Lodestones are naturally
magnetized pieces of the mineral
magnetite.[1][2] They are naturally occurring
magnets, which can attract
iron. The property of
magnetism was first discovered in
antiquity through lodestones.[3] Pieces of lodestone, suspended so they could turn, were the first
magnetic compasses,[3][4][5][6] and their importance to early
navigation is indicated by the name lodestone, which in
Middle English means "course stone" or "leading stone",[7]
from the now-obsolete meaning of lode as "journey, way".[8]
Lodestone is one of only a very few minerals that is found naturally magnetized.[1] Magnetite is black or brownish-black, with a metallic
luster, a
Mohs hardness of 5.5–6.5 and a black
streak.
Origin
The process by which lodestone is created has long been an open question in geology. Only a small amount of the magnetite on the Earth is found magnetized as lodestone. Ordinary magnetite is attracted to a
magnetic field as iron and steel are, but does not tend to become magnetized itself; it has too low a
magnetic coercivity (resistance to demagnetization) to stay magnetized for long.[9] Microscopic examination of lodestones has found them to be made of magnetite (Fe3O4) with
inclusions of
maghemite (cubic Fe2O3), often with impurity metal
ions of
titanium,
aluminium, and
manganese.[9][10][11] This inhomogeneous crystalline structure gives this variety of magnetite sufficient
coercivity to remain magnetized and thus be a
permanent magnet.[9][10][11]
The other question is how lodestones get
magnetized. The
Earth's magnetic field at 0.5
gauss is too weak to magnetize a lodestone by itself.[9][10] The leading theory is that lodestones are magnetized by the strong magnetic fields surrounding
lightning bolts.[9][10][11] This is supported by the observation that they are mostly found near the surface of the Earth, rather than buried at great depth.[10]
History
One of the earliest known references to lodestone's magnetic properties was made by 6th century BC Greek philosopher
Thales of Miletus,[12] whom the ancient Greeks credited with discovering lodestone's attraction to iron and other lodestones.[13] The name magnet may come from lodestones found in
Magnesia,
Anatolia.[14]
The earliest Chinese literary reference to magnetism occurs in the 4th-century BC Book of the Devil Valley Master (Guiguzi).[15]
In the chronicle Lüshi Chunqiu, from the 2nd century BC, it is explicitly stated that "the lodestone makes
iron come or it attracts it."[16][17] The earliest mention of a needle's attraction appears in a work composed between 20 and 100 AD, the Lunheng (Balanced Inquiries): "A lodestone attracts a needle."[18] In the 2nd century BC, Chinese
geomancers were experimenting with the magnetic properties of lodestone to make a "south-pointing spoon" for divination. When it is placed on a smooth bronze plate, the spoon would invariably rotate to a north–south axis.[19][20][21] While this has been shown to work, archaeologists have yet to discover an actual spoon made of magnetite in a Han tomb.[22]
Based on his discovery of an
Olmec artifact (a shaped and grooved magnetic bar) in North America, astronomer John Carlson suggests that lodestone may have been used by the Olmec more than a thousand years prior to the Chinese discovery.[23] Carlson speculates that the Olmecs, for astrological or
geomantic purposes, used similar artifacts as a directional device, or to orient their temples, the dwellings of the living, or the interments of the dead.[23] Detailed analysis of the Olmec artifact revealed that the "bar" was composed of
hematite with
titanium lamellae of Fe2–xTixO3 that accounted for the anomalous
remanent magnetism of the artifact.[24]
"A century of research has pushed back the first mention of the magnetic compass in Europe to
Alexander Neckam about +1190, followed soon afterwards by Guyot de Provins in +1205 and Jacques de Vitry in +1269. All other European claims have been excluded by detailed study..."[25]
Lodestones have frequently been displayed as valuable or prestigious objects. The
Ashmolean Museum in Oxford contains a lodestone adorned with a gilt coronet that was donated by
Mary Cavendish in 1756, possibly to secure her husband's appointment as Chancellor of Oxford University.[26]Isaac Newton's signet ring reportedly contained a lodestone which was capable of lifting more than 200 times its own weight.[27] And in 17th century London, the
Royal Society displayed a 6-inch (15 cm) spherical lodestone (a terrella or 'little Earth'), which was used to illustrate the Earth's magnetic fields and the function of mariners' compasses.[28] One contemporary writer, the satirist
Ned Ward, noted how the terrella "made a paper of Steel Filings prick up themselves one upon the back of another, that they stood pointing like the Bristles of a Hedge-Hog; and gave such Life and Merriment to a Parcel of Needles, that they danc'd [...] as if the devil were in them."[29]
^Brand, Mike; Sharon Neaves; Emily Smith (1995).
"Lodestone". Museum of Electricity and Magnetism, Mag Lab U. US National High Magnetic Field Laboratory. Archived from
the original on 2009-05-01. Retrieved 2009-06-21.
^The
Greek term μαγνῆτις λίθος magnētis lithos (see
Platonis Opera, Meyer and Zeller, 1839, p. 989) means "Magnesian stone". It is uncertain whether the adjective μαγνῆτις "of Magnesia" should be taken to refer to the city Magnesia ad Sipylum in
Lydia (modern-day
Manisa, Turkey) or after the Greek region of
Magnesia itself (whence came the colonist who founded the Lydian city); see, for example,
"Magnet". Language Hat blog. 28 May 2005. Retrieved 22 March 2013. See also: Paul Hewitt, Conceptual Physics. 10th ed. (2006), p. 458.
^The section "Fanying 2" (
反應第二) of The
Guiguzi: "其察言也,不失若磁石之取鍼,舌之取燔骨".
^Dillon, Michael (2017). Encyclopedia of Chinese History. Routledge. p. 98.
ISBN978-0415426992.
^Li, Shu-hua (1954). "Origine de la Boussole II. Aimant et Boussole". Isis (in French). 45 (2): 175–196.
doi:
10.1086/348315.
JSTOR227361.
S2CID143585290. un passage dans le Liu-che-tch'ouen-ts'ieou [...]: "La pierre d'aimant fait venir le fer ou elle l'attire." From the section "Jingtong" (精通) of the "Almanac of the Last Autumn Month" (季秋紀): "慈石召鐵,或引之也]"
^In the section "
A Last Word on Dragons" (亂龍篇Luanlong) of the Lunheng: "
Amber takes up straws, and a load-stone attracts needles" (頓牟掇芥,磁石引針).
^Tom, K. S. (1989). Echoes from Old China: Life, Legends, and Lore of the Middle Kingdom. University of Hawaii Press. p. 108.
^Qian, Gonglin (2000). Chinese Fans: Artistry and Aesthetics. Long River Press. p. 98.
ISBN978-1592650200.
^Curtis Wright, David (2001). The History of China: (The Greenwood Histories of the Modern Nations). Greenwood Publishing Group. p. 42.
^Joseph Needham, Clerks and Craftsmen in China and the West: Lectures and Addresses on the History of Science and Technology. Cambridge: University Press, 1970, p. 241.
^Fara, Patricia (1996). Sympathetic Attractions: Magnetic Practices, Beliefs, and Symbolism in Eighteenth-Century England. Princeton University. p. 23.
ISBN9780691634913.
External links
Look up lodestone in Wiktionary, the free dictionary.