The Lodestone: History, Physics, and Formation

Abstract

The lodestone is an extremely rare form of the mineral magnetite (Fe₃O₄) that occurs naturally as a permanent magnet. It therefore attracts metallic iron as well as fragments of ordinary ‘inert’ magnetite. This ‘magic’ property was known to many ancient cultures, and a powerful lodestone has always commanded a high price. By the eleventh century AD the Chinese had discovered that a freely suspended elongated lodestone would tend to set with its long axis approximately north–south, and utilized this property in the magnetic compass. They also appear to have discovered that this invaluable characteristic could be handed‐on to a steel needle if the latter were contacted with, or stroked by, a lodestone.

The magnetism of the lodestone was scientifically investigated by William Gilbert in the sixteenth century, when he defined its ‘poles' and the well‐known rule that ‘like poles repel, unlike attract’. He also studied ‘inclination’ and ‘variation’, and means to aid the preservation of magnetic power. How to concentrate it by ‘arming’ the lodestone with caps or pole‐pieces of soft iron was discovered in the same century. These methods have been repeated, confirmed, and improved. The lodestone occupies a vital place in the history of magnetism, but little beyond Gilbert's work can be reached by historical studies because vastly improved steel or alloy permanent magnets, and electromagnets, replaced it before quantitative measurements were developed. These techniques have therefore been applied retrospectively to both museum specimens and contemporary natural lodestones. A good source of the latter was found to be the igneous complex known as Magnet Cove, Arkansas, and this material has been used as the ‘type example’. All specimens were discrete, well‐rounded, rusty brown pebbles found near the surface. Their unweathered interiors were black titanomagnetite. No significant trace element or crystallographic differences could be found between the lodestones and the magnetically inert material that always accompanied them. The magnetic moment per unit volume (J _v) of the ‘as‐found’ Magnet Cove lodestones varied between 6.5 and 11.6 emu cm⁻³, which compares poorly with the hundreds of units characteritzing modern permanent magnets. Hysteresis loops gave a saturation intensity (J _s) of 27–51 emu cm⁻³, suggesting that intensity has diminished since formation. This agrees with general experience of magnets, especially in the absence of a ‘keeper’. The initial volume susceptibility of Magnet Cove magnetite was about 0.18 for low fields, and always remained <1. This means that a normal terrestrial magnetic field with a maximum vector <1 Oe is unable to induce even the low magnetic moments we see today, while a field approaching 1000 Oe is required for saturation. These parameters, and the rare occurrence of lodestones as near‐surface fragments, support suggestions that they are the product of a lightning strike upon an exposure of a suitable (titanium‐rich?) magnetite. Transient currents averaging 30 000 A have been measured. This would give rise to a zone of potential magnetic saturation at least 12 cm in diameter, to which some of the ejected fragments would be exposed. An attempt to determine the period elapsed since formation of the Magnet Cove lodestone was made by annealing magnetically saturated specimens at temperatures up to 500°C, and measuring J _v at weekly intervals for 100 days. The decay curves visually resembled exponential functions, but mathematical tests proved that they were not strictly so. Interpretation was therefore difficult, but a pragmatic procedure involving excessive extrapolation suggested an ‘age’ of about 3500 years.

Acknowledgements

I am grateful to the curators of various museums who have kindly allowed me to examine and measure lodestones in their care.

Notes

Wilson and Herroun (note 49).

⁵⁷T.Nagata, Rock Magnetism (Tokyo, 1953, 1961).

⁵⁸Wasilewski (note 51).

⁵⁹Blackman and Lisgarten (note 52).

¹Joseph Ennemoser, Der Magnetismus im Verhältnisse zur Natur und Religion (Stuttgart, 1842). Albert Radl, Der Magnetstein in der Antike: Quellen und Zusammenhänge (Stuttgart, 1988). Gudrun Stecher, Magnetismus im Mittelalter: von den Fahigkeiten und der Verwendung des Magneten in Dichtung, Alltag und Wissenschaft (Kummerle, 1995).

²Petrus Peregrinus, Epistola de Magnete (1269), MS reproduced by Quaritch (London, 1900), trans. by Silvanus Thompson (London: Chiswick Press, 1902). A more accessible source is Mottelay (note 11).

³Agricola, De Natura Fossilium (Basle, 1546), trans. by M. C. and J. A. Bandy, Geological Society of America, Special Paper 63, (1953). Agricola, De Re Metallica (Basle, 1556), trans. by Hoover (London, 1912). J. B. Porta, Magia Naturalis (Naples 1558), trans. as Natural Magic (London, 1658), Book VII: The Magnet. Robert Norman, The Newe Attractive: Containing a Short Discourse of the Magnes or Lodestone … (London, 1581).

⁴William Gilbert, De Magnete (London, 1600), trans. by P. Fleury Mottelay (1893; New York: Dover, 1958). Silvanus P. Thompson, Gilbert of Colchester (London, 1891). E. B. Gilberd and Lord Penny William Gilberd (Colchester: Castle Museum, 1970). The last of these works claims that ‘Gilberd’ is the correct spelling, based on an extant signature.

⁵Marke Ridley, A Short Treatise of Magnetical Bodies and Motions (London, 1613).

⁶William Barlowe, Magnetical advertisements, or, Divers pertinent observations, and approved experiments concerning the nature and properties of the load–stone … (London,1616); microfiche edition, ‘Landmarks of Science’ collection (New York: Readex, 1992).

⁷Athanasius Kircher, Magnes (Cologne, 1643), and Magneticum Naturae Regnum (Rome, 1667).

⁸Dalancé, Traitte de l'Aiman (1687).

⁹Robert Boyle, Experimenta & Observationes Physicae (London, 1691).

¹⁰Servington Savery, ‘Magnetical Observations and Experiments’, Philosophical Transactions, 36 (1730), 295–340.

¹¹P. F. Mottelay, Bibliographical History of Electricity and Magnetism (London, 1922).

¹²Savery (note 10). Gowan Knight, ‘An Account of Some Magnetical Experiments …’ Philosophical Transactions, 43 (1744), 161–66, and 44 (1744), 656–72. J. Michell, A Treatise of Artificial Magnets (Cambridge, 1750). J. Canton, ‘A Method of Making Artificial Magnets Without the Use of Natural Ones’, Philosophical Transactions, 47 (1753), 31–38. J. Fothergill, ‘An Account of the Magnetical Machine Contrived by the Late Dr Gowin Knight’, Philosophical Transactions, 66 (1776), 591–99. Tiberius Cavallo, A Treatise on Magnetism (London, 1800).

¹³David Brewster, ‘A Treatise on Magnetism’, in Encyclopaedia Britannica, 7th ed (1837). J. B. Kramer, ‘The Early History of Magnetism’, Transactions of the Newcomen Society, 14 (1933–34), 183–200. Alfred Still, Soul of Lodestone: The Background of Magnetical Science (New York, 1946). E. N. da C. Andrade, ‘The Early History of the Permanent Magnet’, Endeavour, 17 (1958), 22–30. L. W. McKeehan, Magnets (New York, 1967).

¹⁴McKeehan (note 13).

¹⁵Joseph Needham, Science and Civilisation in China (Cambridge, 1962), IV. C. A. Ronan and J. Needham, The Shorter Science and Civilisation in China (Cambridge, 1986), III, ch.1. In view of the disputed period of discovery it is perhaps ironic to learn that magnetotactic bacteria were using tiny intracellular grains of magnetite to determine direction in the remote geological past. See R. F. Blakemore and R. B. Frankel, ‘Magnetic Navigation in Bacteria’, Scientific American, 245 (December) (1981), 42–49.

¹⁶See Agricola, De Re Metallica (1556) (note 3).

¹⁷See Norman (note 3).

¹⁸Calculated values for the seven elements of the geomagnetic field at any site on any given date are obtainable from the National Geophysical Data Center, Colorado, USA at www.ngdc.noaa.gov. It must be borne in mind that these model values are subject to small and unpredictable variations due to solar activity and local influences.

¹⁹Needham (note 15).

²⁰G. W. C. Kaye and T. H. Laby, Tables of Physical and Chemical Constants (London, 1966).

²¹McKeehan (note 13).

²²Johann von Lamont, Handbuch des Magnetismus (1856, 1867).

²³Kircher, 1667 (note 7), Dalencé, 1687 (note 8), Cavallo, 1800 (note 12).

²⁴P. D. Marianini, ‘Armed Magnets and Some Methods of Magnetization’, Nuovo Cimento, 4 (1856) 231–62.

The usual internal construction of a cased lodestone

²⁵Nehemiah Grew, Catalogue and Description of the Natural and Artificial Rarities Belonging to the Royal Society … (London, 1681).

²⁶A. de la Hire, ‘A New Sort of Magnetical Compass’, Philosophical Transactions, 16 (1687), 344–51.

²⁷Isaac Newton, Principia (London, 1687), Book 3, Prop.6, Theorem 6, Cor.5.

²⁸ Journal Book of the Royal Society, 1702–1714, 10, 373 (for 20 March 1712).

²⁹Francis Hawksbee, ‘An Account of the Experiments Concerning the Proportion of the Power of the Load‐stone at Different Distances’, Philosophical Transactions, 27 (1712), 506–11 and Figure 4 on separate plate.

³⁰Brook Taylor, ‘An Account of an Experiment Made by Dr Brook Taylor Assisted by Mr Hawksbee, in Order to Discover the Law of the Magnetical Attraction’, Philosophical Transactions, 29 (1712), 294–95.

³¹P. Musschenbroek and J. T. Desaguliers, ‘De Viribus Magneticis’, Philosophical Transactions, 33 (1724–5), 370–78.

³²Robert Palter, ‘Early Measurement of Magnetic Force’, Isis, 63 (1972), 544–58.

³³J. K. F. Gauss, Intensitas Vis Magneticae Terrestris (1832), Not easily accessible—see C. C. Gillispie (ed.), Dictionary of Scientific Biography (New York, 1972), V, 298–315. A portion is reproduced in F. Magie, Source Book in Physics (New York, 1935), pp. 519–24.

³⁴J. Deuchar, ‘Large Loadstones’, Memoirs of the Wernerian Natural History Society, Edinburgh, 4 (1821), 386–95 and Plate XII.

³⁵Mary Holbrook, Science Preserved (London, 1992).

³⁶See their website www.mhs.ox.ac.uk

³⁷A. Q. Morton and J. A. Wess, Public and Private Science, (Oxford, 1993). Also NMSI website.

³⁸Paolo Brenni, Gli strumenti di fisica dell'Istituto Tecnico Toscano. Elettricita e Magnetismo (Florence, 2000).

³⁹J. F. Williams, ‘The Igneous Rocks of Arkansas’, in Arkansas Geological Survey, Annual report for 1890. R. L. Erikson and L. V. Blade, ‘Geochemistry and Petrology of the Alkalic Igneous Complex at Magnet Cove, Arkansas’, USGS Professional Paper 425 (1963).

Lodestone (specimen A) from Magnet Cove, Arkansas, attracting small nails to its surface

⁴⁰This site is now closed to public collecting. It is rumoured that both raw and tumble‐polished pieces of ordinary magnetite from various localities have occasionally been artificially magnetized by exposure to a strong magnetic field and then sold as charms.

⁴¹This agrees with Still (note 13), who writes ‘The lodestone found at Magnet Cove in Arkansas is frequently in the form of brown pebbles turned up by the plow.’

⁴²Ward's Natural Science, PO Box 92912, Rochester, NY.

⁴³J. H. Poynting and J. J. Thomson, A Textbook of Physics (London, 1920), IV: Electricity and Magnetism.

Magnetic poles associated with Magnet Cove A. A simple dipole

Magnetic poles associated with Magnet Cove B. A complex multipolar structure

⁴⁴W. H. Newhouse, ‘The Identity and Genesis of Lodestone Magnetite’, Economic Geology, 24 (1929), 62–67.

⁴⁵J. Gruner, ‘The Identity and Genesis of Lodestone Magnetite’, Economic Geology, 24 (1929), 771–75.

⁴⁶C. Davis, ‘Geological Significance of Magnetic Properties of Minerals’, Economic Geology, 30 (1935), 655–62. Repeated in US Bureau of Mines Bulletin, 425 (1941), 362–66.

⁴⁷C. E. N. Bromehead, ‘Ship's Loadstones’, Mineralogical Magazine, 28 (1948), 429–37.

⁴⁸P. Weiss, ‘Magnetization of Crystallised Magnetite’, Comptes Rendus Academie des Sciences, Paris, 122 (1896), 1405–09.

⁴⁹E. Wilson and E. F. Herroun, ‘The Magnetic Properties of Varieties of Magnetite’, Proceedings of the Physical Society, 31 (1919), 299–318.

⁵⁰D. Strangway, History of the Earth's Magnetic Field (New York, 1970).

⁵¹P. J. Wasilewski, ‘Lodestone—Nature's Own Permanent Magnet’, NASA‐GSFC, X691‐76‐110 (1976). P. J. Wasilewski, ‘Magnetic and Microstructural Properties of Some Lodestones’, Physics of the Earth and Planetary Interiors, 15 (1977), 349–62.

⁵²M. Blackman and N. D. Lisgarten, ‘On the Intensity of Magnetisation of Lodestones’, Journal of Magnetism and Magnetic Materials, 30 (1982), 269–72. M. Blackman, ‘The Lodestone: A Survey of the History and the Physics’, Contemporary Physics, 24 (1983), 319–31.

⁵³F. Tyler, A Laboratory Manual of Physics (London, 1959).

⁵⁴D. Owen, ‘To Find the Magnetic Neutral Points of a Bar Magnet in the Earth's Magnetic Field’, in The Science Master's Book, ed. by G. H. J. Adlam, S. R. Humby and G. N. Pingriff (London, 1950), Ser. III, Part I (Physics), pp. 167–69.

⁵⁵G. R. Noakes, A Textbook of Electricity and Magnetism (London, 1947). M. Nelkon, Advanced Level Magnetism and Electricity (London, 1954).

⁶⁰Noakes (note 55).

⁶¹Tyler (note 53).

⁶²Tyler (note 53).

⁶³Andrew Putnis, Introduction to Mineral Sciences (Cambridge, 1992).

⁶⁴E. Köster, ‘Particulate Media’, in Magnetic Recording Technology, ed. by C. D. Mee and E. D. Daniel (New York, 1995), ch. 3.

⁶⁵See Brewster (note 13).

⁶⁶F. Pockels, ‘Über das magnetische Verhalten einiger basaltischer Gesteine’, Annalen der Physikalische Chemie, 63 (1837), 195–201. ‘Bestimmung maximaler Entladungs‐strom‐stärken aus ihrer magnetisirenden Wirkung’, Annalen der Physikalischer Chemie, 65 (1898), 458–75, ‘Uber die Blitzentladungen erreichte Stromstarke’, Physikalische Zeitung, 2 (1900), 306–07.

⁶⁷P. Gamba, ‘Magnetization of Bricks by Lightning’, Atti della Royale Accademia dei Lincei (Roma), 8 (1899), 316.

⁶⁸A. Cox, ‘Anomalous Remanent Magnetization of Basalt’, US Geological Survey Bulletin, 1083‐E (1961), 131–60. R. Thompson and F. Oldfield, Environmental Magnetism (London, 1986). R. F. Butler, Paleomagnetism: Magnetic Domains to Geologic Terranes (Oxford, 1992).

⁶⁹Basil Schonland, The Flight of Thunderbolts (Oxford, 1964).

⁷⁰Martin A.Uman, Lightning (New York, 1969).

⁷¹C. M. Foust and H. P. Kuehni, ‘The Surge‐Crest Ammeter’, General Electric Review, 35 (1932) 644–48.

⁷²See Noakes (note 55).

⁷³See Schonland (note 69).

⁷⁴S. R. Logan, Fundamentals of Chemical Kinetics (London: Longman, 1996).

Decay of J _v with time at various temperatures. Magnet Cove lodestone

⁷⁵A. A. Mills, ‘Armed Lodestones’, accepted by the Bulletin of the Scientific Instrument Society (publication expected 2004).