Observations, Instruments and Theories: the study of terrestrial magnetism in Great Britain, c. 1770s-1830s
Robinson M. Yost
At the beginning of the nineteenth century, Scottish natural philosopher John Robison remarked in the Encyclopaedia Britannica, "We cannot but consider the discovery of the magnetic constitution of this globe as a point of very great importance, both to the philosopher and to society." These sentiments were often heard, both before and after Robison. For a seafaring nation like Great Britain, navigation was of longstanding concern. Given the navigational importance of the compass, understanding the earth's magnetism was a vital practical matter. In the theoretical realm, Isaac Newton had left terrestrial magnetism unsolved– a riddle for future natural philosophers. My talk focuses on the study of terrestrial magnetism in Britain from the 1770s to the 1830s. During this sixty-year period it witnessed many changes– in the collection of observations, in instrumentation and theoretical outlook. Throughout these changes though "scientific" and "practical" aspects remained closely linked.
Observers in the fifteenth and sixteenth centuries had noticed irregularities in compass readings. For example, Christopher Columbus recognized during his voyages to the New World the deviation of the needle from true geographic north (called magnetic variation). In 1581 Robert Norman noticed the vertical tilt of one end of the needle (called the "dip"). Henry Gellibrand in 1635 recognized that magnetic variation changed over long periods of time. During the latter seventeenth century more systematic measurements began. With governmental support, Captain Edmond Halley published a map in 1701 showing lines of constant variation. Twenty years later another Englishmen, George Graham noticed small daily changes in the variation. In the 1770s a French investigator counted vibrations of a needle to determine a new measurement, magnetic intensity. By the nineteenth century it was known that three basic magnetic components- variation, dip and intensity- altered with time, location, and often with the instrument used to measure them. These increasingly complex observations naturally lent themselves to long-term, global-scale investigations.
Perhaps the best known of earlier theories was that of William Gilbert. In the latter sixteenth century he had treated the earth itself as a giant magnet. For Gilbert a small spherical lodestone acted like a miniature earth; compass readings near this terrella, as it was called, were analogous to those on different parts of the earth. A century later, Edmond Halley proposed a four-pole theory where both northern and southern hemispheres contained two magnetic poles which slowly altered positions. Many in eighteenth-century Britain accepted modified versions of Gilbertian-Halleyan macroscopic models. At the microscopic level, the century saw the rise of imponderable fluids to explain not only magnetism but electricity, heat and light as well. Some accepted two magnetic fluids while others believed that one sufficed. Some supposed that navigational mistakes occurred when a compass needle lost its fluid. By the latter quarter of the eighteenth century, it seemed that interest in terrestrial magnetism had leveled off.
In 1772, London instrument maker Edward Nairne described a dipping needle of his own construction. Insuring proper balance was a primary goal when making dipping needles. Ideally, the needle's center of gravity coincided with the center of its axis of rotation. Otherwise, the needle would wobble, producing errors. Using a dipping needle also involved taking multiple sets of readings. Nairne, for instance, removed the needle from his apparatus and reversed its poles, then replaced it taking a new set of measurements. The mean of these sets then rendered the "true" dip. In 1776, Henry Cavendish described the Royal Society's variation compass and dipping needle both constructed by Nairne. The variation compass had a smooth agate cap fixed at the center of the needle. Agate provided durability and less friction between the needle and its pin pivot. Cavendish also placed the instruments in a garden nearly a mile away from the Royal Society's apartments and compared sets of readings taken inside and outside to determine errors caused by ironwork in the buildings.
Observers in the field, in contrast to Nairne and Cavendish, made measurements primarily with ordinary surveying or marine compasses. Military engineer John Macdonald, for instance, made observations from the island of Sumatra during the 1790s. From a small building devoid of iron he recorded magnetic variation, as well as temperature and weather conditions. Macdonald proposed that heat weakened the "magnetic virtue" while cold strengthened it. Combining this notion with a four-pole theory, he concluded that the sun’s differential heating of the two southern poles explained diurnal variation.
For the most part, however, it was officers in the Royal Navy who recorded variation and dip. This is not surprising; they were, after all, global travelers and experienced manipulators of marine compasses. Captain James Cook, for instance, collected magnetic measurements during his famous voyages of the 1780s and 90s. Many of these men lacked formal education in natural philosophy and were uneasy about theoretical speculation. While surveying the coast of Australia in 1802, Captain Matthew Flinders remarked:
I shall leave it to the learned. . . to compare the observations here given with those made by others in different parts of the earth. . . the opinion I have ventured to offer is merely the vague conjecture of one who does not profess to understand the subject.
Although Royal Navy officers collected many measurements, a few natural philosophers made their own. George Gilpin, the secretary of the Royal Society, was from 1786-1805 the most consistent British observer of variation and dip. In 1806 he published measurements made with the same instruments Cavendish had used. Gilpin’s measurements, made from a permanent site at regularly-spaced intervals, were preferable, he wrote, because "experience teaches us, that magnetical observations made for a [limited] period. . . are not sufficient for minute purposes." The true theory would arise, Gilpin said, "from observations made with care, and with good instruments, carefully registered, and properly arranged." Gilpin's work reflects the state of the discipline in Britain. The way in which observations were collected was changing. Another figure illustrating this change was Colonel Mark Beaufoy.
In 1813 Beaufoy established a magnetic observatory at Bushey Heath near London. He remained one of the few British investigators until Arctic expeditions in the later 1810s. Beaufoy commented in 1820:
The only observations which, I believe, have been published are those of the Royal Society, commenced by the late Mr. Gilpin, and continued by the present librarian. . . these observations being made in a room in which iron has been used to strengthen the ceiling (and not in the open air), it is doubtful whether the real variation can be truly ascertained.
In fact, he knew of only two other places where magnetic observations were being made- one in Glasgow and the other in Burgundy. The latter, made by a Frenchman, utilized the technique (originating with Charles Coulomb) of suspending a needle from a fine silk thread. Beaufoy commented that he did not see the advantage of this method.
The turn of the century also saw a changing emphasis between observation and hypothesis. From the latter eighteenth century many agreed that the explanation of terrestrial magnetic phenomena should rely on "actual experience" rather than hypothetical speculation. An investigator in 1790 remarked:
The fact is, that bare hypotheses are seldom useful (often dangerous) to science. . . I am persuaded that we shall never be able to arrive at a true theory of the variation of the needle, without the advantage of numerous observations made at sundry places, at the same, and also at different, times.
By second decade of the nineteenth century, interest in terrestrial magnetism specifically focused on Captain Flinders’ discovery of local attraction. During his surveying voyages, Flinders had recognized that increased amounts of iron in and on ships were causing navigational errors. Before, these errors had been blamed on imperfect compasses, unnoticed ocean currents, or simply bad seamanship. The decades following this discovery saw many seeking out solutions. William Bain in 1817 explained that local attraction was a "fact of much importance to navigation, and consequently to the general interests of the British nation." A reviewer of Bain commented:
The importance of the directive influence of the magnet is . . . great beyond measure, particularly to a maritime nation, and if the various hypotheses and theories which have been formed by some of the most eminent philosophers, have failed. . . that failure must be attributed. . . to the want of a sufficient number of well authenticated facts
Others echoed similar judgements. Local attraction and terrestrial magnetism would require more systematic, accurate observations before they would be truly understood.
Instruments, it seems, did not keep pace with these goals. Captain Edward Sabine reported in 1819 that the dipping needle used for his expedition was of the same construction as Cavendish's. Two years later Sabine complained that no improvements had been made on the dipping needle in nearly fifty years. Increased attention by natural philosophers to magnetism made it desirable, he said, that "a greater degree of accuracy should be obtained in all respects, in observing its various terrestrial phenomena." Sabine also pointed out that dip measurements made by Nairne, Cavendish and Gilpin were in error because of imperfect dipping needles and iron in the buildings where they were performed. For over thirty years, Sabine remained one of the foremost British promoters of terrestrial magnetic studies.
In 1820, the study of the earth's magnetism obtained further impetus when Hans Christian Oersted discovered that a wire carrying electric current deflected a magnetic needle. This long-suspected connection attracted larger numbers of scientifically-trained men into the discussion. British investigators such as Humphry Davy, Michael Faraday, Charles Babbage and John Herschel extended Oersted's work, drawing further speculations from it. Many believed that the forces of nature were interconvertible or perhaps manifestations of a single natural force. By the 1820s-30s a new view came to overshadow the earlier models of terrestrial magnetism. One of this "cosmical" view’s main proponents, Alexander von Humboldt, saw all terrestrial phenomena in terms of "telluric" forces. These forces, emanating from the earth, interacted with extra-terrestrial influences. For proponents of this view the earth was no longer a giant magnet.
Electromagnetism and other discoveries gave adherents to the cosmical view hope for a synthesis. Humboldt himself linked variations in magnetic observations with the motions of thermal, chemical and luminous aspects of electromagnetism. British investigators proposed similar notions. Babbage and Herschel said that atmospheric electricity arose from the thermoelectric interaction of sky and earth, thus producing terrestrial magnetism by induction. Dr.Thomas Traill believed that solar rays converted the earth into "a vast thermo-magnetic apparatus." Peter Barlow, agreeing with Andre Marie Ampère, said that terrestrial magnetism was induced by electrical currents moving through the earth. Samuel Hunter Christie proposed experiments with a copper sphere filled with bismuth to simulate the thermo-chemical reactions that he believed produced the earth's magnetism.
Although still intimately linked to navigational concerns, the study of terrestrial magnetism was becoming an important subject in its own right. In 1819, Norwegian Christopher Hansteen published a mathematical elaboration of a four-pole theory relying on interacting telluric forces. A British reviewer this work commented that the study of terrestrial magnetism:
. . . besides the immediate and valuable application of its discoveries to the purposes of navigation, promises to develope so many curious relations, and to throw so much light over the secrets of electricity, and the other chemical or mechanical powers of Nature, as to demand investigation. . . for its own sake.
The review concluded that the small number of observations and the dispersed manner in which they were collected perpetuated the obscurity which continued to conceal this science.
Officers in the Royal Navy, natural philosophers and instrument makers continued collecting observations and refining their instruments. Some explored methods for making more sensitive, well-balanced and powerful needles, testing needles of different shapes, sizes and compositions. For instance, Captain Henry Kater, after performing many experiments, concluded in 1821 that the optimum material for compass needles was clock spring of sheer steel. He also developed improved methods for communicating magnetism to needles and determined that the best shape was the pierced rhombus. Lieutenant Henry Foster published numerous reports from expeditions to the Arctic and the South Seas. In 1826 he speculated that diurnal variation was caused by solar and lunar influences on the terrestrial magnetic sphere. Working this out, he said, belonged to those more "theoretically conversant" in the subject. By the mid-1820s, suspended needles like those used since the 1770s in France came into use in Britain. The British used these primarily for detecting minute variations or for determining magnetic intensity. Captain Sabine was one of the first British investigators to record intensity using a method developed by Jean-Charles Borda nearly half a century earlier. By 1831, Sabine's vigorous efforts to obtain governmental support had succeeded.
What can be concluded about the British approach to terrestrial magnetism during this period? In the latter eighteenth century there were navigational reasons for collecting observations, but relatively few natural philosophers engaged in the subject. Most observations were recorded by Royal Navy officers under less-than-ideal conditions. Despite the mapping efforts of Halley and others, large gaps and uncertainties existed in the global picture. This picture grew in complexity as the numbers of observations increased.
Flinders' recognition of local attraction gave British investigators further practical incentive to study terrestrial magnetism. Earlier pleas for systematic observation became amplified. Many believed that established facts, not hypothetical speculations, formed the basis from which a true theory of terrestrial magnetism would emerge. Along with practical concerns, the discovery of electromagnetism and emerging notions of the unity or connectedness of natural forces elevated interest in terrestrial magnetism. In these ways and others, both practical and scientific concerns influenced the push for widespread, systematic observations. In the 1830s and 40s the world was to see the establishment of an international network of permanent geomagnetic observatories with British, French, Germans and others participating. Instruments, observations and theories of terrestrial magnetism had changed quite a bit since the days of Sir Henry Cavendish.