Strange Attraction Civity)

HISTORICAL NOTE and then quenched to freeze in the nec essary hardness and magnetic traits (such as improved retentivity and coer Strange Attraction civity). After 1855 it was known that adding tungsten or manganese Each year, several million tOns of Petrus Peregrinus de Maricourt (Peter improved carbon steel as a permanent metal are produced for use as magnetic the Pilgrim) described in detail the use magnet. materials in many everyday devices and of a compass for navigation. He processes common to all of us-from believed that the lodestone derived its Many different families of magnetic radio communication, information stor power from the sky. Apparently on one alloys have come into use since the age, telephone receivers, and stereo of his long voyages, Peter the Pilgrim beginning of this century. At the turn of speakers to electric motors, transform played with a sphere of magnetite, the 20th century Sir Robert Hadfield ers, and generators. All these things exploring its surface with bits of iron. invented iron-silicon alloys, which have would have completely baffled the He discovered that lines of magnetic especially high permeabilities in the ori shepherd Magnes 2500 years ago when, force circled the sphere and seemed to entation in which the alloy passes according to legend, he stepped on a intersect at two opposite places on the through the rolling press. Chrome-steel lodestone (magnetite). Pliny the Elder globe, which he named "poles" in anal alloys were developed in 1917, and until wrote of Magnes, "the nails of whose ogy to the Earth's poles. It wasn't until 1930 the only commercially available shoes and the [iron] tip of whose staff more than three centuries later in 1600, permanent magnets were made of car stuck fast in a magnetick field while he though, that William Gilbert, physician bon-steel, chrome-steel, or tungsten pastured his flocks." to Queen Elizabeth I, proposed that the steel. In 1931 the Japanese developed The Roman philosopher and poet Earth itself was a magnet. cobalt-steel alloys which had improved Lucretius, writing in the first century Gilbert summarized his awe of mag coercivities over other alloys. Alnico B.C., ascribed the name "magnet" to netism by writing: "Magnetick force is magnets came into use in the 1940s, the Greek province of Magnesia in animate, or imitates life; and in many resulting from research by the General Thessaly, where magnetite was first things surpasses human life, [which] is Electric Company. These magnets were mined. In 500 B.C. Thales of Miletius bound up in the organic body." Gilbert hard and brittle and could be machined related that magnetite would pick up also discovered that magnetite lost its only by wet grinding. They proved, small pieces of iron and other bits of magnetic properties when heated to a however, to have high residual flux magnetite. According to Plato, Socrates bright red, but regained them when it densities, high energy products, and claimed that a large enough lodestone cooled. high coercivities. could support by magnetic attraction a Although nickel is a ferromagnetic chain made of iron rings. Magnetite was the only naturally substance, pure nickel itself is a rela occurring magnetic material known to tively poor magnetic material. The addi Magnetite, or lodestone, is a natural early philosophers. But by the end of tion of iron increases its magnetic per magnetic oxide of iron (Fe30 4), a black the 19th century all the known elements meability, and iron-nickel alloys, or opaque mineral of the spinel group with and many compounds had been tested permalloys, resulted in 1920 from the a metallic luster. Today, magnetite is for magnetism. Pierre Curie undertook work of G.W. Elmen. During World found in commercial quantities in Nor the first extended systematic investiga War II, permalloys began to be heat way, Sweden, the Ural Mountains, and tion of the magnetic properties of ele treated in an environment of pure in some parts of the United States. ments. He found that all matter was to hydrogen at 1200-1300°C, which gave The ancients seemed to view mag some extent magnetic, but that the ele the alloy extremely high permeabilities. netite as a plaything and a curiosity. ments fell into three distinct categories. After 1955 thin permalloy films were Lucretius mentioned that "sometimes, Only iron, cobalt, and nickel were used for new applications, such as early too, iron draws back from this stone, for found to have significant magnetic computer technology. it is wont to flee from and follow it in properties, and Curie called these The magnetic properties of rare-earth turn." To explain this phenomena, "ferromagnetic" materials. (In 1935 elements were extensively investigated some of the ancients proposed that the gadolinium was also shown to be ferro in the 1960s due to the increased surface of a magnet was covered with magnetic, and in the 1960s researchers availabilities of such elements and the tiny hooks, while pieces of iron were found that five other rare-earth ele greater ability to work with low temper covered with little rings to catch the ments become ferromagnetic only at atures in the laboratory (because liquid hooks. Other philosophers claimed that low temperatures.) Other elements had helium became easier to obtain). magnetite emitted particles, leaving an very weak magnetic properties, and empty place around the stone into Curie labeled these "paramagnetic" Humans have indeed used magnetic which a piece of iron could move. substances. Most common, though, materials in many applications. In the were the "diamagnetic" materials, mid-1970s, however, scientists discov The term lodestone means "leading which are slightly repelled by magnetic ered the presence of magnetite inside stone," which refers to its ability to be force. some bacteria that have a sense of direc tised as a compass. The ancient Greeks Of the three ferromagnetic metals, tion and can line themselves up to the do not appear to have known of the industrial magnetic applications exten Earth's magnetic field. Magnetite has compass, but some writers suggest that sively use only iron in its elemental also been found in the tissue of higher the Chinese had been using compasses form. However, the creation of some animals known to have an acute sense for land travel as early as the 26th cen alloys greatly improved their magnetic of direction, such as honey bees and tury B.C. The Englishman Alexander characteristics for a wide range of appli homing pigeons. We aren't the only liv Neckam (1157-1217) gives the first cations. The earliest alloy to be made ing things to put magnetic materials to known European reference to a com into a permanent magnet was carbon work for us. pass, and in 1269 the French crusader steel, which was heated to about 800°C KEVIN J. ANDERSON MRS BULLETIN/JUNE 1988 47 .

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