Peter Peregrinus Initiates the Scientific Study of Magnets

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Peter Peregrinus Initiates the Scientific Study of Magnets

Overview

The earliest experimental study of magnetism can be found in a letter written by Petrus Peregrinus in 1269. Peregrinus was the first individual to describe the existence of two magnetic poles in each magnet, to describe the attraction between unlike poles, and to explain the creation of new poles when a magnet is broken in two. A designer of instruments, Peregrinus also described improvements in the magnetic compass, which made it far more useful for navigation on the high seas. Roger Bacon, a Franciscan friar teaching at the universities of Oxford and Paris, popularized the experiments of Peregrinus, including studies now lost. The letter about magnets was copied numerous times and widely circulated. Later it would stimulate the researches of William Gilbert, an English physician whose treatise on magnets would initiate the modern study of electricity and magnetism.

Background

The basic phenomena of magnetism were known to the ancient Greeks. The philosopher Thales (624-546 b.c.) was familiar with lodestone, a naturally occurring magnetic rock, and felt it necessary to attribute to it a "soul" because it was able to cause motion. The Greek medical writer Galen (a.d. 130-200) recommended the use of magnets for "expelling gross humors." Probably the first practical use of magnetism outside the medical area was the magnetic compass, which appeared in Europe in the twelfth century. Scholars are in disagreement about the origin of the compass, which may have been first used by the Chinese or by Norsemen. The magnetization of an iron needle by stroking it with a lodestone was first described in writing by Alexander Neckam (1157-1217), an English monk.

The first detailed experimental study of magnetism is to be found in a letter of Petrus Peregrinus (fl. c. 1269) to one Sygerum de Foucaucourt, a knight and neighbor. Little is known about the life of Peregrinus, whose original name was Pierre de Maricourt, Peregrinus being a title awarded to religious pilgrims and to individuals who served in the crusades. He is believed to have been a university graduate, and, at the time he wrote the letter in 1269, was apparently serving as a sort of military engineer in the army of Charles of Sicily as it lay siege to the Italian city of Lucera. English philosopher and scholar Roger Bacon (c.1214-1292) noted that Peregrinus had a reputation as a scholar prior to the siege, but the Peregrinus's earlier writings have been lost. Since Peregrinus is highly praised in a document Bacon wrote in 1267, but is not mentioned in his earlier writings, it is likely that Bacon met Peregrinus after 1260, possibly at the University of Paris. According to Bacon, Peregrinus was skilled in minerals, metalworking, and agriculture, and he had worked for three years on "burning glasses," lenses that would concentrate the Sun's rays to start a fire.

According to Peregrinus, the letter to de Foucaucourt was intended to be part of a longer treatise on the construction of physical instruments. The letter is divided into two parts, the first on the basic principles of magnetism and the second on the construction of magnetic devices. In the first part, Peregrinus begins with a defense of the experimental method, warning against untested theory and speculation. He then suggests that the best lodestones are those from northern Europe that are somewhat blue in color and that the strength of a lodestone is to be measured by the weight of iron that it can lift.

Peregrinus next describes his construction of a spherical magnet and demonstrated that it had two poles, which could be identified by placing iron needles on the surface and tracing the direction in which the needle pointed. He thus obtained, in effect, lines of longitude that intersected at two points, the two magnetic poles. He found that at these two points fragments of an iron needle can be made to stand on one end. He then describes several studies done with magnets contained in small bowls and allowed to float on water in a larger bowl. He notes that a magnet will orient itself so that its north pole points to the "north pole of the heavens," and establishes that opposites attract each other. He also describes the creation of new poles when a magnet is broken in two, interpreting the attraction between opposite poles as a natural affinity.

Peregrinus considered the possibility that a magnet's north pole might be attracted to a point on Earth but rejected this idea. Instead, he concluded that the magnet poles pointed towards the celestial poles, the points around which the stars in the sky appear to move. At the time, it was commonly thought that the pole star, Polaris, was located exactly at the celestial pole, and it was natural to assume that the magnet pole was attracted to this star. Peregrinus was aware that Polaris, as seen from Earth, actually moves in a small circle about the celestial pole and so had to assume that the magnet's motion was affected by all parts of the celestial sphere.

In the second part of his letter, Peregrinus describes the construction of two types of magnetic compass, both of which represented a significant advance beyond the existing instruments. In one, the magnet was enclosed inside a wooden case and floated on water in a circular vessel, the periphery of which was divided into 360 points. Sighting pins on the case allowed the user to determine the angular position of the Sun, Moon, and stars relative to magnetic north. In the second, the compass points were marked on the transparent lid of a jar, and the magnetized needle rested on a pivot inside the jar. A nonmagnetic bar attached to the magnetized needle indicated the east and west directions. At the end of the letter, Peregrinus describes a "perpetual motion machine" based on the behavior of magnets that he actually constructed. It did not work, but he attributes the failure to his own lack of skill in construction.

Impact

In the universities of thirteenth-century Europe, two routes to reliable knowledge were recognized: divine revelation through the scriptures and argumentation with due deference to ancient authority. The gathering of facts by observation and experimentation was at best suspect. It is possible that Peregrinus was reluctant to have his discoveries more widely known, lest he be accused of witchcraft. Bacon, for instance, had his own experimental studies interrupted by religious authorities and spent 14 years in prison for suspected heresy. He described Peregrinus as one of two perfect mathematicians and as "the only Latin writer to realize that experiment rather than argument is the basis of certainty in science." It is in this sense that Peregrinus is considered by some historians as the first true experimental scientist. More than three centuries would elapse before the so-called Scientific Revolution would establish the experimental method as the ultimate test of scientific truth.

The construction of a reliable ship's compass was a major factor in launching the great age of exploration by sea in the fifteenth and sixteenth centuries. Christopher Columbus (1451-1506) carried a compass like one described by Peregrinus on his journey to the New World and was quite interested in the mechanism by which it functioned. On his journey, Columbus was able to make important observations of the magnetic variation, that is the difference between compass north and geographical north. His crew, noting the deviation with respect to the pole star, became alarmed, but was calmed when Columbus explained that the pole star in fact moved with respect to true north.

The next major advance in the understanding of magnetism after Peregrinus occurred more than 300 years later, when the English physician William Gilbert (1544-1603) conducted the experiments described in his treatise De Magnete, published in 1600. Gilbert's debt to Peregrinus is obvious. Gilbert magnetized a large iron sphere, which he called a "terrella," and by studying the response of a compass needle as it was moved around the sphere, made a convincing case that Earth was itself a large magnet. By Gilbert's time, also, the magnetic variation was familiar to both sailors and scholars, as was the fact that a freely suspended magnetized needle would point or "dip" some degrees below the horizontal. Gilbert demonstrated that these phenomena too were consistent with a magnetized Earth. Gilbert's treatise concludes with a discussion of "magnetic rotation," a type of perpetual motion that he, like Peregrinus, believed would occur if a perfectly magnetized sphere were suspended so that it could move freely.

Although Peregrinus and even his predecessors were aware of the qualitative differences between magnetic attraction and static electricity, it was Gilbert who made the cleanest distinction between the two phenomena. For the following two centuries, magnetic and electric phenomena were considered unrelated. Then in 1800 the primitive battery invented by Italian physicist Alessandro Volta (1745-1827) made it possible to sustain a steady electric current. And a mere 20 years later, Danish physicist Hans Christian Ørsted (1777-1851) discovered that the current flowing through a wire could cause the deflection of a nearby compass needle. Ørsted's discovery triggered a period of intense investigation of electromagnetic phenomena that would last throughout the nineteenth century.

DONALD R. FRANCESCHETTI