We have seen that the magnetic fluid in our steel wire twenty-five inches long was concentrated at its end in a length of two or three inches; that the center of action of each half of this needle was about ten lines from its ends …
Put another way, although it was impossible to isolate a single pole, the poles of a long magnet were very close to its ends. (Ten lines was about ten-twelfths of an inch, or just over two centimeters.) Coulomb hoped that by using a very long magnetized needle or rod to exert a force on the needle of his torsion balance, the effect of one pole would dominate, and the other pole would be so far away that its force would be negligible. By this method he was able to show that both the force of repulsion between like poles and the attractive force between unlike poles also decreased with the inverse of the square of the distance between their “centers of action.”
He rounded off by stating that “the magnetic fluid acts by attraction or repulsion in a ratio compounded directly of the density of the fluid,” although it is not clear how, or even whether, he actually demonstrated this last part.
Coulomb was not finished yet. Being a consummate perfectionist, he could not stop at deriving such important relationships by just one method. What if there was some elusive fundamental flaw in his torsion balance technique? Just in case, he went on to verify all his results again using a quite different kind of experiment, one based on timing oscillations. If an ordinary compass needle is displaced slightly from north and then released, it will swing back and forth at a rate that depends on the strength of the forces tending to restore it to north—in this case, the forces of Earth’s magnetism on the poles of the needle. The stronger the force, the faster are the oscillations. In fact, the square of the frequency—that is, the number of oscillations in one second—is proportional to the force. Using suspended pith balls and magnetized needles similar to those in his electric and magnetic torsion balances, Coulomb reverified his inverse square laws.
All three known fundamental forces of nature—gravity, electrostatic and magnetic—had now been shown to follow an inverse square law. Like Newton’s law of gravitation, Coulomb’s laws of electrostatics and magnetism included elusive constant factors. The determination of these would have to await further developments in the theory of electricity and magnetism.
Lastly, Coulomb turned to the question of the origin of magnetism, and came up with a theory of what happens inside a magnetic material that is still pertinent today.
Coulomb was well aware of the broken magnet paradox—that if you break a magnet in two you make two smaller magnets, each with north and south poles; do the same again and again and again, and while the magnets get smaller and smaller, each will always have two poles, north and south. He was also aware of a growing acceptance in the scientific community that, at the microscopic level, matter was made up of huge numbers of tiny indivisible particles, or molecules. He reasoned that the magnetic fluid, if it existed at all, was confined within individual molecules of magnetic materials such as iron. It could move within a molecule, he theorized, giving it a north pole and a south pole and so turning it into a mini-magnet. But it could not move between molecules. Inside a piece of ordinary unmagnetized iron, the north and south poles of the molecular mini-magnets would normally attract and abut one another, their effects would cancel each other out, and so there would be no overall magnetic effect. On the other hand, when a needle became magnetized—for example, by stroking it with another magnet—the molecular mini-magnets would become aligned, north pole to south pole, all along the needle, so at the ends there would be free north and south poles: the poles of the now magnetized needle.
Coulomb’s work brought to a close the phase in the study of electricity and magnetism that had been concerned only with static charges and poles. Coulomb himself would survive the French Revolution by retiring to the countryside. During this period two sons were born, and apparently some time later he married their mother, who was some thirty years his junior. Around 1800 he was appointed to overhaul the by then public French education system, and he and his family returned to Paris. He died six years later at the age of seventy.
Meanwhile, the world was set for a rush of discoveries and inventions that would establish an intimate relationship between electricity and magnetism—and open up a new range of possible explanations of Earth’s magnetism.
Of Forces and Fields
Ten thousand years from now, there can be little doubt that the most significant event of the nineteenth century will be judged as Maxwell’s discovery of the laws of electrodynamics.
—RICHARD FEYNMAN, 1964
Once again, a stroke of pure luck would launch a new chapter in the story of magnetism. One day in 1791 Luigi Galvani, a professor of anatomy at the University of Bologna, was routinely dissecting a newly dead frog when suddenly and completely unexpectedly the frog’s leg twitched of its own accord. Galvani had touched a nerve with one of his metal instruments.
Having checked that the frog was indeed dead, Galvani carefully repeated the process. The same thing happened. After considerable thought, he decided the twitching must be due to some sort of electric effect in the frog’s nervous system.
Galvani’s report describing his discovery of “animal electricity” was read by a colleague at the University of Pavia, Alessandro Volta, who immediately set about conducting his own experiments. In one he found that by placing a piece of tin foil on top of his tongue with a silver coin underneath he experienced a peculiar acidic taste. Eventually he deduced it was the metals, rather than the animal tissue, that generated the electricity, and in 1800 this led him to construct his famous “voltaic pile”: the world’s first electric battery. The pile consisted of alternate layers of copper and zinc, each separated by a layer of cardboard soaked in a solution of brine.
For a short time after this it was customary to distinguish ordinary electricity, which was generated by the old traditional method of friction or rubbing, from this new voltaic, or chemical, electricity. Ordinary electricity could be stored in a Leyden jar, an early form of capacitor, but when the terminals of the jar were connected together with conducting wires, the jar discharged instantaneously, producing only a single burst of electricity. On the other hand, voltaic electricity could be produced continuously, creating a steady flow of electric charge—a “current.”
One of the first applications of Volta’s battery was the electrolysis of water by Englishman William Nicholson and German Johan Ritter in 1800. When a current was passed through water between electrodes of silver and zinc, bubbles of hydrogen and oxygen gas formed around the electrodes. Other applications of electric currents followed. However, no one could have foreseen the next amazing breakthrough.
There are various accounts as to how it came about. All agree that Hans Christian Ørsted, a Danish professor of physics and chemistry, was giving a demonstration to a group of students during a lecture at his home in Copenhagen. According to some accounts, he was trying to show his students that electricity and magnetism were unrelated phenomena. What happened, though, was that when Ørsted held a wire carrying an electric current over a compass, the compass needle swung around until it was at right angles to the wire. When the current was northward, the compass swung to point west. When the current was reversed, the compass needle also reversed: a southward current made it point east.
It seems Ørsted did not immediately grasp the importance of his discovery: he merely wrote about it, in Latin, in a private letter to a few friends and colleagues. But just a few months later, on September 4, 1820, French physicist François Arago announced Ørsted’s discovery of the magnetic effect of an electric current to a meeting of the Paris Académie des Sciences. At a second meeting the following week, Arago gave an impressive demonstration of Ørsted’s experiments, and several members of the audience immediately seized upon their true significance.
Among them was a gifted mathematician, André-Marie Ampère. Ampère’s life had already been marked by spells of professiona
l and scientific brilliance, interrupted by personal tragedies and periods of severe depression. He had mastered all known mathematics by the age of twelve, but at seventeen he had lost his father to the guillotine during the French Revolution and virtually abandoned his studies. A few years later, when his young wife became ill and died, he was consumed with guilt because his work had kept him away from her for much of their brief marriage. A second marriage would end in separation, and a subsequent affair also proved disastrous, destroying Ampère’s relationships with his children, one from each marriage.
Arago’s announcement of the magnetic effect of an electric current spurred Ampère into action. For the next few weeks, the forty-five-year-old genius scarcely slept, carrying out numerous experiments and presenting demonstrations at the Académie. By the end of the year he had announced a series of important results, as well as his own theory of what he called “electrodynamics.”
Publication took a little longer: it was 1827 before his Memoir on the Mathematical Theory of Electrodynamic Phenomena, Uniquely Deduced from Experience appeared. It has since been praised as the Principia of electrodynamics—the first work to lay out an integrated mathematical theory of electricity and magnetism.
Ampère’s reasoning went along these lines: a current-carrying wire had been shown to behave like a magnet; since two magnets attracted or repelled one another, depending on which poles were brought close together, two current-carrying wires should also attract or repel each other.
His experiments showed that two parallel wires carrying currents in the same direction did indeed attract each other, while parallel wires carrying opposite currents repelled each other. There was, though, a difference: whereas the effects of a magnet or lodestone were concentrated at their poles, Ampère found that with a current-carrying wire the magnetic force was distributed along its whole length.
In the last few months of 1820 two other Frenchmen, a physicist called Jean-Baptiste Biot and his assistant Félix Savart, were also frantically beavering away trying to understand this new phenomenon. Both they and Ampère reported regularly to the Paris Académie so it is unclear who deserves credit for the next discovery. The result is, nevertheless, known as the Biot-Savart Law.
The scientists pictured a current-carrying wire as many tiny segments joined end to end, with the current passing from one segment to the next all along the wire. They found that each such segment produced a magnetic force, and the strength of this decreased with distance from the segment—as the inverse square of the distance. In effect they had discovered yet another inverse square law of nature. The tiny segments did not, of course, exist in isolation: to predict the magnetic force due to the whole current-carrying wire, the scientists had to add together the contributing forces due to each—the mathematical process of integration. Permanent magnets such as lodestones, bar magnets and compass needles, each of which had two poles where the magnetic effect was concentrated, were clearly, then, not the only source of magnetism. A long current-carrying wire had no poles, but it exerted a magnetic force along its whole length. This would prove to be a critical discovery.
Ampère was convinced that the two sources of magnetism were not independent, but intrinsically one and the same. He now set about finding the missing link. First, he imagined a circular coil of wire carrying a current. Picturing this current loop as many tiny segments, and adding up the magnetic forces due to each segment, he used the Biot-Savart Law to show that such a loop had exactly the same magnetic effect as a short bar magnet. In particular, when the current was clockwise it was as if a north pole lay above the loop and a south pole below it. When the current was counterclockwise, the opposite was true. Like a bar magnet, a current loop had two poles: it too was a magnetic dipole.
Ampère then reconsidered Coulomb’s explanation of permanent magnetism. He concluded that at the microscopic level all magnetism must be “electrodynamic” in origin. Perhaps Coulomb’s molecular mini-magnets were actually tiny current loops circulating in the atoms or molecules of magnetic materials.
Meanwhile, 300 kilometers away on the other side of the English Channel, a young scientist at London’s Royal Institution was keenly following the fusion of electricity and magnetism into “electromagnetism.” Although Michael Faraday’s earlier work had been in the field of chemistry, he was about to make discoveries that would revolutionize not just physics but the day-to-day lives of the whole world. His experiments would directly result in the invention of the electric motor, the transformer and, most importantly for geomagnetism, the electric dynamo or generator—and provide an exciting new explanation of Earth’s magnetism.
Michael Faraday, who was born near London in 1791, the son of a blacksmith. He made discoveries in electricity and magnetism that revolutionized not just physics but the everyday lives of people across the world, and led to the dynamo theory of Earth’s magnetism.
Faraday’s story is a classic one: a poor boy, born and brought up on the streets of London, who grows up to develop an unstoppable passion for science, and achieves academic eminence through a career of hard work filled with brilliant discoveries.
Faraday was born in 1791 in Newington Butts near London. His father, James, was a blacksmith who, in search of work, had brought his young family south from Westmorland the previous year. The family was deeply religious, and belonged to a little known Christian sect, the Sandemanians, which had broken away from the Presbyterian Church of Scotland. The Sandemanians strove to live according to the humble principles of Christianity laid out in the Bible, and these principles seem to have stayed with Faraday throughout his life.
James Faraday was often ill and there was rarely enough food to go around. There were no prestigious schools for Michael, who would later write that his education was “of the most ordinary description, consisting of little more than the rudiments of reading, writing and arithmetic at a common day school. My hours out of school were passed at home and on the streets.”
In 1804, at the age of twelve, he left school and was sent to work as an errand boy for a bookbinder. After a year he became an apprentice, and by spending every spare moment reading the books brought in for binding he enhanced his meager education. Apparently these books included an early edition of Encyclopedia Britannica; the chapters on electricity and magnetism fascinated Faraday.
By chance, a customer who had noticed Faraday’s interest in science offered him tickets for a series of lectures to be given by a famous chemist, Sir Humphry Davy, at the Royal Institution in Albemarle Street, Piccadilly. Faraday leapt at the opportunity, not only attending the lectures but scrupulously noting down every detail. Afterwards he neatly transcribed his notes, illustrated them with sketches, bound them and sent them to Davy, requesting consideration should a job arise at the Royal Institution.
Before long a vacancy did arise. Some sources say a fight at the Royal Institution’s lecture theatre led to the dismissal of Davy’s chemical assistant; others tell of a laboratory explosion in which Davy was temporarily blinded, the result being that he needed a secretary and note-taker. Whatever the truth, early in 1813 Davy apparently found himself in need of assistance, and called on the twenty-one-year-old Faraday.
The Royal Institution was, and still is, a unique scientific establishment. It was founded in 1799 by Sir Benjamin Thompson, also known as Count von Rumford, a colorful globe-trotting American whose name crops up all over science, most famously as the person who quashed the theory that heat was a fluid. (As a result of observing the boring of cannons in Austria, Thompson concluded that heat was, in fact, associated with molecular motion at the microscopic level. It was this that eventually led to the concepts of energy and the conservation of energy as it changes from one form to another.)
From the beginning the institution’s goal was to promote a practical, applied approach to science. Its stated mission included “the diffusion of knowledge,” “facilitating the general introduction of useful mechanical inventions” and “teaching … the application
of science to the common purposes of life.” Rumford had installed Humphry Davy as assistant, then lecturer, and finally as professor and director of the laboratory. It was here that Davy made numerous discoveries in chemistry and invented the miner’s safety lamp that bears his name, but even he is reputed to have said that his best discovery of all was Faraday.
Almost straightaway on being hired, Faraday was taken on a grand tour of Europe. This was supposed to be an extended honeymoon for Davy and his new wife but the trio seem to have visited a great many scientists and taken in all the important scientific centers of the day. No wonder Mrs. Davy is reported to have been somewhat grumpy with her husband’s new scientific assistant. Faraday, meanwhile, made the most of his opportunities to meet men such as Volta and Ampère. Both fostered his interest in electricity, so in 1820 he quickly grasped the importance of the news filtering through from Paris.
The apparatus used by Faraday to demonstrate the magnetic forces between magnetic poles and current-carrying wires. Both cups contained liquid mercury and a long magnet fixed at the base. Electric current entered each through the wire at the top. In the left-hand cup, the magnet was pivoted and its upper pole rotated in response to the magnetic force exerted on it by the fixed current-carrying wire. In the right-hand cup, the lower end of the current-carrying wire rotated in response to the force exerted on it by the magnet.
Faraday’s first series of electromagnetic experiments concerned the directions of the magnetic forces exerted between magnets and current-carrying wires: he wanted to find out whether the magnetic effects of currents and magnets were really the same. He figured that the magnetic force produced by a current, as observed by Ørsted and Ampère, should cause a free magnetic pole—if one could be created, or at least approximated—to move in a circle around a current-carrying wire.
North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism Page 8
