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

by Gillian Turner Phd

  Like Coulomb, Faraday used a very long rod-shaped magnet so that its poles were far apart. He fixed the magnet at its lower end in such a way that it could tilt in any direction as its upper pole responded to the magnetic force around a current-carrying wire. A conducting circuit was created by carrying out the experiment in a cup of mercury, as shown on the left of the illustration opposite. Faraday found that the upper pole did indeed revolve in a circle around the end of the wire. He had built the first electric motor.

  Imagining there should be reciprocal magnetic interactions between a current-carrying wire and a permanent magnet, he now investigated the opposite effect. If a current-carrying wire exerted a force on the pole of a magnet, then a permanent magnet should also exert a force on a current-carrying wire. In the right-hand cup of his apparatus he fixed the upper pole of a magnet, while leaving the end of a current-carrying wire free to move. As he had suspected, the wire moved in a circle around the pole of the magnet.

  A further curious fact was that the force on the wire was not in the direction of the current, nor in the direction a free magnetic pole would be expected to move if placed at the location of the wire. Instead it was at right angles to both. This apparently dawned on Faraday on Christmas Day, 1821. Was his mind wandering during the Christmas sermon, or after enjoying Christmas dinner prepared by his new wife in the rooms they shared above the Royal Institution?

  News of Faraday’s discoveries spread, but his achievements were not received warmly in all quarters. His background still counted against him, and more eminent scientists envied his remarkable physical intuition. Over the next few years he would be accused of copying the work of others, and his nomination to become a Fellow of the Royal Society would be opposed by a number of influential scientists—including, surprisingly, Humphry Davy himself. However, he weathered the experience, was finally exonerated, and in 1823 was awarded a fellowship. On Humphry Davy’s retirement two years later Faraday succeeded to the directorship of the Royal Institution. His career was now set to soar.

  Faraday was convinced there was further reciprocity to be found in electromagnetism. Since an electric current was now known to produce a measurable magnetic effect, he reasoned it should be possible to produce an electric current magnetically—by using either a permanent magnet, or the magnetic effect of one current-carrying circuit on another, initially current-free, circuit.

  He seems to have been temporarily sidetracked from this task by his collaboration with another young physicist, Charles Wheatstone, on vibrations related to sound waves. However, the diversion would prove to be serendipitous. The two men had observed that a metal plate set vibrating would induce vibrations in a similar nearby plate without actual physical contact. (We now know that the effect, resonance, is due to sound waves traveling through the air between the plates.) This led Faraday to hypothesize that similar invisible vibrations might explain the action-at-a-distance nature of the magnetic force, and be the means through which the electromagnetic induction he was seeking might take place.

  If an electric current was, as he thought, a sort of wave motion involving tension and strains between charged particles in a conducting material, the space around a current-carrying wire, or a magnet, must be filled with magnetic “lines of force.” Although these lines were invisible and impossible to detect, he believed they were real structures that exerted forces on magnetic objects. Each line followed the direction of the force that it exerted on a free north pole, while the density of the lines indicated the strength of the force. He was no doubt influenced by the familiar pattern traced out by iron filings or iron sand sprinkled around a magnet.

  Faraday was not the only scientist struggling to achieve the electromagnetic induction of a current: the French had been given a head start. Like them and many others, he began by placing magnets close to circuits of conducting wires in which he had incorporated the most sensitive current meters or galvanometers. Again and again, though, this approach proved fruitless. Finally the penny dropped: an electric current was a stream of electric charge in motion. In his earlier experiment in 1821 it had been this moving charge that had exerted force on the free magnetic pole and made it revolve.

  Faraday now deduced that to create a force on the charges in a conductor, and so induce a current in a conducting circuit, he needed to move a magnet in the vicinity of the circuit, rather than simply place one there and hope. Alternatively, and in accord with his resonance experiments with Wheatstone, if he were to change the pattern of magnetic lines of force in the vicinity of a circuit, this should have the same effect.

  In August 1831 he finally struck gold. He later described his apparatus and experiment:

  Two hundred and three feet of copper wire in one length were coiled around a wooden block; another 203 feet of similar wire were interposed as a spiral between the turns of the first coil, and the metallic contact everywhere prevented by twine. One of these coils was connected with a galvanometer and the other with a battery.

  Only at the instants of connecting or disconnecting the battery in the first circuit did Faraday notice a jerk (first one way and then the other) in the needle of the galvanometer in the other circuit; when a steady current flowed in the first coil, the galvanometer stubbornly recorded no current at all in the second. In other words, only during the brief periods when the current in the first coil was increasing and its magnetic lines of force growing, or the current was decreasing and the lines of force decaying away again, was a current induced in the second coil. For a current to be induced, the pattern of the lines of force around, and more importantly threading through, the second coil had to be changing.

  The apparatus through which Michael Faraday discovered the secret of electromagnetic induction. Two coils of wire are wound on the same wooden ring, but insulated from one another by twine. Top: Sketch from Faraday’s notebook. Bottom: Photograph of the actual coils, courtesy of the Royal Institution of Great Britain.

  Having discovered the secret of electromagnetic induction, Faraday now went on to experiment with permanent magnets. He found that moving a magnet into or out of a coil of conducting wire induced pulses of current in the coil. He also found that inserting a core of soft iron inside the two coils of his original experiment greatly enhanced the linkage between the lines of force, and so the induced current. He now had the elements of a basic transformer and all the ingredients of his famous law of electromagnetic induction.

  Today, Faraday’s law of electromagnetic induction is usually rendered in its mathematical form, something like this:

  ε = -dØ/dt

  Astonishingly, though, Faraday did not write as much as a single equation in his laboratory notebooks. His genius was in conceptualizing physical phenomena and explaining them in words. He left the mathematics to his successors.

  In a presentation to the Royal Society in November 1831, he also described the first working dynamo. This was designed to produce an electric current by rotating a copper disc in a magnetic field. In Faraday’s original version, the disc had been rotated between the poles of a magnet, and a current had been produced between brush-contacts on the spindle and the edge of the disc. Good electrical contact had been ensured through the use of liquid mercury. He had subsequently found he could even produce a current using Earth’s magnetic field instead of a permanent magnet.

  Although Faraday’s disc dynamo was conceptually simple, it was inefficient; before long more sophisticated dynamos would be designed for practical applications. But this was not the end. The disc dynamo concept would reappear later, when the race to understand the magnetism of first the sun and then the Earth heated up.

  The Faraday disc dynamo. When the copper disc on the left is rotated between the poles of a magnet (not shown), a voltage is induced between its center and its rim. If a circuit is connected between the center and rim of the disc, the voltage will drive a current through it. This model was made at the University of Aberdeen around the time that James Clerk Maxwell was professor of
natural philosophy there.

  Meanwhile, Faraday’s scientific career, which he would spend entirely at the Royal Institution, forged ahead. He investigated the properties of electrical insulators and various magnetic materials, and the electromagnetic “polarization” of light, as well as formulating the laws of electrolysis. Last but not least, he gave memorable public lectures and scientific discourses. As well as his brilliant descriptions, he was renowned for the experiments with which he illustrated the lectures, bringing science to life for his audiences.

  In 1826 he initiated two popular and enduring series: Friday Evening Discourses and Christmas Letures for a Juvenile Audience. Both continue to this day. (The Christmas Lectures are now broadcast on television.) In all, Faraday would personally deliver nineteen series of Christmas Lectures and 123 Discourses, and organize many more. One Friday evening early in 1846 he was caught short without a lecturer and had to step in at the last minute. The result was his “Thoughts on Ray Vibrations,” published that May in Philosophical Magazine. In this lecture he speculated that the propagation of light, which by then was known to possess wavelike properties, might involve coordinated variations of his lines of magnetic and electric force.

  Late in life Faraday became frustrated by a serious loss of memory. It has been suggested that this resulted from his liberal use of mercury to ensure good electrical contact in many of his experiments. Whatever the cause, in 1867 he resigned his directorship at the Royal Institution in favor of his good friend John Tyndall and declined a knighthood. On August 25 that year he died as plain Mr. Faraday.

  For all his brilliance, Faraday’s one limitation had been his lack of mathematical skills. Before long another scientist would emerge to fill this void and take knowledge of electromagnetism to a new level. James Clerk Maxwell hailed from Edinburgh, where he had been born into a well-to-do family in 1831. In a regrettably short professional life, which oscillated between Scotland and England, this gifted physicist would make pivotal advances in many areas of mathematical physics, from theories of color vision to determining how gases respond to changes in pressure and temperature. Most importantly, he would formalize the language of electromagnetism and lay down its mathematical foundations.

  Much of Maxwell’s early childhood was spent at his family’s country estate, Middlebie, at Glenlair, about twenty miles west of Dumfries in Kirkcudbrightshire. Some time previously the Clerk family had inherited the estate from their relatives the Maxwells—hence the name Clerk Maxwell. From the age of about three James is said to have constantly asked, “Show me how it doos.” Anything mechanical fascinated him, especially the house’s plumbing system and the inner workings of the network of bells used to summon servants.

  Maxwell’s mother died when he was eight, and an aunt helped raise him. He attended the Edinburgh Academy, where it took several years for him to overcome his shyness and excel. At the age of thirteen he presented his first mathematical paper to the Royal Society of Edinburgh; its subject was the geometry of ellipses and related shapes. At sixteen he began to study mathematics, natural philosophy, chemistry and philosophy at Edinburgh University, and in 1851 he moved to Cambridge University, graduating in 1854. He narrowly missed out on the coveted title of “senior wrangler” awarded to the top student in mathematics, but shared the more prestigious Smith’s Prize for an essay based on original research.

  Becoming a Fellow of Trinity College, for the next two years Maxwell continued to study mathematics there while supervising undergraduate students. He might have stayed longer at Cambridge but his father became ill, and in 1856 he returned to Scotland to be closer to him. Before long he was offered, and accepted, the chair in natural philosophy at Marischal College, Aberdeen. He was just twenty-four. A year later he won the coveted Adams Prize offered by St. John’s College Cambridge with a brilliant essay on the rings of Saturn, described by the respected mathematician, geophysicist and Astronomer Royal Sir George Airy as “one of the most remarkable applications [of mathematics] to astronomy that I have ever seen.”

  He married Katherine Dewar, the daughter of Marischal College’s principal, but this did not help him in 1860 when Aberdeen University was restructured. Marischal merged with the other college, King’s, and Maxwell, the junior of the two professors of natural philosophy, lost his job. In another blow, he narrowly missed out on a chair at Edinburgh University;The Edinburgh Courier hinted that despite his quick and brilliant mind and undoubted eminence in academic circles, his lecturing style may not have suited the average Scottish student:

  Professor Maxwell is already acknowledged to be one of the most remarkable men known to the scientific world … There is another quality which is desirable in a professor in a university like ours and that is the power of oral exposition proceeding on the supposition of imperfect knowledge or even total ignorance on the part of pupils.

  Fortunately, Maxwell did not remain unemployed for long. In 1860 he was grasped by King’s College, London, and he and his wife moved south. The next five years would be his busiest and most productive, culminating in the 1865 publication of his paper “Dynamical Theory of the Electromagnetic Field” in Philosophical Transactions of the Royal Society.

  Maxwell followed Faraday in thinking that a magnet or an electric charge somehow modified the space around it, so that when another magnet or charge entered that space it experienced a force. Faraday’s lines of force had given a picture of the forces that could be expected at various points in space. Maxwell developed this idea by calling the region of altered space a magnetic or electric field. The field concept was to become central in many areas of theoretical physics, but Maxwell’s original definition was deceptively simple. A field, he wrote, is “that part of space which surrounds bodies in electric or magnetic conditions.”

  This was just the start. Maxwell now tried to reconcile his fields with Faraday’s “Thoughts on Ray Vibrations” and the famously enigmatic concept of “luminiferous aether” to which Isaac Newton had resorted when his corpuscle theory of light ran into problems. After Newton, a number of scientists —Christiaan Huygens, Thomas Young and Augustin-Jean Fresnel among them—had shown that light propagated more like a wave than a stream of particles, and the concept of “aether” had evolved to describe the medium through which these waves supposedly traveled. Faraday had observed that a magnet could influence the way light traveled, and therefore considered that light itself might involve electric and magnetic vibrations. Maxwell reasoned that if the aether were the medium through which light traveled, it might also provide the support for Faraday’s lines of force and his own magnetic field.

  James Clerk Maxwell, born 1831. Maxwell presented his first mathematical paper to the Royal Society of Edinburgh at the age of thirteen. By his early forties he had developed a mathematical theory to describe all known phenomena of electricity and magnetism. His four famous equations of electromagnetism are known simply as Maxwell’s Equations.

  What, though, was this “aether?” Faraday had doubted it could be matter in the normal sense of the word. “Ponderable matter,” he had written, “is not essential to the existence of physical lines of force.” Nevertheless, Maxwell clung to the notion of a curious thin fluid that pervaded all of space, and even penetrated solid materials. He supposed it could be set in motion by electric currents and magnets, and that the energy of such motion could be transmitted through it by a series of exchanges between what we now call kinetic and potential forms of energy—an elastic wave. This was an important step: it enabled Maxwell to apply the physics of mechanics—in particular of mechanical waves—to problems of electromagnetism.

  The aether would continue to prove elusive. Numerous experiments, most notably those of Americans Albert Michelson and Edward Morley in the 1880s, would fail to find any evidence for its existence, and in 1905 Einstein’s theory of special relativity would finally remove the need to invoke it as anything more than a romantic notion. However, even when the mechanical scaffolding provided by the aether was
dismantled, Maxwell’s results still held true. Today, the field concept has not just survived, but become the backbone of many areas of theoretical physics.

  Maxwell’s next task was to formalize the language of electromagnetism. To do this, he had to define every property clearly and unambiguously, and lay down rules as to how each should be measured. Only then would his mathematics and equations work consistently.

  First and foremost, he needed a more rigorous definition of “field,” one that covered both the strength of the force that would be exerted on a charge or pole placed in the field and its direction. He defined the strength of an electric field as the magnitude of the force that would be exerted on a very small positive charge at a particular point in space, divided by the size of that charge. This imaginary test charge had to be very small so it did not change the overall field it was meant to measure. In the same way, the strength of a magnetic field could be defined in terms of the force on a tiny north pole. To fully understand the effect of a force it was also necessary to know the direction in which it acted. Push or pull? Up or down? To the left or right? Or somewhere in between?

  Maxwell found that to rationalize and describe all the known phenomena of electricity and magnetism, he needed to define not just electric and magnetic fields, but a total of twenty different properties. His mathematical theory incorporated Coulomb’s electrostatic and magnetostatic force laws, Ørsted’s magnetic effect of a current-carrying wire, Ampère’s results on the magnetic interaction between current-carrying wires, Faraday’s force on a current-carrying wire in a magnetic field, and his law of electromagnetic induction.

  Initially, he presented a set of twenty equations, but by 1873, when he published his Treatise on Electricity and Magnetism, he had distilled these to the four that have become universally known as Maxwell’s Equations. Written in the shorthand of mathematical symbols, these equations look deceptively simple. They relate the variations of electric and magnetic fields both in time and space, and are expressed using the mathematical language known as vector calculus. They can be applied to electric and magnetic fields in any situation imaginable—from a single atom to the entire universe—including the deep interior regions of Earth.