Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660 www.elsevier.com/locate/jastp

Geomagnetic research in the 19th century: a case study of the German contribution

Wilfried Schr,oder ∗, Karl-Heinrich Wiederkehr Geophysical Institute, Hechelstrasse 8, D 28777 Bremen, Germany Received 20 October 2000; received in revised form 2 March 2001; accepted 1 May 2001

Abstract Even before the discovery of electromagnetism by Oersted, and before the work of AmpÂere, who attributed all magnetism to the 7ux of electrical currents, A.v. Humboldt and Hansteen had turned to geomagnetism. Through the “G,ottinger Mag- netischer Verein”, a worldwide cooperation under the leadership of Gauss came into existence. Even today, Gauss’s theory of geomagnetism is one of the pillars of geomagnetic research. Thereafter, J.v. Lamont, in Munich, took over the leadership in Germany. In England, the Magnetic Crusade was started by the initiative of John Herschel and E. Sabine. At the beginning of the 1840s, James Clarke Ross advanced to the vicinity of the southern magnetic pole on the Antarctic Continent, which was then quite unknown. Ten years later, Sabine was able to demonstrate solar–terrestrial relations from the data of the colonial observatories. In the 1980s, Arthur Schuster, following Balfour Stewart’s ideas, succeeded in interpreting the daily variations of the electrical process in the high atmosphere. Geomagnetic research work in Germany was given a fresh impetus by the programme of the First Polar Year 1882–1883. Georg Neumayer, director of the “Deutsche Seewarte” in Hamburg, was one of the initiators of the Polar Year. He forged a close cooperation with the newly founded “Kaiserliches Marineobservato- rium” in Wilhelmshaven, and also managed to gain the collaboration of the “Gauss-Observatorium fur, Erdmagnetismus” in G,ottingen under E. Schering. In the Polar Year, the ÿrst automatic recording magnetometers (Kew-Model) were used in the German observatory at Wilhelmshaven. Here, M. Eschenhagen, who later became director of the geomagnetic section in the new Meteorological Magnetic Observatory in Potsdam, deserves special credit. Early hypotheses of geomagnetism and pio- neering palaeomagnetic experiments are brie7y reviewed. The essential seismological investigations at the turn of the 19th to the 20th century are also brie7y described as they underpin the modern theory of the Eartdynamo. c 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Geophysics; Geomagnetic main ÿeld; G,ottingen Magnetic Society; History of geomagnetism; International co-operation; Solar–terrestrial physics

0. Introduction constructed and new aspects of geomagnetic studies were opened, including solar–terrestrial physics. The development of geomagnetic research in the 19th century is discussed in detail. Beginning with the G,ottingen Magnetic Society, scientiÿc activity developed under von 1. Hansteen, Humboldt and the Gottingen Magnetic Humboldt’s in7uence and reached a peak during the First Society International Polar Year (1882–1883). This was a broad international co-operation, for which new instruments were In the ÿrst decades of the 19th century geomagnetism oIered a special opportunity for many scientists as this ∗ Corresponding author. ÿeld of science, which had previously been isolated, be- E-mail address: [email protected] (W. Schr,oder). came linked to electricity. These links became apparent

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c 2001 Elsevier Science Ltd. All rights reserved. PII: S1364-6826(01)00038-4 1650 W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660

Fig. 2. Alexander von Humboldt.

he became acquainted with a promising young physicist, Fig. 1. Christopher Hansteen. Wilhelm Weber, who was Assistant Professor at the Univer- sity of Halle. When the chair of physics became vacant at the through the work of Hans Christian Oersted, who discov- University in G,ottingen, Gauss recommended this inventive ered electromagnetism in 1820, Thomas Johann Seebeck experimenter and attracted him to G,ottingen. Thus Weber’s with his discovery of thermoelectricity in 1821 (called by ambition to improve himself through Gauss’s proximity Seebeck “thermomagnetism”) and Michael Faraday with his was fulÿlled. This was the beginning of a close co-operation researches on electromagnetic induction in 1831. Geomag- which is seldom experienced and it is diOcult to separate netic research, however, had developed even earlier due the individual contributions by the two scientists. Weber to impetuses by Alexander von Humboldt and the Norwe- followed up, and consequently realized, Gauss’s ideas in gian Christopher Hansteen (1784–1873) (see Figs. 1 and later years. The impetus for new research in geomagnetism 2). Humboldt regularly carried out magnetic measurements came from Humboldt’s letter to Weber at the end of 1831. during his trips in America and Russia and determined the After his trip in Russia Humboldt initiated simultaneous geomagnetic horizontal intensity by oscillating a bar magnet magnetic measurements at several locations and requested (these measurements were, nevertheless, only “relative” to co-operation from G,ottingen, too. Gauss had dealt with ge- the weakest ÿeld at the magnetic equator (inclination o◦)). omagnetism theoretically and later published some papers Hansteen published in 1819a monograph entitled “Studies in this ÿeld (Schaefer, 1929). In 1833 Gauss and Weber es- of the Magnetism of the Earth”. He supposed that the geo- tablished a geomagnetic observatory and joined Humboldt’s magnetic ÿeld is due to two bar magnets near to the centre observational network. They designed new geomagnetic of the Earth. He studied intensively the slow movements of instruments, including the uniÿlar magnetometer for decli- the geomagnetic poles (the secular variation) and the daily nation and its variations and the biÿlar magnetometer for variation of the geomagnetic force (which had already been horizontal intensity. The ÿrst published paper was Gauss’s previously observed). He tried to explain the latter in terms famous “Intensitas vis magneticae terrestris ad mensuram of a remote magnetic eIect of the Sun (cf. Hansteen, 1819). absolutam revocata” (the intensity of the geomagnetic force The most signiÿcant and pioneering works in physics of in absolute measure, 1832). Absolute measurements are Carl Friedrich Gauss deal with magnetism. At the confer- contrasted here with the relative measurements as previ- ence of German naturalists and physicians in 1828 in Berlin ously carried out by Humboldt. Gauss established for this W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660 1651 purpose the now well known main positions of a magnet named after him. Both the horizontal intensity and the magnetic moment of the bar magnet used could be exactly determined by this method. Humboldt’s measurements implied a constant magnetic force of the bar magnet—a condition which was not always fulÿlled. Gauss introduced in “Intensitas” a measurement system of three basic units, namely the units of length, mass and time. Such systems were later called absolute systems. The G,ottingen Magnetic Society was founded on Humboldt’s and Gauss’ reputa- tions, and the leadership of the Society was in Gauss’s and Weber’s hands as initiated by Humboldt. The Society was a voluntary association of co-operating scientists from many countries who carried out measurements at predetermined times, often with identical instruments. The G,ottingen Magnetic Society was the model for later programmes of geophysical co-operation, including the First Polar Year 1882–1883 and the International Geophysical Year 1957– 1958. The G,ottingen Observatory and its equipment became prototypes for later, similar, observatories. Weber con- tributed to the publications through a series of annual reports, called “Results” (Wiederkehr, 1964). Beginning in 1836, six volumes were published, together with an “Atlas of Ge- omagnetism”. In the “Results of 1838”, Gauss published his pioneering work “General Theory of Geomagnetism” (see Fig. 3). It remains to this day one of the pillars of the mathematical treatment of the geomagnetic ÿeld. It is not a theory in the present sense of the word, as it does not cover the causes of geomagnetism. Based on the sparse observational material at his disposal Gauss described in this work, using his potential laws and spherical harmonic functions, the geomagnetic ÿeld at the Earth’s surface. The question of whether it is caused by great magnets in the Earth’s interior or by electric currents remained open. The main source was certainly, according to Gauss, within the Earth’s body. Nevertheless, he considered it possible Fig. 3. Front page of the results of the Magnetic Association in the that part of the variations of the geomagnetic force could be year 1938. caused by electric currents in the atmosphere—at that time a remarkable prediction. Weber introduced in the “Results Weber went to England where he met the astronomer and of 1840” the ÿrst absolute electric current unit. This was physicist John Herschel (son of William Herschel), who had the basis for the absolute electromagnetic system which just returned from South Africa, and Weber won his sup- he later developed. He also initiated the presently used port for the Magnetic Society. Herschel had recently been electrical units, the volt, ampere and ohm (cf. Gauss, 1893; made a Baronet and his participation ensured support from Gauss and Weber, 1837–1840; Neumayer, 1892; Schering the whole world. Gauss obtained in 1838 the Copley-medal and Schering, 1886). of the Royal Society. This was at that time the highest sci- The joint work of Gauss and Weber was exceedingly fruit- entiÿc award, being comparable to the present Nobel-prize ful, but it ended sadly in 1837 following the coup d’Ãetat (Wiederkehr, 1985). Weber also met the English geophysi- of the Hanoverian King, Ernst August. Weber belonged cist Edward Sabine (1788–1883). The Magnetic Lobby of to the group of seven professors (the G,ottingen Seven) the British Association (spokesmen Herschel and Sabine), who protested against the despotism of the new ruler and ensured that an expedition was sent to the Antarctic re- who paid for this courageous step with their dismissal. This gion with two ships, “Erebus” and “Terror” led by James protest was a precursor of the revolution of 1848/1849. Clarke Ross (1800–1862) (Wiederkehr, 1985). The project During the following years Weber remained in Gauss’s had political support in England, too. Ross was also given vicinity, and through the support of democratic–patriotic the task of establishing, during his voyage, geophysical citizens, he was able to work for the G,ottingen Magnetic observatories at St. Helena, at the Cape of Good Hope Society and ÿnish some studies started earlier. In 1838 and in Tasmania (Cawood, 1979). The expedition was a 1652 W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660

in the 1840s). About 18 h later, there followed a geomag- netic storm which was identiÿed through large de7ections of the geomagnetic ÿeld. Today we know that an X-ray burst must have emanated from the two bright spots, the eIect of which was then ampliÿed in the ionosphere by electric currents. The solar material ejected during the event (helium nuclei and electrons) propagated with lower velocity and having reached the Earth, caused the geomagnetic storm. Leading physicists, including Lord Kelvin, considered this simultaneity of the observed events as a mere coincidence. In the following years Sabine’s ideas were conÿrmed by auroral research. The frequence of occurrence of geomagnetic storms also changed in parallel with the number of sunspots and followed the 11 years cycle (Schr,oder, 1984, see also: Chree, 1912; Eschenhagen, 1896b; Kohlrausch, 1897; Lamont, 1841; Stuart, 1861; Bartels, 1933; Sabine, 1851, 1852; Schuster, 1886; Wilde, 1894).

3. Lamont’s activity and earth current measurements

Fig. 4. Edward Sabine. After the end of the work of the G,ottingen Magnetic Society, the centre of geomagnetic research in Germany shifted to Munich. von Lamont (1805–1879), director of complete success. They entered the Antarctic continent the Astronomical Observatory in Munich-Bogenhausen, through the later, so-called, Ross Sea and the ships passed started regular geomagnetic observations in 1840 (see close to the southern magnetic pole, just inland on the con- Fig. 5). Lamont substituted smaller magnets for those in tinent. Ross was surprised at the accuracy of the maps given Gauss’ magnetometers—the magnets of which weighed him by Gauss concerning the position of the magnetic pole several kilograms. Weber also constructed smaller mag- (cf. Schering, 1902, 1909; Schering (letters to Neumayer); netometers, the so-called “transportable magnetometers”. Schmidt, 1925; Wiederkehr, 1964). Lamont criticized the large magnets, on the grounds that, due to their inertia, they could not follow rapid changes of the geomagnetic force, H. Lloyd in Dublin was of the 2. The discovery ofsolar–terrestrial connections same opinion. Lamont manufactured with his own hands in his workshop, a magnetic theodolite with which he could Sabine was able to achieve a continuation of the activity determine declination, horizontal intensity and the vertical of the colonial observatories for the following period and component of the geomagnetic ÿeld. Lamont’s versatile England took over the leading role in the study of geomag- instrument was purchased by many institutes and it was netism with signiÿcant ÿnancial investment (see Fig. 4). indispensable for regional surveys for several decades. One result was Sabine’s discovery in 1852 of the parallelism Lamont also substituted for the Gauss deviation method in of the 11 years sunspot cycle with the average intensity of the main positions, a more easily applicable one in which geomagnetic activity (Sabine, 1851, 1852). The discovery, he rotated the deviating magnet until it became perpendicu- more exactly the statement, had nevertheless rather weak lar to the suspended magnet. Lamont published in 1849the supporting evidence. A phenomenon was then observed in very popular “Handbuch des Erdmagnetismus” (Handbook 1859at Kew and Greenwich which would have brilliantly of Geomagnetism). He carried out the ÿrst regional mag- conÿrmed Sabine’s supposition about the solar–terrestrial netic survey of a country in Bavaria and soon West- and connection in geomagnetism had it been correctly identiÿed South-European countries followed his example. In 1838 and interpreted (Chapman and Bartels, 1940). The observer, Steinheil substituted the electric return line of the Gauss– R.C. Carrington in Redhill, was drawing a group of sunspots Weber telegraph by conduction through the soil. Barlow when, suddenly, two bright white spots appeared on the noted, as early as 1849, electric currents on telegraph lines solar disc—a phenomenon never previously observed. which were especially intense during geomagnetic distur- Today, it would be termed a “solar 7are eIect”. Si- bances. In their study, Lamont sank two metal plates in the multaneously, signiÿcant geomagnetic variations were soil and connected them through a galvanometer. He found registered at Kew which hinted at some disturbance the existence of earth currents whose intensity depended on (photographic recording had been introduced in England several factors. Chemically and thermally caused potential W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660 1653

Fig. 6. Georg von Neumayer.

International Polar Year 1882–1883. They unanimously accepted that comprehensive theories in meteorology, geomagnetism (including auroral research), atmospheric electricity and climatic research were impossible without Fig. 5. Johannes von Lamont. the knowledge of processes in polar regions (Schr,oder and Wiederkehr, 1992). What was the impetus for diIerences existed between the metal plates. Rain increased this co-operation of nearly one dozen nations? The the conductivity of the earth, and he also supposed an in- undertaking was initiated by Weyprecht and Neu- 7uence from atmospheric electricity. Using telegraph lines, mayer. During his voyage through the Northern Polar earth currents were measured by several geophysicists in Sea, Weyprecht’s ship, the TegethoI, was the play- the following decades. It proved, however, diOcult to de- thing of the winds and of currents moving the ice, and it was cide if earth currents were a causal generating source of enclosed by ice and slowly crushed. It was pure chance that geomagnetic variations or whether they were merely con- the staI were able to save themselves. Weyprecht suggested sequences of these variations (cf. Van Bemmelen, 1900; that stable observatories should be preferred in the future, Birkeland, 1912; Chree, 1912; Ebert, 1907; Eschenhagen, instead of voyages. He considered international co-operation 1896a–d, 1897; Lamont, 1841, 1861; Messerschmidt, 1905; to be superior to the competition of national expeditions Moidrey, 1917; Stuart, 1861; Terrada, 1916, 1917). and Neumayer (see Fig. 6) expressed similar ideas. Fol- lowing Weyprecht’s and Neumayer’s proposals, the Second International Congress of Meteorologists in Rome in 1879 4. The First Polar Year 1882–1883 and the Marine accepted a recommendation to their governments. In a Observatory in Wilhelmshaven ring of stations around the North Pole, observations were to be carried out according to a common plan, following In the last three decades of the 19th century geomagnetic the example of the G,ottingen Magnetic Society. Germany research was closely connected with polar research. Nine ex- sent two expeditions, one of them to the Kingua-fjord perts of the geophysical disciplines met on October 1, 1879 (Labrador) in the Arctic region, the other to South Georgia at the Marine Station in Hamburg under the chairmanship in the Antarctic region (Neumayer and B,orgen, 1886). Neu- of Georg Neumayer (1826–1909), the director of the Impe- mayer urged the inclusion of the southern polar region, too, rial Institute founded in 1875 to promote marine research but only France sent a further expedition to Orange Bay in and German overseas commerce. The participants included Tierra del Fuego. Before the Polar Year Neumayer asked, the Austrian marine oOcer Carl Weyprecht (1838–1881) in a circular letter, for co-operation from institutes and who had carried out, together with Julius Payer, the Aus- scientiÿc societies in Germany. Final participants were lim- trian North Pole expedition of 1872–1874. These scientists, ited to the Imperial Marine Observatory in Wilhelmshaven from diIerent countries, made detailed plans for the First headed by Carl B,orgen (1843–1909) and the Gauss 1654 W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660

Observatory in G,ottingen headed by Ernst Schering (1833– 1897). The Breslau Astronomical Observatory headed by Johann Gottfried Galle (1812–1910) was ready to participate in a part of the programme (cf. Neumayer and B,orgen, 1886; Neumayer, 1892). Following the war against France in 1870/1871, German commerce increased. In line with the increasing maritime interests of the Empire an observatory for marine studies was founded in 1818 at Wilhelmshaven. Its task was to pre- pare geophysical data for the marine community, to carry out exact time determinations, to compute tides and to per- form compass regulation. Geomagnetism also found there a new research home and Wilhelmshaven occupied until 1989 the role of the central German magnetic station. The Astro- physical Observatory Potsdam, the site of solar observations during the Polar Year, put at Wilhelmshaven’s disposal an automatically recording magnetograph and this instrument was used there for the ÿrst time. This magnetograph was constructed by P. Adie in London and it was purchased not only by Potsdam, but also by other observatories. In England, photography had been used for scientiÿc purposes following its discovery by L.J.M. Daguerre and W.H.F. Talbot, and an automatically recording magnetograph had been developed. Among the constructors Charles Brooke (1804–1879) should be mentioned. There was competition Fig. 7. Ernst Schering. between Greenwich and Kew to construct a better recording apparatus. For magnetometers, photographic recording was an ideal solution, as due to the small magnetic forces, me- Astronomical Observatory since 1868, the Gauss Observa- chanical recording was hardly feasible. In Wilhelmshaven, tory also belonged to this section. Georg Neumayer, as pres- from 1882 onwards, the recording was supervised and copies ident of the German Polar Commission, in a long letter, of the magnetograms were produced, by a young scien- asked the responsible minister for support (letters by Scher- tist, Max Eschenhagen (1858–1901). He mastered this job ing and Schering). As a result an additional subsurface room and so had the opportunity to process and interpret all ge- was built to record declination and the vertical component. omagnetic data from Wilhelmshaven, and later also from Ernst Schering was helped during the First Polar Year by his both expeditions. Eschenhagen did this job in a clear, but younger brother, Karl Schering (1854–1925) who obtained critical way and published several of his own works in his habilitation in G,ottingen in the ÿeld of mathematical the polar report edited by Neumayer and B,orgen (1886). physics. The two brothers constructed new instruments for Whilst at Wilhelmshaven Eschenhagen started the geomag- measuring the perpendicular component of the geomagnetic netic survey of northern Germany and continued this after ÿeld vector, namely the “vertical-de7ector-uniÿlar” and the his appointment to Potsdam in 1889 (cf. Cawood, 1979; “quadriÿlar”. Inclination was determined by an earth induc- Lamont, 1862; Schuster, 1886; Ebert, 1906; Eschenhagen, tor, applying the null-method designed for this instrument. 1897, 1899; Giese and Ambronn, 1886; Lamont, 1861). The Scherings published a paper on the measurements made in 1882–1883 in the Polar Report (Schering and Schering, 1886; Schering, 1902, 1909). 5. Ernst Schering and the Gauss Observatory in the Polar Year 6. Geomagnetic maps by Georg Neumayer As mentioned earlier, the Gauss Observatory in G,ottingen participated, together with Wilhelmshaven, in all geomag- The magnetic material of the First Polar Year, and netic station observations. Georg Neumayer considered it other previously collected measurements, enabled Neu- important to include G,ottingen in the Polar Year as it had mayer to plot magnetic maps for the epoch 1885. They been for some time the centre of magnetic research. After were published as a separate small volume of the large the Prussian-Austrian war in 1866 (the Kingdom of Hanover publication “Physical Atlas” by Heinrich Berghaus. Neu- stood with the losers as an ally of Austria), G,ottingen was mayer also dealt theoretically with geomagnetism. He somewhat neglected by Berlin. On a written request by Ernst estimated again the Gauss coeOcients of the spherical Schering (see Fig. 7), director of Section B of the G,ottingen harmonic expansion to ÿt them to the actual boundary W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660 1655

Fig. 8. Adolf Schmidt. Fig. 9. Arthur Schuster.

conditions of the Earth. The computed values of the in London Stewart developed his ideas and he published a magnetic elements were then compared to the actual summary in the “Supplements to the Annals of Physics and measured ones. The result was not very satisfactory and Chemistry” in 1880 which has been hitherto neglected by this hinted at the necessity for an improvement of the historians (Stewart, 1878, 1880). He compared the Earth Gauss theory (Schr,oder and Wiederkehr, 1992). Other and its atmosphere with RuhmkorI ’s induction apparatus. geomagneticians, such as J.C. Adams, reached the same Changing electric currents in the Earth and in the atmosphere conclusion. Adolf Schmidt (1860–1944) from Gotha mod- were supposed to induce each other, and Stewart wrote of iÿed the Gaussian theory as he developed each compo- an “electromagnetic machine”. He suggested that the higher, nent separately and also took the polar 7attening of the thin layers of the atmosphere—being electrically contact- Earth into account (see Fig. 8). Neumayer enabled him, ing according to Faraday theory on rareÿed gases—move as a teacher in the Ernestinum in Gotha, to publish his by convection through the ÿeld lines of the geomagnetic comprehensive papers in the “Archives of the German ÿeld in a similar way to the armature of an electric gener- Marine Observatory” (cf. Neumayer and B,orgen, 1886; ator. This was in eIect the birth of the dynamo theory of Neumayer, 1892). geomagnetism; an explanation of the self-excitation mech- anism was, however, lacking. Schuster carried out quantita- tive studies in two further steps. In 1886 he proved that the 7. Schuster’s pioneering work to explain daily variations observed ÿeld of the variations consisted of two parts, of ex- ternal and internal origins respectively, the latter being pro- The understanding and the mathematical treatment of pe- duced inside the rotating Earth (Schuster, 1886). He based riodic geomagnetic variations were fundamentally promoted his ideas on the Gauss “General theory of geomagnetism” through two papers by Arthur Schuster (1886, 1889) (see which predicted that an expansion of the ÿeld distribution Fig. 9). Schuster (1851–1934) returned to the fruitful ideas in spherical function harmonics coned decide whether the of Balfour Stewart (1828–1887) who had supposed a con- sources were within or outside the solid Earth. In the second nection between solar irradiation and geomagnetic phenom- step, in 1889, Schuster used Stewart’s hypothesis to explain ena. In the Proceedings of the Royal Society (1877–1879) daily variations (Schuster, 1889; Schmidt, 1925). 1656 W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660

Fig. 11. Louis Agricola Bauer.

Fig. 10. Max Eschenhagen.

8. Eschenhagen and Bauer W. von Bezold, which followed the procedures of the ob- In 1889Eschenhagen (Fig. 10) became head of the section servatory at Parc-St-Maur near Paris. A set of Mascart’s of geomagnetism in Potsdam and in 1896 started his “Fein- sensitive magnetometers was also bought. Recording appa- registrierung” (ÿne recording) of geomagnetic pulsations, ratus was also constructed by Eschenhagen himself, and he the periodic disturbances of very short periods. He called used for this task his experience with the Kew model from them “Elementarwellen” (elementary waves), and this was Wilhelmshaven. In the few years of his activity in Potsdam their very ÿrst recording on a magnetogram. He considered —he died early in 1901—Eschenhagen designed highly them to be a consequence of electric processes (currents) eIective magnetometers and the magnetic theodolite manu- in the upper layers of the atmosphere (Eschenhagen, 1897). factured by the ÿrm Tesdorpf. Louis Agricola Bauer (1865– The hypothesis of electric currents in the upper atmosphere 1932) (see Fig. 11), the well-known American geophysicist, was supported at that time by other physical research and visited Berlin in the 1990s to improve his theoretical knowl- discoveries. The ionisation of gases by UV-light was de- edge. He participated at lectures by Helmholtz, Bezold and tected, the photoelectric eIect was discovered and ion pro- others and in 1895 obtained his doctorate with the work duction by solar radiation was also studied. The atomistic “Contributions to the Knowledge of Geomagnetic Secular character of electricity came to be accepted and in 1896 J.J. Variation”. Any physical theory of geomagnetism had to be Thomas discovered the electron based on previous work. able to explain the phenomenon of secular variation and it Wireless telegraphy, established just before the end of the was even possible that it was the key to the problem. After century, also came into the picture. Kennelly and Heavi- his return to the United States, Bauer became head of the side had postulated around 1900 the existence of an ionised section of Geomagnetism of the Carnegie Institution. He layer in the upper atmosphere where wireless waves are re- founded geomagnetic observatories in previously uncov- 7ected (Bartels, 1933). In the 20th century knowledge of ered parts of the Earth, organised geomagnetic expeditions the ionosphere expanded with previously unsuspected speed to the continents, and geomagnetic surveys of the oceans (Paetzold, 1997). using specially designed research vessels. He also founded The Magnetic Observatory in Potsdam had been con- the international journal, “Terrestrial Magnetism and Atmo- structed before Eschenhagen joined the new Meteorological- spheric Electricity”, a life’s work of American dimensions Geomagnetic Observatory at Telegrafenberg, headed by (Nippolt, 1925). W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660 1657

9. Early hypotheses about the origin ofgeomagnetism data from other disciplines such as geology and seismology. These ÿelds should therefore be brie7y surveyed, too. The The problem of the origin of geomagnetism has always study of rock magnetism led also to discoveries which need interested scientists and many explanations were proposed a theoretical explanation (see also: Larmor, 1929; Paetzold, as new knowledge was obtained in physics. At ÿrst a 1997; Wilde, 1894; v. Zittel, 1899). strong magnet was supposed to exist in the Earth’s interior. Hansteen even supposed two magnets of diIerent strengths tilted against each other to explain the ÿeld observed on the 10. The structure ofthe Earth’s interior Earth’s surface (Hansteen, 1819). AmpÃere, who discovered the in7uence of electric currents on each other, tried to gen- The “Handbuch der Geophysik”, edited shortly before erally reduce magnetism to the eIect of 7owing electricity. 1900 by Siegmund Gunther,, presented many diIerent hy- He supposed that geomagnetism is due to gigantic currents potheses about the structure of the Earth’s interior. Some 7owing in an East–West direction. The currents would be geologists supposed the Earth to be absolutely rigid, others produced by some kinds of huge galvanic elements in the put a hot and liquid metal core below a thin and elastic Earth Earth’s interior; and diIerent metals do occur there. In crust. Even the hypothesis of a gaseous core was considered 1821, Thomas Johann Seebeck, who discovered “thermo- in a state corresponding to the very high temperature and magnetism” (more exactly thermoelectricity), looked for pressure (Gunther,, 1897, 1899). William Hopkins (1793– an origin in diIerently heated metals in the Earth’s inte- 1866)—one of the great British geoscientists, presented an- rior which were in contact. Gauss showed with his phe- other possibility in his publication “Researches in Physical nomenological theory of geomagnetism that the main ÿeld Geology” (1839). According to him, the Earth is covered is certainly of interior origin: As already mentioned, Bar- by a solid crust over a hard core; the two being separated by low detected earth currents in 1849on telegraphic lines hot-liquid material (v.Zittel, 1899). With this model Hop- (Schmidt, 1925, p. 347). Lamont supposed a magnetized kins hinted at recent ideas. Gunther, also published in his compact iron core in the centre of the Earth—an idea also handbook his own model of the structure of the Earth’s in- expressed previously by other geophysicists. Lamont sug- terior. This makes clear how unsure scientists were, even at gested, however, that this core was crossed by electric cur- the turn of the century, of the conditions to be found there. rents of diIerent intensities and directions, an idea worthy In his cross-section through the Earth there are zones of dif- of admiration in his time (Lamont, 1849, 1862). ferent types of material: the crust, a plastic, viscous-liquid Among a great number of hypotheses, and here only a zone and transitional zones from the liquid mass to the core few can be mentioned; there were some which tried to prove consisting of an ionised monatomic gas. His diagram gives a connection with the rotation of the Earth. This seemed the impression that the spherical shells are strictly separated reasonable since the rotation axis makes an angle of only from each other but Gunther, emphasized that the transitions a few degrees with the magnetic axis. In the 1890s Henry are continuous. Wilde (1833–1891) constructed the “magnetarium” which was designed to demonstrate experimentally the origin of the geomagnetic ÿeld and even of the secular variation. It 11. Palaeomagnetism, information about the history of consisted of two mutually rotating concentric globes with the Earth current carrying coils placed around one of them. The axes of the two globes were tilted by 23◦. Oceans were represented In the mid-19th century, geoscientists made discover- on the outer globe by a sheath of iron plates. Although this ies concerning volcanic rocks which were inexplicable to special experiment was hailed by several geomagneticians. geophysicists. Hot 7uid lava is non-magnetic in spite of a Bauer rejected this model (Schmidt, 1925, p. 359). certain content of ferrous minerals. During cooling below The fact that an iron core cannot be ferromagnetic at the Curie-point these minerals, and with them the lava-rock, high temperatures became clear following the investiga- become ferromagnetic and the magnetism existing dur- tions by Pierre Curie at the beginning of the 1990s. Iron ing the time span of cooling is imprinted and ÿxed. This and ferromagnetic minerals lose their ferromagnetic prop- palaeomagnetism, frozen into the lava-rock, is very stable, erties at temperatures above ca. 700◦C (Curie temperature as found by Macedonio Melloni (1798–1854) in the 1850s and Curie–Weiss law). It is curious that the famous geo- (see Fig. 12). This phenomenon is called thermoremanence. magnetician Adolf Schmidt (see Fig. 8) doubted the exis- Ferdinand Carl F,orstemann (?-1873) investigated basalt tence of strong currents in the Earth’s interior, even in the samples from the EiIel Mts. and conÿrmed Melloni’s second decade of the 20th century, as he did not see any results. Ernst Gustav Zaddach (1817–1881) also made mechanism to maintain them. Present dynamo theories of magnetic measurements on basalt samples in the mid-19th the main geomagnetic ÿeld have a diIerent basis and this century and found parallel magnetic “axes” of opposite will be covered later. The modern theory of the “geody- directions of polarity. namo” which has solved, or at least is on the road to solv- Giuseppe Folgheraiter (1856–1913), an Italian sci- ing, the puzzle of the origin of geomagnetism, is based on entist, had the inspiration in the 1890s to measure the 1658 W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660

(MHD). It builds on Joseph Larmor’s original idea from 1919 when he tried to explain the phenomenon of solar magnetism discovered in 1908 by George E. Hale (Larmor, 1929). The theory of MHD was fundamentally created by Hannes OG AlfvÃen in the mid-20th century and is at the in- tersection of hydrodynamics and electrodynamics. The laws of hydro-, thermo- and electrodynamics are linked within its framework and one distinguishes between plasma-MHD and liquid-MHD. The ÿrst is used for the interpretation of geomagnetic pulsations, the second for the explanation of the magnetism of the terrestrial body. One of the essential pre-conditions for the theory is that an electrically conducting moving liquid phase should be present, and the evidence for this was provided by the al- lied disciplines of geology and seismology at the end of the 19th century. Geophysicists today think that the mystery of geomagnetism will eventually be completely solved. Expla- nations for the secular variation and polarity changes of the magnetic ÿeld and of the magnetic axes have to be incorpo- rated in the theory and it is thought that chemical convec- tion must drive the 7ow of the electrically conducting liq- uid (cf. Larmor, 1929; Nippolt, 1925; Schuster, 1886, 1889; Stewart 1878, 1880; Wilde, 1894).

Acknowledgements Fig. 12. Macedonio Melloni. We are grateful to the referees for helpful comments. We thermoremanent magnetism of burnt clay vessels. Starting are also grateful to Prof. N. Skinner for translation help and with ancient vases he was able to follow the secular varia- comments. tion of the inclination until the present day. Folgheraiter can thus be regarded as the initiator of modern palaeomagnetic References and archaeomagnetic research. Information preserved by nature itself of the changing Bartels, J., 1933. Uberblick, uber, die Physik der hohen Atmosph,are. directions of the magnetic ÿeld, throughout the history of Elektrische Nachrichtentechnik, Bd. 10, Sonderheft. Berlin. the Earth provides valuable empirical material for verify- Others ing the theory of continental drift, worked out by Alfred Birkeland, C.H., 1912. Observations of earth-currents at Kaafjord, Wegener as early as 1912. Hot basaltic magma rises along May 1910. The Norwegian Aurora Polaris Expedition, linear zones of mobility such as the Mid-Atlantic ridge, to 1902–1903, Part III, p. 751 (Chapter 1). the sea-7oor and drifts in both directions (sea-7oor spread- Cawood, J. 1979. The Magnetical Crusade, Science and Politics in Early Victorian Britain. Isis, Vol. 70. Philadelphia, ing). Samples from the sea-7oor and modern magnetometer pp. S.493–518. surveys of the oceans conÿrm that many polarity changes Chapman, S., Bartels, J., 1940. Geomagnetism, 2 Vols, Oxford. of the geomagnetic ÿeld have occurred and a timescale for Chree, Ch., 1912. Studies in Terrestrial Magnetism (XI. Magnetic these ÿeld reversal has been deduced (cf. e.g. Melloni, 1854; Storms). Macmillan, London. Neumayer, 1892). Ebert, H., 1906. Uber, Pulsationen von geringer Periodendauer in der erdmagnetischen Feldkraft. Sitz. Ber. der math.-phys. Klasse der k. Bayrischen Akad. der Wiss 36, 527. 12. Mystery ofgeomagnetism is solved? Ebert, H., 1907. Concerning pulsations of short-period in the strength of the Earth’s magnetic ÿeld. Terrestrial Magnetism The birth of the dynamo theory was mentioned in con- 12, 1. nection with Balfour Stewart’s ideas to explain daily geo- Gauss, C.F., 1893. Die Intensit,at der erdmagnetischen Kraft auf absolutes Maass zuruckgef, uhrt,, 1832. Deutsche Ubersetzung,, magnetic variations. However Stewart’s theory excluded, hrsg. von E. Dorn. Ostwalds Klassiker der exakten however, the principle of self-excitation and involved only Wissenschaften, Nr. 13. Leipzig. mutual induction. Modern dynamo theory deals also with Gauss, C.F., Weber, W.Hrsg. (1837–1840). Resultate aus den the excitation of the main part of the geomagnetic ÿeld Beobachtungen des magnetischen Vereins. 6 Jahresbde., 1836 which it tries to explain using magnetohydrodynamics (ersch. 1837), 1837 (1838), 1838 (1839), 1839 (1840), 1840 W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660 1659

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In: Haussner von, R., Gesammelte Werke, Schering. K. (Eds.), 2 Bde., Bd. 2. Lebenslauf, Berlin, The following bibliography contains older papers on ge- S.449–472. omagnetic pulsations and includes related topics (magneto 1660 W.Schr oder," K.-H. Wiederkehr / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1649–1660 tellurics, acoustic waves, etc.) which may be useful for Harang, L., 1932. Observations of micropulsations in the magnetic further studies, and which are often diOcult to ÿnd in the records at TromsH. Terrestrial Magnetism 37, 57. literature. Heathcote, N.H.de.V., Armitage, A., 1959. The Histories of the International Polar Years etc. Annals of the International Geo- Abbe, C., 1898. Eschenhagen’s elementary magnetic waves. Ter- physical Year. Washington. restrial Magnetism 3, 135. Hopkins, W., 1839. Researches in physical geology. Philosophical Angenheister, G., 1913. Uber, die Fortp7anzungsgeschwindigkeit Transactions of Royal Society of London 129, 381–423. magnetischer St,orungen und Pulsationen. Nachr. von der K,on. Lubiger, F., 1924. Uber, die vom Samoa-Observatorium reg- Ges. der Wissenschaften zu G,ottingen. Math.-Phys. Klasse aus istrierten erdmagnetischen Pulsationen und Bayst,orungen. dem Jahre 1913, 565. Jahrbuch der math.-naturwiss. Fak. der Georg-August Univ. Arendt, Th., 1896. Die Beziehungen der elektrischen Erschein- zu G,ottingen. ungen unserer Atmosph,are zum Erdmagnetismus. Das Wetter Nakano, M., 1933. A possible explanation of the phenomenon anal- 13, 241–265. ogous to “beats”, which often occur in rapid periodic variations Arendt, Th., 1903. Erdmagnetische Pulsationen. Naturwissensch. of terrestrial magnetism. Proceedings of Imp. Academy, Japan Rundschau 18, I. Teil 105, II. Teil 117. 9, 235. Birkeland, K., 1901. Sur quelques phÃenomÂenes de magnÃetisme ter- Okuda, M., Kato, Y., 1927. On the pulsation of terrestrial mag- restre dusa  l’action des courantselectriques. à Exp. NorvÃegienne netism. Journal of Faculty of Science Imp. University of Tokyo, 1899–1900 puor letude à des aurores borÃeales. Resultats Seet. 1: I (Part 10), 399. des recherches magnÃetique Videnskapsselskabs Skrifter, I. P,odder, A., 1926. Micromagnetic oscillation as observed at the Mathematisk-naturvidenskabelig Klasse, vol. 1. magnetical section of the observatory of Irkutsk (Zony). 1925. Birkeland, Kr., 1913. The Norwegian Aurora Polaris Expedition, Terrerstrial Magnetism 31, 103. 1902–1903, Vol. I, pp. 625, 756. P,odder, A., 1927. Untersuchung der mikromagnetischen Oszilla- Ehlert, R., 1898. In: Gerland, G. (Ed.), Zusammenstellung, tionen in Zui (Irkutsk) mit Hilfe der Induktionsspule. Gerlands Erl,auterung und kritische Beurtheilung der wichtigsten Seis- Beitraege zur Geophysik 17, 232. mometer mit besonderer Berucksichtigung, ihrer praktischen Rolf, B., 1931. Giant micropulsations at Abisko. Terrestrial Mag- Verwendbarkeit. Beitr,age zur Geophysik, 3. Bd. Leipzig, netism 36, 9. pp. S.350–474. Sandoval, R., 1933. On a remarkable magnetic wave. Proceedings Eschenhagen, M., 1896a. On minute, rapid periodic changes of the of V. Paciÿc Science Congress, Vol. III, Canada, 1861. Earth’s magnetism. Terrestrial Magnetism 2, 105. Van Bemmelen, W., 1901. Pulsations de la force magnÃetique ter- Eschenhagen, M., 1896b. Uber, das Studium der Variationen des restre. Arch. NÃederlandaises des Sciences 6, 382. Erdmagnetismus. Verhandl. d. Ges. f. Naturforschung II Teil, Van Bemmelen, W., 1902. Erdmagnetische Pulsationen, Natu- 1 Heft, p. 35. urkundig Tijdschrift voor Nederlandisch Indie 62, 71. Eschenhagen, M., 1896c. Uber, die Aufzeichnung sehr kleiner Vari- Van Bemmelen, W., 1906a. Erdmagnetische Pulsationen. Meteo- ationen des Erdmagnetismus. Sitz. Ber. d. Berliner Akad. d. rologische Zeitschrift, Hand-Band, p. 268. Wiss. 39, 965. Van Bemmelen, W., 1906b. On pulsationes. Observationes made at Eschenhagen, M., 1896d. Uber, simultan-Beobachtungen erdmag- the royal magnetical and meteorological Observatory at Batavia, netischer Variationen. Terrestrial Magnetism 1, 55. vol. 29, p. 3. Eschenhagen, M., 1897a. Uber, schnelle periodische Ver,anderungen Van Bemmelen, W., 1908. Registration of earth-currents at Batavia des Erdmagnetismus von sehr kleiner Periode. Sitz. Ber. d. for the investigation of the connection between earth-current Preuss. Akad. d. Wiss. zu Berlin 678. and force of earth magnetism. Sitz. Ber. Kon. Akad. Wet. Eschenhagen, M., 1897b. Uber, schnelle periodische Ver,anderungen Amsterdam 10, 512. des Erdmagnetismus von sehr kleiner Amplitude. Sitzungsber. d. Akad. d. Wiss. Berlin, Jg. 1897. Berlin, S.678–686. Unpublished sources Eschenhagen, M., 1899. Uber, erdmagnetische Intensit,atsvario- meter. Verhandl. d. Deutsch. Phys. Ges. im J. 1899, 1. Schering, Ernst und Karl, Briefe an G. Neumayer und Eschenhagen, M., 1896–1901. Internationale magnetische Briefentwurfe, von Neumayer an die beiden Scherings. In: Beobachtungen 1896. Magnetische Beobachtungen Potsdam, Polarakten in der Bibliothek im Bundesamt fur SeeschiI- Anhang I–VIII. , Gerland, G. (1899–1900). Dr. Reinhold Ehlert. Nachruf. Beitr,age fahrt und Hydrographie in Hamburg. Mikroÿche 436a und zur Geophysik (Leipzig) 4, S.105–107. 441a (unpublished sources).