North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism

by Gillian Turner Phd

  Maxwell’s equations predicted something else too, a wave motion consisting of coordinated oscillations of magnetic and electric fields, which travel through empty space at a speed of 300 million meters a second: Faraday’s “ray vibrations.” Maxwell readily acknowledged Faraday’s prior insight:

  The electromagnetic theory of light, as proposed by [Faraday], is the same in substance as that which I have begun to develop in this paper, except that in 1846 there were no data to calculate the velocity of propagation.

  According to Maxwell’s analysis, the speed of his electromagnetic waves depended on just two parameters, electrical permeability and magnetic permittivity, which respectively expressed the electrical and magnetic properties of free space. In the twenty years since Faraday’s “Thoughts on Ray Vibrations,” these had been measured and so, unlike Faraday, Maxwell was able to show that his predicted electromagnetic waves traveled at a speed indistinguishable from the best available measurements of the speed of light.

  Not twenty years after the publication of Maxwell’s equations, a German physicist, Heinrich Hertz, would succeed in generating electromagnetic waves from oscillating electric currents in an antenna, confirming Maxwell’s prediction.

  Unfortunately, Maxwell was no longer alive to see this far-reaching endorsement of his work. In 1865, at only thirty-four, he had “retired” to his estate in the south of Scotland and embarked on writing Treatise on Electricity and Magnetism. During this period he had declined several offers of appointments, but in 1871 he accepted an invitation from Cambridge University to become the inaugural Cavendish professor of experimental physics; his first job was to design and supervise the building of the Cavendish Laboratory. After this he had set himself the task of sorting through and editing the mountains of unpublished paperwork left by the reclusive Henry Cavendish sixty years earlier. He discovered to his amazement that Cavendish had not only built a sensitive electrostatic torsion balance, but had also discovered the inverse square law of electrostatic force—fourteen years before Coulomb.

  “Cavendish,” he reported, “cared more for investigation than publication … it did not excite in him the desire to communicate the discovery to others.” Today Cavendish is best known for his experimental determination of Newton’s universal gravitational constant, work that did see the light of day during his lifetime, and that led to the first estimate of the mass of the Earth and its density.

  Meanwhile, Maxwell’s own research career was all but over. In 1879 he became ill and soon afterwards, like his mother, he died from cancer at the age of forty-eight.

  The Third Element

  The instructions of the Royal Society, and the instrumental means prepared under its direction, provided for the examination, in every branch of detail, of each of the three elements which, taken in combination, represent, not partially but completely, the whole of the magnetic affections experienced at the surface of the globe …

  —EDWARD SABINE, 1857

  While the great nineteenth-century physicists had been laying the experimental and theoretical foundations of electromagnetism, other scientists continued to be fascinated by the intricacies of Earth’s own magnetism. By the time Maxwell published his famous equations, knowledge of the variations and variability of Earth’s magnetic field had increased beyond recognition, and ingenious new mathematical techniques had been invented to describe it in all its complexity.

  By the end of the eighteenth century, mariners were routinely measuring and recording the angles of magnetic declination and inclination at locations throughout their voyages. Sometime during the 1790s the first attempts were made also to measure the third element of Earth’s magnetic field: its strength, or intensity. Coulomb had shown that the strength of the force exerted by one magnet on another could be measured by timing the oscillations of a compass needle. Using a similar idea, mariners now began to time the oscillations of the dip needle with which they already measured the field’s inclination.

  The earliest measurements of the intensity of Earth’s magnetic field may have been made on an ill-fated French expedition led by a naval captain called Jean-François de Galaup—or, as he styled himself, Comte de La Pérouse. In 1785 La Pérouse set sail from France in command of the aptly named frigates l’Astrolabe (the astrolabe) and la Boussole (the compass) on an ambitious scientific expedition to explore Alaska, Japan, Russia and the southern seas. A great admirer of Britain’s Captain James Cook, who had taken scientists and artists on his expeditions, La Pérouse had with him ten scientists and illustrators. There had been keen competition to gain a place on the crew. A sixteen-year-old second lieutenant in the French military, Napoleon Bonaparte, had applied but been turned away. Had his application succeeded, the history of Europe may have been very different.

  The La Pérouse expedition survived three years on the high seas, with members making many scientific observations, measurements and notes, before it arrived in Australia’s Botany Bay in January 1788. After a six-week stay it sailed, planning to be back in France by June 1789. Alas, neither men nor ships were to be seen again.

  In 1791 the French government sent two more ships, la Recherche and l’Espérance, to look for them. The ships were under the command of Joseph-Antoine Bruni d’Entrecasteaux, a noted explorer. He too took a formidable team of scientists with him. Although he failed to find any trace of La Pérouse’s expedition, he managed to carry out a great deal of exploration and make many scientific observations around Australia, particularly at Van Diemen’s Land, today’s Tasmania.

  Drawing of the magnetic dip needle used on the 1791 Pacific expedition of French explorer Bruni d’Entrecasteaux, during which the first recorded measurements of magnetic intensity were made. D’Entrecasteaux died of scurvy, and the expedition was disbanded. When the ships’ records were eventually returned to Paris, the magnetic measurements were published. This illustration appeared in the 1808 report of the expedition.

  After leaving Australia, this expedition also ran into problems. The first were political. While journeying through the Pacific islands, the men picked up news of the ongoing revolution in France. While most officers on board were royalists, the crew was dominated by supporters of the revolution and trouble was never far from erupting. Then there was another crisis: the men were hit by scurvy, and d’Entrecasteaux himself died near the coast of New Guinea. Soon afterwards, in Java, the expedition was disbanded and the officers handed over the ships to the Dutch authorities there to prevent their falling into the hands of French revolutionaries.

  Eventually Elisabeth Paul Edouard de Rossel, one of the most senior officers to survive, made it back to Paris. Here he was reunited with the ships’ records, which included the earliest known measurements of geomagnetic intensity. In 1808 de Rossel published these measurements in Voyage de D’Entrecasteaux, Envoyé à la Recherche de La Pérouse, his two-volume transcription of d’Entrecasteaux’s journals. The information was enlightening. It was clear that the higher the latitude, the faster the dip needle swung and the shorter its period of oscillation (that is, the time for a complete swing, one way and back to center, then the other way and back to center). In particular, the records showed periods of 1.869 seconds and 1.850 seconds in Tasmania in 1792 and 1793, while at locations near the equator the periods of oscillation were 2.404 and 2.429 seconds. Earth’s magnetism was tugging more strongly on the dip needle in Tasmania, making it oscillate faster there than at lower latitudes. It was clear that the strength of Earth’s magnetic field increased with latitude.

  Unfortunately for de Rossel, because of various delays and crises he had been beaten to the publishers by an energetic Prussian explorer and geographer. Born in Berlin in 1769, Alexander von Humboldt had become interested in geology and mineralogy while studying at the University of Göttingen. This interest had taken him to the School of Mines in Saxony, and then to a government job supervising mining. One day, while surveying an outcrop of serpentine rock in the Fichtelgebirge, a mountainous region of Bavaria that
had recently been acquired by Prussia, he noticed his compass behaving strangely. Each time he approached the rock the needle swung in a completely different direction: there seemed to be no sensible pattern to the directions it took up at different locations.

  Von Humboldt had discovered with his compass what Magnus had found with his boots and staff several millennia earlier: the phenomenon of intensely magnetized rocks. In a moment of inspiration he guessed that these rocks, exposed on a mountain top, may have been struck by lightning, and that this might be the origin of their magnetization. Coming twenty years before Ørsted’s discovery of the magnetic effect of an electric current, this was a remarkable insight.

  The incident seems to have sparked in von Humboldt an enduring interest in Earth’s magnetism. An inveterate traveler of independent means, in 1799 he set off from La Coruña in Spain on a scientific expedition to Central and South America in the company of Aimé Bonpland, a French botanist. (He had originally planned an expedition to Egypt followed by a circumnavigation of the world under the French flag, but Napoleon had put an end to that plan.)

  During his five years in the Americas, Von Humboldt collected a multitude of data of all kinds, much of which he would eventually describe in his multi-volume treatise, Kosmos. He was one of the first people to speculate that South America and Africa may once have been joined together, and that the Atlantic Ocean was still in the process of widening. However, he considered the magnetic intensity measurements he made with his dip needle among his expedition’s most important work.

  With the equipment he had available, von Humboldt had been unable to measure the absolute value of the magnetic intensity; instead he had compared intensities from place to place. It was known that the period of oscillation of the dip needle depended inversely on the square root of the intensity. Put more simply, if the period of oscillation of the dip needle at site A was twice that at site B, the intensity of the magnetic field at A was one-quarter that at B. However, without knowing the actual intensity at B it was possible only to make comparisons of this sort.

  Von Humboldt decided to set as a standard the intensity at a location near the equator, where the period of the dip needle was at its maximum value and intensity was, therefore, at its minimum. He chose the town of Micuipampa in northern Peru and defined the intensity there as exactly one unit. From his American expedition, together with his journeys around Europe, he accumulated about 120 measurements of both inclination and intensity (relative to that at Micuipampa) from locations ranging in latitude from 10° S to 52° N, and covering a 105-degree interval of longitude.

  On his return to Paris in 1804, von Humboldt enlisted the help of the French physicist Jean-Baptiste Biot to publish his observations of geomagnetic intensity. Biot (the same man who, with Félix Savart, would later make fundamental discoveries about the magnetic effect of a current-carrying wire) was interested in Earth’s magnetic field. With a chemist friend, Joseph Louis Gay-Lussac, he had made a pioneering ascent in a hot-air balloon to see whether magnetic intensity varied with altitude. The two men had hoped to solve once and for all the question of whether the magnetic field originated inside the Earth or outside it. If the origin were internal, the intensity should decrease as they moved further above the Earth’s surface. If it were external, the intensity should increase. Unfortunately their dip needle had iced up, making measurements of its period of oscillation unreliable, and so their experiment had been in vain.

  Von Humboldt and Biot now produced a chart that was divided into five “isodynamic zones,” four in the northern hemisphere and one in the south. Each zone included a different range of geomagnetic intensity values, and the chart clearly showed a general increase in intensity from the equator towards the poles.

  The first charts to show isodynamic contours, rather than just zones, would be published two decades later, in 1825 and 1827, by Christopher Hansteen, a physicist at Norway’s University of Christiania (now Oslo). It had now also become customary to time the oscillations of a horizontally balanced or suspended compass needle rather than a dip needle, and hence measure the intensity of the horizontal component of the field. As Coulomb had found, by suspending the needle from a low torsion fiber—often silk—it was possible to circumvent the unavoidable friction of a mechanical pivot. This allowed the needle to swing for longer and gave a more accurate and precise result.

  A good needle would swing for ten minutes or more before coming to rest. This allowed an observer to time several hundred oscillations. For his 1825 chart of horizontal intensity, Hansteen recorded the times for 300 oscillations of his horizontally suspended magnet at various locations. They included 753 seconds in Paris, 780 seconds in Oxford, 820 seconds in Edinburgh and 850 seconds in Bergen. The lengthening of the period of oscillation with increasing latitude indicated that, contrary to the total intensity, the horizontal component of the magnetic field decreased from equator to pole.

  This was exactly what Hansteen had expected. He had recognized that the horizontal component of Earth’s magnetic field at any location depended not only on the total intensity, but also on its inclination to the horizontal. At the equator the inclination was zero and the total field was horizontal, so the horizontal component was equal to the total field. At higher latitudes the inclination increased and the field steepened, until at the poles the total field was vertical and there was no horizontal component at all. He therefore knew from earlier measurements of total field intensity and inclination that the horizontal component should decrease from equator to pole. Conversely, having measured the horizontal component and the inclination, he could work backwards to calculate the total intensity. All things considered, this proved to be a more accurate way of obtaining total intensity than the old dip needle method.

  In his 1827 chart Hansteen plotted the total geomagnetic intensity, still using von Humboldt’s practice of referring the values to that at Micuipampa. As expected, the intensity increased steadily from equator to pole.

  Hansteen was also intrigued by the morphology of Earth’s magnetic field, its structure and form, and the origin of its secular variation. His interest had in fact begun when, as a student of Ørsted, he had seen some of Johannes Wilcke’s charts of the southern polar regions. These were based on the declination measurements of James Cook and Tobias Furneaux, who had commanded the Resolution and Adventure respectively, during Cook’s second voyage of discovery to Australia, New Zealand and the southern seas in 1772.

  Hansteen had noticed that the polar magnetic field seemed to separate into two centers: a major one south of Van Diemen’s Land and a smaller one south of Tierra del Fuego in South America. This observation of what appeared to be two south magnetic poles came to dominate Hansteen’s view of terrestrial magnetism. In 1819, just a year before Ørsted’s groundbreaking discovery and the advent of electromagnetism, he had published Untersuchungen über den Magnetismus der Erde (Investigations of the Earth’s Magnetism) and with it won a competition set up by the Royal Danish Academy of Sciences in 1811, which posed the question: “In order to explain the magnetic phenomena of the Earth, is one magnetic axis sufficient or must we assume more?”

  One of the earliest charts of geomagnetic intensity over the surface of the Earth. In line with the convention set by Alexander von Humboldt in the early 1800s, intensities were calculated relative to a value of 1.0 at Micuipampa in northern Peru, and increased (darker shading) by a factor of about two between the equator and the poles. This chart was compiled and published by Edward Sabine, a scientific adviser to the British Admiralty, in 1836.

  This question, of course, went back more than a hundred years to Edmond Halley, who had suggested there were two pairs of magnetic poles: one pair in the Earth’s rigid outer shell and the other fixed in an inner sphere. To explain secular variation, he had supposed that the inner sphere rotated more slowly, lagging further and further behind the shell, so that its poles appeared to drift slowly westward.

  Halley’s idea had received little attenti
on at the time but now, over a century later, Hansteen had become an ardent advocate. First he compiled a 148-page collection of all the declination and inclination observations he could muster, many times more than had been available to Halley. He then examined them all for accuracy and consistency, and plotted charts of inclination for 1600, 1700 and 1780, and of declination for eight different dates between 1600 and 1800.

  Hansteen argued that there was evidence for two “points of convergence,” or focal centers, in the northern polar region, similar to those he had seen in Wilcke’s south polar charts. At northern latitudes, the compass needle pointed towards two distinctly different focal centers: in North America it pointed towards a focus in Hudson’s Bay, and in northern Europe to a focus in Siberia. He supposed that at other locations compasses were affected by both these focal centers, and so pointed towards neither one nor the other but in some intermediate direction.

  Hansteen deduced that his four focal centers must correspond to Halley’s four poles: the American and European north poles and the Asian and American south poles. Using observations made in the polar regions, mainly between 1769 and 1774, he carefully estimated their positions: 70°17′ N, 259°58′ E; 85°47′ N, 101°29′ E; 69°27′ S, 136°15′ E; and 77°17′ S, 236°43′ E.