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

by Gillian Turner Phd

  William Gilbert’s versorium, a kind of compass consisting of a light needle balanced on a pivot. It was sensitive to nearby objects that had become electrically charged through being rubbed—for example with fur.

  Gilbert had designed a sort of electric compass he called a versorium. It consisted of a light metal needle finely balanced on a pivot, and it was much more sensitive to the weak forces of electric attraction than were scraps of paper or chaff. With it, Gilbert found many more materials that displayed the amber effect. He compiled an impressive list of gemstones, including diamonds, sapphires, opals, amethysts and jets, as well as false gems made of glass or crystal, and used the term “electric” to describe any material that, after being rubbed, attracted the versorium’s needle. Materials that did not do so he called “non-electric.”

  What caused this effect? Gilbert believed that an “electric” object, when rubbed, emitted an invisible cloud. This cloud, he supposed, filled the space around the electric material, giving it the ability to attract other objects. He called it an “effluvium” after the Latin effluere, meaning to flow out.

  Strangely, he had shunned the idea of effluvia to explain magnetic forces, mainly on the grounds that a lodestone was able to attract a piece of iron even when solid objects were in the way. In other words, magnetic force could penetrate matter, whereas an effluvium would presumably be stopped short by it.

  Despite his insistence on experimental verification in almost every other situation, Gilbert’s explanation of magnetism remained esoteric. While avoiding the question of exactly how the magnetic force was transmitted through space, he maintained that magnetism was intrinsic to the substance of a lodestone, or a terrella. He had used this to argue that the source of Earth’s magnetism lay within the planet. As he put it:

  The rays of magnetic force are dispersed in a circle in all directions; and the center of this sphere is not in the pole, but in the center of the stone and of the terrella.

  Despite his familiarity with both attractive and repulsive forces in magnetism, Gilbert seems to have missed the phenomenon of electric repulsion. Once again this discovery was left to his successor, the Italian Jesuit Niccolò Cabeo. In the course of his many experiments, Cabeo noticed that a rubbed amber rod would first attract scraps of paper or the needle of a versorium, but as soon as the paper or the versorium needle had touched the amber, they would fly away—repelled in a similar way to two magnetic poles of the same kind. Gilbert’s effluvium theory, which accounted only for electric attraction, was in deep trouble.

  During the next century almost no progress would be made towards understanding electricity and electrical phenomena. In 1720 Willem Jacob’s Gravesande, in an early survey of physics, Physices Elementa Mathematica, Experimentis Confirmata (Mathematical Elements of Natural Philosophy, Confirmed by Experiments), was still describing electricity as:

  that property of bodies by which … (when rubbed) they attract and repel lighter bodies at a sensible distance.

  Then in 1731, out of the blue, came a chance discovery that would propel electricity to the forefront of physics. Stephen Gray, the man responsible, came from Canterbury in England. Born in 1666, he had trained as a cloth-dyer but had developed an interest in astronomy, becoming a friend and colleague of John Flamsteed, the first Astronomer Royal, and working for a time as an astronomer. However, by the time he began experimenting with electricity he was destitute and living on a charitable pension at the London Charterhouse, a home for penniless gentlemen, retired soldiers and former servants of the king or queen.

  Gray’s experiments seem to have been designed more by trial and error than any logical reasoning, but nonetheless he discovered that he could transfer the electrical ability to attract chaff and scraps of paper from one end of a long string or cable to the other, provided the string was made of one of Gilbert’s “non-electric” materials—a metal wire, for example—and was suspended or supported only by “electric” materials. Gray had discovered electrical conduction.

  Gray’s experiments eventually outgrew his rooms at the Charterhouse and he moved them into the paddocks of a mansion house in Kent, where he finally succeeded in conducting electricity through a 245-meter-long line. Gray had no real idea just what was being conducted from one end of his line to the other, and so he continued to experiment in a haphazard manner, testing the electrical conductivity of all sorts of materials from water to live chickens and human beings.

  By the mid eighteenth century, electricity was in danger of becoming more of a fairground sideshow than a scientific endeavor. An event planned as a serious public lecture would often become oversubscribed by an audience attracted by the promise of a charged atmosphere, sparkling displays and maybe a shock or two. However, the spin-off of such frivolity was publicity, and with publicity came growing interest and, in time, financial support for further research. Electricity and magnetism were now set to move to center stage, not only in science but also in industrial and social development.

  Stephen Gray’s demonstration of the conduction of electric charge through a human body—that of a Charterhouse “charity boy.” The boy was suspended from silk threads, and his legs charged by friction. The picture shows scraps of chaff that have been attracted to his chest, his left hand, and to a conducting ball held in his right hand.

  Gray’s discovery of conduction spawned a new generation of theories of electricity. Like contemporaneous ideas about heat, these entailed hypothetical fluids that were supposedly transferred between materials on rubbing—at this time the only known means of electrifying an object.

  In France, Charles-François Du Fay, a multi-talented scientist and the superintendent of Le Jardin du Roi (the Royal Botanic Gardens), and his former student and colleague the Abbé Jean-Antoine Nollet concocted a theory involving two such fluids. Normally an object or material was supposed to contain equal amounts of each fluid, evenly mixed. When rubbed with fur, an amber rod, they said, gained an excess of the “resinous” fluid, while a glass rod when rubbed with silk acquired an excess of the other, “vitreous” fluid. Two bodies carrying excesses of the same type of fluid repelled one another, while ones carrying excesses of different fluids would attract. Add to this the ability of the fluids to conduct through or between non-electric materials, and all the known electrical phenomena of the day could be explained.

  At much the same time, and probably quite independently, a young American named Benjamin Franklin was beginning to take an interest in electricity. Franklin’s later life of political activism, which would lead to his involvement in the Revolutionary War and drafting of the Declaration of Independence, would take him away from science, but not before he had left a lasting mark: the nomenclature of “positive” and “negative” charges, and the direction of “conventional” current trace their origins to Franklin and his theory of electricity.

  Franklin’s “one-fluid” concept was part-way between a fluid theory and a theory of electric particles. His fluid, he reported, consisted of “extremely subtle” electrical particles which repelled one another, but which were attracted to equally subtle particles of normal matter. When a glass rod was rubbed with silk, it became “charged” with an excess, or “positive,” amount of fluid. On the other hand, when an amber rod was rubbed with fur, fluid was transferred out of the rod, leaving it with a deficit, or “negative” charge. Franklin’s positive charge therefore corresponded to an excess of Du Fay’s vitreous fluid, while his negative charge corresponded to an excess of the resinous fluid.

  Franklin’s concept would prove long-lasting, but his terminology of “positive” and “negative” would have an unfortunate impact on the way electrical currents are described to this day. Imagine bringing a normal (uncharged or neutral) conductor into contact with a positively charged glass rod—one carrying an excess of Franklin’s fluid. Some of the excess will flow from the rod on to the conductor—that is, from positive to normal (or neutral). If the same conductor is touched to a negatively charged amber r
od, which has a deficit of Franklin’s fluid, fluid will flow from the conductor on to the rod in an attempt to remedy the deficit. This time, fluid flows from normal (uncharged or neutral) to negative.

  Physicists have chosen to define the direction of “conventional” current as the direction in which Franklin’s conceptual fluid would have moved, namely from positive to neutral, neutral to negative. This is to the perpetual consternation of today’s students, who accept that electric current is, in fact, the flow of negatively charged electrons, which must, therefore, be moving in the opposite direction.

  Research into both electricity and magnetism had now reached a stage where further progress could be achieved only by making quantitative measurements. In his Principia, published in 1687, Isaac Newton had put the science of mechanics on a firm mathematical footing, such that if some properties of a system were known, others could be calculated or predicted.

  For example, Newton had defined the notion of force in a way that meant it could be used to calculate its effect on the motion of a body. A force acting on a body would give the body an acceleration that depended directly on the strength of the force: the bigger the force, the bigger the acceleration. Using this, Newton had shown that the elliptical orbits of the planets around the sun—by then thoroughly investigated by Kepler and Galileo, and almost universally accepted—could be accounted for only if there were a force of attraction between the sun and each planet that decreased as the distance between them increased. To be exact, the force was inversely proportional to the square of the separation. He had argued that this previously unrecognized force of nature— the universal force of gravity—acted between each and every particle or object in the universe according to their masses.

  Newton’s mathematical equation for the gravitational force contained one other factor—the universal gravitational constant, G. Had Newton been able to directly measure the force between two objects, he could have determined this constant. However, the size of the gravitational force between two everyday masses is so tiny compared with the force that the Earth exerts on each—their weight—that the task was impossible with the equipment Newton had available to him. However, he did note that once the constant was known it would be possible to estimate the mass and density of the Earth from the weight of any object. (Another century would pass before this was finally achieved by another Cambridge physicist, Henry Cavendish.)

  At Halley’s instigation, Newton had also begun some experiments on the force between magnetic poles and had seemed to expect a result similar to the force of gravity. However, the task was complicated by the impossibility of obtaining single magnetic poles. The so-called “broken magnet paradox” was well known in Newton’s day: if you break a magnet in two, rather than producing separate north and south poles you get two new magnets, each with north and south poles of their own. To this day, scientists have failed to isolate, or even find evidence of the existence of, magnetic “monopoles.” Probably frustrated by this, Newton did not pursue the matter of magnetic forces, but moved on to other studies.

  Several people now began to wonder if electric forces and magnetic forces could lend themselves to mathematical description. One such person was a Frenchman, Charles-Augustin de Coulomb. Coulomb had been born in 1736 in Angoulême. From an early age he had been fascinated by mathematics and astronomy, and by the age of twenty-one had already presented several papers to the Société Royale des Sciences in Montpellier. Although his mother had planned a career in medicine for him, the meticulous and analytical Coulomb had rebelled and trained as a military engineer. His scientific accomplishments would be sandwiched into a professional life as an engineer and architect of numerous military buildings throughout France and its offshore islands.

  Coulomb had initially been attracted to study magnetism by a competition advertised by the Paris Académie des Sciences in 1773 and again in 1775. Inspired by George Graham’s observations of the daily fluctuations of the compass needle, the Académie sought to find “the best manner of constructing magnetic needles, of suspending them, of making sure they are in the true magnetic meridian, and finally of accounting for their regular diurnal variations.”

  Coulomb’s interest in measuring the mechanical strength of materials, particularly those used in building, had already led him to construct and perfect an extremely sensitive “torsion balance.” The principle of the instrument was quite simple. Originally, Coulomb had been interested in how a thread or wire resisted being twisted, so he had suspended a rod of some sort horizontally on the end of a long thread or wire, all inside a closed glass container. When a controlled force was applied to one end of the rod in a horizontal direction at right angles to the length of the rod, the whole system twisted, until the torsional resistance of the suspending wire counteracted and balanced the applied force. At this point, the angle through which the rod had rotated could be read from a scale mounted around the wall of the container. This gave a measure of the torsional strength of the suspending thread: a torsionally strong thread would rotate through only a small angle, while a weaker thread would twist through a much bigger angle.

  A sketch of Coulomb’s torsion balance, which he used to investigate the force between the poles of two magnets. A magnetized needle was suspended horizontally from a silk thread, and a long magnet was inserted vertically through a hole near the edge of the lid, so that its lower pole approached and repelled one end of the needle. The angle through which the needle twisted depended on the force between the pole of the magnet and the needle.

  Coulomb realized he was now faced with a slightly different situation: the competition required an instrument that would respond to extremely small variations in the force that Earth’s magnetism exerted on a compass needle. If he were to replace the rod of his torsion balance with a long magnetized needle and the wire with a single “silk thread drawn from a cocoon,” he would have just that instrument.

  Coulomb’s new suspended compass won him a share of the Académie’s prize, but it did much more. It became the basis of a generation of instruments that would be the mainstay of magnetic observatories, surveys and laboratories right up until the advent of electronics and computers.

  For five months before submitting his entry, Coulomb took a series of measurements several times a day. In general, he found that the declination increased during the morning, reaching a maximum around one o’clock in the afternoon. It then decreased more slowly until early evening, and remained quite steady overnight. Coulomb agreed with others—including Hiorter and Celsius—that since this regular activity was concentrated during the daytime, its cause was likely to be associated with the sun.

  Some scientists had suggested that, since a magnet lost its magnetization when heated, the heat of the sun was gradually demagnetizing the Earth. Coulomb disagreed, arguing that if this were the case Earth’s magnetism would long since have vanished altogether. Already interested in the effects magnets had on each other, he speculated that the sun was also one of Gilbert’s great astronomical magnets, and “acts on the terrestrial globe as a magnet acts on another magnet.” He also observed, like Graham and Celsius, that, from time to time, the daily variations became large and erratic, and noted that such disturbances were usually followed by spectacular night-time auroras.

  Coulomb’s compulsively analytical streak meant he was not satisfied just to record the daily variations of geomagnetic declination. Interesting as the tiny wanderings of the compass needle were, he was also fascinated in the fundamental nature of electricity and magnetism. Coulomb now set himself the task of finding a simple mathematical description of magnetic and electrostatic forces.

  First, he needed to modify his apparatus to measure the extremely weak forces between electrically charged bodies, and between the poles of magnets. Details of his modifications, experimental methods and results, published in Mémoires de l’Académie Royale des Sciences in 1785, showed that Coulomb was eventually able to measure forces as small as one ten-thousandth of a grain o
f barley. (The grain, based on the mass of a grain of barley, was the basic unit of mass in the British imperial system, and equivalent to about one-fifteenth of today’s gram.)

  For his electrostatic experiments, Coulomb made the rod of his torsion balance of a light insulating material, with a pith ball at one end and a counterweight at the other. The pith ball could be charged, and the force between it and a second charged ball then measured over a range of different separations. A screw knob at the top of the suspension controlled the separation of the two balls.

  In his first series of experiments, Coulomb showed that whether the two pith balls carried unlike charges and attracted each other, or carried like charges and so repelled each other, the force between them decreased as they got further apart. If their separation were doubled, the force decreased to a quarter of what it had been. If the separation were trebled, the force was reduced to one-ninth of the original. In mathematical terms, the measured forces seemed to depend on the inverse square of the separation, just like Newton’s force of gravity. And just as Newton’s gravitational force depended on mass, Coulomb found that as he increased the amount of charge—or, in his words, the “electric mass”—on his pith balls, the electrostatic force between them increased proportionately.

  When it came to magnetism, Coulomb quickly recognized the problem of identifying the exact positions of the poles of a magnet, and the difficulty he would have isolating the effect of one pole while eliminating the effect of the other. He noted however: