For as long as man has speculated about the interior of the Earth, it has been presumed that there exists a central core. Centuries before the rise of modern science, philosophers, and theologians had concluded that the Earth has a hot region at its center, with properties distinct from all other parts of the planet. For nearly as long a time it has been known that the Earth is also magnetic, but the cause of the Earth’s agnetism remained just as mysterious as the nature of the deep interior. Scientific inquiry about the core grew from early investigations of the properties of the geomagnetic field, which began during the era of global exploration. Although the ancient Chinese deserve the credit for discovering Earth’s magnetism, Gilbert (1600) was the first to demonstrate that the compass needle is controlled by a force originating within the Earth (Figure 1). He showed that the pattern of magnetic field lines on a uniformly magnetized sphere approximate the known directions of the compass needle over the Earth’s surface. Three hundred and fifty years later, Sidney Chapman characterized Gilbert’s demonstration as ‘‘the only successful experiment in the history of geomagnetism!’’ Later it was observed that Earth’s magnetic field changes slowly with time. In his famous explanation for this secular variation, Halley (1683, 1692) proposed that the geomagnetic field has its origin near the Earth’s center, in a region separated from the solid crust by a cavernous, fluidfilled shell. Halley (Figure 2) envisioned that both the crust and the central region or core rotate in the prograde sense, but the core spins slightly slower, causing the magnetic field to drift systematically westward as seen at the surface. Thus, two important and long-lasting concepts were born: the basic three-layer model of Earth’s interior (solid crust and mantle, liquid outer and solid inner core), and the association between the westward geomagnetic drift and westward motion of the fluid outer core with respect to other parts of the Earth system. Halley’s model implicitly assumed that the magnetic field originated in a solid inner core (Evans, 1988), akin to Gilbert’s uniformly magnetized sphere. Subsequently, it was shown that Halley’s model is at variance with the ferromagnetic properties of Earth materials, which lose their permanent magnetization at the Curie temperature at depths of a few tens of kilometers beneath the surface (see Chapter 5.06). However, by then the physical connection between magnetic fields and electric currents had been established, providing an alternative explanation for the geomagnetic field that relied on free electric currents rather than permanent magnetization.
