Faraday, Maxwell, and the Electromagnetic Field Page 9
He was prompted to turn to electrostatics by a question that arose naturally during his investigations in electrochemistry. How does the process of electrolysis start? What happens when the battery is first connected to the circuit? Faraday reasoned that immediately before decomposition begins, all the particles in the chemical solution, or electrolyte, must, if only for a moment, be in a polarized state—that is, in a state of tensile strain—with their positively and negatively charged parts being pulled in opposite directions. And, while in this state, they will tend to arrange themselves in chains, each particle held close to its neighbors in a sequence of alternating positive and negative charges. These, he surmised, are the chains along which the chemical exchanges will take place—curved lines of electrical action. This state of strain is maintained only fleetingly in the electrolyte—as soon as the particles begin to decompose, the strain is released, at least in part, and chemical exchanges take place along the curved lines of electrical action, constituting a current.
For Faraday this was more than just an explanation of how chemical decomposition begins in an electrolyte, it was the starting point of a process of thought that completely upended the Newtonian view of the physical world. A grand scheme began to form in his mind. Perhaps the same polarized state of strain that starts the process of electrolysis was actually the source of all electrical action in materials. If the electrolyte were replaced with a metal conductor, the metal would not be able to support the strain in a sustained way and a current would flow freely. On the other hand, if the electrolyte were replaced by an insulating material, the strain would be maintained—though under extreme strain the insulation would eventually break down and there would be a sudden discharge, like a spark through air. In all likelihood, Faraday thought, no materials were actually perfect conductors or perfect insulators; even the best conductors would hold onto a small part of the strain, and even the best insulators would leak a little current. But the idea that insulators could maintain the polarized state of strain, even if not quite perfectly, had momentous implications. This state of strain in insulators seemed to Faraday to be an electrical equivalent of the electrotonic state that he had supposed to exist in and around a wire when it was close to a magnet. Like the electromagnetic type of electrotonic state, the electrostatic one proved impossible to detect directly, though Faraday tried everything he knew to try to reveal it.
There were two principal electrostatic effects that traditional theories attributed to action at a distance. One was the mechanical force between charged bodies. The other was electrical induction: a charged body induced an opposite charge in another body nearby. Faraday could now explain how this happened. There was no mysterious action at a distance; induction occurred along the chains of contiguous polarized particles running from one body to the other through the insulating medium between them. If the first body were positively charged, it would attract the negative parts of the particles of the intervening medium that were next to its surface, so the particles at the far end of the chains next to the surface of the second body would be positive and would attract negatively charged parts of particles on the surface of that body, thus inducing a negative charge there.
If Faraday's idea were correct, the inductive effect between the initially charged body and the other would depend on the propensity of the intervening medium to form the inductive chains, and so it would be expected to vary from one type of insulating substance to another. He made an apparatus to test this very point, using two concentric metal spheres with various substances, including air, shellac, wax, and sulfur, in the space between them, and found that the effect did, indeed, vary widely. Each substance had its own specific inductive capacity—another great discovery. In the course of this experiment, Faraday was able to observe something he already suspected: Induction took time to act through the insulating material—a finding that later posed a problem for the Atlantic telegraph-cable project.
What about the forces between charged bodies? While imagining the paths that the charged fragments of particles followed when traveling between the electrodes in a tank of a chemical solution, Faraday had been reminded of the pattern that iron filings took when sprinkled on a piece of paper over a magnet. It was this pattern that had prompted his idea of lines of magnetic force to explain electromagnetic induction. Now he had envisaged chains of contiguous polarized particles to explain electrostatic induction, and the pattern of curves they formed in his mind also resembled that of iron filings around a magnet. These chains of induction were, surely, lines of electric force. The tension along chains of stretched polarized particles would explain why opposite charges attracted one another.
Faraday also surmised that his lines of electric and magnetic force repelled one another sideways, though the full significance of this property seemed to come to him only slowly—an eminent biographer reports that he was “dimly aware” of it at this time.13 This seems curious, as the repulsion between lines of force so neatly explains that between like charges and between like poles.14 It is not hard to find other examples of Faraday's fallibility. He had earlier toyed with the (correct) idea that the electromagnetic version of his electrotonic state bore a close analogy to momentum in mechanical systems, only to reject it.
In fact, these examples serve to remind us that he was working in completely unknown territory, struggling to make sense of the strange, and sometimes apparently contradictory, findings from his experiments. The wonder is not that he missed the odd point but that he somehow managed, from such confusing evidence, to produce ideas that were so unusual as to be almost impossible to describe in words, yet that turned out to be correct. Some of his ideas were not understood by anyone else until first the great physicist William Thomson (Lord Kelvin) and then James Clerk Maxwell, both Scotsmen from the following generation, expressed them in mathematical language.
By Faraday's scheme, the electric charges that appeared on the objects were simply the end points of lines of induction, which had to be positive at one end and negative at the other so the total net charge was always zero. And because conducting substances like metals cannot support inductive strain, the charge exists only on their surfaces, where they abut the lines of induction. One consequence of these supposed properties of matter was that no induction from outside would penetrate a closed metal container. Faraday carried out many experiments on this and related themes, but the most spectacular one was performed for an astonished audience in the Royal Institution lecture theater. Faraday, the showman, built a wooden-framed cube twelve feet across, coated it with tin foil, and, with the audience present, stepped inside. The metal surface was then charged by an electrostatic generator to many thousand volts. Sparks flew from the corners while Faraday, calmly sitting inside, checked that none of the charge had penetrated inside the box. This was the first demonstration of the Faraday cage. Now everyone travels in cars and airplanes confident that if lightning strikes the vehicle, no harm will come to the passengers.
He chose a simple experiment to show that electrostatic induction does not always act in straight lines, as the action-at-a-distance adherents believed, but can act along curved paths. Faraday placed a brass ball near a negatively charged shellacked rod. Metals did not transmit electrostatic induction, so if the induction acted in straight lines, the ball would have acted as a screen and cast a sharply defined shadow—a region which no induction from the rod would reach. But he found that the negative charge on the rod actually induced a positive charge on objects placed entirely within the supposed shadow. The lines of induction must have curved around the brass-ball screen. This, to Faraday, was confirmation not only that electrostatic induction, and hence electrostatic forces, acted along curved lines but also that they acted from particle to particle—since that was the only way they could follow curves.
By June 1838, he was able to include in his published Experimental Researches in Electricity a ten-point summary of his theory of the nature of static electricity:
The theory
assumes that all the particles, whether of insulating or conducting matter, are as whole conductors.
That not being polar in their normal state, they can become so by the influence of neighbouring charged particles, the polar state being developed at the instant, exactly as in an insulating conducting mass consisting of many particles.
That the particles when polarized are in a forced state, and tend to return to their normal or natural condition.
That being as whole conductors, they can readily be charged, either bodily or polarly.
That particles which being contiguous are also in the line of inductive action can communicate their polar forces one to another more or less readily.
That those doing so less readily require the polar force to be raised to a higher degree before this transference or communication takes place.
That the ready communication of forces between contiguous particles constitutes conduction, and the difficult communication insulation….
That ordinary induction is the effect resulting from the action of matter charged with excited or free electricity upon insulating matter, tending to produce in it an equal amount of the contrary state.
That it can do this only by polarizing the particles contiguous to it, which perform the same office to the next, and these again to those beyond; and that thus the action is propagated from the excited body to the next conducting mass, and there renders the contrary force evident in consequence of the effect of communication which supervenes in the conducting mass upon the polarization of the particles of that body.
That therefore induction can only take place through or across insulators; that induction is insulation, it being the necessary consequence of the state of the particles and the mode in which the influence of electrical forces is transferred or transmitted across such insulating media.15
In brief, static electricity showed itself in material substances as a form of strain that was passed on from each particle to its neighbors. In a closed circuit, when the strain collapsed, a current flowed. Faraday was writing at a time when most physicists still believed that electricity was an imponderable fluid (or two) that exerted forces at a distance along imaginary straight lines. One of his biographers, Sylvanus P. Thompson, has aptly described how he was able to see far beyond the range of his contemporaries:
Living, working and daily investigating in his laboratory, in the presence of all his apparatus, he gave his thoughts free play around the phenomena, incessantly framing theories to account for the observed facts and then testing his ideas by experiment, never hesitating to push these ideas suggested by his experiments to their logical conclusion, no matter how much they may have diverged from the accepted scientific theories of the day…. [H]e worked on and on with a scientific foresight which could be called miraculous. His experiments, even those which at the time seemed unsuccessful, in that they yielded no immediate positive results, have proved to be a deep mine of richness for the scientific minds that followed him.16
Faraday's accomplishments during the 1830s almost defy belief. By 1838, his set of Experimental Researches in Electricity had reached series 14. And beside his duties as director of the Royal Institution, he also lectured at the Royal Military Academy in Woolwich and, of course, in the Institution's own theater. He also broke his earlier resolution by taking on some important analysis and consultancy jobs, some from a strong sense of social or patriotic duty, and some to top up the institution's coffers or to boost his modest salary. One of these was to act as scientific adviser to Trinity House, the organization responsible for Britain's lighthouses. Sarah took him away to the country or the seaside when she could, but the work began to take its toll. The nervous headaches that had occasionally afflicted him in his youth became more frequent; he could no longer keep up the intense concentration that his style of work required; and he suffered increasingly from lapses of memory.
The doctor ordered a month off, then another and another. Faraday went to the laboratory only intermittently and found that even writing to friends was hard work. In one letter, he said his memory was so treacherous that by the time he got to the middle of a sentence he couldn't remember how it started. On his forty-ninth birthday, Faraday felt sure that his days of making great discoveries were over. He was wrong.
Faraday, the compulsive experimenter, did no work in his laboratory for two years. Refreshed by enforced holidays at the seaside and in the mountains, he glowed with physical health, thinking nothing of walking thirty miles in a day over an Alpine pass. He took delight in his family and friends and in the beauty and power of nature—he loved to run out in a thunderstorm—but without experimental work, his life was incomplete. He summed things up in a notebook entry, “I would gladly give half my strength for as much memory—but what have I to do with that? Be thankful.”1
Strictly rationing his mental exertion, Faraday did what work he could. He gave lectures, including the Christmas ones to children in 1841 and 1842; advised Trinity House on ventilation in lighthouses; and led government inquiries into explosions at a gunpowder factory and a coal mine. In 1842, he was tempted to return to the laboratory to investigate William Armstrong's well-publicized discovery that steam issuing from a boiler was electrically charged. After finding that the charge came from frictional contact between water droplets and the vent pipe, he again left the laboratory to Sergeant Anderson's care, not returning until 1844, when he resumed some earlier work on the liquefaction of gases and made a little progress, succeeding with ammonia and with Davy's one-time specialty, the mind-altering nitrous oxide.
In June 1845, Faraday went to the annual meeting of the British Association for the Advancement of Science. He wasn't a regular participant, but this year's gathering was at Cambridge, where William Whewell, who had suggested the words ion, anode, and cathode to him, was master of Trinity College. Between sessions, an engaging young Scot introduced himself—he was a newly elected fellow of Peterhouse, a college at Cambridge—by the name of William Thomson. Now remembered as Lord Kelvin, Thomson was a prodigy who had enrolled at Glasgow University at the age of ten and taken top prizes in all subjects. Mathematics was a particular passion of his, and at the age of seventeen he had shown that Faraday's electric lines of force could be represented by the same equations that Joseph Fourier had derived for the flow of heat in a metal bar. This was a historically significant paper, giving the first indication that Faraday and mathematics were compatible, but, coming from a teenager in his first term at Cambridge, it had attracted little attention. Faraday seems not to have noticed it. All the same, he cannot have failed to be impressed by the young man with the quicksilver mind—everybody was—and when a follow-up letter from Thomson arrived in August, he was inspired to take up the path of discovery once more.
To Thomson, as to Ampère, mathematics was the language of science. Where Faraday had to do his own experiments to understand a topic, Thomson had to write his own equations. His first impression of Faraday's Experimental Researches in Electricity, which gave not a single equation, was that they seemed to be written in a perversely cumbersome foreign tongue, but once Thomson saw the analogy of lines of force with Fourier's mathematical theory of heat flow, he began to take the idea of lines of force seriously—the first person, apart from Faraday himself, to do so. He was intrigued to discover that exactly the same results could be derived from Coulomb's and Ampère's theory of electrostatic forces, so were Faraday's lines of force simply another way of formulating instantaneous action at a distance between point charges? Thomson thought so at first, but he noted Faraday's finding that electrical induction took time to act, rather like Fourier's heat flow; and he found that the using the analogy between lines of force and heat flow actually made some calculations a lot simpler. He began to think that Faraday could be right—that lines of force could have a physical existence and that electrical forces could be the manifestation of some kind of strain in the medium between the charged objects.
Thomson knew that internal strains in a
transparent substance could be detected by shining polarized light through it—that is, light in which the transverse wave vibrations are all lined up in a particular plane rather than being randomly oriented as in ordinary sunlight. Scientists had found that when polarized light passes through a mechanically stressed transparent substance, the alignment of its vibrations, formally the plane of polarization, is altered. Taking Faraday's idea that electric lines of force represented a kind of strain in the medium that carried them, Thomson wondered whether that strain might be detected by subjecting a transparent substance to electrical force, shining a beam of polarized light through it, and observing any change in the plane of polarization. Perhaps Faraday had already tried the experiment; Thomson wrote to ask.
Over the years, Faraday had been plagued by futile suggestions from well-meaning enthusiasts, but this one was different. He had indeed tried the experiment that Thomson had suggested—more than once and to no effect—but it was a good suggestion, and, amazingly, it had come from a young mathematician who had taken the trouble to try to understand his work. This was just the stimulus Faraday needed; he thanked Thomson promptly and resolved to try again.
He first tried sending the polarized light through various liquids undergoing static electrification from an electrostatic generator: distilled water, sulfuric acid, and solutions of copper sulfate and sodium sulfate. He shone the light beam first parallel to the direction of electrification and then across it, but the direction of its transverse vibrations remained unmoved. Undeterred, he tried the effect of electric currents—steady, rising, falling, pulsed—and, finally, sparks. Then he substituted various solid, transparent substances for the liquids—plate glass, quartz, Iceland spar, and others—but he found that he had nothing to show for a fortnight's work.