The history of geomagnetism began around the year 1000 with the discovery in China of the magnetic compass. Methodical studies of the Earth's field started in 1600 with William Gilbert's "De Magnete" and continued with the work of (among others) Halley, Coulomb, Gauss and Sabine. The discovery of electromagnetism by Oersted and Ampére led Faraday to the notion of fluid dynamos, and the observation of sunspot magnetism by Hale led Larmor in 1919 to the idea that such dynamos could sustain themselves naturally in convecting conducting fluids. From that came modern dynamo theory, of both the solar and terrestrial magnetic fields. Paleomagnetic studies revealed that the Earth's dipole had undergone reversals in the distant past, and these became the critical evidence in establishing plate tectonics. Finally, the recent availability of scientific spacecraft has demonstrated the intricacy of the Earth's distant magnetic field, as well as the existence of magnetic fields associated with other planets and with satellites in our solar system.

1. Introduction
2. Early discoveries
3. William Gilbert
4. Halley
5. Coulomb
6. Oersted and Ampere
7. The Lodestone
8. Gauss and Humboldt
9. Explorations and Surveys
10. Faraday's Lines of Force (field lines)
11. Faraday's Disk Dynamo
12. Sunspots
13. The Dynamo Process on the Sun
14. The Earth's Dynamo
15. Dipole Reversals and Plate Tectonics
16. Magnetic Storms and Ring Currents
17. The Magnetosphere
18. Magnetic Reconnection
19. Planetary Magnetospheres
20. Assessment

    This brief history follows two earlier ones on magnetospheric physics [Stern 1989, 1996] and is directed at three audiences: (1) geophysicists seeking to complement their professional expertise with its underlying history (2) students of geology and geophysics, interested in the origins and underpinnings of their disciplines, and (3) historians of science, looking for information about the evolution of the discipline of geomagnetism. It is meant to be a starting point rather than an exhaustive review, and those who seek more details may seek them in the many references. The article supplements a recent web site (home page http://www.phy6.org/earthmag/demagint.htm, by D. Stern) which covers the same subject but at a more popular level. A chronology of the events covered here is given in Appendix A.

    History is an integral part of scientific knowledge. It not only provides the framework of its field, explaining how its concepts arose and developed, but it also provides an insight into the actual path of progress, something scientific articles and texts generally omit. Geomagnetism is of particular interest, because it may well claim to be the oldest discipline in geophysics. In its long history, several threads can be distinguished, now and then intersecting but largely independent. It is this pattern which the article is meant to highlight.