Faraday, Maxwell, and the Electromagnetic Field Page 7
Whatever theories about circular magnetic forces acting through space had begun to form in his mind, he had, so far, kept such speculations to himself—to publish them would be to court ridicule. More investigation was needed, and he devised a typically ingenious experiment, one that seems simple only after you have thought of it. He wound a helical wire coil around a glass tube and connected the coil to a battery, so forming a magnet. With his coil in a fixed horizontal position, he submerged it halfway in water. Then he took a magnetic needle as long as the helix, pushed it through a thin cork, floated the needle-bearing cork on the water, and turned on the current. He now had two magnets: the half-submerged current-carrying coil, which was in a fixed position, and the floating magnetic needle, which could move freely. Unlike poles attract, so the north pole of the needle was attracted to the south pole of the coil magnet and moved toward it. By the accepted theory of magnetic poles, it should have stopped when it reached the south pole of the coil magnet—the mutually attracting north and south poles would then be together. But, instead, it kept on moving right through the glass tube and came to rest with its own north pole beside with the north pole of the coil magnet. And at the other end, the two south poles were together.
Simple yet profound. By Faraday's interpretation, the experiment had shown that magnetism was not simply a matter of poles attracting or repelling one another. Indeed, it had shown that magnetic forces did not begin and end at the poles but ran in continuous loops all the way through the magnet. Despite the strict professional skepticism that Faraday always applied to his own ideas—he spent much time devising experiments that would show errors in his thinking—he was gradually coming to believe that magnetic forces actually had a presence in space. This was another idea that was completely foreign to adherents of the French Newtonian school; they believed that forces were produced by the instantaneous action of one material body on another at a distance, and they didn't concern themselves about what was happening in the intervening space.
There was a new phenomenon that baffled everyone. In Paris in 1825, François Arago had noticed that compass needles were sometimes deflected when a piece of copper moved nearby. To investigate, he suspended a magnetic needle over a rapidly spinning copper disc. Amazingly, the needle rotated, too. Copper was a nonmagnetic material, so what was going on? The answer turned out to depend on a completely new effect that was to be one of Faraday's greatest discoveries. Along with other scientists across Europe, Faraday was intrigued by Arago's result, and, during the long hours working on glass, thoughts on electromagnetism had been gestating at the back of his mind. Sometimes he would pull from his pocket a little, wire-wrapped iron cylinder and gaze at it. One question dominated all others. If electricity could produce magnetism, shouldn't magnetism be able to produce electricity? In his meager free time, he had tried various arrangements of magnets and circuits, so far to no avail. Now he had shaken off the burdensome “common place employment” and could channel all his energy, skill, and experience into solving this mystery.
Humphry Davy had died in 1829 after a debilitating illness. For all Davy's faults, Faraday revered him. They had spent much time together and formed a strong bond. More than just a mentor, Davy had been a friend and an inspiration to Faraday. Faraday felt privileged to have been close to such a great man. When Davy's biographer, John Ayrton Paris, asked to borrow the first letter Davy had sent him, Faraday obliged, with the note:
I send you the original, requesting you to take great care of it, for you may imagine how much I value it.13
The pupil was about to embark on a journey of discovery that surpassed even that of the master. In August 1831, Faraday wrote in his laboratory journal the first words for a new project that was to become his finest work. His Experimental Researches in Electricity, a monumental opus written entirely in words without a single formula, had begun.
The Royal Institution's most treasured possession is something that would not look out of place on a municipal garbage dump—a ragged object resembling a cloth-wrapped deck tennis quoit that has become entangled in wire. It is, nevertheless, priceless—one of the most important pieces of scientific apparatus ever made.
By 1831, everyone knew how to make magnetism from electricity. A current-carrying coil would, if carefully suspended, align itself north–south, and an ordinary piece of iron could be transformed into a permanent magnet by placing it briefly inside the coil. Surely it should be possible to do the opposite: make electricity from magnetism. Many had tried, but all had failed. Faraday's own attempts with various configurations of circuits and magnets had fared no better than anyone else's but his thoughts ran on, aided by the little, coil-wrapped iron cylinder that he carried with him and contemplated from time to time.
Fresh inspiration came from another branch of physics. Faraday's love of music had led to a particular friendship with fellow scientist Charles Wheatstone, whose family business made and sold musical instruments. Wheatstone had invented the kaleidophone, a device that reflected light from the tip of a vibrating metal rod mounted on a wooden board. The moving reflections formed complex patterns when projected onto a screen, which delighted Faraday. Wheatstone introduced him to the work of the German physicist and musician Ernst Chladni, who had demonstrated a similar effect with vibrating plates: he found that if you spread sand thinly on a glass plate and stroke the edge of the plate with a violin bow, vibrations in the plate would form the sand into beautiful patterns—graphic representations of the standing waves in the glass.
Fig. 5.1. Chladni figures—patterns formed in sand spread thinly over vibrating glass plates. (Used with permission from Lee Bartrop.)
Moreover, the patterns could be induced on the sand-strewn plate by stroking another plate a short distance away—vibrations in the first plate produced sound waves in the air that then caused the second plate to vibrate. As always, Faraday tried it for himself and explored every avenue by varying the conditions of the experiment. He was rewarded with an even more vivid demonstration of acoustic induction—when he poured a mixture of egg white, oil, and water on the second plate instead of sand, the vibrations showed up as very fine striations, a kind of crimping of the liquid mixture.
Steeped in musings about acoustic vibrations and waves, Faraday began to think that electricity and magnetism might be transmitted by waves resembling those of sound, or of light. To test the idea, he decided in the summer of 1831 to link two electric circuits magnetically and see whether sending a current through the first circuit would cause some kind of vibration or wave that would act through an iron magnet to induce a current in the second. He needed the strongest possible magnetic link between the circuits and so commissioned a wrought-iron ring, such as his father might have made, six inches in diameter. He also needed coiled circuits with many turns—each extra turn would increase the magnetic effect of a current. So when the ring arrived, he wound two wire coils around it on opposite sides of the ring, each with as many turns as he could fit in on one layer, followed by other layers on top. To insulate each turn of the coils from neighboring turns, he interposed lengths of string; and to insulate each layer of coil from the next, and from the iron ring, he used sheets of cloth. It was not elegant, but handsome is as handsome does.
He connected one coil (A) to a battery and a switch. This formed the primary, or sender, circuit; its role was to magnetize the iron ring. He hoped that some kind of vibration or wave would act through the iron to induce a current in the other coil (B), which was connected by two long wires to a galvanometer (G) with a light, delicately balanced magnetic needle. (The galvanometer worked on the same principle as Oersted's original method of detecting a current, but it had been much refined by Ampère and others.) Coil (B), the long wires, and the galvanometer formed the secondary, or receiver, circuit.
Fig. 5.2. Schematic layout of Faraday's iron ring experiment. (Used with permission from Lee Bartrop.)
If there was even the smallest current, he would see the needle move from its n
ormal north–south position. On August 29, 1831, all was set.
Faraday closed the switch and watched the needle. His heart must have leapt when it moved, but after a brief twitch, it settled back to its resting position. Now a steady current was flowing in the primary circuit but absolutely nothing was happening in the secondary one; the detector needle remained motionless. But when he turned the primary current off, the needle twitched again, this time in the other direction. Strange, indeed, and there was another oddity: the movements of the needle had shown that the first pulse of current in the secondary circuit was in the opposite direction to that in the primary, but the second was in the same direction as the primary current.
What could he make of this? It could be one of the greatest scientific discoveries ever, or it could be some chance result with an elusive but mundane explanation. At any rate, Faraday couldn't contain his excitement and wrote to his friend Richard Phillips: “I am busy just now on electromagnetism and think I have got hold of a good thing but can't say; it may be a weed instead of a fish that after all my labor I may pull up.”1
It was no weed. He repeated the iron-ring experiment many times with minor variations and looked for any possible stray effects that might be causing the needle to twitch. There were none.
No wonder it had taken so long to find a way of generating electricity from magnetism: nothing happened except when the magnetic influence was changed by turning the current on or off. A momentous result, but there was more to do. He had generated electricity from magnetism, but so far only from magnetism that he had in turn made from electricity. Was it possible to do the same using only an ordinary permanent magnet and a wire circuit? Various arrangements of magnets with straight wires, helical coils, and spirals yielded no detectable result, but persistence was rewarded when he put a coil-wrapped iron cylinder between the unlike poles of two similar bar magnets and brought the magnets’ other ends together to make, in effect, a single V-shaped magnet of twice the strength.
Fig. 5.3. Schematic layout of Faraday's V-magnet experiment. (Used with permission from Lee Bartrop.)
At the instant he brought the magnet ends together, the needle of a galvanometer connected to the coil flicked. And when he pulled the magnet ends apart, the needle flicked the other way. He had generated electricity directly from a magnet and soon found out that there was a much easier way to do it: When he connected a coil of many turns to a galvanometer, took an ordinary magnet, pushed it into the coil cavity and pulled it out again, the galvanometer needle swung vigorously, first one way then the other.
Fig. 5.4. Faraday's magnet-and-coil experiment. (Used with permission from Lee Bartrop.)
He had discovered something that would eventually transform people's lives the world over—electromagnetic induction. To generate electricity in a wire, all you had to do, in principle, was to bring it close to a magnet and set the wire and magnet in relative motion. Faraday had opened up the possibility of generating cheap electricity for any purpose. But little blips of electric current were not enough; what was needed was a continuous current. The blips had been produced by the jerky relative motion of a wire coil and a magnet. Was it possible to devise a smooth motion that would produce a steady current? Faraday's thoughts turned again to the curious experiment conducted by François Arago seven years earlier. Arago had made a magnetic needle spin by rotating a nearby copper disc. Nobody had yet explained why this happened, but Faraday now thought he might have the answer. Perhaps the relative motion of Arago's disc and needle had produced electric currents in the disc—if so, the magnetic effect of these currents would have sent the needle spinning.
Faraday decided to try a variation on Arago's experiment. He mounted a copper disc on an axle and set its edge in a narrow gap between the poles of a powerful magnet. He then made an electrical circuit by placing one sliding contact on the edge of the disc, placing another on the axle, and connecting the two contacts by wires to a galvanometer. He hoped that when the disc rotated, its motion relative to the magnet would produce a steady current across the disc that would register on the galvanometer. He spun the disc and watched the needle. It moved, and this time stayed in its new position; there was a feeble but steady current. And when the disc was spun the other way, the needle also reversed its movement. Ten years after making the world's first electric motor, Faraday had made the world's first dynamo.
Fig. 5.5. Schematic layout of Faraday's first dynamo. (Used with permission from Lee Bartrop.)
By the same token, he had explained Arago's result: When Arago spun his disc, its movement relative to the suspended magnet had produced a current in the disc that, in turn, set up magnetic lines of force that caused the magnet to rotate.
As he had done with the electric motor, Faraday was content to have demonstrated the principle of the dynamo and to leave its technological development to others. His job was to try to push forward the frontiers of knowledge about the physical world. He had made startling new discoveries about the way electricity and magnetism behaved, and all his thoughts were now absorbed in trying to explain them.
In fact, he had been thinking hard on the topic for eleven years already. Ever since hearing of Oersted's discovery of the magnetic needle that turned perpendicular to the current, he had been trying to understand the relationship between electricity and magnetism. His ideas came from what he had observed for himself in the laboratory; they owed nothing to mathematics or to the theories of others. Vague and tentative at first, they were now slowly beginning to coalesce, and they were not like anything that had come before. Faraday knew that many orthodox scientists would dismiss his ideas as laughable, or even heretical, and so kept them largely to himself.
On November 24, 1831, he presented his discovery of electromagnetic induction to his colleagues at the Royal Society. The paper was sparsely worded and factual, reporting experimental results with no fanfare and almost no conjecturing. Not yet ready to mount an open challenge to established theories, he chose his wording carefully, invoking only the part of Ampère's model that postulated a connection between electricity and magnetism. He ventured into theoretical territory only briefly, but here he gave just a glimpse of how far away he was from Ampère, or from anyone else. Reporting his experiments with the iron ring, he explained how he believed the current was induced in the secondary circuit:
Whilst the wire is subject to either volta-electric or magneto-electric induction, it appears to be in a peculiar state; for it resists the formation of an electrical current in it, whereas, if in its common condition, such a current would be produced; and when left uninfluenced it has the power of originating a current, a power which the wire does not possess under common circumstances. This electrical condition of matter has not been recognized, but it probably exerts a very important influence in many if not most phenomena produced by currents of electricity. For reasons which will immediately appear, I have, after advising with several learned friends, ventured to designate it as the electro-tonic state.2
Perhaps it is not surprising that few, if any, of Faraday's colleagues had the faintest idea what he was talking about. As we'll see, the electrotonic state went on to play a big part in the development of field theory, but at this stage it was an enigma, even to Faraday. It appeared to be a kind of tension or strain that was present in a wire whenever the wire carried a current, but it revealed itself only while it was being set up and while it was being released—he tried everything he could think of to detect the strain directly, but without success. Faraday's conception of this elusive state was an extraordinary feat of scientific imagination. The most puzzling aspect of the iron-ring experiment was the brief current that flowed in the secondary circuit after the battery in the primary circuit had been disconnected. To Faraday, this seemed to be the release of some kind of strain in the secondary circuit that had been induced by the primary current through the agency of the magnetic force in the iron ring that linked the two circuits. The brief current that had flowed in the reverse directio
n in the secondary after the battery was first connected in the primary circuit was, by the same token, a manifestation of the setting up of this strain. The state of strain continued while the primary current was steady but collapsed when the battery was disconnected, and the brief secondary current then flowed rather in the way that gas is released from a pricked balloon.
Such thoughts sprang from well-prepared soil. Faraday had been thinking deeply along lines different from everyone else's. An electric current might, he imagined, be the result of a rapid buildup and release of a strain in a wire. And every electric current created a circular magnetic force around itself—a force that seemed to have a physical presence in the surrounding space. Similarly, every magnet created a pattern of forces that acted along curved paths in the space around it. Though invisible, these paths needed only a sprinkling of iron filings on a piece of paper held over the magnet to reveal themselves.