A period of rejection of long-held ideas, rapid extension of observation, testing and rejection of new physical theories, and the development of new mathematical approaches to geomagnetism.
The status of nineteenth-century geomagnetic theory was presented by the English natural philosopher John Herschel (1792-1871) in 1840, on the eve of what was called the Magnetic Crusade. He wrote that although in experimental sciences situations can be manipulated to get directly at efficient causes, in observational sciences such as astronomy and the study of Earth's magnetism, theories provide coherence to records otherwise disjointed. In such sciences, he wrote, "the theory is the science" (Herschel, p. 66). Herschel saw geomagnetic theories as temporary structures, scaffolds: "tentative, transient, and empirical conceptions . . ." (p. 67). Terrestrial magnetism in the nineteenth century was no mere inductive science, but its theories were also not solidly established. Not surprisingly, then, ideas about the causes of Earth's magnetism did indeed come and go.
The main theories of Earth's magnetism in 1800 came from two well established traditions: theories of magnetic fluids and theories based on multiple magnetic poles. The first arose in the successful use of imponderable fluids by eighteenth-century investigators of subjects such as electricity, heat, and light. The second was most famously associated with the name of Edmond Halley (1656-1742), although other proponents both preceded and followed him. During the nineteenth century, both of these views came into doubt as new approaches arose. Some of the most important emergent approaches included theories based on discoveries in electrodynamics and electromagnetism by Andr‚-Marie AmpŠre (1775-1836), Michael Faraday (1791-1867), and others, and a new mathematical approach introduced by Carl Friedrich Gauss (1777-1855) in 1839. Gauss's approach, while initially presented as separable from physical theory, ultimately allowed scientists to explore new explanations of Earth's magnetism. Finally, James Clerk Maxwell's (1831-1879) comprehensive treatment of electromagnetic fields provided a basis for new investigations as the twentieth century dawned.
By 1800, magnetism generally and for the first time "occupied its own space on the map of natural philosophy" (Fara, p. 83). Although not due to great discoveries but to complex social developments, this situation rested on certain ideas about magnetism. Gowin Knight (1713-1772) was a foremost proponent of magnetic fluid theory (see Fara, pp. 44-60). Knight in 1748 (pp. 66-77) maintained that the curved lines of iron filings near a magnet indicate the paths of magnetic fluids (Fara, p. 150). His was a fluid of corpuscles, separated by a repulsive force, and present within iron. Meanwhile, Leonhard Euler (1707-1783) and Charles-Fran‡ois Du Fay (1698-1739) -- among others -- based their work on circulating subtle matter, somewhat like that proposed by Ren‚ Descartes (1596-1650). Various magnetic fluids or ethers were actively explored and advocated either side of 1800 by Tiberius Cavallo, Thomas Young, John Robison, and John Dalton (Fara, pp. 247-257).
Franz Aepinus (1724-1802) developed an alternative, detailed, mathematical theory in 1759 which assumed a different sort of magnetic fluid (Home, ch. 4). His was a non-circulating magnetic fluid, its particles attracted to iron, but mutually repelled. Critically, it was the sums of these unexplained forces acting at-a-distance that really mattered. His magnetic theory posited two magnetic states, positive and negative, depending on whether a body had more or less of its "natural" amount of magnetic fluid (Home, pp. 166-167). Aepinus's physical theory was semi-quantitative, but macroscopic. He also argued that Earth has "quite a large" magnetic core, by which he meant a conventional magnet, but which he refused to explain mechanistically, saying rather that it is due somehow to "the immediate action of the creator . . ." (quoted in Home, p. 183). The alignment of the compass needle was often explained as due to the flow of a magnetic fluid through its length; Aepinus explained it instead in terms of the joint action of two opposite poles on the needle (Home, pp. 182-186). He was mostly interested in magnetic laws and action generally, however, and considered secular (and perhaps other) variations of Earth's magnetism to be beyond mathematical analysis, due perhaps to the "generation and destruction of magnetic ores".
A second major variety of magnetic fluid theory supposed that every iron particle (and perhaps those of other materials) contain equal amounts of boreal and austral magnetic fluids. Particles of the one fluid were held to attract those of the other, but to repel those of the same kind. Generally, these fluids were considered inseparable from iron, restricted to motion over only very small distances.
Charles Augustin Coulomb (1736-1806) represents the height of magnetic theory in 1800. Heir to these theories of magnetic fluids, he did not shed them entirely, but he radically transformed their possible meaning. The best historical discussion of his work on magnetism is in C. Stewart Gillmor's biography (ch. 6). Coulomb criticized the notion of circulating aethers, the one-fluid theory of Aepinus, and the various two-fluid theories. None of these accounted adequately, in his view, for some very simple experimental results. Between 1777 and 1799, Coulomb developed a theory in which two magnetic fluids were restricted to movement within individual molecules, whose forces acted at-a-distance (as had those of Aepinus' one-fluid theory) and were subject to calculation (Gillmor, pp. 210-221). These two characteristics differentiated it from previous theories. His theory was further developed in the nineteenth century by, among others, Jean-Baptiste Biot (1774-1862) and Sim‚on Denis Poisson (1781-1840) (Gillmor, p. 218). Poisson's theory of magnetism, presented to the Acad‚mie Royale des Sciences in 1826 was perhaps the highest development of magnetic fluid theory. Herschel and Gauss, among others, knew and used it.
However, some early-nineteenth-century natural philosophers began to doubt fluid models and entertain alternatives. In this, they perhaps unknowingly followed the lead of eighteenth-century writers such as Pieter van Musschenbroeck (1692-1761), who considered that magnetic fluid existed only in certain people's heads (Home, pp. 161-162). Tobias Mayer (the elder, 1723-1762) maintained in 1760 that the fluid theory obstructed progress in magnetic investigation. He called it a "useless and inept" hypothesis, and advocated an approach based only on experiment and calculation (Forbes, pp. 65-67). David Brewster (1781-1868) echoed this view in 1857, stating that circulating fluids such as Euler's or Daniel Bernoulli's (1700-1782) were "useless speculations" (p. 4), although he ignored the fact that they were widely applauded in their time. Neither the objections of von Musschenbroeck and Mayer nor the discoveries of the 1820s and 1830s regarding the magnetic effects of electric currents entirely ended discussion of magnetic fluids, but they did shift the focus to the flow of electricity and its induced magnetic effects.
John Canton (1718-1772) originated an explanation of diurnal variation that was still taken seriously in the mid-nineteenth century. (The daily periodic change of magnetic declination was discovered by George Graham in 1722.) Canton supposed that the Sun's heat acting on the parts of Earth east of the dawn demarcation line somehow weakened their magnetism, thus allowing a compass needle to swing toward the west before dawn. Then in the afternoon, he stated, as the Sun warmed more of the planet west of the observer, the magnetism in that region would be weakened, and the needle would swing eastward (Brewster, p. 6). This was based on the laboratory experience that heat weakens magnets, but Canton provided no clear explanation of how this worked on a terrestrial scale.
The 1820 discovery by Hans Christian Oersted (1777-1851) that electric currents produce magnetic effects is well known, but its immediate effect on theories of terrestrial magnetism is not. Historian John Cawood sketches (pp. 578-581) how Fran‡ois Arago (1786-1853) brought news of the discovery to Paris, where he, Alexander von Humboldt (1769-1859), and others were already active in terrestrial magnetic research. AmpŠre's two-fluid theory of electromagnetism was in part a result of this, and he and Humboldt both speculated on how these discoveries could explain geomagnetism.
The possibility that the relations between electricity, magnetism, and heat would help explain geomagnetism was strengthened by other discoveries of the 1820s and 1830s. Thomas Johann Seebeck (1770-1831) discovered thermoelectricity in 1821. Humboldt and Edward Sabine (1788-1883) were especially impressed at the similarities between the globe's isothermal lines and the lines of equal magnetic intensity. Faraday's discovery in 1831 that magnetism can produce electric currents reinforced ideas about the unity of natural phenomena. Peter Barlow explained geomagnetism in terms of electric currents flowing in Earth's crust. In 1839, Gauss suggested that discussions based on magnetic fluids or on electromagnetism could produce the same appearances (p. 49). John Herschel in 1840 attributed periodic variations such as diurnal variation to electrical currents in either the atmosphere or in Earth's crust, induced by temperature variations (Herschel, p. 95). The relationships between studying magnetism in the global context and in that of the laboratory in the early nineteenth century are problematic, and require much closer historical interpretation.
The history of the understanding of Earth's magnetic poles is a complex topic (see Good, Follow the Needle). Two meanings, sometimes confused, were discussed in the nineteenth century. In the first, a freely suspended magnetic needle pointed vertically downward at Earth's magnetic poles. Other ways to say this were that declination needles converged on this place, or that the horizontal intensity vanished there, although this depended on the theory held. The second meaning considered the magnetic poles to be regions of maximum magnetic intensity, there being such a region in Canada and another in Siberia.
The foremost proponent of the theory that Earth has two magnetic axes (or four poles) in the nineteenth century was Christopher Hansteen (1784-1875). He presented this theory in Untersuchungen ber den Magnetismus der Erde (Investigations concerning the Magnetism of the Earth, 1819). This theory was influential, despite the rejection of Halley's version of it by many eighteenth-century investigators. Hansteen could not accept a single-axis, two-pole theory partly because the magnetic equator (the path along which the magnetic inclination or dip is zero) is not a great circle. Hansteen carefully distinguished between "points of force" and "points of convergence", the former being to him the real magnetic poles and the latter a superficial appearance (Good, Follow the Needle, p. 161). About 1830 Hansteen and others journeyed across Siberia in search of a region of maximum magnetic intensity and James C. Ross (1800-1862) sought the pole of vertical dip in northern Canada, clearly illustrating the practical consequences of different ideas.
The four-pole theory guided not only expeditions but analysis in the 1820s and 1830s. The main distinction between Halley's and Hansteen's theories was that whereas Halley lacked a law of magnetic attraction, Hansteen used quantitative laws and could calculate from his theory the declination and other magnetic variables that should be observed at various locations on Earth (Hansteen, chs. 5 and 6). His calculations were made possible by Henry Cavendish (1731-1810) and Coulomb (Cawood, pp. 553-557). Hansteen did not entirely forswear speculation about the ultimate cause of magnetism, but he carried his application of quantitative law to geomagnetism much further than his predecessors had.
Despite widespread use of and admiration for Hansteen's theory, there were doubts. Biot, who had researched geomagnetism on the ground and in balloons, proposed a detailed mathematical theory based on a single magnetic dipole near Earth's center (Good, Follow the Needle, p. 161; Cawood, pp. 563-564). More importantly, by the 1830s the geomagnetic data were interpreted by some as incompatible with any theory based on one or two dipoles. Barlow concluded in 1833 that every point on Earth has its own poles and own secular magnetic variation. Others agreed, including Herschel and Gauss (Good, Follow the Needle, pp. 162-163).
The ultimate source of geomagnetic phenomena was much debated in the nineteenth century. Hansteen and Brewster were firmly convinced that the real causes lay beyond the Earth: the Sun and Moon. Gauss, like Halley and Mayer, was equally convinced that most geomagnetic phenomena had their origin within the Earth. To Hansteen, the Sun and Moon seemed likely to produce changes in Earth's magnetism, just as they produce the tides. He also speculated that the rotation of his two magnetic axes was due to the precession of the equinoxes (Hansteen, pp. 102-105). Brewster admitted the presence of magnetic metals in Earth's crust, but he thought they were the source of local disturbance, not of the main magnetic phenomena. If the primary source of this magnetism were in the crust or deep in Earth's interior, he wrote, the intensity measured on the ocean, on mountains, or in balloons should be much less than it is. Brewster posited instead that the atmosphere contains metallic vapors, especially iron. This envelope of atmospheric iron, magnetized by induction from an external cause, would, he said, produce the same appearances as a dipole deep in the Earth, but it would also be affected by temperature differences due to the Sun's heat and by electrical discharges. Thus Brewster explained diurnal and magnetic storm variations, lightning, and even auroral noise. The source of all these induced effects, Brewster had no doubt, was the Sun, although he said it was for future research to decide whether it was due to solar heat, light, direct magnetic action, or to some unknown rays (Brewster, pp. 63-68).
Two figures overshadow all others in the history of nineteenth century work in geomagnetism: Alexander von Humboldt and Carl Friedrich Gauss. The former proposed no theory of great importance, but he conducted several important investigations, including that of sudden disturbances of magnetic compasses, called magnetic storms. He also inspired generations of researchers to undertake arduous geomagnetic observational programs. At Humboldt's instigation, a network of magnetic observatories was established first in Europe in the 1830s, and then worldwide in the 1840s (Biermann, passim; Cawood, pp. 583-587). Gauss, working with Wilhelm Weber (1804-1891), achieved breakthroughs in both instrumentation and in geomagnetic theory. In many ways, geomagnetic research followed out the patterns set by Humboldt and Gauss until well into the twentieth century.
Humboldt simultaneously speculated boldly on the causes of geomagnetic phenomena and illustrated the painstaking empiricism required to assess critically the value of such ideas. Largely because of his example, much nineteenth-century geomagnetic research aimed at intricate, quantitative description. This is not to be disparaged. Humboldt's long passage on terrestrial magnetism in the Cosmos (5: 50-156) provides an exhaustive chronology of what researchers learned up to mid-century about the morphology of magnetic variations, magnetic distribution, and magnetic storms. An even longer story could be told about descriptive research done after the Cosmos appeared. It should be noted that although Humboldt explicitly limited the Cosmos to "positive knowledge", he also told the reader that he did indeed entertain and value physical hypotheses.
Gauss's accomplishments are well summarized in G.D. Garland's 1979 article and in Christa Jungnickel and Russell McCormmach's Intellectual Mastery of Nature (1: 63-77). Gauss, like Humboldt, provided some of the stimulus for the establishment of magnetic observatories. He made this possible partly through the design of new magnetic instruments intended for specific variables, and partly through the rigorous mathematical analysis of those instruments and of the measurements taken with them. More importantly, in 1832 he established a method (called the absolute method) by which the intensity of magnetism can be measured in terms of mass, length, and time (Garland, pp. 7-10). Up to then, magnetic intensity was a relative measurement, based on the oscillation of a magnetic needle suspended by a fiber. Gauss's new method made it possible to conduct comparable magnetic intensity measurements around the globe and over long periods of time.
Gauss's longest lasting effect on geomagnetic research was his development of spherical harmonic analysis, a mathematical representation of Earth's magnetism. Although Gauss thought of his theory as calculating the varying density of magnetic fluid, he did not think this physical hypothesis was necessary to his theory (Gauss, p. 47). More importantly, Gauss also did not assume that the Earth had two or four poles, nor did he use any other physical supposition. Rather, he assumed magnetism to be generally distributed within Earth and on its surface. He assumed moreover that the magnetic force at any given point is due to the summation of forces from all other points. That is, he denied any efficacy to particular magnetic poles. Indeed, he denied that the chord between the two poles of vertical dip should even be termed Earth's magnetic axis (p. 45). His method made it possible for the first time to locate the relative contributions of various causes to geomagnetic phenomena (Garland, pp. 14-22; Good, Study of Geomagnetism, pp. 220-221). Another critical development in mid-century was James Clerk Maxwell's synthesis of electromagnetism, embodied in "Maxwell's equations", which made it possible to treat quantitatively Faraday's ideas about electric and magnetic fields. His Treatise on Electricity and Magnetism (1873) epitomized his theory. While it is well known that these equations unified the studies of electricity, magnetism, and light, and that they are part of the story of technologies such as radio, their connection to the study of geomagnetism has yet to be told. Jungnickel and McCormmach offer a suggestive review of reaction to Maxwell (2: 227-245). Maxwell's theory provided another set of mathematical tools, but also a new physical basis that replaced both fluids and action-at-a-distance forces. From the 1880s onwards, geomagnetic theorists wrote invariably of Earth's magnetic field, not of its magnetic forces nor of its magnetic elements.
This was in itself a revolution. A generation of magneticians raised on mathematical physics was coming of age. Arthur Schuster (1851-1934) had worked with Hermann Helmholtz (1821-1894) in Berlin and with Maxwell in Cambridge. Louis Agricola Bauer (1865-1932) did doctoral studies with Max Planck (1858-1947) and Wilhelm von Bezold (1837-1907). Geomagnetic research grew explosively in the 1890s. Existing venues for publication no longer sufficed, leading to the founding of the journal Terrestrial Magnetism in 1896. International organization expanded; national magnetic surveys and permanent observatories multiplied. These institutional developments arose partly in practical concerns, but the increasing theoretical capability of researchers was also a factor.
A better representation of Earth's magnetic field than was possible in Gauss's time was needed to answer several physical questions. First was the question of the location of the cause of the main field, and of various disturbance fields. Gauss had concluded from his analysis that the main field was caused internally (Garland, pp. 18-20). In 1889, Schuster demonstrated that the diurnal variation was very likely caused by external events, but not entirely. His development of Gauss's spherical harmonics showed how the approach could be used to investigate both electrical currents in the upper atmosphere and the electrical conductivity of Earth's crust (Good, Study of Geomagnetism, pp. 223-225). Likewise, with adequate observations to work from, Gauss's theory could reveal whether there was an electrical current flowing vertically through Earth's surface, as well as whether magnetic monopoles exist. Gauss had noted both of these implications (Garland, pp. 21-22), but did not follow up on them. These questions motivated research by Bauer and others (Good, Study of Geomagnetism, pp. 220-221).
The massive accumulation of magnetic data since 1839 allowed Georg von Neumayer (1826-1909), H. Fritsche, and others to enlarge on Gauss's calculations. Whereas Gauss had had to restrict his calculations to a spherical Earth, Schmidt (1860-1944) had adequate data to consider a spheroidal Earth (Chapman and Bartels, chs. 17 and 18). Whereas Gauss included only the terms of the harmonic expansion relating to causes within Earth, both John Couch Adams (1819-1892) and Schmidt could include terms relating to external causes, too. Moreover the precision of the representation was carried much further in the 1880s and 1890s than Gauss could have done. Gauss had included terms to the fourth order of the expansion (i.e., 24 constants), Schmidt to the sixth order, while Adams included 120 constants for causes internal and 120 for causes external to Earth.
Because of the greater precision of these later analyses of Earth's magnetism, it became possible to test certain assumptions of Gauss's theory. Not only had Gauss assumed the permanent part of geomagnetism to be due to internal causes, he also assumed these causes to be referable to a potential. With his more precise calculations Schmidt concluded that another factor was possible: perhaps as much as 1/40th of the total magnetic intensity could be due to an electric current passing vertically from Earth into the atmosphere (Schmidt, passim; Nippoldt, pp. 98-99). Not an easy question to test, this led to several years of active research, as related in 1900 by Alfred Nippoldt (1874-1936) (pp. 99-101).
Just as extensions of Gauss's theory allowed a closer analysis of Earth's permanent magnetic field, investigators also analyzed anew the variations of Earth's magnetism (Good, Study of Geomagnetism, pp. 221-225; Nippoldt, pp. 106-109). Schuster examined spherical harmonic terms related to external causes and demonstrated their connection to diurnal variation; he speculated on the existence of electric currents in the upper atmosphere. Bauer stressed in 1895 that the first term of the Gaussian harmonic corresponds to a uniform magnetization of the Earth, a basic physical interpretation. He also calculated a "residual field", which he related to the diurnal variation field (Nippoldt, pp. 102-104). Meanwhile, V. Carlheim-Gyllenski”ld (1859-1934), Schuster, Arthur Korn (1870-1945), William Sutherland (1859-1911), and others attempted to apply Maxwell's theory and the results of Heinrich Hertz (1857-1894) on electromagnetic induction to various physical models and hypotheses (Nippoldt, pp. 104-106).
Arthur Schuster is a crucial geomagnetic theorist. His numerous articles examined and rejected many proposed theories of geomagnetism, usually because of shortcomings in their mathematics or physics. Among his articles is the neglected "A Critical Examination of the Possible Causes of Terrestrial Magnetism." Published in 1912, it reflects well the state of geomagnetic theory at the turn of the century. In it, Schuster specifically omits theories of the periodic variation and disturbances to concentrate on the origin of the main field. His discussion reflects activity both of theorists and of field and experimental workers intent on testing the various theories. It certainly does not reflect a moribund or unimaginative research community. Indeed, the lively investigations undertaken by this community set the stage for the better known 1919 memoir of Joseph Larmor (1857-1942) on the magnetism of rotating bodies.
Schuster considered and evaluated each of the available theories of the main geomagnetic field. It was a safe starting point, he wrote, to assume that "the near approach between the geographical and the magnetic poles is intimately connected with the ultimate cause of terrestrial magnetism" (pp. 121-122). He judged that geomagnetic writers had prematurely ruled out an explanation based on permanent, normal magnetization of some portion of Earth. Indeed, his laboratory was investigating the effects of pressure on magnetization. He felt that too much hope was placed on explanations based on electric currents circulating within Earth, either as the result of some other energy source or "as a survival of an old state of things," i.e., of an originally much stronger electric current (pp. 123-124). Factors Schuster could not have taken into account later disqualified his objections to this theory. He also argued against the theory that the magnetism of Earth's core is induced by external causes. A group of theories worth much closer scrutiny related rotation of bodies to the generation of magnetism (pp. 125-130). Magnetism could be produced if molecules were magnetic or if they carried an electrical charge. He also considered an intricate relationship between gravity, electrons, and the positive charges in atoms (pp. 133-135). Schuster ultimately, however, drew no firm theoretical conclusion, resting content to have pointed out possibilities, as well as both experimental and theoretical difficulties with each approach. In this, he symbolized geomagnetic theory at the turn of the twentieth century.
The nineteenth century is sometimes considered bereft of serious theories of the origin of geomagnetism and its variations, as a time for gathering of data and for clearing away of fanciful ideas. While there is an element of truth to each of these views, scientists nevertheless proposed and explored numerous geomagnetic theories. That none of these theories provided a complete physical and mathematical explanation of geomagnetism is beside the point. No matter how incorrect, incomplete, or incomprehensible they may now seem, these theories motivated some of the most ambitious large-scale geoscientific research and were entertained by some of the best known scientists of the time. No history of this terrestrial science could be complete without an examination of these theories. They must stand ultimately in juxtaposition to the histories of observing programs and of the cultural and institutional changes that made this science possible.
A starting point for the history of nineteenth-century geomagnetic research is still Chapman and Bartels' Geomagnetism (1940). However, one must be careful not to take this book as a comprehensive reflection of sources, researchers, or issues explored before about 1880. Many earlier investigations and views did not fit the post-Gaussian, post-Maxwellian perspective of the authors. For earlier periods, it's important to consult not only the secondary sources noted above, but also a wide range of primary sources. To transcend positivist historiography and truly read magnetic researchers such as Hansteen in their own context, not ours, is not an easy task. It is especially important that historians examine geomagnetic research in relation to instrument making, the mathematical practitioner community, astronomy, meteorology, and finally, theoretical physics (Good, Geomagnetics and Scientific Institutions).
Gregory A. Good
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