In 1264, the French Pope Urban IV gave southwestern Italy to the French prince Charles of Anjou. There was a catch, however; Charles would have to take it by force. Despite the valiant efforts of a band of Muslim archers from the town of Lucera, he won the Battle of Benevento in 1266. Two years later, the German mayor of Lucera led a revolt against the rule of, now, King Charles I of Sicily, and a year-long siege began. Petrus Peregrinus (a.k.a. Peter of Mericourt and Pierre de Mericourt) was a knight serving under King Charles during the siege of Lucera.
A siege is really a waiting game, so Peregrinus had plenty of time on his hands for scientific experiment and theory, and authoring the earliest known extensive treatise on magnetism, Epistola de magnete, which he sent to his best friend via a letter in 1269. In this letter, he described how to make better compasses (see facing page), discussed the attraction of iron by magnets, the magnetization of iron by permanent magnets, and the ability to reverse the polarity in such induced magnets.
Peregrinus also formed a spherical magnet out of the mineral magnetite (also called lodestone) and used a small compass needle to map out the region in the vicinity of the sphere. He discovered that the lines mapped along the surface of the magnetite ball resembled the meridional lines of a globe and converged at two opposite points of the sphere. He called these antipodal points poles, in analogy with the geographic poles of the Earth, implying that the Earth itself was a large magnet.
By convention, the north pole of a magnetic needle points northward. Thus, Peregrinus noticed that if one models the Earth as a large magnet it is actually the south magnetic pole of the spherical magnet that must correspond to the earth’s north pole. The magnetic axis of the earth is more or less antiparallel to the geographic axis, and the poles are the points where the axis intersects with the surface of the earth.
Peregrinus also came up with a thought experiment that crystalized the principle behind one of the fundamental field equations of electrodynamics. Consider a bar magnet with a north and a south pole. If we break it into two pieces, we get two smaller bar magnets each of which possesses its own north and south magnetic pole. Therefore, Peregrinus reasoned, this could be done no matter how small the magnetic fragment.
Peregrinus’s principle, that neither northness, nor southness, independently exist, still holds true today. Despite the most modern particle accelerators searching in vain for many decades, there have been no reproducible detections of any magnetic monopoles. For the next 500 years, no other original work on magnets matched the scientific insight of the brilliant knight from Mericourt.
We do not know what happened to Sir Peter of Mericourt after his letter of 1269. Presumably he would have completed more scientific works had he returned to Picardy, but one never knows. He may have fallen in battle, quietly joined a monastery, or settled down somewhere in Italy.
What we do know, however, is that in 1270 the king granted a number of the knights funds to build a church and hospital in Naples (St. Eligio) to tend to the wounded. We also know that the king brought as much French culture as possible to southwestern Italy, and personally oversaw the building of two Cistercian monasteries in the French Gothic style.
After the siege of Lucera, King Charles taxed the Islamic city heavily as punishment for their revolt, but otherwise let them live in peace for the rest of his reign. Alas, his son (King Charles II) sold the Muslims of Lucera into slavery in a deliberate attempt to ethnically cleanse his kingdom.
After Peregrinus’s friend published Epistola de magnete, the work greatly influenced later scholars, both directly and indirectly—especially William Gilbert. Gilbert reproduced, and expanded upon, the experiments, and in turn published his own treatise. Gilbert’s work influenced other scientists, whose work influenced others, and so on, until some of Peregrinus’s ideas eventually influenced Michael Faraday .
Among his many accomplishments, Faraday reformulated Peregrinus’s thought experiment in terms of his concepts of magnetic field, magnetic flux, and lines of magnetic force—showing “that the lines of force are continuous through the body of the magnet, and with that continuity gives the necessary reason why no absolute charge of northness and southness is found.”
 Pierre de Maricourt, a.k.a. Petrus Peregrinus, Epistola de magnete (1269) published as The Letter of Petrus Peregrinus On The Magnet, A.D. 1269, trans. Brother Arnold, M.Sc. Principal of La Salle Institute, Troy (New York: McGraw Publishing Company, 1904).
 Caroline A. Bruzelius, “‘ad modum francia’: Charles of Anjou and Gothic Architecture in the Kingdom of Sicily,” Journal of the Society of Architectural Historians Vol. 50, No. 4 (1991), 402-420.
 Taylor, Julie, Muslims in Medieval Italy: The Colony at Lucera, (Lexington Books, 2005)
 William Gilbert of Colchester, On the Magnet, Magnetick Bodies also, and on the great magnet the earth; a new Physiology, demonstrated by many arguments & experiments. (Latin version: London: 1600, 1628, 1633; English translation: London: Chadwick, 1900)