Lodestone
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Lodestones are naturally magnetized pieces of the mineral magnetite.<ref name="Dana">Template:Cite book</ref><ref name="Bowles">Template:Cite book</ref> They are naturally occurring magnets, which can attract iron. The property of magnetism was first discovered in antiquity through lodestones.<ref name="Tremolet">Template:Cite book</ref> Pieces of lodestone, suspended so they could turn, were the first magnetic compasses,<ref name="Tremolet" /><ref>Template:Cite web</ref><ref>Template:Cite book</ref><ref>Template:Cite book</ref> and their importance to early navigation is indicated by the name lodestone, which in Middle English means "course stone" or "leading stone",<ref>Template:Cite encyclopedia</ref> from the now-obsolete meaning of lode as "journey, way".<ref>Template:OED: 'Literally 'way-stone', from the use of the magnet in guiding mariners.'</ref>
Lodestone is one of only a very few minerals that is found naturally magnetized.<ref name="Dana" /> Magnetite is black or brownish-black with a black streak, with a metallic luster and a Mohs hardness of 5.5–6.5.
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, or resistance to magnetization or demagnetization.<ref name="Livingston">Template:Cite book</ref> 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.<ref name="Livingston" /><ref name="Wasilewski">Template:Cite journal</ref><ref name="Warner">Template:Cite book</ref> This inhomogeneous crystalline structure gives this variety of magnetite sufficient coercivity to remain magnetized and thus be a permanent magnet.<ref name="Livingston" /><ref name="Wasilewski" /><ref name="Warner" />
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.<ref name="Livingston" /><ref name="Wasilewski" /> The leading theory is that lodestones are magnetized by the strong magnetic fields surrounding lightning bolts.<ref name="Livingston" /><ref name="Wasilewski" /><ref name="Warner" /> This is supported by the observation that they are mostly found near the surface of the Earth, rather than buried at great depth.<ref name="Wasilewski" />
History
One of the earliest known references to lodestone's magnetic properties was made by 6th century BC Greek philosopher Thales of Miletus,<ref>Template:Cite web</ref> whom the ancient Greeks credited with discovering lodestone's attraction to iron and other lodestones.<ref>Template:Cite book</ref> The name magnet may come from lodestones found in Magnesia, Anatolia.<ref>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, Template:Cite web See also: Paul Hewitt, Conceptual Physics. 10th ed. (2006), p. 458.</ref> The ancient Indian medical text Sushruta Samhita describes using magnetic properties of the lodestone to remove arrows embedded in a person's body.Template:Citation needed
The earliest Chinese literary reference to magnetism occurs in the 4th-century BC Book of the Devil Valley Master (Guiguzi).<ref>The section "Fanying 2" (反應第二) of The Guiguzi: "Template:Lang".</ref>
In the chronicle Lüshi Chunqiu, from the 2nd century BC, it is explicitly stated that "the lodestone makes iron come or it attracts it."<ref>Template:Cite book</ref><ref name=Li54>Template:Cite journal
From the section "Jingtong" (Template:Lang) of the "Almanac of the Last Autumn Month" (Template:Lang): "Template:Lang]"</ref> 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."<ref>In the section "A Last Word on Dragons" (Template:Lang Luanlong) of the Lunheng: "Amber takes up straws, and a load-stone attracts needles" (Template:Lang).</ref> 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.<ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref>Template:Cite book</ref> While this has been shown to work, archaeologists have yet to discover an actual spoon made of magnetite in a Han tomb.<ref>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.</ref>
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.<ref name="Carlson1975">Template:Cite journal</ref> 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.<ref name="Carlson1975"/> 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.<ref>Evans, B. J., Magnetism and Archaeology: Magnetic Oxides in the First American Civilization, p. 1097, Elsevier, Physica B+C 86-88 (1977), S. 1091-1099</ref>
"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..."<ref>Needham, Clerks and Craftsmen, p. 240.</ref>
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.<ref>Template:Cite web</ref> Isaac Newton's signet ring reportedly contained a lodestone which was capable of lifting more than 200 times its own weight.<ref>Template:Cite book</ref> And in 17th century London, the Royal Society displayed a Template:Convert spherical lodestone (a terrella or 'little Earth'), which was used to illustrate the Earth's magnetic fields and the function of mariners' compasses.<ref>Template:Cite web</ref> 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."<ref>Template:Cite book</ref>