Magnetism

Magnetism is a force of nature that attracts and repels, and only some materials, mostly metals such as iron and nickel, can be magnetized and affected by magnetic force. A magnet carries a magnetic pole at each end, a north pole and a south pole, and like poles repel while opposite poles attract. The magnetic force around a magnet forms a magnetic field composed of lines of force that run from the north pole to the south pole. The Earth itself is a giant magnet, which is why it has north and south magnetic poles.

The earliest recorded reflections on magnetism belong to Greek philosophy. Thales of Miletus, who lived around 624 to 546 BCE, held that the magnet has a soul in it because it moves iron. Empedocles of Akragas proposed that iron is borne toward the lodestone by effluvia emanating from both materials. Plato's Ion records that the lodestone not only attracts iron rings but imparts to them the power of attracting other rings, showing that chains of magnetic induction were known in the 5th century BCE. In the ancient world the basic principles of magnetic induction were known and referenced as early as the 5th century BCE.

The first compasses used for navigation appeared in China by 1088 during the Song dynasty, as described by Shen Kuo, made of iron needles magnetized by striking them with a lodestone. The first recorded use of a compass in Western Europe occurred around 1190 and in the Islamic world in the 13th century.

William Gilbert was born in 1544 and became the most distinguished man of science in England during the reign of Elizabeth I; he was a pioneer researcher into magnetism. After nearly two decades of experimentation, Gilbert completed in 1600 his magnetism opus De Magnete, On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth. In 1635 Henry Gellibrand established that magnetic declination changes over time by comparing new measurements with ones he had obtained 12 years earlier. In 1699 Edmond Halley carried out the first magnetic declination survey and published a chart of his findings two years later.

In 1825 William Sturgeon developed and exhibited the first practical electromagnet, strong enough to support 20 times its own weight.

In 1850 William Thomson, later Lord Kelvin, introduced the concepts of magnetic susceptibility and permeability. In 1852 Edward Sabine discovered a correlation between the sunspot cycle and magnetic activity on Earth. Maxwell's first essay on electricity, On Faraday's Lines of Force, related Faraday's conception of lines of force to the flow of a liquid and derived equations for electric and magnetic phenomena. That paper showed that a few relatively simple equations could express the behaviour of electric and magnetic fields and their interrelation.

Michael Faraday's careful copies of Humphry Davy's 1812 Royal Institution lectures became his passport to a scientific career.

Gilbert was the first to use the terms electric attraction, electric force and magnetic pole, and is often considered the father of electrical studies.

In 1820, Hans Christian Ørsted discovered that a wire carrying an electric current produces a magnetic field. His result was reported to the Academy in Paris on 4 September 1820 by Arago, who repeated the experiment at an Academy meeting a week later.

André-Marie Ampère attempted a combined theory of electricity and magnetism after hearing of Ørsted's experimental results. Before the end of September 1820 he had discovered electrodynamical forces between linear wires, and that November he reported his law of addition of these forces to the Academy in Paris. He formulated a circuit force law and treated magnetism by postulating small closed circuits inside magnetised matter, and he showed that parallel wires carrying currents attract each other when the currents flow in the same direction and repel each other when they flow in opposite directions. In 1822 Ampère formulated his fundamental law for the relationship between a magnetic field and the current that is its source.

Biot and Savart formulated a law for calculating the magnetic field at a given distance from the electric current that is its source.

In 1821 Faraday discovered that electricity can produce rotary motion and built one of the first primitive electric motors.

Gauss built a new magnetic observatory, completed in 1833 and free of all magnetic metals, and his changes to Humboldt's procedures obtained more accurate results with less effort; in 1834 he established the Göttingen Magnetic Union, a network of magnetic observatories mostly located in Europe.

During one of his lectures, Ørsted noticed that the magnetic needle of a compass aligns itself perpendicularly to a current-carrying wire. A decade later, in 1831, Faraday made what is often called his most important discovery, electro-magnetic induction, by showing that a magnet could induce an electrical current in a wire; Faraday further observed that a current in one circuit can excite a current in a second circuit when the current in the first changes, the basis of Faraday's law of induction, enabling the conversion of mechanical energy into electrical energy and the construction of the first dynamo.

Joseph Henry discovered the principle of self-induction and, with his improved electromagnet design, lifted more than a ton of iron. In 1832 Paul Erman invented a simple magnetometer and used it in the first large-scale survey of the Earth's magnetic field. In 1833 Gauss and Weber built a telegraph line in Göttingen almost a mile long that used a galvanometer as a receiver. In 1856 Weber and Kohlrausch measured the ratio of electrostatic to electromagnetic units and found it analogous to the accepted speed of light.

Around 1862 Maxwell calculated that an electromagnetic field propagates at approximately the speed of light and proposed that light is therefore an electromagnetic phenomenon. He concluded that light consists in transverse undulations of the same medium that is the cause of electric and magnetic phenomena. The complete set of Maxwell's equations appeared in 1864 in his paper On a Dynamical Theory of the Electromagnetic Field. In 1865 Maxwell showed that there is really one electromagnetic field and that electricity and magnetism are two aspects of the same phenomenon, with light itself waves of electromagnetic radiation moving through space.

Carl Friedrich Gauss took up the study of terrestrial magnetism after Alexander von Humboldt sought his help in making a grid of magnetic observation points around the Earth. By 1840 Gauss had written three important papers on terrestrial magnetism, treating absolute measure for magnetic force and giving an empirical definition of the phenomenon. He proved that the Earth can have only two magnetic poles, and he determined the intensity of the horizontal component of the magnetic force together with the angle of inclination. In his calculations Gauss used the Laplace equation and specified a location for the magnetic South pole.

Wilhelm Weber arrived in Göttingen in 1831 as professor of physics; Gauss had known him since 1828 and supported his appointment. Earth's magnetic field strength was measured by Gauss in 1832 and has been repeatedly measured since, showing a relative decay of about 10% over the last 150 years.

The SI unit of flux density is the tesla, and the gauss may also be used, with 1 gauss equal to 100 microtesla. The magnetic fields generated by currents and calculated from Ampere's Law or the Biot-Savart Law are characterized by the magnetic field B measured in tesla. B is sometimes called the magnetic flux density or the magnetic induction, and the quantity M in these relationships is called the magnetization of the material. B and H are related by B = mu H, where mu is the product of the relative permeability and the magnetic permeability of space.

Ferromagnetic materials respond strongly to magnets and can themselves be magnetized, whether induced to become magnetic or made into permanent magnets. Ferromagnetism is associated with iron, cobalt, nickel and some alloys or compounds containing them, and also occurs in gadolinium and a few other rare-earth elements. Such materials are magnetized easily, and in strong magnetic fields their magnetization approaches a definite limit called saturation. When the magnetizing field is applied and then removed, the magnetization does not return to its original value, a phenomenon called hysteresis. The magnetism itself is caused by the alignment patterns of the constituent atoms, which act as elementary electromagnets.

The magnetic fields of permanent magnets arise from nuclear spins, electron spins and the orbits of the electrons themselves. In many materials these fields point in random directions and cancel each other out, so there is no permanent magnetism. In ferromagnets such as iron, cobalt and nickel, by contrast, the spins and orbits of the electrons line up so that the material becomes magnetic. Within a ferromagnetic material the regions called domains act like small bar magnets, and in response to an external field the domains may grow to millimeter size. Alloys of the rare-earth elements are used as strong permanent magnets, a popular one being neodymium.

For ferromagnetic materials there is a well-defined temperature, the Curie temperature, above which thermal motion disrupts the domains and the material cannot be magnetized. The Curie temperature of iron is 1043 K (770 degrees C), well above room temperature.

Electromagnetism is the use of electric current to make magnets; these temporarily induced magnets, called electromagnets, are used from scrap-yard cranes to medical imaging machines.

The LHC electromagnets use a current of 11,080 amperes, and a superconducting coil lets the high currents flow without energy loss to electrical resistance. The LHC has 1232 main dipoles, each 15 metres long and weighing 35 tonnes. Without superconducting magnets the 27 km accelerator would have to be 120 kilometres long for the same energy.

Earth is surrounded by a giant bubble of magnetism called the magnetosphere, which deflects most solar material sweeping toward the planet at 1 million miles per hour or more. A magnetosphere is the region around a planet dominated by the planet's magnetic field, and Earth has the strongest magnetosphere of all the rocky planets. It is a vast, comet-shaped bubble that shields the planet from solar and cosmic particle radiation and from erosion of the atmosphere by the solar wind, the constant flow of charged particles streaming off the Sun. Without the magnetosphere, solar particles could strip the Earth of protective layers that shield the planet from the Sun's ultraviolet radiation. Mars lost its magnetosphere about 4.2 billion years ago, and the solar wind is thought to have stripped away most of its atmosphere. Earth's magnetosphere is generated by the convective motion of charged, molten iron far below the surface in the planet's outer core.

The solar wind compresses the dayside of the magnetosphere to about six to ten Earth radii, while the nightside magnetotail stretches hundreds of Earth radii, far past the Moon's orbit. Earth's magnetic intensity tends to decrease from the poles toward the equator, with a minimum in the South Atlantic Anomaly and maxima over northern Canada, Siberia and the coast of Antarctica south of Australia. The past magnetic field is recorded mostly by strongly magnetic minerals, particularly iron oxides such as magnetite, that can carry a permanent magnetic moment. The most recent full reversal of Earth's magnetic field, the Brunhes-Matuyama reversal, occurred about 780,000 years ago.

Auroras light up when electrons from the magnetosphere collide with oxygen and nitrogen in the upper atmosphere, exciting the atoms and molecules so that they release energy as light. The aurora typically forms 80 to 500 km above Earth's surface, guided by the magnetic field into two ovals approximately centered at the magnetic poles. During major geomagnetic storms the auroral ovals expand away from the poles and the aurora can be seen over most of the United States.

The USGS Geomagnetism Program monitors the Earth's magnetic field with ground-based observatories and provides continuous records of magnetic field variations. Magnetic storms can generate electric fields in the Earth that can interfere with electric power transmission grids. USGS research into the nature of geomagnetic variations serves scientific understanding and hazard mitigation.

Paul Lauterbur discovered how to create a two-dimensional picture by introducing gradients in the magnetic field, making it possible to image structures that other methods could not visualize. In 1973 Lauterbur described how adding gradient magnets to the main magnet allowed a cross section of tubes of ordinary water surrounded by heavy water to be visualized. Peter Mansfield used gradients to sharpen differences in the resonance and showed how extremely rapid imaging could be achieved by fast gradient variations, a technique that became useful in clinical practice a decade later.

In 1988 Albert Fert and Peter Grünberg independently discovered giant magnetoresistance, in which very weak magnetic changes give rise to major differences in electrical resistance. The first read-out head based on the GMR effect was launched in 1997 and soon became the standard technology for reading data on hard disks. A hard disk stores information as microscopically small magnetized areas, and the information is retrieved by a read-out head that scans the disk and registers the magnetic changes.