Geomagnetic Research in the 19Th Century: a Case Study of the German Contribution

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 pioneering 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 1364-6826/01/$ - see front matter 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

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