5 Geomagnetism and paleomagnetism It is not known when the directive power of the magnet 5.1 HISTORICAL INTRODUCTION - its ability to align consistently north-south - was first recognized. Early in the Han dynasty, between 300 and 5.1.1 The discovery of magnetism 200 BC, the Chinese fashioned a rudimentary compass Mankind's interest in magnetism began as a fascination out of lodestone. It consisted of a spoon-shaped object, with the curious attractive properties of the mineral lode­ whose bowl balanced and could rotate on a flat polished stone, a naturally occurring form of magnetite. Called surface. This compass may have been used in the search loadstone in early usage, the name derives from the old for gems and in the selection of sites for houses. Before English word load, meaning "way" or "course"; the load­ 1000 AD the Chinese had developed suspended and stone was literally a stone which showed a traveller the pivoted-needle compasses. Their directive power led to the way. use of compasses for navigation long before the origin of The earliest observations of magnetism were made the aligning forces was understood. As late as the twelfth before accurate records of discoveries were kept, so that century, it was supposed in Europe that the alignment of it is impossible to be sure of historical precedents. the compass arose from its attempt to follow the pole star. Nevertheless, Greek philosophers wrote about lodestone It was later shown that the compass alignment was pro­ around 800 BC and its properties were known to the duced by a property of the Earth itself. Subsequently, the Chinese by 300 Be. To the ancient Greeks science was characteristics of terrestrial magnetism played an impor­ equated with knowledge, and was considered an element tant role in advancing the understanding of magnetism. of philosophy. As a result, the attractive forces of lode­ stone were ascribed to metaphysical powers. Some early 5.1.2 Pioneering studies in terrestrial magnetism animistic philosophers even believed lodestone to possess a soul. Contemporary mechanistic schools of thought In 1269 the medieval scholar Pierre Pelerin de Maricourt, were equally superstitious and gave rise to false concep­ who took the Latin nom-de-plume of Petrus Peregrinus, tions that persisted for centuries. Foremost among these wrote the earliest known treatise of experimental physics was the view that electrical and magnetic forces were (Epistola de Magnete). In it he described simple laws of related to invisible fluids. This view persisted well into the magnetic attraction. He experimented with a spherical nineteenth century. The power of a magnet seemed to magnet made of lodestone, placing it on a flat slab of iron flow from one pole to the other along lines of induction and tracing the lines of direction which it assumed. These that could be made visible by sprinkling iron filings on a lines circled the lodestone sphere like geographical merid­ paper held over the magnet. The term "flux" (synony­ ians and converged at two antipodal points, which mous with flow) is still found in "magnetic flux density," Peregrinus called the poles of the magnet, by analogy to which is regularly used as an alternative to "magnetic the geographical poles. He called his magnetic sphere a induction" for the fundamental magnetic field vector B. terrella, for "little Earth." One of the greatest and wealthiest of the ancient It was known to the Chinese around 500 AD, in the Greek city-colonies in Asia Minor was the seaport of Tang dynasty, that magnetic compasses did not point Ephesus, at the mouth of the river Meander (modern exactly to geographical north, as defined by the stars. The Kucuk Menderes) in the Persian province of Caria, in local deviation of the magnetic meridian from the geo­ what is now the Turkish province of western Anatolia. In graphical meridian is called the magnetic declination. By the fifth century Be the Greek state of Thessaly founded the fourteenth century, the ships of the British navy were a colony on the Meander close to Ephesus called equipped with a mariner's compass, which became an Magnesia, which after 133 BC was incorporated into the essential tool for navigation. It was used in conjunction Roman empire as Magnesia ad Maeandrum. In the vicin­ with celestial methods, and gradually it became apparent ity of Magnesia the Greeks found a ready supply of lode­ that the declination changed with position on the globe. stone, pieces of which subsequently became known by the During the fifteenth and sixteenth centuries the world­ Latin word magneta from which the term magnetism wide pattern of declination was established. By the derives. end of the sixteenth century, Mercator recognized that 281 282 Geomagnetism and paleomagnetism declination was the principal cause of error in contempo­ properties of magnets (e.g., the concentration of their rary map-making. powers at their poles) had been established, but the accu­ Georg Hartmann, a German cleric, discovered in 1544 mulated observations were unable to provide a more fun­ that a magnetized needle assumed a non-horizontal atti­ damental understanding of the phenomena because they tude in the vertical plane. The deviation from the horizon­ were not quantitative. A major advance was achieved by tal is now called the magnetic inclination. He reported his Charles Augustin de Coulomb (1736-1806), the son of a discovery in a letter to his superior, Duke Albrecht of noted French family. who in 1784 invented a torsion Prussia, who evidently was not impressed. The letter lay balance that enabled him to make quantitative measure­ unknown to the world in the royal archives until its dis­ merits of electrostatic and magnetic properties. In 1785 he covery in 1831. Meanwhile, an English scientist, Robert published the results of his intensive studies. He estab­ Norman, rediscovered the inclination of the Earth's mag­ lished the inverse-square law of attraction and repulsion netic field independently in 1576. between slTIaII electrically charged balls. Using thin, mag­ In 1600 Willianl Gilbert (1544-1603), an English scien­ netized steel needles about 24 inches (61 em) in length, he tist and physician to Queen Elizabeth, published De also established that the attraction or repulsion between Magnete, a landmark treatise in which he summarized all their poles varied as the inverse square of their separation. that was then known about magnetism, including the Alessandro Volta (1745-1827) invented the voltaic cell results of about seventeen years of his own research. His with which electrical currents could be produced. The studies extended also to the electrostatic effects seen when relationship between electrical currents and magnetism some materials were rubbed, for which he coined the name was detected in 1820 by Hans Christian Oersted "electricity" from the Greek word for amber. Gilbert was (1777-1851), a Danish physicist. During experiments the first to distinguish clearly between electrical and mag­ with a battery of voltaic cells he observed that a magnetic netic phenomena. His magnetic studies followed the work needle is deflected at rightangles to a conductor carrying of Peregrinus three centuries earlier. Using small magnetic a current, thus establishing that an electrical current pro­ needles placed on the surface of a sphere of lodestone to duces a magnetic force. study its magnetic field, he recognized the poles, where the Oersted's result was met with great enthusiasm and needles stood on end, and the equator, where they lay par­ was followed at once by other notable discoveries in the allel to the surface. Gilbert achieved the leap of irnagina­ same year. The law for the direction and strength of the tion that was necessary to see the analogy between the magnetic force near a current-carrying wire was soon for­ attraction of the lodestone sphere and the known mag­ mulated by the French physicists Jean-Baptiste Biot netic properties of the Earth. He recognized that the Earth (1774-1862) and Felix Savart (1791-1841). Their compa­ itself behaved like a large magnet. This was the first triot Andre Marie Ampere (1775-1836) quickly under­ unequivocal recognition of a geophysical property, pre­ took a systematic set of experiments, He showed that a ceding the laws of gravitation in Newton's Principia by force existed between two parallel straight current­ almost a century. Although founded largely on qualitative carrying wires, and that it was of a type different from the observations, De Magnete was the most important work known electrical forces. Ampere experimented with the on magnetism until the nineteenth century. magnetic forces produced by current loops and proposed The discovery that the declination of the geomagnetic that internal electrical currents were responsible for the field changed with time was made by Henry Gellibrand existence of magnetism in iron objects (i.e., ferromagnet­ (1597-1637)'1 an English mathematician and astronomer, ism). This idea of permanent magnetism due to con­ in 1634. He noted, on the basis of just three measure­ stantly flowing currents was audacious for its time. ments made by William Borough in 1580'1 Edmund At this stage, the ability of electrical currents to gener­ Gunter in 1622 and himself in 1634, that the declination ate magnetic fields was known, but it fell to the English had decreased by about 7° in this time. From these few scien tist Michael Faraday (1791-1867) to demonstrate observations he deduced what is now called the secular in 1831 what he called "magneto-electric' induction. variation of the field. Faraday came from a humble background and had little Gradually the variation of the terrestrial magnetic mathematical training. Yet he was a gifted experimenter, field over the surface of the Earth was established. In and his results demonstrated that the change of magnetic 1698-1700 Edmund Halley, the English astronomer and flux in a coil (whether produced by introducing a magnet mathematician, carried out an important oceanographic or by the change in current in another coil) induced an survey with the prime purpose of studying compass vari­ electric current in the coil.