Introduction

The science of geomagnetism developed slowly. The earliest writings about compass navigation are credited to the Chinese and dated to 250 years B.C. (Figure 1.1). When Gilbert published the first textbook on geomagnetism in 1600, he concluded that the Earth itself behaved as a great magnet (Gilbert, 1958 reprint) (Figure 1.2). In the early nineteenth century, Gauss (1848) introduced improved magnetic field observation techniques and the spherical harmonic method for geomagnetic field analysis. Not until 1940 did the comprehensive textbook of Chapman and Bartels bring us into the modern age of geomagnetism. The bibliography in the Appendix, Section B.7, lists some of the major textbooks about the Earth's geomagnetic field that are currently in use.

For many of us the first exposure to the concept of an electromagnetic field came with our early exploration of the properties of a magnet. Its strong attraction to other magnets and to objects made of iron indicated immediately that something special was happening in the space between the two solid objects. We accepted words such as field, force field, and lines of force as ways to describe the strength and direction of the push or pull that one magnetic object exerted on another magnetic material that came under its influence. So, to start our subject, I would like to recall a few of our experiences that give reality to the words magnetic field and dipole field.