OERSTED, HANS CHRISTIAN

(b. Rudkøbing, Langeland, Denmark, 14 August 1777; d. Copenhagen, Denmark, 9 March 1851)

physics.

Oersted was the elder son of an apothecary, Søren Christian Oersted, and his wife, the former Karen Hermansen. The demands of his father’s business and his mother’s superintendence of a large family forced his parents to place Hans Christian and his younger brother, Anders Sandøe, with a German wigmaker and his wife while they were still young boys. It was there that Oersted learned German by translating a German Bible and speaking with the couple. The brothers’ intellectual abilities were soon apparent, and neighbors did what they could to stimulate and educate them. In this way they picked up the rudiments of Latin, French, and mathematics. When Oersted was eleven, he began to serve as his father’s assistant in the pharmacy, thereby gaining a practical knowledge of the fundamentals of chemistry.

This was not much formal education; but when the two brothers arrived in Copenhagen in 1794, they were able to pass the entrance examination for the university with honors. At this point they parted intellectual company; Anders went on to become a jurist and Hans Christian pursued a career in natural philosophy. The most important of Oersted’s courses for his intellectual development was that offered on Kant and the critical philosophy. Oersted became a passionate Kantian and defender of Kant’s philosophical views, which were to be of fundamental importance to his scientific development. They were even to be the agent that led him to his most important discovery, electromagnetism.

At the University of Copenhagen, Oersted studied astronomy, physics, mathematics, chemistry, and pharmacy. In 1797 he received his pharmaceutical degree with high honors. The following year he became a member of the editorial staff of a new periodical, Philosophisk repertorium for faedrelandets nyeste litteratur, which was devoted to the propagation and defense of Kantian philosophy. Although short-lived, the journal provided Oersted with an opportunity to mature his philosophical thinking. An unpublished article that he wrote for it served as the starting point for his doctoral dissertation. In 1799 he received his doctorate with a thesis entitled “Dissertatio de forma metaphysices elementaris naturae externae,” which states Oersted’s appreciation of the importance of Kantian philosophy for natural philosophy and, in addition, provides a clue to the two areas in which he was to apply his scientific training: electromagnetism and research on the compressibility of gases and liquids.

After a brief stint as the manager of a pharmacy, Oersted set out in the summer of 1801 on a journey that was to complete his scientific education. The scientific world was in ferment over the recently announced discovery of the voltaic pile (1800), and Oersted eagerly pursued information relating to galvanism and its relation to chemistry. A small voltaic battery of his own invention gained him entry to others’ laboratories, and he gathered knowledge and ideas as he visited Berlin, Göttingen, and Weimar. Again the influences at work on him were twofold. At Göttingen he was given an introduction to Johann Ritter, who was then publishing on the chemical effects of current electricity. Ritter focused Oersted’s attention on the forces of chemical affinity and their relationship to electricity. Ritter’s highly unorthodox ideas on matter and force also stimulated Oersted to develop his own concepts. At Berlin he attended lectures on Naturphilosophie and met such Naturphilosophen as Henrik Steffens and Franz von Baader. He read Schelling and heard Friedrich Schlegel. As a result he developed his philosophical insights by comparing his own metaphysics with those of the Naturphilosophen. Since both Oersted and the Naturphilosophen drew their inspiration and basic ideas from Kant, it is no coincidence that Oersted’s later philosophy closely resembled Naturphilosophie.

Oersted was saved from the extravagances of a Schelling by his basic respect for empirical fact. Nevertheless, during this trip it was his philosophical penchant that dominated, for although he was suspicious of Schelling’s system-building, he swallowed as fact what were only wild guesses by Ritter and the Hungarian chemist J. J. Winterl. Indeed, it was as a defender of Winterl and Ritter that Oersted made his scientific debut in Paris.

The result was disastrous. Winterl’s “system” rested on two archetypal substances—Andronia and Thelycke—the essences of acidity and basicity. From these Winterl developed a chemistry of conflicting opposites which, because of its philosophical beauty, completely seduced Oersted. The French chemists, however, were scornful; and Oersted was blasted in the Annales de chimie et de physique. It was a valuable lesson. Henceforth, Oersted tended increasingly to hold his philosophical enthusiasms in check at least until he had some evidence for their plausibility. The lesson was driven home by his championing of Ritter’s work. To his dismay, he discovered that many of the experimental results his friend reported in the journals were, like Winterl’s Andronia, mere figments of his imagination. The pain of having made a scientific fool of himself taught Oersted the critical attitude necessary for the successful pursuit of scientific knowledge.

Oersted returned to Denmark in 1804, preceded by his reputation as an uncritical enthusiast. He had hoped for a professorship in physics but was disappointed by the failure of the warden of the University of Copenhagen to nominate him. He turned, instead, to public lectures, which became so popular that he finally gained an extraordinary professorship in 1806. He then began his own scientific work in earnest. A series of sober publications, among which was an excellent paper on acoustical figures (1810), gradually erased his earlier reputation. He began a steady advance in the academic hierarchy and in reputation. In 1824 he founded the Society for the Promotion of Natural Science and in 1829 became the director of the Polytechnic Institute in Copenhagen, a position that he held until his death. Oersted was a superb teacher and, almost single-handed, raised the level of Danish science to that of the major countries of Europe. He was also an ardent popularizer of science, writing articles and reviews for popular journals. Some of these writings, collected and published as The Soul in Nature, reveal his deepest philosophical and scientific beliefs.

There is a unity in Oersted’s scientific work that is rarely found in the results of someone whose researches ranged from the forces of chemical affinity, electromagnetism, and the compressibility of fluids and gases to the new phenomenon of diamagnetism. This unity was drawn from Oersted’s philosophy, inspired by his reading of Kant. Most Kantian scholars today would insist that Oersted totally misread Kant and came to conclusions to which Kant would have objected. That charge is probably correct; but what is important is that Oersted, and a number of other philosophers and scientists of the time, misread Kant in the same way. Basically, what Oersted thought Kant was saying was that science was not merely the dis-covery of Nature; that is, the scientist did not just record empirical facts and sum them up in mathematical formulas. Rather, the human mind imposed patterns upon perceptions; and the patterns were scientific laws. That those patterns were not arbitrary was guaranteed by the existence of Reason. Human reason corresponded to the Divine Reason, for man was made in the image of God. And, inasmuch as God had created Nature, it too shared in the Divine Reason. Thus human reason, unaided, could construct the laws of nature by virtue of its congruence with the Divine Reason. “Was der Geist versprecht, leistet die Natur” is a misquotation from Schiller’s Columbus—“Mit dem Genius steht die Natur in ewigem Bunde, Was der Eine Verspricht, leistet die andre gewiss”—that Oersted used more than once in The Soul in Nature. It represents the basic position of Naturphilosophie.

Oersted’s reading of Kant led him to more than an attitude toward nature. It also gave him what he felt was a firm metaphysical foundation for his beliefs. In a now neglected treatise, Metaphysische Anfangsgründe der Naturwissenschaft (1786), Kant had abandoned some of his agnosticism expressed in the antinomies in the Critique of Pure Reason. More particularly, whereas in the Critique he had argued that it was impossible for reason to decide between an atomistic or a plenist concept of matter, in the Metaphysische Anfangsgründe he came down on the side of the antiatomists. He argued that we experience only force; that force manifests itself in matter as the force of attraction that defines the limits of a body and the force of repulsion that gives a body the property of impenetrability. These two forces Kant called Grundkräfte (basic forces). Other forces, such as electricity, magnetism, heat, and light, he hinted, were merely modifications of the Grundkräfte under different conditions.

Oersted read both the Critique of Pure Reason and the Metaphysische Anfangsgründe while still at the university. His doctoral dissertation is a defense of the Metaphysische Anfangsgründe and an attempt to have it accepted in Denmark as a basic philosophical treatise. As early as 1800 it is possible to discern the two elements that were fundamental in Oersted’s later scientific work: the clear enunciation of the doctrine of forces and the disbelief in atoms. The first was to lead him, through the convertibility of forces, to the discovery of electromagnetism; the second seems to have been the stimulus behind his work on compressibility, for if solid, incompressible atoms existed, there ought to come a point when further compression of a gas or fluid was impossible.

In 1800, however, Oersted’s ideas were only half-formed. He was far more au courant in philosophy than he was in science. This is why his journey to Germany and France was so crucial. It acquainted him with men who were at the frontiers of science and forced him to bring his philosophical speculations down to earth.

The reentry was a difficult one. The “new” chemistry of Lavoisier and the other French chemists left him unmoved because it turned its back on the very questions, such as elective affinity and the true nature of acids and bases, that fascinated Oersted. Winterl’s system, on the other hand, was just what he was looking for. Instead of some thirty-odd elements, defined only empirically as the last products of a laboratory analysis, Winterl offered two fundamental and opposed substances. Andronia and Thelycke could be viewed as materializations of the Grundkräfte and chemistry could then, it was hoped, be seen as a Kantian science. Similarly, Ritter’s work in electro-chemistry appeared to Oersted as a development of Kantian thought and all of a piece with his own philosophy of forces. It was only when his philosophical theories and the empirical facts refused to fit together in repeatable experiments that Oersted’s critical faculties were awakened. It is significant that, at this point, he did not reject his philosophical faith. Instead, he rejected the physical systems of Winterl and Ritter. His first real scientific achievement was to create his own system, based upon his own experiments. The results appeared in German in 1812 and in a French translation in 1813. The title of the latter, Recherches sur l’identité des forces chimiques et électriques, indicates its purpose. From the Grundkräfte, Oersted hoped to deduce a system of chemistry that would be in accordance with the results of experiment.

The Recherches is an undeservedly neglected work of theoretical chemistry. The standard histories of chemistry barely mention it, yet it tried to come to grips with some of the major problems of the day. Specifically, it sought to make some sense of the various chemical reactions involved in combustion and the neutralization of acids and bases. By 1813 Lavoisier’s theory of acids and of combustion could be severely criticized. Humphry Davy’s work on chlorine showed that oxygen was not the only supporter of combustion. The fact that hydrochloric acid contained no oxygen also proved that oxygen was not, as Lavoisier had claimed, the principle of acidity. Oersted now tried to show how one could create a new chemistry based on forces, not elements.

According to Oersted, the Kantian Grundkräfte of attraction and repulsion manifest themselves in chemistry as combustibles and combusters. These forces are in conflict and when allowed, in combustion, to come to grips with one another, so to speak, produce the light and heat that are so preeminently the effects of combustion. But these two forces do not annihilate one another chemically; instead, they produce a higher synthesis—the acids and bases. Acidity and basicity, in turn, are opposites which unite to form the neutral salts. The supposedly Hegelian triad of thesis-antithesis-synthesis is here clear and is a standard aspect of Naturphilosophie. Although this analysis of fundamental chemical processes did provide a conceptual unity where before there was chaos, it left little impression upon Oersted’s colleagues. Nor did his final chapters, in which he examined the convertibility of other forces. By 1813 everyone admitted the chemical role of electricity, but Oersted’s treatment of it seemed to be of little help. What is of interest, at least to historians of science, is his discussion of the possibility of the conversion of electricity into magnetism.

It is important to stress that electromagnetism was not an effect to be expected according to the orthodox, corpuscular theories of the day. Coulomb seemingly had proved in the 1780’s that electricity and magnetism were two entirely different species of matter whose laws of action were mathematically similar but whose natures were fundamentally different. The conversion of one into the other was, literally, unthinkable. Hence, those who accepted Coulomb’s findings simply did not look for a magnetic effect.

For Oersted the situation was quite different. The Kantian doctrine of Grundkräfte led directly to the idea of conversion of forces. All that was necessary was to discover the conditions under which such conversions took place. The particular conditions for the conversion of electricity into magnetism were deduced by Oersted from the nature of electricity. Electricity to him was a conflict of the positive and negative aspects of magnetism, which conflict spread out in wave fashion in space. When the electric conflict was confined in a rather narrow-gauge wire, the result was heat. When the conflict was restricted still further by decreasing the diameter of the wire, light was produced. So, Oersted suggested in his treatise on the identity of chemical and electrical forces, the magnetic force should be produced when the electrical conflict is still further confined in a very narrow-gauge wire. In 1813, therefore, he had already predicted the existence of the electromagnetic effect. He was wrong, of course, on the conditions; and this error, together with his increasing teaching duties in the years that followed, prevented him from bringing his prediction to reality. The actual discovery was made in the early spring of 1820 and may best be given in Oersted’s own words.

Electromagnetism itself was discovered in the year 1820, by Professor Hans Christian Oersted, of the University of Copenhagen. Throughout his literary career, he adhered to the opinion, that the magnetical effects are produced by the same powers as the electrical. He was not so much led to this, by the reasons commonly alleged for this opinion, as by the philosophical principle, that all phenomena are produced by the same original power. … His researches upon this subject, were still fruitless, until the year 1820. In the winter of 1819–20, he delivered a course of lectures upon electricity, galvanism, and magnetism, before an audience that had been previously acquainted with the principles of natural philosophy. In composing the lecture, in which he was to treat of the analogy between electricity and magnetism, he conjectured, that if it were possible to produce any magnetical effect by electricity, this could not be in the direction of the current, since this had been so often tried in vain, but that it must be produced by a lateral action. This was strictly connected with his other ideas; for he did not consider the transmission of electricity through a conductor as an uniform stream, but as a succession of interruptions and reestablishments of equilibrium, in such a manner that the electrical powers in the current were not in quiet equilibrium, but in a state of continual conflict.… The plan of the first experiment was, to make the current of a little galvanic trough apparatus, commonly used in his lectures, pass through a very thin platina wire, which was placed over a compass covered with glass. The preparations for the experiments were made, but some accident having hindered him from trying it before the lecture, he intended to defer it to another opportunity; yet during the lecture, the probability of its success appeared stronger, so that he made the first experiment in the presence of the audience. The magnetical needle, though included in a box, was disturbed; but as the effect was very feeble, and must, before its law was discovered, seem very irregular, the experiment made no strong impression on the audience [“Thermo-electricity,” in Edinburgh Encyclopaedia (1830), XVIII, 573–589; repr. in Oersted’s Scientific Papers, II, 356].

Oersted could not be sure that the effect was the one he had anticipated, and therefore he deferred working on it for some three months. In July he resumed his researches and made certain that a current-carrying wire is surrounded by a circular magnetic field. The results appeared in a short paper, written in Latin, sent to the major scientific journals in Europe. The “Experimenta circa effectum conflictus electrici in acum magneticam,” dated 21 July 1820, opened a new epoch in the history of physics. From it followed the creation of electrodynamics by Ampere and Faraday’s Experimental Researches in Electricity.

Oersted’s second major area of research involved the compressibility of gases and fluids. It may be, as his biographer Kirstine Meyer implies, that he became interested in this problem by noting inconsistencies in the experiments of previous investigators. There may also, however, be a matter of theoretical importance involved. In all his experiments on compressibility, especially the compressibility of fluids, Oersted was intent upon proving that the reduction in volume was proportional to the pressure. If this were so, then the law of compressibility would provide a smooth pv curve. The existence of incompressible atoms, occupying space, would force a discontinuity in this curve if and when the point could be reached when the atoms were packed tightly together. Oersted’s system of forces permitted continual compression, and it seems plausible that his experiments on compressibility were intended to test the atomic hypothesis. The results were inconclusive, but his apparatus and critical acumen in detecting sources of error were of basic importance for later investigations of compressibility.

Oersted’s last scientific researches were on the phenomena of diamagnetism. He tried to account for diamagnetic substances by assuming reverse polarity and reverse inductive effects in substances that were repelled from, rather than attracted to, a magnetic pole. This work, in the late 1840’s, was made obsolete by Faraday’s investigations, which showed that the concept of polarity could not be applied to diamagnetics.

In his last years Oersted returned to his first love, philosophy. In a series of articles, published together in The Soul in Nature, he considered the relation between beauty and science. He still saw the hand of God in both. Beauty in art and music was the Divine Reason manifested in the harmonies of sight and sound. “Spirit and nature are one, viewed under two different aspects. Thus we cease to wonder at their harmony.” Oersted’s last work, The Soul in Nature, was left unfinished when he died on 9 March 1851. It was intended to express, in final form, the faith that had guided his entire scientific career.

BIBLIOGRAPHY

I. Original Works. There is an autobiography, in Danish, in Kofod’s Konversationslexikon, XXVIII (Copenhagen, 1828), but it deals only with Oersted’s earlier years. The published primary sources are H. C. Ørsted, Scientific Papers. Collected Edition With Two Essays on His Work by Kirstine Meyer, 3 vols. (Copenhagen, 1920); and Correspondance de H. C. Orsted avec divers savants, H. C. Harding, ed., 2 vols. (Copenhagen, 1920). There is also a considerable amount of unpublished MS material at the Royal Academy of Sciences in Copenhagen. Oersted’s views on philosophy, nature, and aesthetics are found in The Soul in Nature (London, 1852; repr. 1966).

II. Secondary Literature. The only biography to deal with Oersted’s entire scientific life is that by Kirstine Meyer, which introduces the Scientific Papers.

There are a number of specialized studies on Oersted. Bern Dibner, Oersted and the Discovery of Electromagnetism (Norwalk, Conn., 1961), is a study of Oersted’s most important work. Robert C. Stauffer’s “Speculation and Experiment in the Background of Oersted’s Discovery of Electromagnetism,” in Isis, 48 (1957), 33 ff.; and “Persistent Errors Regarding Oersted’s Discovery of Electromagnetism,” ibid., 44 (1953), 307 ff., first drew scholarly attention to the importance of Naturphilosophie for an understanding of Oersted’s scientific career.

L. Pearce Williams

Hans Christian Oersted

The Danish physicist Hans Christian Oersted (1777-1851) was the first to notice the interaction of electric current and the magnetic needle, thereby initiating the study of electromagnetism.

Hans Oersted was born on Aug. 14, 1777, in Rudköbing on the island of Langeland. Hans received his education from some friendly towns-people, but what was lacking in the way of competent teachers was amply supplemented by Oersted's extraordinary thirst for knowledge. In 1794 he went to Copenhagen and matriculated in science at the university in the fall of that year. The completion of his training in pharmacy came in 1797. Two years later he received his doctorate for a dissertation in which a new and competent theory of alkalies was proposed.

Oersted began his teaching career at the University of Copenhagen as lecturer in pharmacy. In 1801 he went abroad and sought out some of the best philosophic and scientific minds in Germany, the Netherlands, and France, where he spent the winter of 1802/1803. One of the things he immediately realized was the excitement created everywhere by Alessandro Volta's development of the electric battery 2 years earlier. Oersted's attention to this advance was evidence of his ambition to occupy eventually the chair of physics at his university. In 1803 his application was rejected. Clearly he was more of a chemist or pharmacist than physicist. But he kept experimenting and publishing, not only in chemistry but also in physics. His ingenious analysis of Chladni's acoustic figures finally secured for him the position of professor extraordinarius (associate) of physics in 1806. Three years later he published the first volume, dealing with mechanics, of a longer work planned to cover all areas of natural philosophy (physics).

During 1812 and 1813 Oersted visited in Germany and France. While in Berlin he published in German his Views of the Chemical Laws of Nature Obtained through the More Recent Discoveries. It came out the next year in French translation under the revealing title Researches on the Identity of Chemical and Electrical Forces. Oersted spoke in his Researches about the identity of magnetism and electricity with such assurance that he seemed predisposed to be the discoverer of electromagnetism. In the eighth chapter he noted the close analogies between the properties of magnetic and electric fields, their equally universal presence in nature, and certain reciprocal actions between them such as the loss of magnetism in steel due to rise in temperature and the simultaneous increase of the metal's electrical conductivity. He concluded, "An attempt should therefore be made to see whether electricity, in its most latent stage, has any effect on the magnet as such."

Oersted was also aware of the fact that lightning often resulted in the magnetization of pieces of iron, even to the point of altering the polarity of compass needles. He spent years in search of an elusive goal as he systematically vitiated his work by expecting the magnetic effect to be in the direction of the flow of the current. During those years of search, Oersted blossomed into a most sought-after lecturer both within and outside the university. In recognition of his consummate versatility in scientific matters, he was appointed the leader of a geological survey party charged with the exploration of the island of Bornholm.

In the spring of 1820 Oersted was giving a series of lectures on the interaction between electricity and magnetism before an advanced group of students. The responsiveness of the audience proved to be stimulating, and he was prompted to demonstrate the experimental evidence in support of one of his conjectures. It concerned the possible action of electric discharge on a magnetic needle placed near the circuit. As he expected a discharge through incandescence to be most effective, he inserted in the circuit a very thin platinum wire right above the magnetic compass. "The effect," he wrote, "was certainly unmistakable, but still it seemed to me so confused that I postponed further investigation to a time when I hoped to have more leisure." He resumed the experiments in July, carefully repeating all the steps in the presence of a group of colleagues and students. On July 21 he dispatched to scientists, universities, and learned societies throughout Europe the account of his findings in a four-page essay written in Latin, Experimenta circa effectum conflictus electrici in acum magneticam (Experiments about the Effects of an Electrical Conflict [Current] on the Magnetic Needle).

In the essay Oersted noted the dependence of the extent of the needle's motion on the strength of the battery, on the direction of the current in the wire, and on the needle's position with respect to the wire. He found that no effect was noticeable when the wire was perpendicular to the plane of the magnetic meridian. From the dip of the needle, he concluded that the magnetic effect existed in closed circles and not in spirals around the wire. He also found that neither metal plates nor wood nor stoneware would, when interposed between the wire and the needle, screen the effect. This meant that "the transmission of effects through all these matters has never before been observed in electricity and galvanism [current]. The effects, therefore, which take place in the conflict of electricity [current] are very different from the effects of the [static] electricities."

In his account of his discovery given in 1821, Oersted merely cared to correct the belief that the magnetic needle was in its actual position accidentally. It was only 10 years after the event that he emphasized, in the third person, in an article prepared for the Edinburgh Encyclopedia that "In composing the lecture, in which he was to treat of the analogy between magnetism and electricity, he conjectured, that if it were possible to produce any magnetical effect by electricity, this could not be in the direction of the current, since this had been so often tried in vain, but that it must be produced by a lateral action." In the same article Oersted also expressed his surprise over the fact that he failed to resume his experiments for 3 months and that those present were not impressed at all as the needle made the historic movement. One of the witnesses later claimed it was by chance that the compass needle was almost under the wire in the desired position.

The impact of Oersted's discovery on the scientific world was enormous. According to Oersted's own count, more than a hundred scientists published their comments and researches on electromagnetism during the first 7 years following its discovery. Oersted was showered with honors and awards. The Royal Society of London gave him the Copley Medal, and the French Academy awarded him a prize of 3,000 gold francs. But his greatest satisfaction was undoubtedly the spectacular growth of a new branch of physics, electromagnetism, which was to have revolutionary impact on modern culture.

Oersted was 43 when he made his great discovery. For the rest of his life he held the position of a leader in science. He had a major role in the establishment of the Royal Polytechnic Institute in 1829, of which he became the first director.

Further Reading

The most authoritative and exhaustive biography of Oersted is the book-length essay by Kirstine Meyer in her edition of H. C. Oersted: Scientific Papers (3 vols., 1920). A detailed biography by one of Oersted's contemporaries is in Bessie Zaban Jones, ed., The Golden Age of Science: Thirty Portraits of the Giants of 19th-Century Science by Their Scientific Contemporaries (1966). For additional material on Oersted's work and the general historical background see Edmund Taylor Whittaker, A History of the Theories of Aether and Electricity (1910; 2 vols., rev. ed. 1951), and Bern Dibner, Oersted and the Discovery of Electromagnetism (1961; 2d ed. 1963). □

Hans Christian Oersted

1777-1851

Danish Physicist

In 1820 Hans Oersted described the action of an electric current on a magnetized needle, demonstrating a connection between electric and magnetic forces unexpected by the majority of researchers in electricity and magnetism. His observation stimulated further work on electromagnetic phenomena by numerous investigators, including André Ampère (1775-1836) and Michael Faraday (1791-1867), leading to a grand synthesis in 1873 by English physicist James Clerk Maxwell (1831-1879). Maxwell's equations explained the behavior of light as an electromagnetic wave, led to the discovery of radio waves, and posed questions concerning the velocity of light which would lead to Einstein's theory of relativity.

Oersted received a broad education in scientific subjects at the University of Copenhagen, receiving a degree in pharmacy in 1797. Two years later he received his doctorate for a thesis that argued for the importance of the philosophy of Immanuel Kant (1724-1804) for the study of nature. In 1801 Oersted built his own version of the Voltaic pile, invented the year before by Alessandro Volta (1745-1827), and used it to gain entrance to laboratories throughout the German-speaking world. At the same time, he was able to attend lectures on the "Naturphilosophie" being developed by a number of German scholars. Always interested in philosophical simplicity, Oersted was suspicious of the new chemistry introduced by Antoine Lavoisier (1743-1794) with its numerous chemical elements, preferring the "chemistry of opposites" proposed by Hungarian chemist J. J. Winterl. Drawing from Kant the notion that there are only two basic forces, attraction and repulsion, Oersted came to believe that the various forces apparent in nature were all versions of the same basic two.

By 1820 the majority of physicists believed that electricity and magnetism were unrelated, although they had some superficial similarities. Oersted, in contrast, believed that the same electric current that could produce light or heat or chemical effects depending on the size and surroundings of the wire that carried it must also be the source of magnetic effects, though not in the direction in the current. By sending a current down a wire passing by a magnetic compass, he was able to produce a slight deflection of the compass needle. Refining the experiment over the next few months, he was able to demonstrate that the lines of the magnetic field run in circles about a current carrying wire.

Building on Oersted's result, the French physicist Ampère , and later Biot and Savart, was able to determine the detailed magnetic interaction between electrical circuits. Ampère also provided an explanation for the existence of permanent magnets as materials with built-in electrical currents. Seeing that an electrical current could produce magnetic effects, the logical question arose as to whether a magnetic field could produce an electrical effect. English physicist and chemist Michael Faraday provided the correct connection about 10 years later by the discovery of electromagnetic induction, that is, the creation of a voltage by changing the magnetic fluxthrough a surface. The results of Ampère and Faraday attracted the attention of the James Clerk Maxwell, who had the mathematical skills needed to express the experimental conclusions in the form of differential equations. Maxwell then noted that his equations could take the form describing transverse waves in empty space, waves that traveled with a speed related to the force constants appearing in the Coulomb electrostatic force law and the Ampère law for the magnetic force. Numerically, the speed of Maxwell's waves turned out to be three hundred million meters per second, the known speed of light in vacuum, thus suggesting that light was an electromagnetic wave. The creation of lower frequency radio waves, and Einstein's theory of relativity, needed to explain how the speed of Maxwell's waves could be the same as measured by all observers soon followed Maxwell's discovery.

Oersted remained fascinated with philosophy and at the time of his death was writing articles for a book entitled The Soul in Nature. While Oersted's bias against atomic theory and the growing list of chemical elements was certainly incorrect in hindsight, the search for a unified description of the forces of nature remains an important theme in modern physics.

DONALD R. FRANCESCHETTI

Oersted, Hans Christian (1777–1851) Danish physicist and professor at Copenhagen University. He took the first steps in explaining the relationship between electricity and magnetism, thus founding the science of electromagnetism. The oersted unit of magnetic field strength is named after him.