MAGNETISM

phenomenon associated with the motion of electric charges. It involves magnetic fields, which are regions wherein a force is exerted on a current-carrying medium or magnetic body, and the effects of such fields. The magnetic properties of matter are largely determined by the behaviour of the negatively charged electrons that orbit the nuclei of atoms. The magnetic dipole moment of a single orbital electron has two components, one resulting from the spin of the electron about its own axis, the other from its orbital motion about the nucleus. Both kinds of motion may be considered as tiny circular currentsmoving chargesthus linking electric and magnetic effects at a fundamental level. For most atoms and molecules, the sum total magnetic moment of all the orbital electrons is zero. When these materials are placed in a magnetic field, a small magnetic moment is induced by the interaction of the field with atomic electrons; this induced moment is opposite in direction to the applied field, and substances that exhibit this effect are called diamagnetic. In some materials whose atoms have an incomplete electron shell, each atom has a net magnetic moment which while ordinarily randomly ordered, will react to an applied field to exhibit a bulk net magnetism; such materials are called paramagnetic. In some highly ordered crystalline materialsiron, nickel, and cobalt, for instancethe spins of some of the orbital electrons in adjacent atoms become coupled, creating local magnetic domains in which the magnetization is unidirectional. Adjacent domains are magnetized in different directions, so that there is no resultant bulk magnetization. When an external field is applied, however, those domains aligned with the field grow at the expense of those that are not, and a very strong permanent magnetization results; materials exhibiting this property are called ferromagnetic. Related phenomena are antiferromagnetism and ferrimagnetism (qq.v.). Magnetic fields exert forces on moving charges. Examples of this include the bending of an electron beam in a cathode-ray tube, the Hall effect in a semiconductor or conductor, and the motor force on a current-carrying conductor. The converse occurs when a conductor is moved through a magnetic field; it experiences an electromotive force which causes a current to flow through it. The magnetic properties of the black metallic mineral magnetite, an oxide of iron found in igneous rocks, were known to the ancient Greeks. The practical application of those properties in the magnetic compass was accomplished possibly as early as the 26th century BC by the Chinese, though historians remain uncertain on this point. The first serious study of magnetism was made by Petrus Peregrinus de Maricourt in the late 1260s. He established the existence of magnetic poles and stated that like poles repel and unlike attract; he also specified the construction of a mariner's compass in detail. William Gilbert, physician to Queen Elizabeth I, observed that the Earth is a huge magnet and thus explained why a magnetic needle tends to dip its north-pointing end downward in the Northern Hemisphere. The forces between the poles of magnets were first investigated experimentally by the French physicist Charles-Augustin de Coulomb in 1785 and were found to follow an inverse square law. (Coulomb's finding corroborated the observation made by the English physicist Joseph Priestly some years earlier.) In 1824 Simon-Denis Poisson, a French mathematician, presented a mathematical model of magnetism which still provides a sound basis for the calculation of the forces between permanent magnets. A connection between electricity and magnetism had long been suspected, and in 1820 the Danish physicist Hans Christian rsted showed that an electric current flowing in a wire produces its own magnetic field. Andr-Marie Ampre of France immediately repeated rsted's experiments and within weeks was able to express the magnetic forces between current-carrying conductors in a simple and elegant mathematical form. He also demonstrated that a current flowing in a loop of wire produces a magnetic dipole indistinguishable at a distance from that produced by a small permanent magnet; this led Ampre to suggest that magnetism is caused by currents circulating on a molecular scale, an idea remarkably near the modern understanding. The English chemist and physicist Michael Faraday demonstrated the motor action of a current-carrying conductor in a magnetic field in 1821 and the induction of a current in a moving conductor in a magnetic fieldthe dynamo effectin 1831; he coined the term magnetic field in 1845. The electromagnet, in which an iron core enhances the field generated by a current flowing through a coil, was invented by William Sturgeon in England during the mid-1820s. It later became a vital component of both motors and generators. The unification of electric and magnetic phenomena in a complete mathematical theory was the achievement of the Scottish physicist James Clerk Maxwell. Maxwell's field equations, published in 1864, predicted the existence of electromagnetic waves, subsequently verified by Heinrich Hertz of Germany, and showed that light was also such a wave. Today magnetism finds many technical applications, from the humble magnetic door catch to medical imaging devices and superconducting magnets for use in high-energy particle accelerators. phenomenon associated with the motion of electric charges. This motion can take many forms. It can be an electric current in a conductor or charged particles moving through space, or it can be the motion of an electron in atomic orbit. Magnetism is also associated with elementary particles, such as the electron, that have a property called spin. Frank Neville H. Robinson Eustace E. Suckling Edwin Kashy Additional reading P.C.W. Davies, The Forces of Nature, 2nd ed. (1986), is an interesting, readable account. David N. Schramm and Gary Steigman, Particle Accelerators Test Cosmological Theory, Scientific American, 258(6):6672 (June 1988), discusses the fundamental constituents of nature. Edward M. Purcell, Electricity and Magnetism, 2nd ed. (1985), is superbly illustrated and treats key principles and phenomena with remarkable insight. Many examples and problems on magnetism, as well as elementary discussions of vectors and other aspects of physics, are found in David Halliday and Robert Resnick, Fundamentals of Physics, 3rd ed. (1988). Useful physics textbooks with illustrations, examples, and problems include Richard Wolfson and Jay M. Pasachoff, Physics (1987); and Francis W. Sears, Mark W. Zemansky,and Hugh D. Young, University Physics, 7th ed. (1987). Harald A. Enge, Introduction to Nuclear Physics (1966), provides an overview of the magnetic properties of matter. The magnetic properties of solids are discussed in Charles Kittel, Introduction to Solid State Physics, 6th ed. (1986). Robert Eisberg and Robert Resnick, Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles, 2nd ed. (1985), broadly treats quantum mechanical effects in various phenomena, including magnetic properties such as ferromagnetism. Reference books include Reference Data for Engineers: Radio, Electronics, Computer, and Communications, 7th ed. (1985), with data and discussion about magnetic properties of matter; and CRC Handbook of Chemistry and Physics (annual), an indispensable handbook. Edwin Kashy Sharon Bertsch McGrayne