A Medieval Study of Magnetism: The Letter of Petrus Peregrinus
An experimental scientific study from the Middle Ages is revealed by a letter written in 1269.
Who was Petrus Peregrinus?
Petrus Peregrinus was also known as Pierre Pelerin de Maricourt, or Peter the Pilgrim of Maricourt. (Maricourt is in Picardy in northern France.) Beyond his famous letter, almost nothing is known about him.
His letter ends with “finished in camp at the siege of Lucera on the eighth day of August, AD 1269.”1 The title “Peregrinus” (pilgrim) probably comes from the fact that this siege was part of a papally sanctioned war of Charles of Anjou, King of Sicily, against the Italian city of Lucera.
What does his letter say?
Peregrinus addresses his letter to a friend, telling him “I will write in a simple style about things entirely unknown to the ordinary individual.”2 He will explain what he has discovered about the lodestone from his experiments, and its application in practical devices.
Known since antiquity, lodestone is magnetite (an iron ore) that has been naturally magnetized. The lodestone was always considered a mysterious phenomenon, since the source of its power was hidden (or “occult”). Peregrinus refers to his studies as “occult experimentation,” but notes that he is focusing only on the “manifest properties of the lodestone.”
Written in Latin, the letter is divided into two parts, the first on the properties of the lodestone, and the second on its practical applications. Beginning with instructions on how to find a good piece of lodestone, and the effect it has on nearby iron, Peregrinus then describes a method for precisely locating the two poles for any given lodestone, using small elongated pieces of iron placed around the stone. This may have been the first time the word “pole” (polus) was used to refer to parts of a magnet.
The letter then explains how to identify the north versus the south pole, by mounting the stone on a small floating platform in a bowl of water, so that it can rotate into alignment with the north-south meridian line. This “wet” or “floating compass” was a mechanism used by ship navigators.
The letter goes on to explain basic magnetic phenomena: unlike poles attract, and like poles repel; breaking a lodestone into two (between the poles) creates new poles at the break; when rejoined, the original lodestone is restored with all its same powers; lodestone can transfer its power to pieces of iron.
Peregrinus then asks what is the source of this power to point north. People in Italy thought that a lodestone points to the place where it was mined, since many iron mines were in northern Europe. But this view contradicted the known fact that lodestones all over the world pointed north.
Peregrinus concludes that “the poles of the lodestone derive their virtue from the poles of the heavens.”3 These are the points around which the heavens rotate and all the celestial meridians intersect. Moreover, he concludes, if correctly mounted and aligned with the rotating sphere of the heavens, a lodestone should itself rotate along with the heavens, completing one rotation every twenty-four hours (and serving as an accurate clock).
These conclusions strike modern readers as odd, but we must note that Peregrinus was unaware of two magnetic phenomena discovered by Europeans centuries later: declination and inclination.
Declination (variation) is the effect on a compass from the difference between the earth’s magnetic pole and the actual north pole. Inclination (dip) is the deviation from the horizontal in a balanced compass needle. In Europe, for example, the north end of a compass needle points downward at an angle, into the earth. This was discovered by instrument maker Robert Norman and described in his treatise The New Attractive of 1581. Norman’s discovery was key in leading William Gilbert to the conclusion, in his De Magnete of 1600, that the Earth itself is a magnet.
Part two of Peregrinus’ letter describes three applications of the lodestone:
A floating compass with precise divisions of the circle, and with vertical rods for measuring the azimuthal position of celestial bodies such as the sun
A pivoted “dry” compass (the first known such description)
A perpetual motion machine
Peregrinus’ perpetual motion machine is a wheel that will supposedly rotate continuously, based on the action of a lodestone mounted on an arm, with many magnetized iron nails mounted around the edge of a circle, so that the lodestone is alternately attracted and repelled by the nails. This is apparently the first proposed perpetual motion machine based on magnetism. Over the centuries, a variety of such machines were proposed, and it wasn’t until the nineteenth century that the science of thermodynamics proved they are impossible.
What is the significance of Petrus Peregrinus’ letter?
According to historian Edward Grant, this letter “ranks as one of the most impressive scientific treatises of the Middle Ages.”4 It is the first extant work devoted solely to magnetism, experimentally demonstrating all of the basic behaviors of the lodestone, and providing designs for its practical applications. Peregrinus may have been the European originator of the “dry” compass, which came into use around this time.
Peregrinus’ letter was widely copied and became well known. (There are currently over thirty-one surviving manuscript copies.) It was clearly influential on the work on William Gilbert, whose treatise De Magnete was one of the early masterpieces of the Scientific Revolution.
References for Further Reading
Petrus Peregrinus, The letter of Petrus Peregrinus on the magnet, A.D. 1269 Translated by Brother Arnold (New York, McGraw Publishing, 1904)
Edward Grant, "Peter Peregrinus," Dictionary of Scientific Biography (New York: Scribners, 1975)
Endnotes
Peregrinus, p. 34
Peregrinus, p. 3
Peregrinus, p. 20
Grant, p. 537