Three Stories about Electromagnetism

It is easy to forget that the ``old-fashioned'' physics we learn today was once new and exciting and that it was discovered by people of incredible imagination. I want to tell you the stories of three of the people who made important contributions to electromagnetism -- John Priestley, Hans Christian Ørsted, and Michael Faraday. While I will stop along the way to discuss some hard-core physics, the primary purpose is to learn who these men were and, perhaps, to get an idea of what led them to their most important discoveries.

Joseph Priestley

Priestley is probably best known as a chemist. His work with gases, particularly his discovery of oxygen, is well known. But Priestley was an extremely prolific writer who made contributions to education, moral philosophy, theology, metaphysics, political economy, history, and physical science. In particular, in 1766 he performed an experiment of great beauty to verify Coulomb's Law (i.e., the force between two electric charges is inversely proportional to the square of the distance between them).

Joseph Priestley was born at Fieldhead in the northern English county of Yorkshire on March 13, 1733. He was the oldest of the six children -- four sons and two daughters -- of Jonas Priestley, a dresser and finisher of cloth and Mary Swift, a farmer's daughter. Mary sent Joseph as an infant to live at his maternal grandfather's farm some miles away. He returned to his father's house after his mother's death only to be adopted by his childless Aunt Sarah (Mrs. John Keighley, his father's sister) when he was nine. It was at the Keighly household that he was exposed to discussions of theological questions and to liberal political attitudes for many of the dissenting ministers of the neighborhood were welcome there. He remained with his aunt until her death in 1764. During his boyhood, Joseph went to the local schools where he learned Greek and Latin at an early age, and during school holidays, Hebrew. In his mid teens he fell seriously ill with tuberculosis, was forced to drop out of school, and temporarily abandoned his plan of entering the ministry. As he gathered strength after his illness, he taught himself French, Italian, and German and learned Chaldean, Syrian and Arabic. He also taught himself the rudiments of geometry, algebra and mathematics. Soon he was ready to pursue the goal of his boyhood, the ministry. By this time, however, he had started to question some of the orthodox tenets of the Calvinist faith. He decided not to go to the Academy at Mile End, where he would have been required to profess his loyalty every six months to ten printed articles of the faith. He went instead to the more liberal Daventry Academy.

At this time, Dissenting Academies were the center of liberal education since the doors of the great universities were closed to nonconformists. The academy at Daventry offered Joseph the opportunity of learning traditional subjects as well as natural and experimental philosophy. It was here that his interest in natural phenomena and experimentation was encouraged. But his dominant inclination was still towards theology. After Daventry, Priestley accepted his first position with a poor congregation at Needham Market in Suffolk where he remained for three years. He was not very successful. His preaching was hampered by a serious speech impediment, and his popularity was diminished by his Unitarian tendencies. He was invited to preach at Nantwich in Cheshire. The congregation there was more accepting of his unorthodox theology, and he felt more welcome. His responsibilities expanded to that of schoolmaster and private tutor. The increased income enabled him to buy instruments that he needed for his research. His success as a teacher in Nantwich opened the door to a post as tutor in ``polite'' and classical languages at a new Dissenting Academy in Warrington where he spent six happy years. For the first time since his academic days, he found himself in sympathetic surroundings. His colleagues held the same opinions and the same ideals as he did. If disagreement arose on any point, they looked on controversy as a means of discovering the truth and not as a sign of moral reprobation.

It was there in 1762 that Priestley married Mary Wilkinson, daughter of John Wilkinson, one of the leading figures of the emerging Industrial Revolution in England. He described his marriage as ``a very suitable and happy connexion, my wife being a woman of an excellent understanding, much improved by reading, of great fortitude and strength of mind, and of a temper in the highest degree affectionate and generous; feeling strongly for others, and little for herself.'' A month before his wedding, in anticipation of impending financial obligations, he applied for and was conferred ordination to the Dissenting ministry. His lectures on History and General Policy were his most important work while at Warrington. Through his lectures he attempted to introduce his students to wider realms. He expected his students to become familiar with political theory, laws, grammar, oratory and criticism. He acquainted them with Shakespeare, Milton, and other writers and poets of that time.

During his annual month-long visits to London, he joined a group of men who were known for their liberal politics and their rational dissent. Foremost among these were Richard Price (whose pamphlet, Civil Liberty, is said to have influenced the Declaration of Independence of the American Colonies) and Benjamin Franklin, whose friendship was crucial to his emerging scientific career. He was introduced to scientific society and encouraged by Franklin to write The History and Present State of Electricity, which included original experiments and illustrations in copperplate. This led to his election as a Fellow of the Royal Society in 1766. Priestley's History was found to be too difficult for ordinary readers so he prepared a version that was more popular. This necessitated the inclusion of some drawings, but he could not find anyone who could do the work. So he learned the rules of perspective drawing. Since there was no book to help him, he published one himself. In the course of this work, he stumbled on the use of India rubber as an eraser of lead-pencil marks. The preface to his book contains the first printed reference to India rubber erasers.

What did Franklin suggest? Franklin was a politician, the representative of the Colony of Pennsylvania to the English court. He was also a self-trained scientist of real ability. Franklin figured out an important consequence of Coulomb's Law: an electric charge placed inside a spherically symmetric distribution of charge will feel no force. The argument is as follows. Place an electric charge on a perfectly conducting sphere. The charge will spread uniformly to get a surface charge density of $\rho = Q/ 4 \pi R^2$. ¹ Now consider a test charge q inside the sphere. Draw a line through q and consider two little cones of solid angle $d\Omega$ about this line. These cones will intersect the sphere twice at distances r₁ and r₂. The charge on the sphere contained in each cone will exert a force on q of equal magnitude and in opposite directions. The charge at r₁ will be $\rho \, d\Omega \, r_1^2$, and the corresponding force on q will be $\rho \, d\Omega \, r_1^2/r_1^{\gamma}$. Similarly, the force due to the charge at r₂ will be $-\rho \, d\Omega \, r_2^2/r_2^{\gamma}$. If Coulomb's Law is true, $\gamma$< will be exactly 2. The geometrical factors of r_1,2² will cancel exactly against the dynamical factors of 1/r_1,2², and the net force on q will be zero. This idea of balancing dynamics against geometry is amazing! Franklin suggested this experiment to Priestley. Priestley performed the experiment and found that there was, indeed, no force on the charge inside the sphere. Coulomb's law was thus verified with high precision and real elegance! Can you imagine any politician today having such a profound idea in theoretical physics with such clear experimental consequences? I cannot.

Soon after his work on electromagnetism and confronted with the obligations of a growing family, Priestley decided to accept the invitation to minister to the congregation at Mill Hill. In 1767, he moved his family to Mill Chapel in Leeds, close to his birthplace.

Joseph Priestley is probably best known as the discoverer of oxygen. His experiments on air began in 1767 and peaked in 1774. The proximity of his house to a public brewery set the stage for many experiments on fixed air (carbon dioxide). Access to an abundant source of fixed air eventually led to an understanding of the nature of the effervescence found in mineral waters such as those of Spa, a resort in Belgium. Restorative sparkling beverages and baths were simply water containing fixed air. His publication on the impregnation of water with fixed air won him the prestigious Copley Medal of the Royal Society. The carbonated beverages of today trace their origin to Priestley's initial experiments.

Experiments on air in the eighteenth century posed challenges to the natural philosopher. Today we unhesitatingly regard air as a combination of various gases and confidently understand the chemical and physical properties that distinguish one colorless gas from another. However, centuries ago there was considerable confusion. In Priestley's time air was subjected to only simple tests of appearance, odor, and solubility. Any differences could have been real or, depending on the purity of the samples, caused by contamination. Modern gaseous elements and compounds were known as types of air: nitrous air (NO), phlogisticated air (N₂O), acid air (HCl), and reduced fixed air (CO). The ``goodness'' of air, a measure of its respirability, interested Priestley. In 1771 he noted the restoration of ``injured'' or depleted air by green plants. He wrote, ``the injury which is continually done to the atmosphere by the respiration of such a large number of animals ...is, in part at least, repaired by the vegetable creation.'' This balance between animal and plant kingdoms is particularly relevant to present environmental concerns over global warming and rainforest destruction.

Joseph Priestley simplified experimental techniques for the preparation and collection of gases. His pneumatic trough of 1772 was an major development. Gases soluble in water, previously difficult to collect, were collected successfully over mercury. In 1774 Priestley focused sunlight through a lens in order to heat a sample of mercuric oxide (red calx). The resulting gas supported the burning of a candle with a vigorous flame, was essentially insoluble in water, and accommodated a mouse under glass for some time. In Priestley's own words, ``I have discovered an air five or six times as good as common air.'' This ``good'' air was, of course, oxygen.

Priestley subsequently accepted a position with William Petty, Earl of Shelburne as librarian and tutor to his sons. He was offered a generous compensation which provided for protection in case of his patron's death or of a separation, the use of a house at Calne, the Shelburne residence at Wiltshire during the summers, and as intellectual-in-residence, freedom and resources to engage in his many interests. He published his two major philosophical works, Disquisitions Relating to Matter and Spirit, a materialistic view of man and Experiments and Observations, which contained his major chemical achievements. Priestley accompanied Lord Shelburne on a tour of the continent. In Paris he met other members of the scientific world. With Lavoisier he spoke of his recent isolation of the gas from the red oxide of mercury, unaware of the importance of his discovery. This was the missing clue from which Lavoisier developed the grand conceptual scheme of the role of oxygen in burning, leading to the overthrow of the phlogiston theory and the revolutionizing of the whole science of chemistry.

Priestley withdrew from his position with Lord Shelburne amicably, with an annuity secure for the rest of his life. Wishing to resume his active ministry he settled at Fairhill, on the outskirts of Birmingham with his family, which now included three sons and a daughter. Among his benefactors were his brother-in-law John Wilkinson, who provided a house for his family, and Josiah Wedgwood, master potter, who supplied him with the funds he needed for his experiments. Also in Birmingham were Matthew Boulton (manufacturer of buckles and buttons) and James Watt, who together were preparing to manufacture the steam engine, Erasmus Darwin, grandfather of Charles Darwin, who is responsible for the theory of evolution, and William Small, a tutor of Thomas Jefferson at the College of William and Mary.

When Priestley arrived in Birmingham in the autumn of 1780, he joined the Lunar society which met at the home of James Keir. The Lunar Society was an informal group of a dozen men, sometimes referred to as the ``Lunatics'', who were interested in natural science and literature. Benjamin Franklin was a frequent guest. The society met once a month on the Monday nearest the full moon. This time was chosen so that the members could return to their homes by moonlight. These meetings were described in a note Darwin wrote to Boulton: ``...what inventions, what wit, what rhetoric, metaphysical, mechanical, and pyrotechnical, will be on the wing, bandied like a shuttlecock from one to another of your troop of philosophers!''

Among Priestley's accomplishments during his years in Birmingham were the publication of the first part of Letters to a Philosophical Unbeliever, an attempt to defend natural religion against the skepticism of David Hume; a History of the Corruptions of Christianity, a direct attack on the central tenets of orthodox religion, particularly the doctrine of the Trinity; and a History of the Early Opinions Concerning Jesus Christ, where he set out to prove that the doctrine of the Trinity was not according to Scripture. These three works created storms of controversy. Priestley was attacked in pamphlets and periodicals, denounced in pulpits and in the House of Commons, and considered as an agent of the Devil because of his unorthodox views.

Under the Test and Corporation Acts in England, Dissenters were deprived of the rights of citizenship, and by law the Unitarian Church was not even tolerated. When the French Revolution broke out, the sympathies of the Dissenters lay with those who were struggling under the yoke of corruption, tyranny and oppression. Two years later, festivities were planned to celebrate the auspicious event. On the 11th of July, 1791 a local newspaper printed an advertisement for a dinner to be held at a leading hotel on July 14, ``to commemorate the auspicious day (Bastille Day) which witnessed the Emancipation of Twenty-six Millions of People from the yoke of Despotism''.

The dinner was attended by eighty-one men and ended without incident. Dr. Priestley had declined to attend, to the disappointment of the crowd that had gathered to demonstrate their contrary views to Priestley's revolutionary and heretical writings. By evening, the crowd reconvened and, fueled by liquor, sacked and burned the New Meetinghouse where Priestley preached. The Old Meetinghouse was sacked and burned, too. Warned of the murderous multitude, Priestley and his wife left Fairhill with ``nothing more than the clothes we happened to have on''. They did not realize the magnitude of the danger they were in and stopped at a friend's house a mile away. There they received information that the mob was now at their house looking for them. They moved again a little farther away but not far enough for Priestley to be spared the sight of the holocaust that engulfed his home, his laboratory and especially his library which contained precious manuscripts of works, some of which were as yet unpublished.

Priestley fled to London and never returned to Birmingham. He moved to Tottenham and then to Hackney. During the following months, Priestley was verbally attacked in the House of Commons, burned in effigy, portrayed in caricatures, denounced in pulpits and subjected to threatening letters. Priestley had by now become an honorary citizen of France which was at war with England. He was snubbed by the Royal Society and was forced to resign his membership when several of his colleagues turned against him. His sons were unable to find work in the area and decided to emigrate to America. He and his wife decided to join them. They sailed from Gravesend on the Samson on April 7, 1794, two weeks after Priestley's 61st birthday.

While the Priestleys were on their journey to America, Laviosier met his death at the guillotine in the Place de la Revolution. The Priestleys landed in New York and proceeded to the capital, Philadelphia. He refused the offer of a chair in chemistry at the University of Pennsylvania, choosing instead to join his son, Joseph, and friend, Thomas Cooper, who were establishing a colony for English Dissenters in central Pennsylvania. He moved 130 miles to the north and settled in the small town of Northumberland on the banks of the Susquehanna River. Within the year, the youngest Priestley son, Harry, died, as did Joseph's wife. Priestley remained active, writing, preaching and experimenting in his newly established laboratory, but the old fire and cheerfulness were gone.

Joseph Priestley missed his circle of friends and decided to spend the winter months in Philadelphia. As the founder of the first Unitarian church in America, his sermons were attended by then Vice President John Adams and other luminaries. When Adams became president, Priestley sided with the Jeffersonian opposition. Jefferson greatly admired Priestley and even consulted him for advice on the curriculum for the University of Virginia which he was planning to found. It was in Philadelphia, during Jefferson's term, in 1801, that he suffered his first serious illness and nearly died.

During his last journey to Philadelphia he was honored by the American Philosophical Society in a testimonial dinner. He offered a benediction to his scientific colleagues in a manner that proved to be prophetic: ``Having been obliged to leave a country which has been long distinguished by discoveries in science, I think myself happy by my reception in another which is following its example, and which already affords a prospect of its arriving at equal eminence.'' Although he never fully recovered his health, he continued work on his latest manuscript even when he could no longer rise to dress and shave himself. On February 5, 1804, he had all the children brought to his bedside, and after prayers spoke to each of them separately. The next morning, he asked his son and Mr. Cooper to bring him the pamphlets they had been working on and dictated clearly and distinctly the additions and alterations he wished to have made. He objected to Mr. Cooper's putting the corrections in his own language. He then repeated over again what he had said before and when done he said, ``That is right; I have now done.'' Half an hour later he was dead. The date was February 6, 1804.

Hans Christian Ørsted

Hans Christian Ørsted was born in Rudkøbing i 1777. His father, Søren, was the local apothecary and had a great many business interests. Among these was a monopoly for the distillation of spirits and the gathering of medicinal plants to be sold in Copenhagen. There was little time for instruction at home, and Rudkøbing had no school. Thus, Hans Christian and his brother Anders Sandøe were sent to a German wigmaker and his wife for instruction. The brothers proved to be intelligent, and the whole town soon joined in their education. As you can imagine, the result was not particularly systematic. For the rest of his life, Hans Christian had serious gaps in his knowledge which modern physicists would find shocking. Mathematics and even Newtonian mechanics remained mysteries for Ørsted. Nevertheless, the brothers enrolled in Københavns Universitet in 1795. From the start Ørsted's interests were remarkably broad. His first academic success was a medal for the University's prize question in esthetics (1796) for paper entitled ``Hvorledes kan det prosaiske Sprog fordærves ved at komme det poetiske for nær og hvor er Grænserne mellem det poetiske og prosaiske Udtryk?''. The next year he won a prize for a question posed by the medical faculty on ``Modervandets Oprindelse og Nytte''.

In the same year Ørsted finished his pharmaceutical education and started work on his doctoral thesis which he defended in 1799. The subject of this thesis, entitled ``Grundtrækkene af Naturmetaphysiken'', was nothing less than the naturphilosophie of Immanuel Kant (1724-1804) as expressed in his ``Metaphysical Foundations of Natural Science'' (1786). Ørsted was particularly taken by Kant's rejection of the notion of point-like atoms in favor of a dynamical theory in which two fundamental forces -- attraction and repulsion -- could be associated with every point in space. According to Kant, the ``conflict'' between these two fundamental forces could then provide an explanation for all phenomena observed in nature including those traditionally associated with matter. The bewildering variety of forces actually seen in nature -- electricity, magnetism, heat, and light among others, should all have a similar origin. If this were true, it seemed natural that these various manifestations of the fundamental forces of attraction and repulsion could be converted into one another. There should, for example, be a connection between electricity and magnetism. Kant's ideas captured the imagination of the young Ørsted. As a result, his dissertation was devoted to the rewriting and rearranging Kant's metaphysical scheme both to increase Danish awareness of Kant's contributions and to create the foundations of a similar a priori basis for Ørsted's own field of chemistry. Kant's focus on forces rather than atoms was of interest to a remarkable number of people since his ideas seemed to suggest the possibility of breaking down the barriers between the physical and spiritual worlds. Thus, the English poet Samuel Taylor Coleridge (1772-1834) actually went to visit Kant. Upon his return to England, Coleridge shared his enthusiasm for naturphilosophie and the prospect of creating a more universal science, broad enough to encompass both God and the material universe, with his friend Sir Humphry Davy (1778-1829) and with Davy's scientific assistant, Michael Faraday. (See below.)

We can only speculate on why Ørsted was so taken with German romantic ideas. My guess is that his enthusiasm was related to his lack of knowledge of physics and mathematics. It was probably very appealing to believe that physics had to be ``reinvented'' along Kant's lines and that he did not need to bother to learn the many things that he did not know.

Soon after the completion of his university studies, Ørsted received a legacy for travels in Germany and France. During his time in Germany, he met the philosophers Schelling, Fichte, and Schlegel. How did a young man get to meet all these famous people? The Italian Allesandro Volta (1745-1827) had invented the Voltaic cell in 1800 and opened new possibilities for the study of electrical currents and the role of electricity in chemical phenomena. This new invention was very exciting and not yet well known or understood.² Before leaving Copenhagen, Ørsted made a pocket version of a Voltaic cell which he offered to demonstrate wherever he went. It was this offer that gave him access to scholars in Germany.

In Germany, Ørsted started a warm scientific friendship with the chemist Johann Wilhelm Ritter (1776-1810). Ritter was a chemist of some distinction and shared Ørsted's interests in naturphilosophie. His scientific accomplishments were real enough and include the discovery of invisible ultraviolet radiation (1801). Ritter's invention of the storage battery in 1802 was an important technical advance. Unfortunately, Ritter's romantic philosophical convictions often outstripped his experimental skills and scientific judgment. His belief that electricity and magnetism were merely two manifestations of Kant's fundamental forces led him to claim that the earth had two electric poles just as it has two magnetic poles. Similarly, Ritter claimed with some enthusiasm that a needle made half of zinc and half of silver would behave like a compass needle and align itself with the earth's magnetic field.³ Before continuing on to Paris in 1802, Ørsted encouraged Ritter to prepare a written summary of his electrical discoveries.

The reason was simple. Late in 1800, Volta had demonstrated his discovery for Napoleon in Paris. The First Consul immediately decided to create an annual prize of 3,000 francs and a gold medal for a galvanic discovery. Volta was the first recipient. Soon after, Napoleon created a single prize of 60,000 francs for a galvanic discovery of particular importance. Ørsted intended to get one of these prizes for his friend Ritter. The plan did not succeed. Empirical truth was the rule in French science, and there was little sympathy for German romantic philosophy in Paris. While Ritter's storage battery was real enough, his other claims were soon recognized to be pure fantasy. In spite of Ørsted's unselfish efforts, there was no prize for Ritter. Indeed, Ørsted's own reputation among the scientists of Paris suffered from his advocacy of Ritter's questionable discoveries and of similarly dubious chemical discoveries made by the Hungarian chemist Jakob Joseph Winterl (1732-1809). The 27-year-old Ørsted returned to Copenhagen in 1804 preceded by rumors of his of uncritical enthusiasm. As a result, his application for a professorship in the faculty of medicine was unsuccessful.

It was at this point, still filled with Kant's romantic philosophy but burdened by a growing reputation for poor scientific judgment, that Ørsted embarked on an ambitious program of careful measurements of literally hundreds of acoustic figures. Acoustic figures are made by stroking a metal plate with a violin bow. The plate is covered with sand or powder, and a ``picture'' of the motion of the surface of the plate results from the powder which remains on nodal lines. They had been introduced and studied with some care by Ernst Florens Chladni some 30 years before. Ørsted's purpose was partly to verify and refine Chladni's results. Early in his investigations, Ørsted decided to replace the coarse sand used by Chladni with fine lycopodium powder in order to obtain more precise figures. This was a sound technical decision since it led him to the discovery that Chladni's straight nodal lines were really narrow hyperbolic surfaces. But Ørsted also had deeper and more original goals. He hoped to find evidence that the mechanical motions associated with the production of sound could produce electrical effects, and lycopodium was known to be sensitive to static electricity. This was not merely idle speculation since Ritter had informed Ørsted of his own experiments in which a sound was produced when the ear was given a substantial electrical shock from a Voltaic pile.<sup>4</sup> In a letter to Ritter in 1804, Ørsted wrote that ``I believed that I would also be able to discover electrical phenomena in the production of the acoustic figures and therefore chose semen lycopodii to strew on the glass plates ...in the hope that the dust would adhere to the positively charged places and fall off the negatively charged ones''. This proved to be the case, and some of the powder remained on the hyperboles even when the plate was inverted. This effect was turned to good use. By pressing the inverted plate on a piece of black paper coated with a solution of gum Arabic, it was possible to preserve the acoustic figures in a manner which facilitated precise measurement. In the process, Ørsted had inadvertently invented the first Xerox machine. More profoundly, Ørsted regarded this effect as evidence for the desired connection between mechanical and electrical effects. His Kantian spirit was excited, and he was quick to speculate that mechanical oscillations, sound, and even light were all the same phenomenon merely observed at increasing frequencies. Thus, writing to the French physicist Marc Auguste Pictet (1752-1812) in 1805, Ørsted was confident in endorsing Ritter's erroneous belief that ``in each sound there are as many alternatives of positive and negative electricity as there are oscillations, but the union of the two electricities produces a shock. The perceptible effect of the union of all these imperceptible shocks is sound''. Ørsted extended these thoughts to include light by suggesting that the sense of sight should be thought of as ``an octave'' of the sense of hearing in the grand scale of sensations.

By today's standards, the immediate results of Ørsted's labors (which continued until 1807) seem to provide insufficient justification for his efforts. He developed an elaborate and fanciful theory of the propagation of vibrations in solids in order to explain the results he found puzzling. His theory offered an explanation of the accumulation of small piles of lycopodium on moving parts of the plate. The English physicist Michael Faraday (1791-1867) reproduced Ørsted's results in 1831 and found that the unexpected piles of powder were merely due to the effects of air currents above the plate. It is not accidental that Faraday's work on acoustic figures coincided with his own discovery of electromagnetic induction. Acoustic figures helped Faraday understand the nature of his own electromagnetic discovery. (See below.) The difficulties that Ørsted encountered in explaining how vibrations could pass through the motionless nodal lines reflected his failure to understand that the standing waves seen in acoustic figures are merely the superposition of traveling waves moving in opposite directions.⁵ He posed instead the Kantian rhetorical question of whether it ``might not be possible that the external oscillatory motion, changed into a penetrating internal motion, passed also from a mere mechanical motion into a generation of force''. If nothing else, this work kept alive Ørsted's conviction of the unity of the forces of nature. In this sense, his work on acoustic figures maintained the basis for his discovery of the connection between electricity and magnetism in 1820.

Whatever the scientific merits of his acoustic research, Ørsted reaped significant professional rewards from it. His work was first reported to Videnskabernes Selskab in 1807 and was awarded the Society's silver medal in 1808. Later that year, Ørsted was made a member of the Society. The work was published in Germany in 1809, and Ørsted was immediately made a corresponding member of the Academy of Sciences in Munich. In spite of the recognition of this work by the scientific community, the underlying motivation for his studies -- the search for evidence of the unity of mechanical and electrical phenomena -- was largely passed by in silence. The positive reaction from his scientific contemporaries was limited to the evident technical merits of the work that Ørsted had performed.

The period from 1808-20 was largely filled with chemical research which need not concern us. Note, however, that Ørsted was also extremely busy with teaching duties. In 1815 he became the Secretary of Videnskabernes Selskab, and he was appointed professor ordinarius in 1817. But none of this gave any indication of what was to happen in 1820. During a University lecture, Ørsted discovered that an electrical current from a powerful Voltaic cell resulted in motion of a compass needle. Remarkably, he waited until June 1820 before looking at this effect more carefully.⁶ Ørsted performed a very complete set of beautiful experiments, described the results in a short Latin article, and sent it off to scholars throughout Europe and America on 20 July 1820. The reaction was immediate.

By the first week of September, Biot and Arago could report to colleagues in Paris that they had confirmed Ørsted's results. New experimental results came in every week, and on the first Monday in December, Ampère gave his own mathematical description of the effect in the way we understand it today. The entire process took only four months! Because of his lack of mathematics, Ørsted never appreciated or understood Ampères contribution to electromagnetism. With uncharacteristic sarcasm, Ørsted later wrote: ``Den Klygt, hvormed den sindrige franske Mathematiker har vidst efterhaanden at omdanne og udvikle sin Theorie, saaledes, at den lader sig forene med en Mangfoldighed af stridige Kjendsgjerninger, er mærkværdig.''

Even though (because?) Ørsted was now world famous, people claimed that his discovery was not original and/or that he had simply been ``lucky''. He could dismiss the former criticisms as groundless. The latter irritated him enormously. He liked to refer to an article from 1813 in which he had vaguely suggested that ``it must be determined whether electricity in its most latent [bound] state has any action upon the magnet as such''. Given the extreme simplicity of the experiment, I think we must conclude that Ørsted was lucky. Given a Voltaic cell and a compass needle, it is almost impossible to fail to discover electromagnetism! The crucial thing is that Ørsted, with his belief in Kant and his conviction that all of nature's forces are related, was prepared to see and understand such an effect. The French physicists were not. A friend wrote to Ampère early in 1821 asking why a Dane rather than a Frenchman had discovered electromagnetism. Ampère replied that ``Coulomb had assured us that there was no connection between the electric and magnetic fluids''. The French were not prepared to discover electromagnetism.

During the years 1822-23, Ørsted took a triumphal tour of Germany, France, and England. In England, he found literally hundreds of small societies aimed at bringing science to ordinary people. Upon his return, Ørsted helped found a similar society, Selskab for Naturlærens Udbredelse, in Denmark. He now lectured five evenings each week on physics and chemistry in addition to his normal University teaching and was responsible for arranging similar lectures in the provinces. But there was still time for research. Until 1825, he was concerned with additional electromagnetic experiments. These included an elegant experiment ``inverse'' to his original discovery. He made a very small Voltaic cell (just as he had in 1803), suspended it from a string, and demonstrated that a strong iron magnet would cause it to change its orientation. He also began a long series of technically beautiful experiments aimed at measuring the compressibility of water and other fluids. This is a difficult experimental task since the compressibility of liquids is not much different from that of their container. Ørsted found a solution to this problem which is still used today: He placed the vessel containing the liquid to be studied inside a water-filled cylinder. The fluid to be studied was isolated from the water by a drop of mercury in a capillary tube, and the height of the mercury provided a precise measure of the volume of the liquid.⁷ Why did he perform these experiments? The answer is that Ørsted wanted to show that the compression of a fluid was linearly proportional to the applied pressure. (This is correct except for ultra high pressures which can cause a phase transition.) He believed that this result was inconsistent with an underlying atomic picture. Thus, a linear proportionality between pressure and fluid density would serve to disprove the existence of atoms. Ørsted was still seeking to verify Kant's scheme. Ørsted also continued his chemical researches and was the first to prepare metallic aluminum in 1825.

>From roughly 1830 on, Ørsted's scientific production declined markedly as a consequence of his many public duties. In addition to SNU, he was now the director of the Polyteknisk Læreanstalt (now Danmarks Tekniske Universitet) and had even more teaching to do -- an additional 10 hours per week. His advice was also frequently sought on many questions of public policy. Questions regarding patents and monopolies were frequently sent on to him. He developed new rules for issuing patents and introduced the notion that some inventions were of such great importance (e.g., the telegraph) that their patents should belong to the state. But there was still time for good physics. Soon after Faraday's discovery of diamagnetism, Ørsted reported a number of careful experiments on the subject (1847-48).

In view of Ørsted's many contributions to both science and Danish society, it is hardly surprising that thousands of Danes took part in a torchlight parade at his death in 1851.

Michael Faraday

Michael Faraday, the discoverer of electromagnetic induction, electromagnetic rotations, the magneto-optical effect, diamagnetism, and field theory (in a certain sense), was born in Newington Butts (the area of London now known as the Elephant and Castle) on 22 September 1791. His father, James, was a blacksmith and a member of the Sandemanian sect of Christianity.⁸ James Faraday had come to London a year or so earlier from Northwest England. Very little is known of the first few years of Faraday's life. In an autobiographical note Faraday recalled that he had attended a day school and had learned the ``rudiments of reading, writing, and arithmetic''.

In 1805 at the age of fourteen Faraday was apprenticed as a bookbinder to George Riebau of Blandford Street. During his seven year apprenticeship Faraday developed his interest in science and in particular chemistry. He read⁹ Jane Marcet's ``Conversations on Chemistry''<sup>10</sup> and the scientific entries from the ``Encyclopedia Britannica''¹¹. He was also able to perform chemical experiments and he built his own electrostatic machine¹². But, more importantly, Faraday joined the City Philosophical Society in 1810. In this society, which was devoted to self-improvement, a group of (youngish) men met every week to hear lectures on scientific topics and to discuss scientific matters. It was here that Faraday gave his first scientific lectures.

Towards the end of his apprenticeship, in 1812, Faraday was given, by one of Riebau's customers, William Dance (one of founders of the Royal Philharmonic Society), four tickets to hear Sir Humphry Davy's last four lectures at the Royal Institution. Faraday attended these lectures, took notes, and later in the year presented them to Davy asking for a position as a scientist or technician. Davy interviewed Faraday but said that he had no position available. Early in 1813 there was a fight in the main lecture theater of the Royal Institution between the Instrument Maker and the Chemical Assistant which resulted in the dismissal of the latter. Davy was asked to find a replacement for him; he remembered Faraday and called him for a second interview. As a result, Faraday was appointed Chemical Assistant at the Royal Institution on 1 March 1813.

Faraday, in effect, started a second apprenticeship in chemistry. For most of the 1810s and 1820s he worked under Davy's replacement as Professor of Chemistry, William Thomas Brande. However, between October 1813 and April 1815, he accompanied Davy, as his assistant, on a scientific tour of the Continent. Davy had been given a passport by Napoleon for himself, his wife, her maid and a valet. Faraday, very reluctantly, agreed to play the role of servant in order to use the passport. This led to tension between Faraday and Jane Davy who chose to regard him as a servant -- which he assuredly was not. Davy sought to keep the peace between his relatively new wife and Faraday. This probably says as much about the state of the Davys's marriage as it does about his high opinion of Faraday. On the tour they visited Paris (where Davy confounded the French chemists by demonstrating electrochemically the elementary nature of iodine), Italy (where they met the aged Volta, visited Vesuvius where Davy was able to decompose a diamond into carbon by using the Duke of Tuscany's great optical lens), Switzerland, and Southern Germany. Davy had intended to continue into the Turkish Empire to visit Athens and Constantinople, but whether due to the tensions in the party or to Napoleon's escape from Elba, they returned to England in April 1815.

Back in England, Faraday resumed his position as Chemical Assistant at the Royal Institution and continued to learn his science from Brande as well as occasionally helping Davy as with the Miner's Safety Lamp in 1816 and 1817. Between 1818 and 1822 he worked with the surgical instrument maker James Stoddart in improving the quality of steel. One of the reasons why this sort of work was carried out at the Royal Institution was that it had the best-equipped laboratory in England and one of the best in Europe.

The year 1821 was in many ways one of the most important in Faraday's life. On 21 May 1821 he was promoted in the Royal Institution to the position of Superintendent of the House. On 2 June he married Sarah Barnard, who was a member of one of the leading Sandemanian families in London, and on 15 July Faraday made his Confession of Faith in the Sandemanian Church. The year also marked his first major contribution to science.

As we have mentioned, Ørsted had discovered electromagnetism in 1820. His Latin article was quickly translated into the major scientific languages of Europe. It was immediately evident that Ørsted had made a major discovery, but his paper contained views which many readers found strange.¹³ Writing later Faraday commented that ``I have very little to say on M. Oersted's theory, for I must confess I do not quite understand it''. What was clear was that Ørsted had opened up a major field of scientific enquiry which was exploited by scientists all over Europe. Faraday was a little late getting started on electromagnetism. Ørsted's Latin article had been sent to Davy, and Davy did not bother to show it to Faraday. By the time Faraday read the English translation, Ampère had already made his contribution. Still, Faraday was part of the effort to explore electromagnetism, and on 3 and 4 September 1821 in his basement laboratory at the Royal Institution, he performed a set of experiments which culminated in his discovery of electromagnetic rotation -- the principle behind the electric motor. Apart from the practical significance of this discovery, it was important as Faraday's interpretation of the phenomenon indicated that he was not a Newtonian in supposing that forces had to act along straight lines.

During the following decade, Faraday's opportunity for doing original research was severely circumscribed, although he was able to liquefy chlorine in 1823 and discover bicarbuet of hydrogen (later renamed benzene by Eilhard Mitscherlich) in 1825. At Davy's instigation he became the first secretary of the newly founded Athenaeum Club in 1824. In the late 1820s he undertook an extensive project on making optical glass for a joint committee of the Royal Society and Board of Longitude. In addition in 1826 he founded the Friday Evening Discourses and, in the same year, the Christmas Lectures for juveniles. In total, Faraday gave 123 Friday Evening Discourses between 1826 and 1862 and 19 series of Christmas lectures between 1827 and 1861. These and other lectures that he gave served to establish his reputation as the outstanding scientific lecturer of the time. Both the Friday Evening Discourses and the Christmas lectures continue to this day. The latter series is televised each year.

It was almost exactly ten years after his discovery of electromagnetic rotations that Faraday was able to resume his work on electromagnetism, when he discovered on 29 August 1831, electromagnetic induction. In 1830 Faraday had been looking again at Ørsted's acoustic figures. By studying them in vacuum, he determined that some of Ørsted's more puzzling results were simply a consequence of air currents. In the process, he noted that one vibrating plate could ``induce'' the same vibrations in another similar plate. Thus, Ørsted's work from 1804-07 gave Faraday the idea for electromagnetic induction. Faraday's experimental observation is almost as simple as Ørsted's discovery of electromagnetism. Imagine that you have a closed electrical circuit (made with a coil of wire with N turns). Define the magnetic flux through this circuit as

$$\Phi = N \int \, d{\vec \sigma} \cdot {\vec B}$$ (1)

where $d{\vec \sigma}$ is an element of area, ${\vec B}$ is the magnetic field, and N is the number of turns in the coil. The integral is performed over the area of the loop. Faraday discovered that any change in the flux¹⁴ resulted in an electromotive force (i.e., an electric potential) and thus in a current of electricity in the loop. The current was found to be proportional to $d\Phi /dt$. This led immediately to the construction of a generator. This discovery, more than any other, allowed electricity to be turned, during the nineteenth century, from a scientific curiosity into a powerful technology.¹⁵ During the remainder of the 1830s Faraday worked on developing his ideas on electricity. He produced a new theory of electrochemical action (1832-34). This led him to introduce a series of new words (now familiar) including electrode, electrolyte, anode, cathode, and ion. In the later half of the 1830s Faraday worked on a new theory of static electricity and electrical induction. This work led him to reject the traditional theory that electricity was an imponderable (i.e., massless) fluid. Instead, he proposed that electricity was a form of force that passed from particle to another.

In 1836 Faraday was appointed Scientific Adviser to Trinity House, a post which he held until 1865. Trinity House is responsible for the safety of navigation round the shores of England and Wales. In his capacity as Scientific Adviser, Faraday sought to make light houses more efficient in the fuel they consumed and in the light they produced. In the 1840s he invented a chimney for oil burning lamps which allowed much more of the products of combustion to be taken away from the lamp. Although Faraday did not patent anything himself, this chimney was patented by his brother Robert. As well as being installed in all lighthouses, it was also used in the Athenaeum and Buckingham Palace. Faraday also spent a considerable amount of time, especially in the early 1860s, working on various systems of electric light that were proposed. These systems were installed and tested in the Tynemouth and South Foreland lighthouses.

Faraday's work for Trinity House was not the only example of his scientific expertise being used for practical purposes. Between 1830 and 1851 Faraday was Professor of Chemistry at the Royal Military Academy in Woolwich. During his tenure generations of officers of the Royal Engineers and Royal Artillery learned their chemistry from him. The Admiralty frequently sought his advice on matters as diverse as the quality of oats at sea to the best way to attack Cronstadt during the Crimean War. In 1844 he and the geologist Charles Lyell were asked by the Home Office to attend an inquest regarding the explosion at Haswell Colliery. The report they produced stated that better ventilation of mines would reduce explosions. However, the government and mine owners ignored their conclusions.

In the early 1840s Faraday suffered a breakdown in health and also became an Elder of the Sandemanian Church. These two events resulted in a sharp decline in the quantity of Faraday's scientific work (in both research and lecturing) during the early 1840s compared with the 1830s. He did not stop working. In 1843 Faraday posed the question of whether empty space is a conductor of electricity. Faraday showed that space can sometimes conduct electricity. To explain this phenomenon, Faraday suggested that atoms should be viewed as centers of force where lines of force met. One problem with this view was that magnetism was known to be specific to only three types of metallic elements -- iron, cobalt, and nickel. The solution to this problem followed from a conversation that Faraday had with the twenty-one year old William Thomson (later Lord Kelvin) at the 1845 meeting of the British Association in Cambridge. Thomson asked Faraday if he had ever investigated whether light was affected when passing through an electrolyte. Faraday said he had tried this experiment but had not found any effect. He promised to try again. When he repeated this experiment he still found no effect. It then occurred to him to see what would happen to light passing near a powerful magnet. He thus placed a piece of heavy glass on the poles of a powerful electromagnet; then he passed polarized light through the glass. When he turned the electromagnet on, he found that the angle of polarization of the light changed.

This experiment told Faraday two things. First, that light could be affected by magnetic forces -- the magneto-optical effect, which later became known as the Faraday Effect. The second thing it told Faraday was that glass had been affected by magnetic force. Faraday wanted to demonstrate this second effect directly -- not just through the agency of light. On 4 November 1845 he hung a piece of heavy glass between the poles of an electromagnet and observed that the glass aligned itself along the lines of force of the magnet. He then experimented with many other substances, all of which displayed similar phenomena -- diamagnetism. Faraday concluded that magnetism is an inherent property of matter. This gave him the confidence to reassert strongly his views on the nature of matter in a lecture entitled ``Thoughts on Ray-vibrations'', which he delivered in April 1846. This lecture laid the basis for the field theory of electromagnetism which Faraday developed in the following years. This theory was taken up and cast in a more mathematical form by Thomson, and, at Thomson's instigation, by James Clerk Maxwell. The result -- Maxwell's four equations -- represents precisely the way we understand all electrical and magnetic phenomena today.

Although Faraday continued working in science and for Trinity House, ill health eventually took its toll. In 1858 he was given a Grace and Favour House at Hampton Court where he increasingly spent much of his time. Between 1860 and 1864 he was again an Elder of the Sandemanian Church. He died at Hampton Court on 25 August 1867 and was buried in the Sandemanian plot in Highgate Cemetery five days later.