Petrus (Peter) Peregrinus de Maricourt

fl. 1269

French Physicist and Engineer

Petrus Peregrinus de Maricourt conducted the first systematic experiments on magnetism and invented improved nautical compasses. He was one of the few medieval scientists to have conducted experimental inquires.

Little is known of Peregrinus's life. His real name was Petrus de Maharncuria (Pierre de Maricourt), indicating he was from Méharicourt in Picardy, France. It appears he was of noble birth. The only event of his life that can be dated with certainty is the completion of Epistola de magnete (Letter on the magnet), which describes his experiments with magnets. He signed this document, "Completed in camp, at the siege of Lucera, in the year of our Lord 1269." Based on this it has been suggested that Peregrinus was an engineer in the army of Charles of Anjou, King of Sicily, who at the time was directing the siege of Lucera in southern Italy. Since the Church officially declared the assault a crusade, and any who participated in a crusade could be awarded the honorific "Peregrinus" (the Pilgrim), it is likely that he earned his cognomen as a result of services rendered during the siege.

Peregrinus's Epistola is the earliest extant experimental treatise on magnetism. Though the north-south orientation of magnets had been known and used since at least the eleventh century in China and twelfth century in Europe, Peregrinus's treatise provides the earliest extant account of magnetic polarity. He also described various methods for determining the poles of magnets. This in turn allowed him to outline the laws of magnetic attraction and repulsion.

One method for determining the poles that Peregrinus described requires placing an iron needle on a spherically shaped magnet. After the needle aligns itself, one draws a line along its length, dividing the sphere in half. Repeating this process with the needle at different positions, one finds that the lines converge at two points opposite each other on the sphere. Since the network of lines resembles Earth's lines of longitude, which pass through the North and South Poles, the points of convergence on the magnet are called the "poles" of the magnet. Peregrinus appears to have been the first to apply the term polus to these points. Furthermore, he stated that when a magnet is floated in a vessel of water its north pole aligns with the north celestial pole while its south pole aligns with the south celestial pole.

Peregrinus described the mutual attraction of opposite poles when brought together and the repulsion of like poles. He further noted that when a magnet is broken in two the parts function as separate magnets with their own north and south poles. When the pieces are rejoined, the resulting magnet behaves like the original. He also observed that strong magnets can reverse the polarity of weaker ones.

The Epistola was intended as part of a larger work on instruments. Accordingly, the second part deals with the construction of devices exploiting magnetic effects. Along with an ill-conceived perpetual motion machine, Peregrinus described two improved compasses. His wet compass involved enclosing an oval magnet in a wooden case and floating it on water in a circular container. Markings were placed along the container rim, and a rule with perpendicular sights was attached to the cover. This allowed mariners to determine the direction of their ships as well as the azimuth of the Sun, Moon, and stars. His dry compass consisted of a magnetized needle on a pivot within a circular jar. As with the wet compass, the transparent cover for this instrument was marked and provided with a perpendicular rule for sighting. These improvements greatly increased the utility of the compass for navigation.

STEPHEN D. NORTON