Iron, Loadstones and Terrestrial Magnetism

Abstract

William Gilbert (1544–1603) was born in Colchester, County Essex, England. A physician in the service of Queen Elizabeth, Gilbert carried out extensive studies on the properties of loadstones and other magnetic substances. In 1600, he published De Magnete, wherein he offered a comprehensive review—and a sharp critique—of his contemporaries’ understanding of magnetism. Of particular note in Gilbert’s De Magnete is the careful distinction that he draws between electrical and magnetic phenomena. Previously, this distinction had been either greatly muddled or entirely unrecognized. According to Gilbert, electrical attractions, such as those exhibited by precious stones or amber when rubbed, were attributed to a material cause—a corporeal effluvium emitted by the substance which is “awakened by friction.” Magnetic attractions and repulsions, on the other hand, such as those exhibited by loadstones and magnetized iron, Gilbert attributed to a formal cause—a native strength or vigor which ferruginous substances share with the earth itself. The language of material and formal causes which Gilbert employs, though uncommon today, was typical of the intellectual heirs of the ancient greek philosopher Aristotle. But despite this similarity, Gilbert rejected Aristotelian thinking in at least two very important ways. First, Gilbert explicitly rejected Aristotle’s geocentrism in favor of a Copernican worldview. Gilbert’s heliocentrism was later shared by the famous German astronomer Johannes Kepler, who—explicitly invoking Gilbert’s De Magnete—would attribute Earth’s motion about the sun to some type of magnetic force. After all, Kepler reasoned, the earth (and perhaps the sun also) is a huge magnet—and magnets are known to attract one another. Second, Gilbert rejected the theory of the magnetic compass needle, or versorium, which was propounded by Aristotle’s intellectual heirs, the so-called “elementarian philosophers.” It is here that we begin our reading of Gilbert’s De Magnete. The text selections that follow are extracted from Book iii of the English translation by P. Fleury Mottelay. As you consider this text—and are thereby transported back in time, as it were, to the birth of the modern science of magnetism—you might permit yourself to naively consider the following simple questions: how, exactly, is a magnetic stone different than other stones? And what might be the hidden cause of such strange and wonderful behavior?