The Spinning Magnet

by Alanna Mitchell

  Finally, Karine Sellegri, director of research of the National Center for Scientific Research’s Laboratory of Physical Meteorology and an expert in cloud chemistry, among other things, came to meet us. She was the team leader.

  “Kornprobst!” my companion declared, bowing slightly. He explained that we were trying to help the world understand the significance of Brunhes’s bold finding that the magnetic field had reversed. Sellegri was apologetic. One small conference room was named after him at the lab, but nobody was told why. His name was not mentioned in courses at the university in Clermont-Ferrand. However, there had been a small lecture on reversals, she recalled, brightening up a little, and she, of course, knew of Brunhes’s experiments.

  “They were not experiments!” Kornprobst barked. “They were observations!”

  Later, over espresso and a biscuit as we were waiting for the train to take us back down the volcano, Kornprobst was reflective. In retirement, he was scientific adviser to Vulcania, known as the European park of volcanism—an amusement park set up to help the public learn about volcanoes and other inner-Earth mysteries. The park was another sign of just how forcefully volcanoes had shaped the psyche of the region, partly spurred by Kornprobst’s own fiery determination as head of the observatory to make Clermont-Ferrand an international center of excellence in the discipline. Just a few kilometers from the Dôme, Vulcania’s tourist season was launching that evening to great fanfare, and Kornprobst was to be a guest of honor.

  As he sipped his espresso, he lovingly tracked the science of the discovery of magnetism back through time, fluidly quoting not only the main figures—Peregrinus and Gilbert, among others—but also landmark papers that had advanced the field. Brunhes was a fulcrum. There was the science of magnetism before him and there was the science of magnetism after, and they were not the same. The next day, we would take the most important journey of all: a trip to try to find the very same seam of rock Brunhes took his fabled samples from, the ones that changed the history of science.

  CHAPTER 8

  the greatest scientific undertaking the world had ever seen

  On June 12, 1634, just as the sun was entering the critical phase of its summer solstice, Henry Gellibrand transported two foot-long magnetized needles and two quadrants to the garden of Mr. John Welles in Deptford, just west of London, England. He used the instruments to take five sets of measurements in the morning—the sun marking true north, the needle pointing to magnetic north—as the sun marched up in the sky. He took six sets of measurements in the afternoon as the sun marched back down the sky. Then, Gellibrand, a young mathematician who held the coveted position of professor of astronomy at Gresham College in London, did a few sums.

  The results transformed the understanding of magnetism. Even Gellibrand, who has been described as a “plodding, industrious mathematician without a spark of genius,” could see that right away. The angle of declination in (or at least, near) John Welles’s garden in Deptford had shifted by more than 7 degrees in just fifty-four years. That meant declination changed over time even if you stood on the same spot of the Earth and used the same instruments.

  It ran against everything the navigators and academics of the day thought they knew. It was one thing to understand that declination changed at sea, where there were so many variables, including the motion of the waves, cloud cover that masked the celestial guideposts, and dodgy geographical coordinates. It was a different kettle of fish to know that right there, in a garden near London, declination was changing. And not only changing but changing fast. Subsequent measurements have shown that the angle of declination in London, then one of the best-measured cities in the world, was on a fast canter, shifting from 11 degrees east in the late sixteenth century all the way to 24 degrees west by 1820.

  The incredulity the discovery spawned is almost impossible to fathom, catapulting magnetism into one of the greatest scientific puzzles of the day, right after gravity. In our era, it would be like waking up in the morning to find that time had begun to run backward: A staple of the universe that you had taken for granted no longer worked the same way.

  For one thing, Gellibrand’s finding meant that Gilbert’s idea that the Earth’s magnetic field was like a simple permanent magnet could not be correct. The field was in motion. For another, it meant that every single measurement of declination studiously recorded over the years was useless unless the date had also been recorded. It meant that a magnetic coordinate contained within it not only declination and inclination but also the dimension of time. It meant that you couldn’t take a measurement and be satisfied that it would be the same years later.

  There had been hints before Gellibrand. He was building on the work of his predecessor as professor of astronomy at Gresham College, Edmund Gunter, the brilliant mathematician who invented the slide rule. Gunter had taken what must have seemed to him to be routine measurements of declination, likely in Deptford, in 1622, aiming to verify those taken forty-two years earlier in the same place by William Borough, a seaman and enthusiastic butcher of pirates who became comptroller of the Queen’s Navy. Gunter was astonished to realize that there was a discrepancy of more than 5 degrees in the sets of measurements. He put aside the unthinkable: that the magnetic “soul” of the Earth was not fixed. But he carefully recorded his measurements. Gellibrand took up the post when Gunter died in 1626. Eventually, Gellibrand took note of the discrepancy and carted Gunter’s own needle to John Welles’s garden to repeat the measurements in 1634. He published his findings the following year, naming the phenomenon “secular variation,” after the Latin saeculum, or age. If we were to name it today, we would call it long-term variation.

  In that single academic publication, the discipline of magnetism had undergone a revolution. From the localized, inconstant property of lodestone that the ancient Greeks knew to the perpetual “natural instinct” of Peregrinus’s magnet to the permanent global phenomenon Gilbert described, magnetism was now akin to an invisible, capricious, living force within the planet. It meant magnetism must always be on the move, whether you measured it from second to second or across millions of years. What secret power could be buried inside the Earth to make this happen? Where would it lead?

  The logical implication to the scientific and seafaring minds of the day was that if magnetic readings changed, there must be a way to calculate how they would change. In other words, there must be a rationale for the changes, and therefore a mathematical formula that would at last allow navigators to use declination to read longitude. This hypothesis meant taking more measurements and trying to figure out what the formula was. The impulse to solve this problem in an orderly fashion was overwhelming.

  This imperative set the stage for the fabled exploits of Edmond Halley and for the direct intervention of the British monarchy in the growing urgency to solve the problems of navigation. A physicist who became Britain’s second Astronomer Royal, Halley is most famous for the comet that bears his name. He correctly calculated the comet’s path through the heavens and predicted that it would reappear in 1758. But being fascinated with the celestial realm at that time meant also being fascinated with navigation and with magnetism. So, in the final years of the seventeenth century, Halley, the son of a wealthy soap maker, took to the seas on the first ocean-bound expeditions undertaken solely for the sake of science, at the pleasure of the Crown.

  Despite a dreadful, truncated first voyage on his ship the Paramore, a flat-bottomed little pink commissioned specially for him by royal decree, Halley came triumphantly back to London after the second voyage with a list of declination measurements—he didn’t measure inclination—stretching across the Atlantic Ocean and down past the tips of both Africa and South America. He had ventured farther into Antarctic waters than anyone before him, calling it the “Icey Sea.”

  All London awaited his findings. But how to express what he had found in any way that could make sense to the men who steered ships?

 
; Halley had a brain wave. Instead of a table of numbers, he would make a map. He plotted his measurements of declination on the map, and where the numbers showed the same angle he drew curved lines to join them, just as he had plotted the comet’s elliptical orbit. He ended up with a document that once more radically changed the way people thought about magnetism. And not only how they thought about it, but also how they saw it.

  “A New and Correct CHART Shewing the Variations of the COMPASS in the WESTERN & SOUTHERN OCEANS as Observed in ye Year 1700 by his Ma.ties Command by Edm. Halley,” shows swooping new lines laid overtop the familiar grid of longitude and latitude. Some lines indicated that the compass was to be adjusted eastward and others westward, depending on where you were at sea. It was the first published visual image of the Earth’s magnetic lines, evidence of what we now know to be a force field of magnetic energy pulsing from the core, spouting from one pole to the other in elastic-band lines, and enveloping the planet. Somehow, the magnetic force now had to be thought of as a volatile, evolving, and constantly moving power that touched everything on the planet.

  Seamen needed a little explanation. Up in the top left corner overtop “Canada New-France” and directly underneath “Hudson Bay,” Halley wrote: “The Curve Lines which are drawn over the Seas in the Chart, do shew at one View all the places where the Variation of the Compass is the same; The Numbers to them, shew how many degrees the Needle declines either Eastwards or Westwards from the true North; and the Double Line passing near Bermudas and the Cape de Virde Isles is that where the Needle stands true, without Variation.” Halley had apparently found Ground Zero of declination. It didn’t run along any known longitudinal line, as theorists had lovingly imagined, but instead bisected the Atlantic Ocean between the western bulge of Africa and the eastern flank of South America before veering off underneath Bermuda and right to the shore above Florida.

  Today, those same types of cartographical markings are known as contour lines on terrestrial maps, where they indicate different topographical heights. For example, at the base of a mountain, a loop might indicate 100 meters above sea level. Farther up, another loop might indicate 200 meters, and so on to the top. On meteorological maps, these curved lines are called isobars or isotherms, depending on whether they show barometric pressure or temperature. Then, they were called Halleyan lines, and they were an instant hit. Halley later expanded them to parts of the Pacific Ocean with other mariners’ data and updated things when he got new information. They were published in some form until the nineteenth century.

  Alas, despite Halley’s claim to the contrary, his chart was incorrect as soon as he published it. The Earth’s magnetic field had already shifted. And the chart was all but useless for finding longitude, except when sailing where Halley’s curved lines ran parallel to a coast, and only until the field moved on once more.

  As he explored the changeability of the magnetic field, Halley showed that some of its components appeared to be moving to the west, a phenomenon known as “westward drift.” To explain it, he developed an intriguing model of the Earth’s interior, suggesting a liquid core surrounded by empty space—which he thought might have been home to unknown creatures—contained within an outer shell. He proposed that the inner core had its own pair of poles, in addition to the shell’s two, for a planetary total of four. The inner pair rotated more slowly than the outer pair. In simplistic terms, the pairs of poles were fighting each other for dominance, pulling this way and that on the magnetic lines.

  Although wrong in several fundamental ways, this model represented an attempt to uncover a comprehensive explanation for the variation in the Earth’s magnetic impulse, a step that took Gellibrand’s finding that magnetic readings changed over time dramatically further: This was the “why.” Halley’s new planetary model was also a prophetic attempt to place the cause for the variation within the Earth’s liquid core. It was not universally accepted. But suddenly, the quest was on for a hypothesis to describe the magnetism of the whole planet, and what caused it in the first place.

  Halley, who died in 1742, made an uncannily accurate prediction: “It will require some Hundreds of years to establish a complete doctrine of the Magnetical system,” he wrote.

  When he died, a big piece of the geomagnetical puzzle remained unsolved. There was declination, which varied tremendously over time, particularly over the Atlantic Ocean and less over the Pacific. There was inclination or dip, the pulling down or pushing up of the magnetic force on a magnetized needle, compared to the horizon. It changed over time too, but seemingly not as much. Both measurements gave information about the direction of the magnetic force. But what about its strength? In the language of physicists, it was like having knowledge of one component of the vector but not both. It was akin to knowing that a vehicle was heading northwest but not knowing whether it was going 10 kilometers an hour or 100 or 1,000.

  Some explorers of exotic latitudes had already used their dip instruments to figure out that the push and pull of the compass needle was stronger the closer you got to the poles. You could tell by applying a mathematical formula to the period of the oscillation of the dip needle as it was pushed or pulled and then returned to where it had begun. But that just measured intensity compared to the strength of the magnet you were using, not actual intensity. The German naturalist and geologist Alexander von Humboldt decided to establish a standard for comparison anyway. Just as the nineteenth century was dawning, he went on an extended scientific trip to Central and South America. As he roamed, collecting unknown creatures to take back to Europe, he also took readings of magnetic intensity. The weakest field he found was in the town of Micuipampa in northern Peru and on that basis established the value of the global field in that town as one unit of intensity, as Gillian Turner explains. That meant future measurements of intensity could use the Peruvian unit as a reference point. It was a start, and could provide a snapshot of the relative strength of the field for at least a few points on Earth. Von Humboldt began to dream of a worldwide network of magnetic observation stations that would measure declination, inclination, and intensity relative to Peru.

  In 1828, he met the German mathematician Carl Friedrich Gauss. Gauss, the son of illiterate parents, learned numbers before he learned speech, it was said. A famous story tells of the precocious three-year-old correcting his father’s payroll sums and supplying him with the right answers. Today he is known as the prince of mathematics. Gauss figured out an elegant formula for calculating absolute magnetic intensity and published it in 1832. He also devised an instrument, the first magnetometer. It involved one magnet pulling at right angles to another, allowing him to calculate the strength of the magnet measuring the dip, and thus of the Earth’s force. It swept the scientific world, becoming the standard for magnetic observatories. Today, we have accurate measurements of the intensity of the Earth’s field going back to 1840. Suddenly, the ability to measure the whole magnetic vector was complete. Not only that, but by 1838, Gauss had proved mathematically that the main part of the Earth’s magnetic field was generated within the Earth itself. Finally, Gilbert’s bold experimentation from more than two hundred years earlier could be shown to be correct.

  In the meantime, von Humboldt pressed on with his determination to establish a global network of magnetic observatories. He was in search of the big picture, and to do it systematically he needed observatories across the hemispheres using standardized instruments and collecting information at the same moment. He enlisted Gauss and many others around the world in the effort—including Tsar Nicholas I of Russia. In 1834 the Göttinger Magnetische Verein (the Magnetic Union of Göttingen) was born, named for the city in Germany where Gauss was based. It was the beginning of what came to be known as the magnetic crusade—the first global scientific collaboration and the predecessor to CERN, the European organization for nuclear research that is examining the characteristics of the tiniest parts of atoms.

  Longitude still figured int
o the urgency. Technically, the problem of longitude had been solved by 1759 when the Yorkshire-born clockmaker John Harrison finished his masterpiece: a handheld watch that could keep time at sea. It was known as Harrison’s H4, because it was the fourth and most perfect of his mariner’s clocks, the fruit of three decades of work for Harrison and his son.

  The longitude problem was so consuming throughout the eighteenth and nineteenth centuries that the British Parliament set up a Commission of Longitude and passed longitude acts, offering lavish rewards to whoever solved the problem for mariners. The act of 1714, for example, offered as much as £20,000, which is more than US $4 million in today’s funds. Finding longitude was imperative for the “Safety and Quickness of Voyages, the Preservation of Ships and the Lives of Men,” the “Trade of Great Britain,” and, not least, “the Honour of [the] Kingdom,” the act’s authors said. It caused a stampede of theories, most of which were bunk.

  But Harrison engineered a clock that could keep time at sea, and time, as any sailor knew, was also distance, which was also longitude, because the Earth rotates 360 degrees in twenty-four hours. While Harrison’s fourth clock, remorselessly tested against celestial longitude readings in overseas voyages first to Jamaica in 1761–62 and then to Barbados in 1764, could keep time beautifully, it was not easily replicable. Instruments of the day were rare and expensive. Noodle through any museum in the world that contains compasses or quadrants or sextants or other navigational instruments of that period and feast on intricately engraved, lovingly burnished, precisely made works of art. This was not the era of the cheap knockoff.

  The Longitude Commission, ultimately known as the more self-important Board of Longitude, wanted a solution that every steersman could lay his hands on. Harrison eventually won the reward, after years of controversy. But lingering questions remained about whether reading the moon or the sun or the stars to find longitude could prove cheaper and more widely accessible. In fact, there was a vibrant and powerful camp within the European scientific community still arguing that the true solution to longitude lay in the celestial sphere. Even at that time the Astronomer Royal—a job Halley once had—was considered the British Empire’s prime expert on longitude. The holder of that position ruled over the Greenwich Observatory, founded in 1675 by Charles II for the express purpose of gathering astronomical data in order to find longitude at sea. Astronomy and longitude were inextricably linked. And that meant paying attention to the Earth’s magnetic force.