Proceedings 6        29 January 1988    the Danson Room, Trinity College

MAGNETIC THEORY FROM DE MARICOURT TO GILBERT, a paper illustrated with slides by Dr Willem Hackmann, Assistant Curator, the History of Science Museum, Oxford, and member SHMTS.

Dr Crombie alluded to the speaker's work in the history of electricity and medicine, and welcomed him on behalf of the meeting.


Dr Hackmann explained that his paper would address the period from de Maricourt and Gilbert in the context of earlier history.

From about 1250, Petrus Peregrinus ("Peter the Pilgrim") also known as Pierre de Maricourt from his native district in France. was in Paris where Roger Bacon was among his pupils and praised him highly: "What others strive to see dimly and blindly, he gazes   at in the full light of day, because he is a master of experiment." In 1269 he wrote his Epistola de magnete while at the siege of Lucera with the army of Charles of Anjou, probably as an engineer. Heir to a tradition originating with the ancient observation that the lodestone attracted iron, Petrus wrote the key work on magnetism before William Gilbert's De Magnete of 1600, which he influenced.

A proper appreciation of early developments in magnetism became possible only after the appearance of Joseph Needham's Science and Civilization in China, vol. iv (1962). The lodestone (mineral magnetite, Fe3 04) had various ancient names, among them lithos magnetis, i.e. "stone of Magnesia" a town in Lydia. The Old English lodstone from Anglo -Saxon lad, "journey" derived from its compass properties. Chinese names for the magnetic needle were "the tadpole" and "the fish", the lodestone was the "mysterious stone".

The fundamental properties recognized in the lodes tone were [1] attraction of iron over a distance [2] attracted pieces adhered [3] it could induce attractive power in iron [4] this power remained for some time [5] the magnetic influence acted through substances other than iron [6] some magnets would repel some pieces of iron. Lucretius, who introduced the concept of magnetic repulsion described all these properties in his De Rerum Natura. The classical knowledge on magnetism was transmitted by Pliny to medieval Europe, where there was at first particular interest in medical and alchemical applications. From the 7th c. on Islamic scholars such as Jabir ibn Haiyan, or "Geber" (fl.770s) showed considerable interest in magnetic attraction. Knowledge of magnetic phenomena existed from China to Europe but whether it was transmitted must remain a matter of conjecture.

In the 11th-12th century knowledge of a completely new magnetic phenomenon appears: magnetic orientation. Dr Hackmann traced its origins to Chinese geomancy and divination techniques, which involved the throwing of small models symbolic of celestial bodies onto prepared boards and which may in turn have derived from magical board games, enveloped in astronomical symbolism. The most significant of the ancient boards, the shih comprised a square base earth-plate and an upper, rotatable disc-shaped heaven plate. Also used was a lodestone 'spoon' which, resting on a highly-polished earth plate could rotate in response to the torque of the earth's magnetic field. Later a wooden fish (a lodestone in its belly) was floated in a bowl of water; in the 7th-8th c. this came to be replaced by a magnetized iron needle. The Chinese regarded needle or stone as South pointing, but realized magnetization could reverse polarity.

The Chinese knew of magnetic declination as well as polarity by the 9th c. (600 years before Europe). Sometime before the 11th c. they also discovered that iron leaf could be made magnetic by quenching from red heat in the earth's magnetic field. This led to the common form of a needle floating in water. The 1st clear account of a mariner's compass was a Chinese description (1110s) of events of 1086.

Although transmission from east to west has been assumed, there is no evidence. Persian, Arabic and Indian references to fish shaped compasses post date European usage. The Arabic word alkonbas indicates derivation from the west. This could argue for an overland westward transmission (through the peoples of the steppe or along the Silk Route) of the astronomer's compass followed by an independent western application to marine use.

The first known mention of the compass in a western source is in Alexander Neckam's De Utensilibus of 1187--this was probably a floating needle compass, later a dry-mounted type is described. Since the founding of the Crusader kingdom of Jerusalem in 1099, Italian cities had expanded regular trade with the Levant; among them the ships of Amalfi carried magnetic iron ore from the mines on Elba. The city was the early centre of compass making.

After Neckam there was a flood of references: e.g. Vitry spoke of the magnetic needle pointing to the pole star; Bruno Lantini of a magnetic mountain beneath the speculated on the possibility pole star. Magnetic repulsion was ignored until Gilbert because it did not fit the conceptual framework.

Petrus's Epistola de magnete provided a compendium of medieval knowledge on magnetism and introduced certain improvements. Possibly he was inspired to write by watching compass makers at Amalfi during his campaigning in Italy.

Petrus [1] dismissed the pole star theory of magnetic orientation because the star was on the meridian only twice a day. [2] rejected the theory of magnetic mountains. [3] offered a description of magnetic polarity.

He described 3 INSTRUMENTS {A}--A floating lodestone compass in a light wooden container, with degrees around the edge; together with an alidade for taking azimuth readings of celestial objects (this was possibly never more than a theoretical concept){B}-A magnetized needle rotating in a box with a transparent top -its degree scale in 4 quadrants and the needle of softish' iron as it required repeated magnetization by bringing a lodestone near it. The degree scale was perhaps influenced by contemporary astrolabe practice--this was the first illustration of a European compass. {C}--Magnetic perpetual motion machines: a globular lodestone mounted without friction parallel to the celestial axis so that it would rotate once a day (diurnal motion) and, if mounted with a map of the heavens, would provide an automatic armillary sphere for astronomical observations, and also a perfect clock. This idea may have come from William of Auvergne's De Universo Creatarum (l230s) who used the analogy of magnetic induction to explain the motion of the celestial spheres. A second device of Petrus comprised a wheel mounted with lodestone to rotate, it was hoped, in a frame set with iron nails. The Epistola circulated widely but was not printed until c.1520 (falsely attributed to Raymond Lull).

From the practical point of view it is unlikely the floating compass could be used in foul weather. It was most likely used to check wind direction rather than establish bearings. With the advent of the dry-mounted compass it became easier to take magnetic bearings of objects and places from ships and there is evidence that this was well established in the 13th century.

Little was known about the design of medieval compasses: some were in portable sundials with meridian lines and four cardinal points.  Next came the fly' or compass rose which derived no doubt from the graduated circle at the edge of the edge of the compass box, transformed into a graduated card attached to the needle, and rotating with it. The windrose was perhaps added by Flavio Gioja of Amalfi, ca.1300, while the stella maris began to be the traditional -aecoration of the compass rose later in the 14th century.

Crude drawings on 15th century maps depict round-box compasses with pivoted compass roses. The rose with 32 divisions marking the principal wind directions or rhumbs became standard These rhumb or bearing lines were drawn on charts from the early 14th century. The sailing master used a ruler over the network of rhumb lines from the port of destination to his estimated position; identified the nearest parallel rhumb line; traced this back to the windrose and set course accordingly. Classic instances of these charts were the Genoese MS Atlas of Pietro Visconti (1318) and a Venetian portolan chart of Britain and the Atlantic coast of France, Spain and North Africa (14th century)

Above: Forging compass needle in Earth's magnetic field; from Gilbert's De Magnete (1600).

Left: Compass with quadrant rose, needle and alidade from early 14th-c. copy of de Maricourt's 'Epistola'

The next development was the azimuth compass to resolve magnetic variation. In the 15th century it began to be realized that the compass needle did not point true north. Early in the 16th century the Portuguese produced treatises on the subject and in 1537 Pedro Nunez devised an instrument to determine the variation of the compass needle from the meridian. It was perhaps mounted in gimbals (known from the notebooks of Villard, and suggested for the compass by Leonardo): In England William Barlow, who was indebted to Nunez’s ideas developed a compass of variation mounted in gimbals and built for him by Richard Norman, a professional compass maker. The last magnetic movement identified was dip from the horizontal, effectively discovered by Norman in 1581.

Thus by 1600, the year in which Gilbert published his De magnete, 5 magnetic movements were known in Europe: attraction; orientation; variation from the meridian; variation from the horizontal; diurnal rotation. Gilbert devoted one book to each movement and a chapter to distinguishing the "amber effect" (or electricity) from magnetic attraction.

Born in Colchester in 1544 the son of the town recorder, Gilbert studied medicine at Cambridge, settled in London about 1573 and became a successful doctor, being appointed court physician to Elizabeth I shortly after the publication De magnete. He died probably of the plague, in 1603.

Dr Hackmann ended with a problem: How, if at all, did the Vikings fit into the story? There are large deposits of magnetite in Scandinavia, but no hard evidence that the Vikings used the compass. The suggestive gloss in the Saga of Floki (discovered Iceland 874) is a later interpolation. However, the Pivoting bronze fish pedestal of the Sutton Hoo hanging bowl could have been used as compass--or, the speaker surmised, in geomancy.(early 7th century).

The Speaker concluded: The Chinese, long acquainted with the directive property of the magnet, first used it in navigation only in the late 11th century, 100 years before the first known European compass. Magnetic lore may have been transmitted over the Asian landmass, but from the 1200s European craftsmen and scholars made rapid advances in magnetism, as exemplified by the Epistola of Peregrinus. Their magnetic technology overtook the Chinese and in the 16th c. accelerated, no doubt thanks to the powerful western combination of craftsman and scholar.

The President thanked Dr Hackmann for his wide-ranging talk and especially remarked on his identification of the twin themes of craftsman and scholar in the emergence of natural experimental philosophy in the west. The meeting closed after a diverse and stimulating discussion from the floor.

Summary GHS