Michael Faraday
1791 -1867
Electromagnetic Induction and Laws of Elcctrolysis
BY THE EARLY nineteenth century physics had advanced appreciably, particularly in the branch of mechanics, and one needed a strong background in mathematics to follow, professionally, this rapidly growing science. Yet the most prolific experimental physicist of that period, in fact, of any period in the history of physics, was a man whose formal schooling did not extend beyond the primary grades. He defied tradition in still another respect; by comparison with most of his predecessors, his greatest contributions were made relatively late in life. The two may bear some relation to each other, perhaps linked by a third characteristic; Faraday dedicated his adult life completely to the cause of science, allowing nothing to interfere with his singleness of purpose.
The son of a blacksmith who earned barely enough to support his family, Michael Faraday was born in Newington Butts, near London, on September 22, 1791. The Industrial Revolution was in full swing, as was another, more brutal transition, the French Revolution. It was largely because of Faraday's electrical discoveries that industry progressed so rapidly during the latter half of the nineteenth century. Faraday received what formal education he had between the ages of five and thirteen. By his own account this "... was of the most ordinary description, consisting of little more than the rudiments of reading, writing, and arithmetic at a common day-school." At thirteen he became errand boy for a bookbinder, and the following year was apprenticed to him for the usual seven-year period. He became deeply interested in books and took to educating himself by reading everything he could find, especially scientific books. These stimulated him so that he attended several lectures on chemistry delivered by Sir Humphry Davy (1778-1829). He was determined somchow to make science his career, and when his apprenticeship expired in 1812 he shortly applied to Davy for a position, presenting as evidence of his capabilities some notes he had taken of Davy's lectures. Davy, who was then president of the Royal Society, engaged Faraday as his assistant at a salary of twenty-five shillings per week plus living quarters at the top of the Royal Institution. Judging from all accounts, Faraday would have been happy to work without com-pensation, or with enough only to provide the barest essentials, so passionate was his desire to work in science.
His early work with Davy was largely in the field of chemistry; his first publication, in 1816, was an article describing an analysis he had made of a sample of caustic lime from Tuscany. During the next four years he published nearly forty articles and notes, climaxed by his discovery, in 1820, of two chlorides of carbon. He trained himself painstakingly, both in science and in the art of lecturing, becoming equally proficient in both. He began to lecture at the Royal Institution in 1827, a series that continued for more than thirty years, and marked him a brilliant popular lecturer. It has been suggested that Faraday's meager scientific background, particularly in mathematics, compelled him to seek simple explanations; hence his success in lecturing to the general public. This is perhaps an oversimplification. No doubt his lack of training had some influence on the manner in which he approached those problems on which he worked, but his success, either in research or on the lecture platform, could hardly be attributed to this deficiency.
Faraday became interested in electromagnetic phenomena following Oersted's discovery of the magnetic effects of a current in 1820 and Ampère's discovery, shortly afterward, of the action of currents upon one another. He performed experiments that led to the principie of the electric motor. These involved arranging magnets and current-carrying conductors so that either the conductors or the magnets were free to rota te continuously. Publication of his results involved Faraday in an unfortunate misunderstanding with Davy, who charged that Faraday had taken the idea from some work of William Wollaston (1766-1828) and Davy in the same área. Over Davy's opposition, Faraday was elected to fellowship in the Royal Society in 1824; yet the following year, on Davy's recommendation, he was appointed director of the laboratory at the Royal Institution. In his new position he was instrumental in reorganizing the Institution activities. He originated a series of evening meetings which became known as the Discourses, and which are still continued, and he started, in 1826-1827, the very popular Christmas Courses of Lectures, which were designed for juvenile audiences and continue to attract them in large numbers.
Following his brief experience with electrical phenomena, Faraday returned to chemical investigations. He liquified chlorine, which resulted in another dispute with Davy, and discovered the compound now known as benzene. By then very well known, Faraday was offered a number of gov-ernment positions. He became a lecturer at the Royal Military Academy, a member of the Scientific Advising Committee of the Admiralty, and scien-tific adviser to Trinity House. He returned to his electrical experiments from time to time in an effort to discover the induction of electricity, but failed to observe any effect until 1831, when he improved the coupling between his coils by means of an iron core. Faraday was forty years old when he discovered electromagnetic induction, yet he continued to make important contributions for many more years. Compared with many other notable contributors in physics, both before and after Faraday, this was a relatively advanced time of life for fundamental discovery.
Two years later he showed the identity of the different sorts of electricity: from voltaic cells, frictional effects, and electromagnetic induction. Faraday then turned to electrochemistry, where he extended the work of Davy, Berzelius, and others to formulate his well-known laws of electrolysis, together with a new terminology that is still in use. His fertile mind carried him from one problem to the next with remarkable agility. In 1834 he again investigated induction, including self-induction, which he discovered independently of Joseph Henry (1797-1878), and the induction of static electricity. He tried to account for the action at a distance of electromagnetic effects in terms of Unes of force, fictional lines which proved use-ful in explaning these phenomena, and which, by emphasizing the role of the médium in such effects, led eventually to the concept of field.
Ill health kept him from active work for a number of years, but he resumed his researches in 1844 with a study on the liquefaction of gases. The following year found him investigating the effect of a magnetic field on light, during which he discovered the rotation of the plane of polarization of a beam of light in a magnetic field (Faraday effect) and diamagnetism. Year after year he poured out a steady stream of important discoveries. And the honors heaped upon him failed to slow his enormous pace. He put aside ali activities that might interfere with his research. Twice he refused to accept the presidency of the Royal Society. He resigned his professorship in the Royal Institution in 1861, at the age of seventy, but continued his research for another year. His last investigation was a search for the split-ting of a light beam in a magnetic field. He was unsuccessful in this, but the effect was later discovered by Pieter Zeeman (1865-1943). Faraday died in 1867, ending a career that would have taxed the energies of several ordinary men.
His electrical discoveries were reported to the Royal Society and later published as Experimental Researches in Electricity, both in book forni and in the Philosophical Transactions. He was in the habit of numbering each paragraph consecutively throughout his work and of referring to earlier entries by paragraph number.
Of the extracts that follow, the first (through paragraph 120) was read before the Society on November 24, 1831, and appeared in Philosophical Transactions (1832), page 125. The second, on static electrical induction, was written in the form of a letter to R. Phillips, Esq., and appeared in Philosophical Magazine, 22 (1843), page 200. The last extract, beginning with paragraph 661, appeared in Philosophical Transactions (1834), page 77. Ali may be found, as well, in Faraday's Experimental Researches in Electricity (London: R. and J. E. Taylor, 1839-1855), volumes 1 and 2.
James Clerk Maxwell
1831-1879
The Electromanetc Field
OF THE major achievements in physics two may be singled out for the manner in which they served to synthesize great bodies of knowledge. To-ward the end of the seventeenth century Newton published his famous Principia (see Chapter 4), in which he unified, in terms of a few simple laws, ali that was then known of dynamics. Except for the corrections re-quired to accord with the theory of relativity for bodies traveling at very high speed, rarely encountered except in subatomic phenomena, Newton's laws of motion remain the foundation for the science of mechanies. Little was known in Newton's time about the nature of light; electrostatic and magnetostatic phenomena had been observed by Franklin, Gilbert, and others, but the connection between electricity and magnetism, or between these and light, as it later turned out, went unsuspected. Nearly two centuries later Maxwell did for electromagnetic phenomena what Newton had accomplished for mechanies. He summed up everything that was then known concerning light, electricity, and magnetism. But this was not ali. He formulated the mathematical strueture, now known as MaxwelVs equa-tions, that pointed up the unity of the "ether" and formed the basis for ali of electromagnetic theory. He predicted the existence of electric waves propagating through space, discovered later by Hertz, and he contributed, no less successfully, to other branches of physics, notably to the kinetic theory of gases.
James Clerk Maxwell was born to a well-to-do family in Edinburgh, Scotland, on June 13, 1831, a time when Faraday was in the midst of his most important electrical discoveries and only a few years before Lenz carne upon his principie of electric induetion. He grew up in a period noted chiefly, in physics, for progress in electricity, thermodynamics, and kinetic theory, and for the first clear formulation, by Hermann von Helmholtz (1821-1894), of a general principie of energy conservation.
Maxwell was an inquisitive but not precocious youngster who was tutored privately for a time before entering Edinburgh Academy at the age of ten. There, after a slow start, he began to display extraordinary talents, not only in mathematics, in which he excelled, but also in the writing of English verse, a practice by which he delighted his friends ali his life. After spending six years at the academy and three years at the University of Edinburgh Maxwell went on to Cambridge, where he was elected a scholar in Trinity College and received his degree with high honors in 1854.
He remained at Trinity for another two years, studying Faraday's works and engaging in his own researches on mathematics, geometrical optics, and a theory of color. In 1856 he was elected to the Professorship in Natural Philosophy at Marischal College, Aberdeen, where he completed the first of his many remarkable contributions in mathematical physics. This was his Adams Prize essay1 on the stability of the rings of Saturn, a work that placed him among the front ranks of his contemporaries. It was here also that he became interested in the kinetic theory of gases and solved the prob-lem of the distribution of velocities among the molecules of a gas, known generally as the Maxwellian distribution. While his formal proof of this importam law did not go unchallenged, there is no doubt of the correctness of the final result.
In 1860 Maxwell was appointed Professor of Natural Philosophy in King's College, London, where he remained for the next five years, a period that was his most creative. He completed his work on the theory of color, developed his theory of electricity and magnetism, contributed further to .the kinetic theory of gases, and investigated experimentally the viscosity of air at different temperatures and pressures. The last of these formed the subject of a Bakerian Lecture which Maxwell presented to the Royal Society early in 1866. The same year he published a paper on the Dynamical Theory of Gases, in which certain errors in his earlier work on kinetic theory, pointed out by Rudolph Clausius (1822-1888), were corrected. It was also during this period that he took active part, together with B. Stewart and F. Jenkin, in experiments to determine the value of the ohm in absolute measure.
Maxwell resigned his professorship at the end of the 1865 academic session to devote his time more fully to his scientific studies, and to the study of English literature, which he greatly enjoyed. During the next few years he completed the major part of his classic treatise on electromagnetic theory, although it was not published until 1873.2 While in retirement Maxwell partly inspired and lent active support to a movement to establish a chair in experimental physics and a physical laboratory in Cambridge University. In 1871 the university approved such a chair and Maxwell was appointed professor of experimental physics and director of the newly es-tablished Cavendish Laboratory, named for one of the most distinguished experimenters ever associated with Cambridge, Henry Cavendish.
Maxwell gave much of his time during the next few years to the building and furnishing of the new laboratory, which was officially opened in 1874 and soon became one of the leading physical research laboratories in the world. His own interests during this period were given chiefly to lecturing and to the task of editing the papers of Henry Cavendish,3 whose unpub-lished work on theoretical and experimenta] electricity impressed Maxwell by its originality and by the fact that it anticipated several discoveries later made by others. MaxwelFs crowning achievement was, of course, his theory of the electromagnetic field, in which, among other things he showed light to be an electromagnetic phenomenon, in the same sense as electricity and magnetism, and pictured the propagation of electric waves through the ether. Unfortunately, he did not live to see experimental confirmation of his prediction, for he died in the prime of his career, at the age of forty-eight, in November 1879, eight years before Hertz demonstrated the exist-ence of electric waves. It was then that Maxwell's genius was fully recog-nized and his lasting fame assured.
MaxwelFs contribution to the development of physics goes far beyond the solutions that he found to particular problems. As Einstein pointed out in commemoration of Maxwell's birth:
We may say that, before Maxwell, Physical Reality, in so far as it was to represent the processes of nature, was thought of as consisting in material particles, whose variations consist only in movements governed by partial differential equations. Since MaxwelPs time, Physical Reality has been thought of as represented by continuous fields, governed by partial differential equations, and not capable of any mechanical interpretation. This change in the conception of Reality is the most profound and the most fruitful that physics has experienced since the time of Newton.
Maxwell gave mathematical form to Faraday's conceptions of electrical phenomena. He derived a set of equations relating ali known electric and magnetic phenomena; that is, the equations give quantitative relations between the electric and magnetic fields, and the charges, currents, and time-varying currents producing these fields. They contain Coulomb's law of force between electric charges, as well as his corresponding law for magnetic poles , Oersted's discovery of the magnetic effect of an electric current , Ampère's work in electrodynamics, Ohm's law relating the current in a conductor to the potential difference across it, Faraday's law of electromagnetic induction , and, of course, Lenz's law (Chapter 11). They contain ali this and more, for they include MaxwelTs hypothesis that electric waves should proceed from oscillating electric currents and travei through free space with the velocity of light. Given the electric and magnetic forces everywhere in space at some initial time, Max-well's equations permit one to calculate them for ali future time. Faraday found it useful to regard action at a distance, typical of electric and magnetic phenomena (and gravitational as well), in terms of Unes of force, which were thought of somewhat as mechanical linkages in the médium surrounding a magnetic body or an electric charge. The médium thus be-came the seat of the electric and magnetic field, and since the effects could be transmitted through empty space, it was clear that the médium could not be of ordinary material form. The concept of an elastic ether as the médium for transmission of electric and magnetic forces grew by analogy with the physics of fluids, for it was difficult to conceive of a médium with-out some material form of its own. In fact, there is a similarity in structure and range of application between MaxwelFs equations and the fundamental equations of fluid dynamics. Despite the fact that the ether envisoned by Maxwell is no longer considered a useful physical concept because of in-consistencies disclosed by the theory of relativity, the idea of an electro- , magnetic field associated with the observed phenomena remains an im-portant conceptual feature of electromagnetism.
Beyond the purely electrical consequences of Maxwell's theory may be found several distinctive features. He assumed the continuity of current; that is, that ali currents flow in closed circuits. He introduced the idea that energy resides throughout the electromagnetic field, rather than in the conductors alone. He showed the identity of the electromagnetic médium with the luminiferous ether (the médium by which light is propagated) and concluded thereby that light is an electromagnetic phenomena. And he introduced the concept of displacement current, which in free space implies that when electric charge flows through a médium there results a current in addition to that represented by the motíon of the charges. This current he interpreted as being connected with a "displacement" of the electromagnetic médium, which accorded with his view of the ether as an elastic médium subject to stresses and strains; these giving rise to electric and magnetic forces.
The extract that follows is taken from an early paper of MaxwelTs, written during his last year at King's College, entitled A Dynamical Theory of the Electromagnetic Field, Philosophical Transactions, vol. 155 (1865), page 459.
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