A group of phenomena associated with magnetic fields. Whenever an electric current flows a magnetic field is produced; as the orbital motion and the spin of atomic electrons are equivalent to tiny current loops, individual atoms create magnetic fields around them, when their orbital electrons have a net magnetic moment as a result of their angular momentum. The magnetic moment of an atom is the vector sum of the magnetic moments of the orbital motions and the spins of all the electrons in the atom. The macroscopic magnetic properties of a substance arise from the magnetic moments of its component atoms and molecules. Different materials have different characteristics in an applied magnetic field; there are four main types of magnetic behaviour:
(a) In diamagnetism the magnetization is in the opposite direction to that of the applied field, i.e. the susceptibility is negative. Although all substances are diamagnetic, it is a weak form of magnetism and may be masked by other, stronger, forms. It results from changes induced in the orbits of electrons in the atoms of a substance by the applied field, the direction of the change opposing the applied flux. There is thus a weak negative susceptibility (of the order of –10 −8 m 3 mol −1) and a relative permeability of slightly less than one.
(b) In paramagnetism the atoms or molecules of the substance have net orbital or spin magnetic moments that are capable of being aligned in the direction of the applied field. They therefore have a positive (but small) susceptibility and a relative permeability slightly in excess of one. Paramagnetism occurs in all atoms and molecules with unpaired electrons; e.g. free atoms, free radicals, and compounds of transition metals containing ions with unfilled electron shells. It also occurs in metals as a result of the magnetic moments associated with the spins of the conducting electrons.
(c) In ferromagnetic substances, within a certain temperature range, there are net atomic magnetic moments, which line up in such a way that magnetization persists after the removal of the applied field. Below a certain temperature, called the Curie point (or Curie temperature) an increasing magnetic field applied to a ferromagnetic substance will cause increasing magnetization to a high value, called the saturation magnetization. This is because a ferromagnetic substance consists of small (1–0.1 mm across) magnetized regions called domains. The total magnetic moment of a sample of the substance is the vector sum of the magnetic moments of the component domains. Within each domain the individual atomic magnetic moments are spontaneously aligned by exchange forces, related to whether or not the atomic electron spins are parallel or antiparallel. However, in an unmagnetized piece of ferromagnetic material the magnetic moments of the domains themselves are not aligned; when an external field is applied those domains that are aligned with the field increase in size at the expense of the others. In a very strong field all the domains are lined up in the direction of the field and provide the high observed magnetization. Iron, nickel, cobalt, and their alloys are ferromagnetic. Above the Curie point, ferromagnetic materials become paramagnetic.
(d) Some metals, alloys, and transition-element salts exhibit another form of magnetism called antiferromagnetism. This occurs below a certain temperature, called the Néel temperature, when an ordered array of atomic magnetic moments spontaneously forms in which alternate moments have opposite directions. There is therefore no net resultant magnetic moment in the absence of an applied field. In manganese fluoride, for example, this antiparallel arrangement occurs below a Néel temperature of 72 K. Below this temperature the spontaneous ordering opposes the normal tendency of the magnetic moments to align with the applied field. Above the Néel temperature the substance is paramagnetic.
A special form of antiferromagnetism is ferrimagnetism, a type of magnetism exhibited by the ferrites. In these materials the magnetic moments of adjacent ions are antiparallel and of unequal strength, or the number of magnetic moments in one direction is greater than those in the opposite direction. By suitable choice of rare-earth ions in the ferrite lattices it is possible to design ferrimagnetic substances with specific magnetizations for use in electronic components.
Most people are familiar with magnets primarily as toys, or as simple objects for keeping papers attached to a metal surface such as a refrigerator door. In fact the areas of application for magnetism are much broader, and range from security to health care to communication, transportation, and numerous other aspects of daily life. Closely related to electricity, magnetism results from specific forms of alignment on the part of electron charges in certain varieties of metal and alloy.
How It Works
Magnetism, along with electricity, belongs to a larger phenomenon, electromagnetism, or the force generated by the passage of an electric current through matter. When two electric charges are at rest, it appears to the observer that the force between them is merely electric. If the charges are in motion, however—and in this instance motion or rest is understood in relation to the observer—then it appears as though a different sort of force, known as magnetism, exists between them.
In fact, the difference between magnetism and electricity is purely artificial. Both are manifestations of a single fundamental force, with "magnetism" simply being an abstraction that people use for the changes in electromagnetic force created by the motion of electric charges. It is a distinction on the order of that between water and wetness; nonetheless, it is often useful and convenient to discuss the two phenomena as though they were separate.
At the atomic level, magnetism is the result of motion by electrons, negatively charged subatomic particles, relative to one another. Rather like planets in a solar system, electrons both revolve around the atom's nucleus and rotate on their own axes. (In fact the exact nature of their movement is much more complex, but this analogy is accurate enough for the present purposes.) Both types of movement create a magnetic force field between electrons, and as a result the electron takes on the properties of a tiny bar magnet with a north pole and south pole. Surrounding this infinitesimal magnet are lines of magnetic force, which begin at the north pole and curve outward, describing an ellipse as they return to the south pole.
In most atomic elements, the structure of the atom is such that the electrons align in a random manner, rather like a bunch of basketballs bumping into one another as they float in a swimming pool. Because of this random alignment, the small magnetic fields cancel out one another. Two such self-canceling particles are referred to as paired electrons, and again, the analogy to bar magnets is an appropriate one: if one were to shake a bag containing an even number of bar magnets, they would all wind up in pairs, joined at opposing (north-south) poles.
There are, however, a very few elements in which the fields line up to create what is known as a net magnetic dipole, or a unity of direction—rather like a bunch of basketballs simultaneously thrown from in the same direction at the same time. These elements, among them iron, cobalt, and nickel, as well as various alloys or mixtures, are commonly known as magnetic metals or natural magnets.
It should be noted that in magnetic metals, magnetism comes purely from the alignment of forces exerted by electrons as they spin on their axes, whereas the forces created by their orbital motion around the nucleus tend to cancel one another out. But in magnetic rare earth elements such as cerium, magnetism comes both from rotational and orbital forms of motion. Of principal concern in this discussion, however, is the behavior of natural magnets on the one hand, and of nonmagnetic materials on the other.
There are five different types of magnetism—diamagnetism, paramagnetism, ferromagnetism, ferrimagnetism, and antiferromagnetism. Actually, these terms describe five different types of response to the process of magnetization, which occurs when an object is placed in a magnetic field.
A magnetic field is an area in which a magnetic force acts on a moving charged particle such that the particle would experience no force if it moved in the direction of the magnetic field—in other words, it would be "drawn," as a ten-penny nail is drawn to a common bar or horseshoe (U-shaped) magnet. An electric current is an example of a moving charge, and indeed one of the best ways to create a magnetic field is with a current. Often this is done by means of a solenoid, a current-carrying wire coil through which the material to be magnetized is passed, much as one would pass an object through the interior of a spring.
All materials respond to a magnetic field; they just respond in different ways. Some non-magnetic substances, when placed within a magnetic field, slightly reduce the strength of that field, a phenomenon known as diamagnetism. On the other hand, there are nonmagnetic substances possessing an uneven number of electrons per atom, and in these instances a slight increase in magnetism, known as paramagnetism, occurs. Paramagnetism always has to overcome diamagnetism, however, and hence the gain in magnetic force is very small. In addition, the thermal motion of atoms and molecules prevents the objects' magnetic fields from coming into alignment with the external field. Lower temperatures, on the other hand, enhance the process of paramagnetism.
In contrast to diamagnetism and paramagnetism, ferro-, ferri-, and antiferromagnetism all describe the behavior of natural magnets when exposed to a magnetic field. The name ferromagnetism suggests a connection with iron, but in fact the term can apply to any of those materials in which the magnitude of the object's magnetic field increases greatly when it is placed within an external field. When a natural magnet becomes magnetized (that is, when a metal or alloy comes into contact with an external magnetic field), a change occurs at the level of the domain, a group of atoms equal in size to about 5 × 10−5 meters across—just large enough to be visible under a microscope.
In an unmagnetized sample, there may be an alignment of unpaired electron spins within a domain, but the direction of the various domains' magnetic forces in relation to one another is random. Once a natural magnet is placed within an external magnetic force field, however, one of two things happens to the domains. Either they all come into alignment with the field or, in certain types of material, those domains in alignment with the field grow while the others shrink to nonexistence.
The first of these processes is called domain alignment or ferromagnetism, the second domain growth or ferrimagnetism. Both processes turn a natural magnet into what is known as a permanent magnet—or, in common parlance, simply a "magnet." The latter is then capable of temporarily magnetizing a ferromagnetic item, as for instance when one rubs a paper clip against a permanent magnet and then uses the magnetized clip to lift other paper clips. Of the two varieties, however, a ferromagnetic metal is stronger because it requires a more powerful magnetic force field in order to become magnetized. Most powerful of all is a saturated ferromagnetic metal, one in which all the unpaired electron spins are aligned.
Once magnetized, it is very hard for a ferro-magnetic metal to experience demagnetization, or antiferromagnetism. Again, there is a connection between temperature and magnetism, with heat acting as a force to reduce the strength of a magnetic field. Thus at temperatures above 1,418°F (770°C), the atoms within a domain take on enough kinetic energy to overpower the forces holding the electron spins in alignment. In addition, mechanical disturbances—for instance, battering a permanent magnet with a hammer—can result in some reduction of magnetic force.
Many of the best permanent magnets are made of steel, which, because it is an alloy of iron with carbon and other elements, has an irregular structure that lends itself well to the ferromagnetic process of domain alignment. Iron, by contrast, will typically lose its magnetization when an external magnetic force field is removed; but this actually makes it a better material for some varieties of electromagnet.
The latter, in its simplest form, consists of an iron rod inside a solenoid. When a current is passing through the solenoid, it creates a magnetic force field, activating the iron rod and turning it into an electromagnet. But as soon as the current is turned off, the rod loses its magnetic force. Not only can an electromagnet thus be controlled, but it is often stronger than a permanent magnet: hence, for instance, giant electromagnets are used for lifting cars in junkyards.
Finding the Way: Magnets in Compasses
A north-south bar magnet exerts exactly the same sort of magnetic field as a solenoid. Lines of magnetic run through it in one direction, from "south" to "north," and upon leaving the north pole of the magnet, these lines describe an ellipse as they curve back around to the south pole. In view of this model, it is also easy to comprehend why a pair of opposing poles attracts one another, and a pair of like poles—for whom the lines of force are moving away from each other—repels. This is a fact particularly applicable to the operation of MAGLEV trains, as discussed later.
A magnetic compass works because Earth itself is like a giant bar magnet, complete with vast arcs of magnetic force, called the geomagnetic field, surrounding the planet. The first scientist to recognize the magnetic properties of Earth was the English physicist William Gilbert (1544-1603). Scientists today believe that the source of Earth's magnetism lies in a core of molten iron some 4,320 mi (6,940 km) across, constituting half the planet's diameter. Within this core run powerful electric currents that ultimately create the geomagnetic field.
Just as a powerful magnet causes all the domains in a magnetic metal to align with it, a bar magnet placed in a magnetic field will rotate until it lines up with the field's direction. The same thing happens when one suspends a magnet from a string: it lines up with Earth's magnetic field, and points in a north-south direction. The Chinese of the first century B.C., though unaware of the electromagnetic forces that caused this to happen, discovered that a strip of magnetic metal always tended to point toward geographic north.
This led ultimately to the development of the magnetic compass, which typically consists of a magnetized iron needle suspended over a card marked with the four cardinal directions. The needle is attached to a pivoting mechanism at its center, which allows it to move freely so that the "north" end will always point the user northward.
The magnetic compass proved so important that it is typically ranked alongside paper, printing, and gunpowder as one of premodern China's four great gifts to the West. Prior to the compass, mariners had to depend purely on the position of the Sun and other, less reliable, means of determining direction; hence the invention quite literally helped open up the world. But there is a somewhat irksome anomaly lurking in the seeming simplicity of the magnetic compass.
In fact magnetic north is not the same thing as true north; or, to put it another way, if one continued to follow a compass northward, it would lead not to the Earth's North Pole, but to a point identified in 1984 as 77°N, 102°18′ W—that is, in the Queen Elizabeth Islands of far northern Canada. The reason for this is that Earth's magnetic field describes a current loop whose center is 11° off the planet's equator, and thus the north and south magnetic poles—which are on a plane perpendicular to that of the Earth's magnetic field—are 11° off of the planet's axis.
The magnetic field of Earth is changing position slowly, and every few years the United States Geological Survey updates magnetic declination, or the shift in the magnetic field. In addition, Earth's magnetic field is slowly weakening as well. The behavior, both in terms of weakening and movement, appears to be similar to changes taking place in the magnetic field of the Sun.
Magnets for Detection: Burglar Alarms, Magnetometers, and Mri
A compass is a simple magnetic instrument, and a burglar alarm is not much more complex. A magnetometer, on the other hand, is a much more sophisticated piece of machinery for detecting the strength of magnetic fields. Nonetheless, the magnetometer bears a relation to its simpler cousins: like a compass, certain kinds of magnetometers respond to a planet's magnetic field; and like a burglar alarm, other varieties of magnetometer are employed for security.
At heart, a burglar alarm consists of a contact switch, which responds to changes in the environment and sends a signal to a noisemaking device. The contact switch may be mechanical—a simple fastener, for instance—or magnetic. In the latter case, a permanent magnet may be installed in the frame of a window or door, and a piece of magnetized material in the window or door itself. Once the alarm is activated, it will respond to any change in the magnetic field—i.e., when someone slides open the door or window, thus breaking the connection between magnet and metal.
Though burglar alarms may vary in complexity, and indeed there may be much more advanced systems using microwaves or infrared rays, the application of magnetism in home security is a simple matter of responding to changes in a magnetic field. In this regard, the principle governing magnetometers used at security checkpoints is even simpler. Whether at an airport or at the entrance to some other high-security venue, whether handheld or stationary, a magnetometer merely detects the presence of magnetic metals. Since the vast majority of firearms, knife-blades, and other weapons are made of iron or steel, this provides a fairly efficient means of detection.
At a much larger scale, magnetometers used by astronomers detect the strength and sometimes the direction of magnetic fields surrounding Earth and other bodies in space. This variety of magnetometer dates back to 1832, when mathematician and scientist Carl Friedrich Gauss (1777-1855) developed a simple instrument consisting of a permanent bar magnet suspended horizontally by means of a gold wire. By measuring the period of the magnet's oscillation in Earth's magnetic field (or magnetosphere), Gauss was able to measure the strength of that field. Gauss's name, incidentally, would later be applied to the term for a unit of magnetic force. The gauss, however, has in recent years been largely replaced by the tesla, named after Nikola Tesla (1856-1943), which is equal to one newton/ampere meter (1 N/A·m) or 104 (10,000) gauss.
As for magnetometers used in astronomical research, perhaps the most prominent—and certainly one of the most distant—ones is on Galileo, a craft launched by the U.S. National Aeronautics and Space Administration (NASA) toward Jupiter on October 15, 1989. Among other instruments on board Galileo, which has been in orbit around the solar system's largest planet since 1995, is a magnetometer for measuring Jupiter's magnetosphere and that of its surrounding asteroids and moons.
Closer to home, but no less impressive, is another application of magnetism for the purposes of detection: magnetic resonance imagining, or MRI. First developed in the early 1970s, MRI permits doctors to make intensive diagnoses without invading the patient's body either with a surgical knife or x rays.
The heart of the MRI machine is a large tube into which the patient is placed in a supine position. A technician then activates a powerful magnetic field, which causes atoms within the patient's body to spin at precise frequencies. The machine then beams radio signals at a frequency matching that of the atoms in the cells (e.g., cancer cells) being sought. Upon shutting off the radio signals and magnetic field, those atoms emit bursts of energy that they have absorbed from the radio waves. At that point a computer scans the body for frequencies matching specific types of atoms, and translates these into three-dimensional images for diagnosis.
Magnets for Projecting Sound: Microphones, Loudspeakers, Car Horns, and Electric Bells
The magnets used in Galileo or an MRI machine are, needless to say, very powerful ones, and as noted earlier, the best way to create a super-strong, controllable magnet is with an electrical current. When that current is properly coiled around a magnetic metal, this creates an electromagnet, which can be used in a variety of applications.
As discussed above, the most powerful electromagnets typically use nonpermanent magnets so as to facilitate an easy transition from an extremely strong magnetic field to a weak or nonexistent one. On the other hand, permanent magnets are also used in loudspeakers and similar electromagnetic devices, which seldom require enormous levels of power.
In discussing the operation of a loudspeaker, it is first necessary to gain a basic understanding of how a microphone works. The latter contains a capacitor, a system for storing charges in the form of an electrical field. The capacitor's negatively charged plate constitutes the microphone's diaphragm, which, when it is hit by sound waves, vibrates at the same frequency as those waves. Current flows back and forth between the diaphragm and the positive plate of the capacitor, depending on whether the electrostatic or electrical pull is increasing or decreasing. This in turn produces an alternating current, at the same frequency as the sound waves, which travels through a mixer and then an amplifier to the speaker.
A loudspeaker typically contains a circular permanent magnet, which surrounds an electrical coil and is in turn attached to a cone-shaped diaphragm. Current enters the speaker ultimately from the microphone, alternating at the same frequency as the source of the sound (a singer's voice, for instance). As it enters the coil, this current induces an alternating magnetic field, which causes the coil to vibrate. This in turn vibrates the cone-shaped diaphragm, and the latter reproduces sounds generated at the source.
A car horn also uses magnetism to create sound by means of vibration. When a person presses down on the horn embedded in his or her steering wheel, this in turn depresses an iron bar that passes through an electromagnet surrounded by wires from the car's battery. The bar moves up and down within the electromagnetic field, causing the diaphragm to vibrate and producing a sound that is magnified greatly when released through a bell-shaped horn.
Electromagnetically induced vibration is also the secret behind another noise-making device, a vibrating electric doorbell used in many apartments. The button that a visitor presses is connected directly to a power source, which sends current flowing through a spring surrounding an electromagnet. The latter generates a magnetic field, drawing toward it an iron armature attached to a hammer. The hammer then strikes the bell. The result is a mechanical reaction that pushes the armature away from the electromagnet, but the spring forces the armature back against the electromagnet again. This cycle of contact and release continues for as long as the button is depressed, causing a continual ringing of the bell.
Recording and Reading Data Using Magnets: from Records and Tapes to Disk Drives
Just as magnetism plays a critical role in projecting the volume of sound, it is also crucial to the recording and retrieval of sound and other data. Of course terms such as "retrieval" and "data" have an information-age sound to them, but the idea of using magnetism to record sound is an old one—much older than computers or compact discs (CDs). The latter, of course, replaced cassettes in the late 1980s as the preferred mode for listening to recorded music, just as cassettes had recently made powerful gains against phonograph records.
Despite the fact that cassettes entered the market much later than records, however, recording engineers from the mid-twentieth century onward typically used magnetic tape for master recordings of songs. This master would then be used to create a metal master record disk by means of a cutting head that responded to vibrations from the master tape; then, the record company could produce endless plastic copies of the metal record.
In recording a tape—whether a stereo master or a mere home recording of a conversation—the principles at work are more or less the same. As noted in the earlier illustration involving a microphone and loudspeaker, sound comes through a microphone in the form of alternating current. The strength of this current in turn affects the "recording head," a small electromagnet whose magnetic field extends over the section of tape being recorded. Loud sounds produce strong magnetic fields, and soft ones weak fields.
All of this information becomes embedded on the cassette tape through a process of magnetic alignment not so different from the process described earlier for creating a permanent magnet. But whereas the permanent magnetization of a natural magnet is difficult to reverse, reversal of a tape's magnetization—in other words, erasing the tape—is easy. An erase head, an electromagnet operating at a frequency too high for the human ear to hear, simply scrambles the magnetic particles on a piece of tape.
A CD, as one might expect, is much more closely related to a computer disk-drive than it is to earlier forms of recording technology. The disk drive receives electronic on-off signals from the computer, and translates these into magnetic codes that it records on the surface of a floppy disk. The disk drive itself includes two electric motors: a disk motor, which rotates the disk at a high speed, and a head motor, which moves the computer's read-write head across the disk. (It should be noted that most electric motors, including the universal motors used in a variety of household appliances, also use electromagnets.)
A third motor, called a stepper motor, ensures that the drive turns at a precise rate of speed. The stepper motor contains its own magnet, in this case a permanent one of cylindrical shape that sends signals to rows of metal teeth surrounding it, and these teeth act as gears to regulate the drive's speed. Likewise a CD player, which actually uses laser beams rather than magnetic fields to retrieve data from a disc, also has a drive system that regulates the speed at which the disk spins.
Maglev Trains: the Future of Transport?
One promising application of electromagnetic technology relates to a form of transportation that might, at first glance, appear to be old news: trains. But MAGLEV, or magnetic levitation, trains are as far removed from the old steam engines of the Union Pacific as the space shuttle is from the Wright brothers' experimental airplane.
As discussed earlier, magnetic poles of like direction (i.e., north-north or south-south) repel one another such that, theoretically at least, it is possible to keep one magnet suspended in the air over another magnet. Actually it is impossible to produce these results with simple bar magnets, because their magnetic force is too small; but an electromagnet can create a magnetic field powerful enough that, if used properly, it exerts enough repulsive force to lift extremely heavy objects. Specifically, if one could activate train tracks with a strong electromagnetic field, it might be possible to "levitate" an entire train. This in turn would make possible a form of transport that could move large numbers of people in relative comfort, thus decreasing the environmental impact of automobiles, and do so at much higher speeds than a car could safely attain.
Actually the idea of MAGLEV trains goes back to a time when trains held complete supremacy over automobiles as a mode of transportation: specifically, 1907, when rocket pioneer Robert Goddard (1882-1945) wrote a story describing a vehicle that traveled by means of magnetic levitation. Just five years later, French engineer Emile Bachelet produced a working model for a MAGLEV train. But the amount of magnetic force required to lift such a vehicle made it impractical, and the idea fell to the wayside.
Then, in the 1960s, the advent of superconductivity—the use of extremely low temperatures, which facilitate the transfer of electrical current through a conducting material with virtually no resistance—made possible electromagnets of staggering force. Researchers began building MAGLEV prototypes using superconducting coils with strong currents to create a powerful magnetic field. The field in turn created a repulsive force capable of lifting a train several inches above a railroad track. Electrical current sent through guideway coils on the track allowed for enormous propulsive force, pushing trains forward at speeds up to and beyond 250 MPH (402 km/h).
Initially, researchers in the United States were optimistic about MAGLEV trains, but safety concerns led to the shelving of the idea for several decades. Meanwhile, other industrialized nations moved forward with MAGLEVs: in Japan, engineers built a 27-mi (43.5-km) experimental MAGLEV line, while German designers experimented with attractive (as opposed to repulsive) force in their Transrapid 07. MAGLEV trains gained a new defender in the United States with now-retired Senator Daniel Patrick Moynihan (D-NY), who as chairman of a Senate subcommittee overseeing the interstate highway system introduced legislation to fund MAGLEV research. The 1998 transportation bill allocated $950 million toward the Magnetic Levitation Prototype Development Program. As part of this program, in January 2001 the U.S. Department of Transportation selected projects in Maryland and Pennsylvania as the two finalists in the competition to build the first MAGLEV train service in the United States. The goal is to have the service in place by approximately 2010.
Where to Learn More
Barr, George. Science Projects for Young People. New York: Dover, 1964.
Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.
Hann, Judith. How Science Works. Pleasantville, NY: Reader's Digest, 1991.
Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998.
Molecular Expressions: Electricity and Magnetism: Interactive Java Tutorials (Web site). <http://micro.magnet.fsu.edu/electromag/java/> (January 26, 2001).
Topical Group on Magnetism (Web site). <http://www.aps.org/units/gmag/> (January 26, 2001).
VanCleave, Janice. Magnets. New York: John Wiley &Sons, 1993.
Wood, Robert W. Physics for Kids: 49 Easy Experiments with Electricity and Magnetism. New York: Tab, 1990.
Definition: charm, attractiveness
Antonyms: repugnance, repulsion
Magnetic Poles, Forces, and Fields
Any object that exhibits magnetic properties is called a magnet. Every magnet has two points, or poles, where most of its strength is concentrated; these are designated as a north-seeking pole, or north pole, and a south-seeking pole, or south pole, because a suspended magnet tends to orient itself along a north-south line. Since a magnet has two poles, it is sometimes called a magnetic dipole, being analogous to an electric dipole, composed of two opposite charges. The like poles of different magnets repel each other, and the unlike poles attract each other.
One remarkable property of magnets is that whenever a magnet is broken, a north pole will appear at one of the broken faces and a south pole at the other, such that each piece has its own north and south poles. It is impossible to isolate a single magnetic pole, regardless of how many times a magnet is broken or how small the fragments become. (The theoretical question as to the possible existence in any state of a single magnetic pole, called a monopole, is still considered open by physicists; experiments to date have failed to detect one.)
From his study of magnetism, C. A. Coulomb in the 18th cent. found that the magnetic forces between two poles followed an inverse-square law of the same form as that describing the forces between electric charges. The law states that the force of attraction or repulsion between two magnetic poles is directly proportional to the product of the strengths of the poles and inversely proportional to the square of the distance between them.
As with electric charges, the effect of this magnetic force acting at a distance is expressed in terms of a field of force. A magnetic pole sets up a field in the space around it that exerts a force on magnetic materials. The field can be visualized in terms of lines of induction (similar to the lines of force of an electric field). These imaginary lines indicate the direction of the field in a given region. By convention they originate at the north pole of a magnet and form loops that end at the south pole either of the same magnet or of some other nearby magnet (see also flux, magnetic). The lines are spaced so that the number per unit area is proportional to the field strength in a given area. Thus, the lines converge near the poles, where the field is strong, and spread out as their distance from the poles increases.
A picture of these lines of induction can be made by sprinkling iron filings on a piece of paper placed over a magnet. The individual pieces of iron become magnetized by entering a magnetic field, i.e., they act like tiny magnets, lining themselves up along the lines of induction. By using variously shaped magnets and various combinations of more than one magnet, representations of the field in these different situations can be obtained.
The term magnetism is derived from Magnesia, the name of a region in Asia Minor where lodestone, a naturally magnetic iron ore, was found in ancient times. Iron is not the only material that is easily magnetized when placed in a magnetic field; others include nickel and cobalt. Carbon steel was long the material commonly used for permanent magnets, but more recently other materials have been developed that are much more efficient as permanent magnets, including certain ferroceramics and Alnico, an alloy containing iron, aluminum, nickel, cobalt, and copper.
Materials that respond strongly to a magnetic field are called ferromagnetic [Lat. ferrum = iron]. The ability of a material to be magnetized or to strengthen the magnetic field in its vicinity is expressed by its magnetic permeability. Ferromagnetic materials have permeabilities of as much as 1,000 or more times that of free space (a vacuum). A number of materials are very weakly attracted by a magnetic field, having permeabilities slightly greater than that of free space; these materials are called paramagnetic. A few materials, such as bismuth and antimony, are repelled by a magnetic field, having permeabilities less than that of free space; these materials are called diamagnetic.
The Basis of Magnetism
The electrical basis for the magnetic properties of matter has been verified down to the atomic level. Because the electron has both an electric charge and a spin, it can be called a charge in motion. This charge in motion gives rise to a tiny magnetic field. In the case of many atoms, all the electrons are paired within energy levels, according to the exclusion principle, so that the electrons in each pair have opposite (antiparallel) spins and their magnetic fields cancel. In some atoms, however, there are more electrons with spins in one direction than in the other, resulting in a net magnetic field for the atom as a whole; this situation exists in a paramagnetic substance. If such a material is placed in an external field, e.g., the field created by an electromagnet, the individual atoms will tend to align their fields with the external one. The alignment will not be complete, due to the disruptive effect of thermal vibrations. Because of this, a paramagnetic substance is only weakly attracted by a magnet.
In a ferromagnetic substance, there are also more electrons with spins in one direction than in the other. The individual magnetic fields of the atoms in a given region tend to line up in the same direction, so that they reinforce one another. Such a region is called a domain. In an unmagnetized sample, the domains are of different sizes and have different orientations. When an external magnetic field is applied, domains whose orientations are in the same general direction as the external field will grow at the expense of domains with other orientations. When the domains in all other directions have vanished, the remaining domains are rotated so that their direction is exactly the same as that of the external field. After this rotation is complete, no further magnetization can take place, no matter how strong the external field; a saturation point is said to have been reached. If the external field is then reduced to zero, it is found that the sample still retains some of its magnetism; this is known as hysteresis.
Evolution of Electromagnetic Theory
The connections between magnetism and electricity were discovered in the early part of the 19th cent. In 1820 H. C. Oersted found that a wire carrying an electrical current deflects the needle of a magnetic compass because a magnetic field is created by the moving electric charges constituting the current. It was found that the lines of induction of the magnetic field surrounding the wire (or any other conductor) are circular. If the wire is bent into a coil, called a solenoid, the magnetic fields of the individual loops combine to produce a strong field through the core of the coil. This field can be increased manyfold by inserting a piece of soft iron or other ferromagnetic material into the core; the resulting arrangement constitutes an electromagnet.
Following Oersted's discovery the various magnetic effects of an electric current were extensively investigated by J. B. Biot, Félix Savart, and A. M. Ampère. Ampère showed in 1825 that not only does a current-carrying conductor exert a force on a magnet but magnets also exert forces on current-carrying conductors. In 1831 Michael Faraday and Joseph Henry independently discovered that it is possible to produce a current in a conductor by changing the magnetic field about it. The discovery of this effect, called electromagnetic induction, together with the discovery that an electric current produces a magnetic field, laid the foundation for the modern age of electricity. Both the electric generator, which makes electricity widely available, and the electric motor, which converts electricity to useful mechanical work, are based on these effects.
Another relationship between electricity and magnetism is that a regularly changing electric current in a conductor will create a changing magnetic field in the space about the conductor, which in turn gives rise to a changing electrical field. In this way regularly oscillating electric and magnetic fields can generate each other. These fields can be visualized as a single wave that is propagating through space. The formal theory underlying this electromagnetic radiation was developed by James Clerk Maxwell in the middle of the 19th cent. Maxwell showed that the speed of propagation of electromagnetic radiation is identical with that of light, thus revealing that light is intimately connected with electricity and magnetism.
See D. Wagner, Introduction to the Theory of Magnetism (1972); D. J. Griffiths, Introduction to Electrodynamics (1981); R. T. Merritt, Our Magnetic Earth (2010).
Property of some materials to attract or repel others.
Magnetic attraction or repulsion.
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