essentially limited to the observation of natural phenomena and endless philosophizing as to their causes. Without the modern elements of prediction, experimentation and testing, alternative theories such as animist, atomist and effluvia could not be evaluated against each other in any substantial way, and so little progress was made.
At the same time as the Greeks were holding sway in the Mediterranean, an advanced civilization was thriving in China. While science there was also inextricably mixed up with mysticism, divination and religion, technology reached a degree of sophistication that would be unparalleled in the West until the Renaissance of the fifteenth and sixteenth centuries.
The earliest recorded compass, a Chinese divining instrument, probably dated from the first century AD, although it could have been in existence as much as 300 years earlier. From early Chinese writings studied and described by the English historian of science Joseph Needham, we know that this compass was used to determine the directions favored by the winds and waters, and so was a guide to laying out villages, building houses, plowing fields, orienting tombs and much more—the ancient art of feng shui.
The instrument consisted of a spoon-shaped piece of lodestone, known in China as tzhu-shih or “loving stone,” which represented the star constellation of Ursa Major, the Great Bear. This was delicately balanced on a circular “heaven” plate made of bronze or wood, which was itself placed on top of a square “Earth” plate. Both the heaven plate and the Earth plate were intricately engraved with astronomical symbols and directions. The “spoon” took on a natural magnetization along its length so that, when balanced, its handle came to rest pointing to the south. Interestingly, the early Chinese routinely chose south as the prime cardinal direction.
A reconstruction of the earliest Chinese magnetic compass, which was used to determine the favored directions of feng shui. A south-pointing lodestone spoon is balanced on a bronze Earth-plate, which is engraved with Chinese astrological symbols. Kaifeng, Henan Province, China.
The Chinese seem never to have questioned the nature of the force that aligned their compass. To them, as to the Greeks, such things lay in the lap of the gods. There is, however, documentary evidence that they recognized discrepancies between the compass’s south and true south. Between about AD 720 and 1086, Chinese compasses appear to have deviated by up to 15° east of true south, while all later records show the deviation to have been to the west of true south. Indirect evidence of this deviation is to be found in the streets of many ancient Chinese towns and cities, including Beijing and Nanking. A plan of the southern part of the township of Shandan in Gansu province on the Old Silk Road shows two distinct street orientations. The older is due north–south, but the younger deviates by eleven degrees, trending from 11° west of south to 11° east of north. Presumably the streets were aligned to the favorable directions of the winds and waters as determined by the spoon-shaped compass, and between the two periods of building the compass had shifted to the west by eleven degrees.
These early Chinese were not great seafarers or travelers—had they been, the compass would almost certainly have become a navigational tool much earlier. As it is, the earliest reference to a mariner’s compass comes from the beginning of the twelfth century. By then the Chinese had perfected techniques for magnetizing a fine iron needle by stroking it with a piece of lodestone and balancing it on a finely made pivot, floating it on water, or suspending it from a fine silk thread in order to minimize the effect of friction and improve its overall performance. Beautiful floating fish and turtle-shaped pivoted compasses originate from this period.
A plan of part of the Chinese town of Shandan, showing streets aligned in two different orientations, one due north–south, the other trending from 11 degrees west of south to 11 degrees east of north. The difference is thought to have come about because of a change in orientation of the magnetic compass between the two periods in which the town was built.
Following the so-called Dark Ages, the compass eventually surfaced in Europe in the writings of an Englishman, Alexander Neckam. Born in St. Albans in Hertfordshire in 1157 on the same night as Richard I, Neckam had grown up with the future king as a foster brother. He went on to teach arts at the newly emerging University of Paris and later returned to St. Albans School before becoming a canon and abbot of the Augustinian abbey at Cirencester. Neckam’s interests were far-ranging, from theology to natural philosophy, but he is remembered mainly for his two books, De Nominibus Utensilium (On Instruments), published around 1180, and De Naturis Rerum (On the Natures of Things), around 1200.
Each contained an article on nautical navigation. In the first, Neckam explains the use of a magnetic compass needle for navigation at sea, while in the second he extols the advantages of a pivoted needle:
The sailors moreover, as they sail over the sea, when in cloudy weather they can no longer profit by the light of the sun, or when the world is wrapped up in the darkness of the shades of night, and they are ignorant as to what point of the compass their ship’s course is directed, they touch the magnet with a needle. This then whirls round in a circle until, when its motion ceases, its point looks direct to the north.
How the compass had reached Europe is something of a mystery. Neckam probably came across it first in Paris, but the tenor of his writing suggests that by the end of the twelfth century it was already in common use by mariners. This is at odds with the suggestion that it was Marco Polo who brought it back to Europe from China. Polo did not visit China until 1275 and returned to Venice in 1295, a whole century after Neckam’s descriptions.
Another more persuasive theory is that the compass arrived in Europe courtesy of Arab traders. The presence of ancient Chinese objects through the Persian Gulf and Red Sea regions and along the east coast of Africa indicates that, from the eighth century onwards, there was busy trading between the Arab inhabitants and the Chinese. It is hard to imagine that the compass would not have eventually become an item of trade, and found its way north and west to Europe. The timing—one hundred years from the compass’s invention in China to its appearance in Europe—is plausible. However, to complicate the theory, the earliest Arabic references to the compass also seem to post-date Neckam’s. It is not until the mid thirteenth century that Arab documents and stone tablets mention sailors finding their way by means of floating compasses fashioned from fish-shaped pieces of iron rubbed with a magnet.
Perhaps the likeliest explanation is that the European compass was developed independently. This is supported by the difference in prime direction: to this day Chinese compasses are made with the prime end of the needle pointing south, while European compasses have always pointed to the north.
North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism Page 2
