Faraday, Michael (1791–1867)
Faraday, Michael (1791–1867)
Michael Faraday, the British chemist and physicist, came from a poor family and had no formal schooling beyond the elementary level. While a bookbinder's apprentice, he became interested in chemistry and electricity. Faraday took notes on a series of lectures given by Sir Humphry Davy, the leading British chemist, presented them to Davy, and soon afterward, at the age of twenty-one, was appointed laboratory assistant to Davy at the Royal Institution (London). He became director of the laboratory in 1825 and Fullerian professor of chemistry at the institution in 1833. His early scientific work in chemistry included the discovery of several new compounds and the liquefaction of chlorine and other gases. In 1831, Faraday discovered electromagnetic induction, or the creation of electric currents in a conductor by changing currents or moving magnets in the vicinity; this phenomenon is the basis of the electrical generator. This was followed by a series of investigations demonstrating with greater certainty than had been previously achieved the identity of the nature of the electricity generated by friction, voltaic cells, electromagnetic induction, and other means. Extensive experiments in electrochemistry led Faraday to the enunciation of his laws of electrolytic decomposition in 1833. The source of the power of the voltaic pile, or battery, was the object of his subsequent research. He investigated the electrical properties of insulators, or dielectrics, in 1837. In 1845 he discovered that the plane of polarization of light was rotated on passing through a transparent diamagnetic substance in the direction of externally applied lines of magnetic force. At the same time he began his investigation of diamagnetism. In his last years he suffered from loss of memory, and he ceased his researches in 1855. He was a member of a small Christian sect, the Sandemanians, and was noted for his gentleness of character.
Faraday is generally regarded as one of the greatest of all experimental scientists. The truth of this, adequately attested to in the three-thousand-odd paragraphs of the Experimental Researches in Electricity, should not be allowed to obscure the fertility of his imagination and conceptualizing powers and the guiding role of theoretical principles in sustaining his persistent research. His most important contribution to physics is probably the concept of lines of force, which was the beginning of the development of field theory. The accepted approach to electrodynamical phenomena at the time was to express the forces between charges mathematically as direct actions at a distance, an approach that was to prove unfruitful. Faraday was not trained in the mathematics necessary for this tradition. In order to represent the action of electromagnetic induction, he envisaged the space surrounding magnets to be filled with lines of magnetic force representing everywhere the direction of the force that would be experienced by a magnetic pole introduced from outside in the manner of the lines formed by iron filings sprinkled on a paper resting on a magnet. The lines of magnetic force have not only direction but also sense—that is, a north magnetic pole is pushed one way along them, and a south pole is pushed in the opposite sense; furthermore, their concentration near a given point represents the intensity of the magnetic force at that point. Each such line forms a closed loop, beginning or ending nowhere, but in the case of a magnet passing through its substance from one pole to the other. In these terms the law of electromagnetic induction may be expressed: "The quantity of electricity thrown into a current is directly as the amount of curves intersected" (Experimental Researches in Electricity ). In James Clerk Maxwell's famous words:
Faraday, in his mind's eye, saw lines of force traversing all space where the mathematicians saw centres of force attracting at a distance: Faraday saw a medium where they saw nothing but distance: Faraday sought the seat of the phenomena in real actions going on in the medium, they were satisfied that they had found it in a power of action at a distance impressed on the electric fluids. (Preface to the first edition of the Treatise on Electricity and Magnetism )
In most of Faraday's researches the concept of lines of force was used merely as a "representative aid" and was not meant to include "any idea of the nature of the physical cause of the phenomena." This cautiousness was a mark of Faraday's methods; in the choice of terminology to describe new phenomena, for example, he carefully attempted to avoid suggesting anything more than they warranted. However, at times Faraday allowed himself to speculate, and in 1852 he considered "the possible and probable physical existence of such lines" ("On the Physical Lines of Magnetic Force"). On the basis of arguments that can be characterized only as suggestive (such as that the magnetic lines are curved), he hypothesized that magnetic lines of force have physical existence and contrasted this with gravitation, where there was no evidence that the lines of force are anything but abstract and ideal. In a charming talk published in 1846 ("Thoughts on Ray-Vibrations") Faraday speculated that the atoms of matter might be simply point centers of force, as Roger Joseph Boscovich had suggested in the eighteenth century, or, in Faraday's terms, points from which lines of force spread into space. The extension of the atom may be identified with the extent of these lines, so that each atom would occupy all space and atoms would be mutually penetrable. Light might consist of vibrations in these lines, possibly obviating the need for an ethereal medium for its propagation; on the other hand, he suggested elsewhere that the lines might represent a condition of the ether, "for it is not at all unlikely that, if there be an ether, it should have other uses than simply the conveyance of radiations."
Faraday's geometric-intuitive representation was in particular rejected by the Continental electrodynamicists, and in 1846 Wilhelm Weber developed a theory of forces acting directly at a distance between charges that included the phenomena of electromagnetic induction. The validity of the lines of force concept was vindicated by the theoretical researches of William Thomson and particularly of Maxwell, who regarded his task to be putting Faraday's ideas into mathematical notation. It was with this motive that Maxwell developed his electromagnetic field theory, which, reinterpreted in quantum terms, remains the accepted theory of electromagnetic action and which was the prototype of all the field theories that dominate physics today. Maxwell's early representation of the lines of magnetic force as vortexes in the ether spinning about these lines as axes was in part suggested by Faraday's discovery of the magnetic rotation of the plane of polarization of light. The existence of this magnetic effect upon light had confirmed others in their speculations that light was some sort of propagated electromagnetic phenomenon, and the rotation of the plane of polarization suggested to Thomson and Maxwell that magnetism was in some way a rotatory effect, or, in contemporary terminology, the magnetic field is a pseudovector field.
Faraday was one among many who gave adumbrations of the generalized principle of conservation of energy, the clear expression of which is credited to Julius Mayer, James Joule, and Hermann von Helmholtz. His convictions regarding the interconvertibility of forces led him, from Hans Christian Ørsted's generation of magnetism by an electric current, to seek that generation of a current from magnetism that he found. In a lecture of 1834, Faraday spoke explicitly of this mutual convertibility, but he did not proceed further to specify how he conceived of the "forces" or "powers" that might be conserved or to discover quantitative relations. In connection with his investigations in 1840 of the source of the action of the voltaic pile he cited this principle against the contact theory, according to which the mere contact of two metals was the source of the current so that there would be a "creation of power" out of nothing, and in favor of the chemical theory, which found the source in the chemical actions occurring in the pile.
Faraday's work in electricity, including electrochemistry, is to be found in his Experimental Researches in Electricity, 3 vols. (London: R. and J. E. Taylor, 1839, 1844, 1855); his chemical work is collected in Experimental Researches in Chemistry and Physics (London: R. Taylor and W. Francis, 1859). Of particular interest are the speculative papers "Thoughts on Ray-Vibrations," in Philosophical Magazine 28 (1846): 345–350, reprinted in the Experimental Researches in Electricity, Vol. III, pp. 447–452, and "On the Physical Lines of Magnetic Force," in Royal Institution Proceedings June 11, 1852, reprinted in Experimental Researches, pp. 438–443. The standard biography is H. Bence-Jones, The Life and Letters of Faraday, 2 vols. (London: Longmans, Green, 1870). See also L. P. Williams, Michael Faraday (New York: Basic, 1965). A brief and lucid treatment of the development of his researches is given by John Tyndall in Faraday as a Discoverer (London: Longmans, Green, 1868).
Arthur E. Woodruff (1967)