Faraday, Maxwell, and the Electromagnetic Field Read online

  Perhaps the change in the plane of polarization had been too small to detect. He decided to try the effect of magnetic forces, which could be made much stronger than electric ones—he had powerful electromagnets in his Royal Institution laboratory and could call on an even bigger one from the Royal Military Academy in Woolwich. Once again, he varied the conditions of the experiment, changing the position and strength of the magnet and shining polarized light through all the transparent substances again, all to no avail. Then he thought of the small sample of special heavy glass, made of boro-silicate of lead, left over from his onerous work on the glass project of the 1820s.

  For two weeks he had been looking into a lens and seeing nothing but black. The source of light was an oil lamp, and the light passed first through a polarizing prism, then through the test substance, and finally through an analyzer, before arriving at the eyepiece. The apparatus was arranged so that only if the light's plane of polarization were altered by the action of electric or magnetic force on the test substance would any light reach the eyepiece. At first, he put the piece of heavy glass between the north pole of one magnet and the south pole of another. Nothing happened. Nor did it when he tried several other configurations. But when he placed the piece of glass alongside the same two poles, shone the beam, and looked into the eyepiece—there was a faint image of a flickering flame.

  Fig. 6.1. Arrangement of magnet and heavy glass in Faraday's magneto-optic experiment. (Used with permission from Lee Bartrop.)

  Faraday once more scented discovery. He had opened the door a crack but needed more powerful equipment. That was easily arranged; he sent for the great Woolwich electromagnet and took a four-day break in pleasant anticipation, planning what he would do when it arrived.

  On September 18, he repeated the experiment with the sample of heavy glass, this time using the Woolwich magnet. The image of the flame was there, much brighter than before, but it took a second or so to rise to full brightness. This was the best demonstration yet of something Faraday had observed before—electromagnets take time to reach their full intensity of magnetization. He then verified that the light's plane of polarization did indeed rotate as it traveled through the magnetized glass, determined the direction of rotation, and found that the angle of twist was proportional to the strength of the magnet (which could be altered by varying the number of turns of wire coiled around the iron that was connected to the battery). Working with all the energy of his youthful days, he carried out test after test, noting the results in his laboratory log along with such expressions as “very fine effect” and “effect was best yet.” After filling twelve pages, he closed the log with the words, “An excellent day's work.”2

  With the big Woolwich magnet at hand, Faraday now retested the various transparent substances that had earlier failed to produce any discernible rotation in the light's plane of polarization. They all now did so, and Faraday felt that his belief in the deep-lying unity of all nature's forces was being borne out. He had shown not only that light and magnetism were in some way connected but also that glass and a variety of other transparent substances, hitherto believed to be nonmagnetic, were affected by magnetism. The results prompted the question, are all substances in some way magnetic? This wasn't a new thought; he had actually already tried the effect of a magnet on many substances with no result. But now the muse was with him and he resolved to have another go.

  The most direct demonstration of magnetism in a supposedly nonmagnetic substance would be to put a sample of it between the poles of a strong magnet and get it to move, like a compass needle. The most promising substance was his piece of heavy glass, which was in the shape of a bar that was two inches long and half an inch square, but when he put it in a small paper sling and suspended it by a silk thread between the poles of the Woolwich magnet, there was no discernible effect. This disappointment didn't deflect Faraday from his task. Perhaps an even stronger magnet was needed; he procured half of a huge iron link from a ship's anchor chain and had it made it into a gigantic electromagnet, wound with 522 feet of wire and weighing, in all, 238 pounds.3 While this monster was in preparation, he tried many more experiments with the equipment he already had in the laboratory. All were designed to find further evidence of connections between nature's forces and all gave negative results, but in one experiment he narrowly missed detecting what later became known as the Kerr effect, in which the polarization of light is modified by reflection from a magnetized metal surface.

  On November 4, the new magnet was ready. When Faraday suspended his bar of heavy glass between its north and south poles, the bar swung and finally aligned itself at right angles to the lines of magnetic force. He had shown that glass had a new kind of magnetic property—one that didn't depend on light. The door was open to yet another great discovery. He substituted many other supposedly nonmagnetic substances for the glass, and all behaved in the same way. Crystals, powders, and various liquids (in thin containers), wood, beef, apple, bread, and even most metals aligned themselves at right angles to the magnetic lines of force. Iron, cobalt, and nickel were the exceptions that aligned themselves parallel to the lines of force. Summarizing the results vividly in his Experimental Researches in Electricity, Faraday wrote:

  If a man could be suspended with sufficient delicacy…and placed in the magnetic field, he would point equatorially, for all the substances of which he is formed, including the blood, possess this property.4

  Just as significant, in its way, as the great new discovery is Faraday's use here of the term magnetic field, which made its first appearance in his notebook a little earlier. The idea that space itself could be the seat of forces was now encapsulated in a word that would become indispensable to physicists—the field.

  Faraday was so taken up with the new work that he didn't want to leave the laboratory to attend the Royal Society meeting on November 20, at which his paper “On the Action of Magnets by Light” was to be presented—someone else had to read it for him. His spectacular results drew him to conclude that all solid and liquid substances react to magnetic forces—probably gases, too, though for the present he couldn't think how to prove it. He found that the substances that aligned themselves across the magnetic field, rather than parallel to it, were repelled by any magnetic pole, whether north or south. The great majority of substances were in this category, and he needed a new word to describe them. With William Whewell's help, he chose diamagnetic. A new word was now also needed to describe the minority of substances that aligned themselves parallel to the field—they had been called simply magnetic but that would no longer do now that all substances were believed to have magnetic properties. The name Faraday chose for materials such as iron, nickel, and cobalt was paramagnetic.

  Heartily encouraged by these successes, Faraday went on with his search for ways of unifying nature's forces. He now believed that magnetism was a universal property of matter, and he knew that it could affect a ray of light. What about the converse: Could light be made to electrify or magnetize objects? On a bright day, he shone a beam of sunlight down the length of a helical coil. No effect. He put a bar of unmagnetized steel inside the coils, but there was still no effect, even when he tried rotating the steel bar. This was one of hundreds of failed attempts at finding connections between one type of force and another—in some others he tried to connect electricity and magnetism with gravity. Scientists are still looking for that link, as part of their search for a single theory that will unite the four forces known today—the electromagnetic force, the weak nuclear force, the strong nuclear force, and gravity. (The first two of these have been unified in something called the electroweak force.)

  The Royal Institution's Friday Evening Discourses had by now become an institution in their own right. The lecture on April 3, 1846, turned out to be a historic occasion, although none of the audience recognized it as such and the whole thing happened by chance in rather bizarre fashion. Charles Wheatstone was to have been the latest in a long line of distinguished speakers,
but he panicked and ran away just as he was due to make his entrance. Although amply confident in his professional dealings as scientist, inventor, and businessman, Wheatstone was notoriously shy of speaking in public, and Faraday had taken a gamble when engaging him to talk about his latest invention, the electromagnetic chronoscope—a device for measuring small time intervals, like the duration of a spark. The gamble had failed, and Faraday was left with the choice of sending disappointed customers home or giving the talk himself. He chose to talk, but he ran out of things to say on the advertised topic well before the allotted hour was up.

  Caught off-guard, he did what he had never done before and gave the audience a glimpse into his private meditations on matter, lines of force, and light. In doing so, he drew an extraordinarily prescient outline of the electromagnetic theory of light, as it would be developed over the next sixty years. In his vision, shared by nobody else at the time, the universe was crisscrossed by lines of force—electric, magnetic, and possibly other kinds. The points where these lines met were the points at which we perceive matter to exist; his “atoms” were merely the centers of forces that extended through all space. When disturbed, the lines of force vibrated laterally and sent waves of energy along their lengths, like waves along a rope, at a rapid but finite speed. Light, he suggested, was probably one manifestation of these vibrations. He was emphatic that the vibrations were vibrations of the lines of force themselves, not of the supposed luminiferous aether—the imponderable medium that was thought necessary to transmit light waves. Faraday was doubtful that such an aether existed; he commented that it would have to be “destitute of gravitation and infinite in elasticity.”

  As if all this were not enough, he suggested that gravity, too, might be propagated by lines of force:

  The propagation of light and therefore of all radiant action occupies time; and a vibration of the line of force should account for the phenomena of radiation, so it is necessary that such vibration should occupy time also. I am not aware whether there are any data by which it has been, or could be ascertained, whether such a power as gravitation acts without occupying time or whether lines of force being already in existence, such a lateral disturbance of them at one end…would require time, or must of necessity be felt at the other end.5

  To his contemporaries, the idea that gravity could act through lines of force and might be somehow connected to electricity and magnetism seemed bizarre. In their eyes, gravitation was, by the law of Newton, a rectilinear force that acted instantaneously at a distance; electricity and magnetism were fluids; and light was a vibration of an imponderable substance. All this could be explained in elegant mathematics, yet here was a mathematical illiterate putting forward ideas that, if taken seriously, would threaten to overturn the established laws of the physical world. From today's perspective, it is clear that this was a historic moment. Faraday, the bold theorist, was making an advance announcement of a scientific transformation that has given us not only electromagnetic theory but also special relativity, radio, television, and much more besides.

  But that was not how things seemed at the time. Faraday may have regretted that he had let these private thoughts escape, so opening himself to ridicule, but the damage had been done. To limit it, he recapitulated the talk in an article for Philosophical Magazine called “Thoughts on Ray-vibrations,” which closed with the passage:

  I think it likely that I have made many mistakes in the preceding pages, for even to myself my ideas on this point appear only as a shadow of a speculation, or as one of those impressions on the mind which are allowable for a time as guides to thought and research. He who labors in experimental inquiries knows how numerous these are, and how often their apparent fitness and beauty vanish before the progress and development of real natural truth.6

  The general scientific opinion was that Faraday had ventured far out of his depth. His peers recognized that he was a superb experimentalist but felt that, having no mathematics, he was simply not equipped for any kind of theorizing. His latest outpouring seemed amply to confirm this view. Even Faraday's supporters were embarrassed. His first biographer, Henry Bence Jones, dismissed the “Thoughts on Ray-vibrations” paper in half a line; his third, John Hall Gladstone, didn't mention it at all; and his second, John Tyndall, who succeeded him at the Royal Institution, described it as “one of the most singular speculations that ever emanated from a scientific man.”7 Yet, speculative as they were, Faraday's ideas were essentially correct, as Maxwell and his followers would show.

  Whatever they thought of Faraday's ability as a theorist, scientists everywhere were stunned by his discovery of diamagnetism. Why did substances behave in this unexpected way? Hans Christian Oersted, now a grand old man of science, suggested reverse polarity: a magnetic pole induces a pole of the opposite kind in a paramagnetic substance like iron but one of the same kind in a diamagnetic substance like bismuth. Edmond Becquerel, whose father had worked with Ampère, came up with an ingenious theory analogous to Archimedes's principle in hydrostatics: All substances had magnetic power, but the more powerful ones (the paramagnetics) always tended to displace the weaker (the diamagnetics), just as a dense liquid displaces a less dense body like an air bubble. A neat answer, but to explain why diamagnetics were repelled by magnetic poles even in a vacuum Becquerel had to invoke an all-pervading aether that had its own degree of magnetism: less than the paramagnetics but more than the diamagnetics.

  In the course of his own investigations, Faraday, as was his way, carefully examined everyone else's theories and experiments. Oersted's reverse-polarity hypothesis was first on his list. He sprinkled filings of bismuth on paper over a magnet, reasoning that if Oersted was right, they would arrange themselves in the familiar pattern that iron filings did—each little filing would be the wrong way around, as it were, but the pattern would look the same. In other words, if the bismuth filings showed the same pattern as iron filings, they would reveal the presence of poles. However, Faraday's bismuth filings showed no signs of lining up. Oersted seemed to be wrong; there was no sign of reverse polarity.

  Then he heard about a brilliant experiment by the German physicist Wilhelm Weber, at that time a professor in Leipzig, that made him think again. It was a clever variation on Faraday's own iron-ring experiment, with a primary coil and a secondary one, but without the ring. A helical primary coil with a soft iron bar inside it was connected to a battery, forming a strong electromagnet. The secondary coil was another helix, coaxial with the first and a short distance away. It was wound on a wooden tube so that a bar, either of iron or of bismuth, could be pushed into the coil and pulled out again. The secondary circuit was completed by connecting the ends of the coil to a galvanometer, which would indicate the direction of any current. When Weber pushed the iron bar into the secondary coil, the galvanometer needle twitched briefly to the right, and when he pulled it out, the needle did the same but this time to the left. All as expected. But when he substituted the bismuth for the iron, the needle twitched the other way—a clear demonstration of reverse polarity.

  Fig. 6.2. Schematic layout of Weber's experiment. The wooden tube on the right held either a bar of iron or a bar of bismuth. (Used with permission from Lee Bartrop.)

  Faraday thought of a way to explain Weber's experiment. He had long believed that magnetism acted along lines of force. Was it not likely that some substances offered easier passage to the lines than others? He had shown that each substance had its own inductive capacity for static electricity; should it not similarly have its own capacity for conducting magnetic lines of force? Everything began to fall into place. Paramagnets conducted lines of force much better than the surrounding air did, so the lines converged on them. Diamagnets, on the other hand, conducted the lines more poorly than air, so the lines diverged from them.

  Fig. 6.3. Magnetic lines of force converge through a paramagnetic substance (left) and diverge through a diamagnetic one (right). (Used with permission from Lee Bartrop.)

  Consequently, in
air, paramagnets tended to move to regions where the lines were denser and the magnetic force stronger, and diamagnets to regions where the lines were more sparse and the magnetic force weaker. In general, the magnetic behavior of a material object would depend on the medium that surrounded it. If it conducted lines of force better than the surrounding medium, it would act like a paramagnet; and if it conducted lines of force less well than the surrounding medium, it would act like a diamagnet. So simple. There was no poles at all, in the sense of centers of force. There was no attraction or repulsion, and certainly no action at a distance; all that happened was that bodies reacted to the patterns of lines of force in their own localities—in other words, to the field.

  Weber's experimental result was simply a consequence of the grand design. When the iron was pushed into the secondary coil, the lines of force converged on it, thus cutting the wires in the coil and inducing a current. When, instead, the bismuth was pushed in, the lines of force diverged from it, again cutting the coil but this time inducing a current in the opposite direction.

  Diamagnetism had to wait for atomic theory and quantum mechanics for a full explanation. Broadly speaking, all substances are diamagnetic, but some are also paramagnetic and their paramagnetism swamps the feeble diamagnetism. Iron, cobalt, and nickel possess a further quality, now called ferromagnetism, which swamps both the other forms of magnetism and enables the metal to form a permanent magnet. Although their theories were deficient, Oersted, Becquerel, and Weber all emerge with credit. Faraday's theory turned out to be remarkably close to Becquerel's hydrostatic analogy, and he enjoyed the collegial rivalry with Weber, a fellow seeker of truth. When he had finished his paper, Faraday wrote to Oersted, promising to send him a copy and making a delightful observation on the whole episode: