Inclination, or magnetic dip, was a later discovery. Measurements began sporadically in England in 1576, and the idea gained prominence after the publication in 1581 of The Newe Attractive, a pamphlet written by the Elizabethan mariner Robert Norman. After having spent a couple of sharp-eyed decades at sea, Norman became a master instrument maker in London. His revolutionary work on magnetic dip stemmed from years of experimentation with the sea compasses he was making. He discovered that if you have a compass needle moving freely in a sphere and you point it at the horizon, it will be pulled either up or down by the Earth’s magnetic force, depending on where you are compared to the magnetic equator. In London, Norman measured the dip at an angle of 71 degrees, 50 minutes. At the equator it doesn’t dip at all, remaining horizontal. At what we call today’s North Pole, it will point straight down, and at the South Pole straight up.
The fact that things changed so much implied that the Earth’s magnetic force was a vast entity moving to its own inscrutable rhythms. Scientists of the nineteenth century were determined to crack the code. One goal was to measure regional variations in magnetic fields over time, to reconstruct the Earth’s magnetic life back through the centuries. That’s where Brunhes entered the picture.
The first of the three critical papers that caught Brunhes’s attention was by Macedonio Melloni, an Italian physicist who founded the Vesuvius Observatory just outside Naples in 1848, the same year revolutions were bubbling over in cities around Europe. That meteorological observatory was set up in part to monitor the activity of Mount Vesuvius, the volcano that had erupted in 79 CE, destroying the Roman settlements of Pompeii and Herculaneum, killing at least 1,000. Today Vesuvius is one of the few active volcanoes in Europe. Melloni selected the observatory’s site, designed its building, and chose the instruments. But months after it opened, Melloni was fired and only just avoided being banished from Naples. He was caught up in a mass ouster of academics following an insurrection against King Ferdinand II of Naples. A military letter recently uncovered in the Naples state archives disclosed that political leaders had deemed Melloni “bad” because he was friendly with some of Europe’s ultra-liberal thinkers and radicals. Among them were famous scientists, including the great British physicist Michael Faraday.
Melloni was no stranger to bitter scientific controversy. He had been drummed out of Paris’s scientific circles earlier in his career for findings that linked heat and light. By 1834, that same work captivated London and Melloni became one of the most celebrated physicists in Europe.
After being fired from the Vesuvius Observatory for being a radical liberal, he took himself to Portici, just outside Naples, and reignited an earlier interest in measuring the magnetism of volcanic rocks from Italy and Iceland. Melloni had developed a simple process reminiscent of the early Chinese compasses: a pair of needles, nine centimeters in length, magnetized and hung one above the other by silk thread. When he passed a piece of lava near the upper one, he could measure whether it attracted the needle away from the orientation of the lower needle, and by how much.
He found that all the lavas made the needle move. And he went on to make a bold contention: The lavas had captured the precise magnetic coordinates of the spot on the Earth where they were when they cooled. The idea was that a piece of lava laid down in Italy would have different magnetic coordinates from one in Bolivia. We could say that the electrons had developed a magnetic memory, aligning in a sort of magnetic fingerprint that helped identify the declination and inclination and intensity of where they were located.
Melloni went further. In his lab, he heated lava rocks until they were red hot, at which point they lost their original magnetic memory. When they cooled, they acquired a new one. The flaw in his research was that he didn’t systematically determine whether a batch of lava showed the same clear magnetic orientation across its flow. His results were fascinating but not conclusive. He died in a cholera epidemic in 1854.
By 1899, Giuseppe Folgheraiter had taken Melloni’s findings to another level. This was the second key paper Brunhes read. Based in Rome, Folgheraiter examined archeologically dated terracotta clay pots from ancient Greek, Roman, and Etruscan civilizations and found that they retained a strong magnetic orientation, even over many centuries. He surmised that they held the coordinates of the magnetic field from the time they were baked.
The third important paper was by Pierre Curie, the French physicist who went on to win the Nobel Prize with his wife, Marie Curie, and Henri Becquerel for their work on radioactive substances. Pierre Curie discovered in 1895 that any solid heated to a high enough temperature loses its magnetic properties. The temperature depends on the material and is in the hundreds of degrees Celsius. In a nutshell, the unpaired electrons become excited and confused by all that heat and refuse to line up in the same direction. In certain and relatively rare materials like terracotta and lava, when the atoms cool down again, their unpaired electrons line up once more in a field, taking on the coordinates of whatever field they are in at the time. The temperature that suspends their magnetism is known as the Curie point, and today is an undisputed rule of physics.
Brunhes, sitting in his Rabanesse tower a few years later, put all these pieces together. He was in the perfect place to do it. There he was, nestled in the remains of a string of ancient volcanoes in central France. Obviously, there had been hot lava. Some places also had natural terracotta laid down in a layered sedimentary bed. Terracotta contains iron-based molecules, and Folgheraiter had shown that it retained a magnetic signature. What Brunhes needed was an undisturbed seam of terracotta that had been heated up when lava poured over it, Kornprobst explained to me. It was hard to find. But there was some in Boisséjour, near Clermont-Ferrand. So he traveled there, likely by mule, where the Gravenoire volcano had erupted 60,000 years earlier, and collected a few samples.
One of the car salesmen came out to see what Kornprobst was doing. Kornprobst explained that he was retracing the steps of a famous French physicist who had hacked a small piece out of this hill a century before. The salesman shrugged and went back inside the dealership. In the end, the tiny cube of terracotta clay Brunhes cut out of the hill in Boisséjour didn’t tell him much. But, more determined than ever to find more magnetic evidence, he set about trying to find a much better volcanic site.
CHAPTER 6
the earth’s magnetic soul
Like so many who have studied magnetism over the ages, William Gilbert did it in his spare time. He was an accomplished medical doctor, and by the time he published his towering work, De Magnete (On the Magnet), in 1600, he was at the peak of his profession. That year, he was physician to the British monarch Elizabeth I, then nearing the end of her life.
Magnetic theory had come a long way from the thirteenth century, when Peregrinus had developed his idea that the magnet carried a replica of the heavens—with its north and south poles—within its very body. In Peregrinus’s understanding, those heavens were perfect and unchangeable and therefore the magnet was too. In the centuries after him, some mathematicians, troubled by the odd variability of compass readings, had begun to propose that the source of the planet’s magnetism was not celestial but terrestrial. It was a quantum leap in imagination. It implied that the Earth was not just a lumpen facsimile of something else, but might have its own unique qualities. Could this mysterious source be a magnetic mountain or a lodestone island? Could it lie in the Arctic?
There were good reasons for the renewed attention to magnetism in Gilbert’s day. For Peregrinus, parsing the magnet stemmed from intellectual and perhaps practical curiosity. By the time Gilbert began thinking about the magnet, it had become an urgent problem.
The Age of Sail had begun, and with it international trade, battles at sea, and the colonizing of continents far from Europe. It meant traversing the open ocean. No longer could seafarers navigate by keeping an eye out for the coastline, or by sounding the ocean’s bottom. It meant a far greater reli
ance on the magnetic compass. However, as sailors were discovering, the compass was fickle at sea. Not only did declination change from spot to spot, it also changed from year to year. Those trying to explain the discrepancies pointed fingers at shoddy instruments, slapdash steersmen, the heave of the waves during readings, the scent of garlic on a seaman’s breath when he passed the compass (a flogging offense because it was thought to interfere with a magnet), and even changes to the magnets within compasses themselves. But whatever the reasons, over the course of a lodesman’s career, charts could change enough to make a big difference. Lives, fortunes, and reputations depended on knowing where a ship was and where it would be. The primary way of doing that was describing one’s position by the geographic coordinates of latitude and longitude. But that was proving impossible.
The great conundrum was longitude. By contrast, latitude was easy. It was just a conceptual set of parallel lines running east and west around the body of the planet, never intersecting. The only one that cuts the globe in half is the equator, and it can be used therefore as the natural reference point. You could find your latitude pretty accurately at sea by using the sun and stars to guide you with instruments such as an astrolabe, quadrant, or cross-staff. And if you were careful, you could sail straight across the ocean along a single latitude, barring island barriers.
But all longitudinal lines run in great north-south circles around the globe, intersecting at each pole, each one dividing the globe into equal halves. Which was the prime meridian, the reference point? Lines of latitude are the same physical distance apart except at the poles, where the slightly flattened shape of the Earth makes them a little farther apart. So, in the main, each minute of latitude is 1 nautical mile. Sixty minutes, which is 1 degree, is 60 nautical miles or 68 statute miles or 110 kilometers. But lines of longitude are different distances apart, depending on where you are on the planet. On the equator, 1 minute of longitude is the same as 1 minute of latitude: 1 nautical mile. At the poles, where the lines of longitude converge, it is 0.
Because the Earth spins on an axis at the same rate every day, changes in longitude as you sail represent changes in both distance and time. The Earth is a sphere rotating 360 degrees per day. That means every hour it turns 15 degrees. Assuming you know the longitude of your home port and what time it is there, as well as what time it is where your ship currently is, you can tell how many degrees of longitude you have traveled each day. If you know your latitude too, you can also tell how many kilometers or miles you’ve gone. In those days, sailors had to have superb math and geometry skills. They also had to be able to read the stars.
The problem was that clocks of that era kept time by pendulum, and they could not do it accurately on a moving ship. It was an intractable problem for four centuries, bedeviling the finest minds in Europe. Therefore, navigators and scientists were trying to use the heavens to navigate more reliably. That meant reading the difference between the stars that pointed to the geographic poles and the magnetic pole the compass pointed to. To them, that meant finding the longitudinal prime meridian. They believed that someone traveling around the world at the same latitude taking declination measurements would find two spots on precisely opposite sides of the globe with a declination angle of 0. Halfway between the two points, always at the same latitude, the declination should be 90 degrees. In other words, if you knew the prime longitudinal meridian and could read declination properly, you should be able to figure out longitude too. At least, in theory. The principle assumes a regularity in the Earth’s magnetic field lines that was wildly wrong, as it turned out. The Earth’s field lines stretch and contort from pole to pole like elastic bands, not at all in straight lines.
Failing a reference point, they thought, if you had a complete survey showing declination all over the world, you should still be able to find longitude. All it took was a great deal of determined measuring all over the globe, plus math, laid over some pretty good maps. The race for a longitudinal formula was on.
At the time Gilbert began his magnetic research in the 1580s—just as William Shakespeare was launching his career as a playwright in London—measurements had been pouring in for years. The problem was, the more data there were, the more confusing the case became. It was a beautiful challenge for Gilbert. He was in full, choleric revolt against the teachings of Aristotle, the ancient Greek philosopher whose ideas then held powerful sway at the universities, including Cambridge University, where Gilbert had studied. Aristotle, who died in 322 BCE, taught that the Earth was the dull and unchanging center of a glorious heaven. Four uniform fundamental elements made up the Aristotelian planet Earth: air, fire, water, and earth. By contrast, the heavenly bodies, which revolved around the sun, were made of a superior substance, the fifth essence, or “quintessence.” Those bodies had souls or supernatural intellects, unlike the dreary Earth.
Gilbert had been reading Peregrinus and knew of his experiments with magnets. He embraced the bold idea that if you ran yet more complex experiments, you could make more observations about what was going on in the world. This was in direct opposition to the ruling doctrine that if you knew your Aristotle, you knew everything there was to know about the world. You didn’t need to look at the world around you, you just needed to read about it in ancient texts. Empiricism, at that time, was next to heresy.
And then there was the folklore. Assertions about the mysterious powers of magnets had expanded over the centuries. Gilbert decried the idea that a lodestone placed under the head of a sleeping woman would force her from her bed if she had committed adultery. Nor could a magnet make a woman like her husband better or induce one to become melancholic or smooth-tongued. It could not cure stab wounds. The blood of a male deer did not restore the strength of a weak magnet. Nightfall did not cancel a magnet’s powers. The only magnetic truths could be found in experimentation, Gilbert said.
He rolled up his sleeves and took to his laboratory. His great innovation was to develop little models of the Earth—he called them “terrellae”—that could reproduce the physical phenomena that he observed on the Earth itself. The models were magnetized spheres, likely made of magnetite, and had poles, equators, and even mountainous excrescences. The Oxford University historian of science Allan Chapman notes that today the idea of making a model and experimenting on it is the essence of much scientific practice. Back then, it was outrageous.
That was bad enough. But to make a model of the Earth and then to discover, as Gilbert did, that magnetism worked differently on different parts of it was to contradict Aristotle’s contention that the Earth was uniform and unchanging. Gilbert went much further. His careful experimentation convinced him that the Earth itself—“our common mother,” as he called it—is a great magnet. The origin of its magnetic force lay in the core of the planet, not in the heavens or on the Earth’s surface. Instead, he said the Earth-magnet created an invisible, permanent force that skittered across the planet, interrupted in places by irregularities such as the iron-rich continents. Not only did the Earth have a soul, like the heavenly bodies, but it had a magnetic soul at that, Gilbert averred. Not inert, it could attract; it could repel. It had power and perhaps even its own inexorable, unimagined strategy.
This was utterly novel. The source of a magnet’s vigor had moved from the heavens to the Earth to within the Earth itself. And while Peregrinus had written three centuries before about a magnet’s “natural instinct,” implying that it was a constant property, Gilbert was saying that the whole Earth itself carried this fundamental power, and that the power was inextricably bound to the Earth’s core. The implications were staggering. Gilbert was conscious of being a pioneer, a detective hot on the trail of the planet’s unexplored internal secrets “which, either through the ignorance of the ancients or the neglect of moderns, have remained unrecognized and overlooked.” He wrote that for the first time in scientific history he was penetrating the “innermost parts of the Earth.”
Gilbert’s s
ense of outrage at Aristotle’s casting of the Earth as inferior is palpable in his writing:
It is surely wonderful, why the globe of the earth alone with its emanations is condemned by him and his followers and cast into exile (as senseless and lifeless), and driven out of all the perfection of the excellent universe. It is treated as a small corpuscle in comparison with the whole, and in the numerous concourse of many thousands it is obscure, disregarded, and unhonoured. . . . Let this therefore be looked upon as a monstrosity in the Aristotelian universe, in which everything is perfect, vigorous, animated; whilst the earth alone, an unhappy portion, is paltry, imperfect, dead, inanimate, and decadent.
Gilbert shows all the signs of one in love with his subject. He seems to have viewed magnetism as a noble property, almost as if it gave purpose to the Earth, as he thought the soul did to the body. One chapter of his six-book tome, written in rather tortured Latin, is devoted to comparing electricity to magnetism. It smacks of the need to deal with electricity in order to dismiss it. Gilbert looked at amber and jet stones and the fact that if you rubbed them, they would attract straw or chaff. Today, we would call that type of energy static electricity. In fact, Gilbert gave electricity its name; the word is derived from the Greek electrum, which means “amber.” But Gilbert takes pains to explain that amber’s pull is transitory, unlike that of the steadfast magnet. “Magneticks” and “electricks” were not at all the same thing, he declared. “All magneticks run together with mutual forces; electricks only allure,” he scoffed. While Gilbert’s conclusions would be found wanting a few centuries later, it’s still instructive that this influential champion of experimental discovery would consider magnetism and electricity in the same breath, even if only to dismiss any similarity.
The Spinning Magnet Page 5
