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4. Halley
The only solution left, proposed ingeniously by Edmond Halley [Bullard, 1956; Evans, 1988; Chapman, 1941, 1943; Bauer 1896 (1990), 1913 (1990), Clark, 2000] was that the interior of the Earth consisted of concentric spherical shells, each magnetized differently, and that some rotated differently from others. Halley was so proud of his theory that when at age 80 he had his portrait painted, he appeared in it next to a diagram of his spherical shells. (The 1943a article by S. Chapman includes poems by Halley honoring Newton and the inventor of the compass, both on p. 231. The 1943b article is the same as 1943a, but without the second poem.)He thought he could locate 4 distinct magnetic poles, belonging to two different layers. The modern theory of the Earth's field actually suggests that the solid inner core of the Earth might rotate at a slightly different rate [Buffett and Glatzmaier, 2000] but this is a completely different process and permanent magnetism is not involved. The name of Halley (1656-1742) is nowadays most commonly associated with that of a periodic comet whose return he predicted. However, he was also one of the main pillars of the British scientific community around 1700--the second Astronomer Royal, an active member of the "Royal Society," a man without whose help and encouragement, Newton's "Principia" might not have been published. He was as well the leader of the earliest global magnetic survey. Feeling the need for more accurate magnetic charts of the Atlantic Ocean, their Lordships of the British Admiralty lent Halley a small sailing ship, the 52-foot "Paramore" (or "Paramour"), and instructed him to carry out a magnetic survey of the Atlantic Ocean and its bordering lands [Thrower, 1981]. Perhaps considering this task an insufficient justification of the expedition, they also gave him a second one--"to stand soe farr into the South, till you discover the Coast of the Terra Incognita, supposed to lye between Magelan's Streights and the Cape of Good Hope" [Bullard, 1956]. The "Paramore" set out in October 1698, but was troubled by both leaks and by a personal conflict between Halley and the navy officer in charge of the ship, Lieutenant Harrison. Harrison countermanded Halley's orders and spoke insultingly of him before the crew, until Halley had the man arrested and turned the ship back to England, where a court of inquiry upheld him and gave him sole command of the ship. It turned out that Harrison had published a small book "Idea Longitudinis", proposing a way of helping navigation at sea. The book was presented to the Admiralty and the Royal Society but received an unfavorable report, and Halley was among its reviewers. The "Paramore" set out again in September 1699 and soon encountered a severe storm, in which the cabin boy was swept overboard, never to be found. By February 1st, 1700, the ship reached southern latitude 520 40'. Some highly unusual wildlife was seen and later, when Halley's magnetic map appeared, it showed in that region two rather unusual creatures, with the legend "The sea in these parts abounds with two sorts of Animalls of a Middle Species between a Bird and a Fish, having necks like Swans and Swimming with their whole Bodyes always under water only putting up their long Necks for Air."(click here for that part of the map). But there was no "Terra Incognita." Three strange islands appeared on the horizon, and Halley sketched them in his logbook, but next day he realized they were nothing but enormous floating icebergs. Then a dense fog descended, and dodging the ice became a dangerous game:
After this encounter the ship continued to Tristan da Cunha, St. Helena, Brazil, Barbados, Bermuda, Newfoundland and finally, at the end of August, back to England. From Halley's survey the first magnetic map of the Atlantic was compiled with contour lines connecting points of equal declination, the first known use of contour lines: for the next century, such lines were known as "Halleyan lines." By incorporating observations made by others Halley in 1702 extended his chart, and it was reprinted and revised many times. 5. Coulomb It is only natural that when scientists develop any method of measurements, they push it to its limits. Careful observation of the position of the tip of a long compass needle by George Graham (1675-1751), a London clockmaker and instrument builder, showed in 1722 that the direction of the magnetic force in London underwent a 24-hour cycle, a diurnal variation. In 1741 he and Anders Celsius in Uppsala, Sweden also observed simultaneous perturbations due to the polar aurora [Chapman and Bartels, 1940, sect. 26.9; Beckman, 2000]. The 24-hour "diurnal" magnetic variation was barely observable, and in 1773 the Paris Academy of Sciences offered a prize for finding "the best manner of constructing magnetic needles, of suspending them, of making sure that they are in the true magnetic meridian, and finally, of accounting for their regular diurnal variations." The offer was renewed in 1775, and was claimed in 1777 by a French military engineer, Charles Augustin Coulomb [Gillmor, 1971;p. 59 in Shamos, 1959].
Coulomb's instrument, known as the "torsion balance," served as model for magnetic instruments over nearly two centuries. A magnetic needle was suspended from a long thin twistable wire--long enough and thin enough that even a small torque produced a notable twist. That twist, furthermore, could be accurately measured by attaching a small mirror just above the needle, and observing the shifts of a spot of light reflected from it (Figure 5). Indeed, the instrument was so sensitive, that Coulomb not only had to place it inside a glass enclosure to shield it from stray air currents, but also found that static electric charges sometimes interfered with his magnetic observations. As it turned out, this sensitive instrument allowed much more to be measured than the magnetic diurnal variation. The twisting moment (torque) of the suspension wire is proportional to the angle of twist, and can be measured, for instance, by suspending from its end a non-magnetic bar and timing its back-and-forth oscillation around the equilibrium position. If next a long magnetic needle is suspended, and one end is placed near a magnetic pole of the same kind, the end of the needle will be repelled to a new position. However, the knob from which the wire is suspended can now be twisted to restore the needle to its previous direction. Now the extra torque (deduced from the extra twist of the knob) can tell the strength of the repulsion. By methods like this Coulomb showed that the magnetic repulsion between magnetic poles--and also their attraction--varied inversely with the square of the distance. How much like gravity, indeed! Replacing the suspended magnet with a small straw covered with wax, carrying a pith ball at its end (counterbalanced by some object at the other end of the straw) Coulomb repeated the experiment with electrical forces. He charged a similar pith ball on an insulating stand inside the enclosure, made the two balls share their charge by touching each other, and again measured the force against the twist of a wire. It turned out--again, so much like gravity!--that the strength of electric forces also decreased like the inverse square of the distance. Finally, in 1796, Henry Cavendish used a similar torsion balance (an experiment possibly proposed by the Rev. John Michell) to measure the gravitational attraction between massive spheres, a much weaker force and much more difficult to measure. This demonstrated that Newton's law of planetary attraction--inversely proportional to the square of the distance--held in the laboratory, as well. For a while all nature seemed in harmony--three fundamental forces, all obeying the inverse-squares law, of which only gravity differed, by always attracting and never repelling. 6. Oersted and Ampére This nice symmetry was upset in 1820 by an unexpected connection between magnetism and electricity. This is not the place to tell how Luigi Galvani and Allessandro Volta introduced an entirely new way of generating electricity--not as a static charge, generated by rubbing, but as a continuous flow of electric charge, an electrical current generated by a chemical process [see e.g. Segré, 1984, Vershuur; 1993]. Volta's "voltaic cell" and "voltaic pile" were the ancestors of today's "dry cells" and car batteries. Hans Christian Oersted (1777-1851) was born in Rudkoebing in southern Denmark, a town so small it lacked a school [Dibner, 1962]. Nevertheless, Hans and his younger brother Anders found willing teachers among local citizens and acquired enough education to be accepted in 1793 by the University of Copenhagen. Anders studied law and later became quite famous in his field; Hans took up medicine and science, but his interests must have been much broader, for in 1797 he won a gold medal for an essay on "Limits of Poetry and Prose." After some travels he joined the university in 1806 and became a regular professor in 1817. Two years later he struck a friendship with a poor boy of 14 who had just arrived in the city--this was Hans Christian Andersen, later a writer of folk tales. Their friendship continued for the rest of Oersted's life.
Oersted's interest centered on electricity and chemistry and on what then was still a novelty, the electric battery. In the spring of 1820 he arranged at his home a lecture on electricity and magnetism, before a group of friends and/or students. Accounts differ as to what exactly happened, but they all agree that Oersted's equipment included a magnetic compass, as well as an electric battery and a thin metal wire, and that one demonstration involved heating the wire by an electric current from the battery. Most probably it was only by accident that the wire passed over the compass or near it, although Oersted later hinted that he had put it there deliberately, acting on a long-standing suspicion that a link existed between electricity and magnetism. Accidentally or not, whenever the wire was connected to the battery and a current flowed, the magnetic needle moved, and whenever the current ceased, it returned to its old position. No one else noticed and Oersted said nothing, but in the months that followed he conducted many experiments, unsuccessfully trying to understand what had happened. Oersted would have been less puzzled if the wire had attracted the needle, the way a magnet would do, but no, the needle tried to turn at right angles to the electric current! And when the connections to the battery were exchanged so that the current flowed in the opposite direction, the compass needle followed suit and also reversed. There was surely a message here, but try as he would, Oersted could not decode it. Still, it was no mean feat: here was the first clear-cut evidence connecting electricity and magnetism, and by July 21 Oersted announced it to the world in a 4-page report, written in Latin just like Gilbert's book [p. 121-7, Shamos, 1959]. Perhaps Oersted intended to provide a text from which translations could be made, for Latin certainly was no longer a universal language of scholars, as it was in Gilbert's time. The experiment was easy to repeat, and the best scientific minds of Europe turned at once to exploit and explore this new "electro-magnetism". A report of Oersted's discovery (and of its confirmation by de la Rive in Geneva) reached Paris on Monday, September 11, 1820, and was discussed at a meeting where among others André-Marie Ampére (1777-1836) was present. In a tremendous feat of imagination and insight, Ampére solved much of the riddle within one week, and soon afterwards, in a series of elegant experiments, confirmed a completely new view of magnetism [Williams, 1989; Segré, 1984] The fundamental interaction in magnetism, Ampére said in essence, had nothing to do with the attraction or repulsion of magnetic poles. Instead, the basic ingredient of magnetism was the electric current. Magnetism, Ampére implied, would have existed even if there were no permanent magnets, because its basic feature was the force between electric currents. When two currents flowed in the same direction along parallel wires, the wires attracted, and when the currents flowed in opposing directions, they repelled each other. Ampére then showed that an electric current circulating around a wire loop acted like a short magnet. A current flowing in a coil of 1000 turns produced magnetism 1000 times stronger, because the magnetic forces of all its loops added up, and two coils with the same axis attracted or repelled, depending on whether their flows were parallel or opposed. The magnetism of iron, Ampére implied, may have come about because iron atoms contained small circulating currents, which could be lined up so that they reinforced each other [see also Livingston, 1996]. Inserting an iron core in a current-carrying coil increased its magnetic pull, by lining up the magnetic domains of the iron, but iron was in no way essential. 7. The Lodestone The study of magnetism began with the lodestone, and at this point one can finally look back and ask--what created those strange minerals in the first place? Lodestones are an iron-rich ore, magnetite. Some such ores can support magnetism, but only in the presence of an active source--a permanent magnet or a coil with current circulating in it. Their magnetism, like that of soft iron, is "soft" and disappears again when the source is removed. Peter Wasilewski, who currently studies these matters at NASA, found that a certain transformation to a fine-grained structure was needed, occurring only when the ore underwent certain geological changes, under high temperature and pressure [Wasilewski, 1977; Wasilewski and Kletetschka, 1999]. Even magnetically "hard" substances, however, become magnetic only if exposed to a sufficiently strong magnetic field. The exposure may last no more than a brief instant--like the instant when a magnetic tape or disk rapidly passes in front of the recording head--but the field strength must exceed a certain minimum. Exposing a "latent lodestone" to the weak field of the Earth, even for millions of years, will not accomplish the same result. Dr. Wasilewski believes (as had been proposed before) that the magnetization was produced by the strong electric current which briefly flows when lightning strikes an outcrop of suitable rock. He exposed latent lodestones to natural lightning at the Langmuir Laboratory for Atmospheric Research in New Mexico, built atop a peak where lightning often strikes, and found they became magnetized. Interestingly, William Gilbert had a clue to this process but (not surprisingly) missed its significance. In"De Magnete" (book III, chapt. 12) he cited the following passage from a book published in Italy:
In hindsight we would guess that the church tower had been hit by lightning, which had magnetized the iron (and possibly also bent it). Gilbert however attributed it to long-term exposure to the Earth's magnetism, "by the turning of its extremities towards the poles for so long a time." |
