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The basic dipole shape of Earth's magnetic field (bottom) and the teardrop shape of the magnetosphere created by the solar wind (top).
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The work of Oersted and Ampére drew to the study of magnetism one of the sharpest minds of Europe, that of Carl Friedrich Gauss (1777-1855). In their magnetism lab, Gauss and Weber constructed the first magnetic telegraph, using it until lightning knocked down the wire.
But perhaps the most lasting contribution was the use of a precise mathematical method to represent the global magnetic field of the Earth and to combine observations at many locations.
It took the ingenuity of Gauss and Weber to devise the tools for conducting a global survey of the magnetic field of the Earth.
Alexander Von Humboldt then stepped in and asked for the support of the British Royal Society [Cawood, 1979].
Humboldt had earlier noted large scale magnetic disturbances (possibly already observed by George Graham) and gave them the name they still bear--"magnetic storms." The world-wide net of magnetic observatories confirmed that such "storms" were world-wide phenomena. The British participation was greatly helped by three dedicated explorers--Rear Admiral John Ross (1777-1856), his nephew James Clark Ross (1800-62), and an artillery officer named Edward Sabine (1788-1883). The Rosses were denied any further support by the British admiralty, but in 1829 they found an independent backer--Felix Booth, owner of Booth's distillery (Booth's gin is still being made). When Gauss and Weber began organizing a global magnetic survey, the Rosses and Sabine (later Sir Edward Sabine) became their leading lobbyists and supporters, and soon were the de-facto leaders of a "magnetic crusade." By 1841 much of the world-wide network was actually in operation.
The new tool of spherical harmonic analysis provided the first quantitative description of the Earth's magnetic field, both its direction and strength. The analysis has shown, as expected, that the 2-pole ("dipole") part of the field greatly exceeded all others. Even though the measured strength of the magnetic field is decreasing, it would be more accurate to say that it is becoming more complex, rather than weaker.
If all the Earth's magnetism comes from its liquid core (whose radius, about half the Earth's, is known from seismology), the non-dipole components of the field are much stronger there, because they diminish much faster with distance R, like higher powers of R. So--if we wait 1500-2000 years, is it possible that the dipole part will become very small, while the overall structure of the field becomes complex, with perhaps more than one pair of magnetic poles? Faraday (1791-1867) was the son of an English blacksmith, apprenticed by his parents to a bookbinder. Starting as little more than Davy's servant, Faraday rose to make a name for himself in the study of chemistry and magnetism, discovering (for instance) the basic laws of electrolysis. Those "lines of force" (today we call them field lines) spread out from the south pole of the terrella, arch around its middle and converge again near the north pole (Figure 6). Magnetic field lines in space thus assume yet another role--defining the direction in which ions and electrons travel most easily, as do electric currents and heat carried by them. This led to machines--once called "dynamos," though today "generators" is the more common term--in which conductors are whirled around and around, through the fields of magnets, producing electric currents.
Using these principles, Faraday devised the simplest of all dynamos, the so-called "Faraday disk." Imagine the space between the poles of a magnet (Figure 7), with straight magnetic field lines connecting one pole to the other.
Because of the symmetric arrangement of the disk and magnetic field, the current also tends to flow symmetrically (a symmetry upset by the placement of the sliding contacts and external circuit)--from the axle to the rim, or vice versa, depending on the direction of rotation and on the magnetic polarities of the two magnet faces.
While Faraday's disk dynamo illustrates the principle of dynamo action, it is not a practical generator of electricity, unless one requires huge currents driven by tiny voltages.
The basic "dynamo conditions"--a magnetic field and a closed conducting path, part of which moves across the magnetic field while another part stays fixed--also occur under other conditions. Faraday had also experimented with electric currents flowing through glass containers from which most air had been removed. When checked, Shutterstock's safe search screens restricted content and excludes it from your search results. The Website was developed in part with the support of UCAR and NCAR, where it resided from 2000 - 2010. It leads to the title in accordance with the Wikipedia naming conventions for capitalisation, or it leads to a title that is associated in some way with the conventional capitalisation of this redirect title.
Gauss was a professor of mathematics at the German university of Göttingen and rarely traveled away from home, but in 1828 he attended a conference in Berlin, and stayed there as house guest of Alexander von Humboldt [Dunnington, 1955].
During this visit he showed Gauss his collection of magnetic instruments and encouraged him to apply his talents to magnetism.


They devised a new suspension for observatory magnets--big magnets, slow to respond, later replaced by more nimble instruments. That was spherical harmonic analysis, previously used for analyzing gravitational fields in celestial mechanics. The force is three-dimensional, but because of its simple structure, a single mathematical expression, known as the harmonic potential, can describe it all. It took crafty politics and the collaboration of many individuals to establish the world-wide network of observatories which actually conducted such a survey.
The participation of the British Empire was essential, because it alone had outposts around the globe, and its involvement was helped by the curious fact that due to a kinship of ruling families, Göttingen too was under British rule until 1837. The change in the magnetic attraction of the Earth was quite small, rarely exceeding 1% of the total value--but its pattern across the world was remarkably similar, suggesting a large-scale phenomenon.
For many years the British navy had searched for a "Northwest Passage" to the Pacific Ocean, an east-west sea lane among the icy islands of northern Canada, but all its efforts were blocked by polar ice.
They bought a 150-ton paddle steamer, the "Victory," and with favorable weather sailed it to the eastern shore of a long peninsula jutting northwards from North America.
In the following summer it managed to move only a few miles, and after the third winter the explorers abandoned it, moved on by sledge and were fortunate to be picked up by a whaling ship.
Since then, magnetic surveys have been carried out repeatedly [Good 1985, 1988, 1994] across land, aboard special non-magnetic ships and most recently by satellites whose orbits sweep above the entire Earth every day. In other words, the 2-pole terrella has always been a good approximation to the actual field, although--if one wants to be a stickler for accuracy--the magnetic center, the location of the dipole which fits observations best, is offset a few hundred kilometers from the Earth's center. As the dipole part of the field is weakening, the more complex parts (4-pole etc.) are gaining strength. Theory suggests that the total magnetic energy cannot easily change as quickly as the observed changes in the field.
And could it be that 2000 years after that, the dipole part will have continued to grow in the opposite direction, until it again dominated the global structure of the magnetic field, the way it does now--but with opposite polarity?
Just measuring it accurately was challenging enough--and in the process, proving that it originated inside the Earth, not outside it.
Faraday took advantage of his job to read books brought to the bindery, among them a volume of the Encyclopaedia Britannica with an article about electricity. What he lacked in formal schooling and mathematical analysis he made up in intuition and insight. They started out as no more than a visualization aid, like lines of latitude and longitude, which have no real existence but merely help visualize positions on the globe.
Much of space above the Earth's atmosphere (and indeed, throughout the universe) contains plasmas--gases hot enough to contain free electrons and positive "ions," atoms stripped of some electrons.
To carry such currents, very massive conductors would be needed, and the copper wiring used in conventional machinery would be completely inadequate. In those rarefied gases he observed glows somewhat similar to the polar aurora or "northern lights." He therefore speculated that the Gulf Stream, a broad flow in the Atlantic Ocean along the US Eastern seaboard, may similarly generate a voltage, and if its circuit was closed through the high atmosphere, it might generate there the light of the polar aurora.
The field is strongest close to the magnet and becomes weaker as you get farther from the magnet. Humboldt (1769-1859) was a naturalist who had earlier explored the jungles of South America and later the author of "Kosmos," a 5-volume encyclopaedic compendium of the natural sciences, which greatly helped spread public interest in science. That Gauss did, together with his young assistant Wilhelm Weber (1804-91), contributing greatly to the understanding of the Earth's magnetic field [Garland, 1979]. In 1832 Gauss and Weber also devised a clever method of using an auxiliary magnet to measure not only the direction of the Earth's magnetic force, but also its intensity. According to Sidney Chapman, it was introduced to geomagnetism by the French mathematician Simeon Denis Poisson (1781-1840) [Chapman, 1964, p. This leaves 8 obtuse corners, and by cutting off these in a suitable way, one is left with 16 corners, each making an angle only slightly smaller than 180°. Humboldt also got the Russian Czar to support a chain of observatories across Siberia and even one in Sitka, Alaska, which operated from 1842 to 1864. The Rosses, with Sabine conducting magnetic observations, tried to get through in 1818 and failed.
The surprising feature is that since the time of Gauss, the strength of this "dipole" component has steadily declined by about 5% per century, the decline accelerating somewhat after 1970.
In other words--as the strength of the field associated with the largest spatial scale (dipole) decreases, that associated with smaller scales increases. Calculations indeed confirm that the sum-total of that energy has changed very little--just that the complicated parts have grown at the expense of the dipole, and since those decrease faster with distance, the field at the surface has weakened too. The geomagnetic record suggests this is not likely to happen, and that the trend will probably change. Science aroused his interest and as a young man he attended free scientific lecturers by Humphry Davy, Britain's leading chemist [Knight, 2000], taking careful notes of what he heard.
He had a remarkable talent of expressing complicated scientific ideas in simple words, and was noted for his lucid lectures and writings.


It also tells about the strength of that force: where the lines crowd together, the force is strong, where they are spaced widely apart it is weak. Faraday, however felt [Baggott, 1991] that space in which magnetic forces could exist was itself modified, and such space later became known as a "magnetic field." Faraday's younger contemporary, James Clerk Maxwell--a good friend and outstanding physicist, but unlike Faraday, also a skilled mathematician--extended that notion and showed that an "electromagnetic field" could support a wave motion which spread at the velocity of light and had the observed properties of light. The Sun consists of plasma, space around Earth is filled with plasma, and in such plasmas, only very occasionally do free electrons re-combine with positive ions, usually not for long. In a rarefied plasma, magnetic field lines are like the grain in a piece of wood, which outlines the "easy" direction in which the wood readily splits. However, reversing the process--winding a conducting wire around a magnet--did not produce any electric current.
The shaft and disk form the moving part of the circuit: the magnet and the wires that complete the circuit are stationary, and the wires are electrically linked to the disk by two sliding contacts, one on the axle, one near the periphery. But in a large volume filled with conducting material--for instance, the liquid core of the Earth, believed to be largely molten iron, or on the Sun--such flows are quite possible, and one notes that instead of a solid wheel, a rotating fluid eddy would serve just as well.
His speculation was completely wrong, since the actual atmosphere is too poor a conductor of electricity to complete the circuit between the ocean to the upper atmosphere; but otherwise the mechanism was in principle possible. Today undergraduate physics students use the Gauss method for measuring the strength of the Earth's field as a standard lab experiment, not realizing its historical significance. As long as all magnetic forces could be ascribed to a collection of magnetic poles, combinations of such harmonic potentials could describe them as accurately as was desired (this holds in any simply-connected current-free region). Incidentally, to fully describe the field two distinct expansions are needed--one (the main one, in inverse powers of R) for fields originating inside the Earth and another (with positive powers) for those originating on the outside. However, while these observatories covered Europe quite well, most of the world was left uncovered.
An 1819 expedition, under Parry, reached a bit further before ice stopped it--but Sabine had sailed with it, and his observations convinced him he had actually passed north of the magnetic pole. In the summer of 1831, aided by local Eskimos (as desolate as the area was, it was inhabited) he set out towards that spot.
Many scholars--most recently, Jeremy Bloxham [1992; Bloxham and Gubbins, 1989] and Andrew Jackson--also collected earlier compass observations and tried to derive from them models of the field before the era of Gauss, as well as of its slow "secular" variation. However, it also tells us (as discussed in a later section) that the above scenario cannot be ruled out, and that such reversals have apparently happened many times during the Earth's magnetic past.
When Davy dismissed his assistant for brawling and advertised for a replacement, Faraday applied, sending his notes as proof of his interest--and ended up with the job. Light, radio waves, x-rays, microwaves--all belong to the same family, making the concept of electromagnetic fields one of the foundations of modern science and engineering.
This is quite unlike the familiar environment near the surface of the Earth, where free ions and electrons are rare and neutral atoms and molecules are the rule. All these concepts sprang from the visual and intuitive imagination of Faraday, from a simple idea which grew into an extremely useful and versatile one.
Instead, Faraday discovered that electric currents were generated in a closed conducting circuit only if that circuit sensed a changing magnetic field. That is usually done by metal rings attached to the rotating shafts (but electrically insulated from them), connected to the rotating wires while at the same time touching sliding contacts mounted on the non-rotating part. That idea ultimately became the foundation of modern dynamo theory, briefly described in section 14, which seeks to explain how the magnetic fields of the Sun and the planets--including Earth--are generated. The magnetic field was provided by the Earth, the moving conductor was the flowing water, and the circuit was closed by a non-moving wire strung across Waterloo Bridge, dipping into the water at both banks (Figure 8).
Actually it made possible for the first time a global network of magnetic observatories, because now every instrument could be calibrated locally, independently of any others. The method used by Gauss also showed that at least 99% of the field originated inside the Earth.
Today's models of the Earth's magnetic field, derived largely from satellite data, carry those expansions to hundreds of terms [e.g.
In most of space, plasmas are so rarefied that collisions are rare, and ions and electrons spiral around magnetic field lines, guided by them the way a bead is guided by a wire on which it is strung [e.g.
The change could come from variations in the strength of the magnetic source, or it could arise from relative motion between the source and the conductor.
The idea was sound, but the voltage generated was too small to be detected, in the presence of voltages due to chemical interactions of metals. He was lucky to find the magnetic pole so far south: because of the gradual change in the Earth's magnetic field, the pole changes its position, and in 1831 it was near the end of a long southward excursion [Dawson and Newitt, 1982, Figure 1].



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