Bubble memory is a sophisticated method of storing data that gets rid of the need for moving parts such as are required in tape machines and disk drives. Data is stored as tiny magnetic fields, in a medium that is made from magnetic film and semiconductor materials.

Bubble memory makes use of all the advantages of magnetic data storage, as well as the favorable aspects of electronic data storage. Advantages of electronic memory include rapid storage and recovery, and high density (a lot of data can be put in a tiny volume of space). Advantages of magnetic memory include non volatility (it can be stored for a long time without needing a constant current source), high density, and comparatively low cost.

Bubble memory seems to go through phases. Just as it is declared obsolete, someone comes up with a new and improved way to make it work. Check the Internet to find out its current status; enter “bubble memory” or “magnetic bubble memory” into a search engine.

Since the advent of the personal computer, ever-more compact data-storage systems have evolved. One of the most versatile is the magnetic disk.

Hard disks, also called hard drives, store the most data, and are generally found inside of computer units. Diskettes are 8.9 cm (3.5 in) across, and can be inserted and removed from recording/playback machines called diskette drives. In recent years, magnetic diskettes have been largely supplanted by nonmagnetic compact disc recordable (CD-R) and compact disc rewritable (CD-RW) media.

The principle of the magnetic disk, on the microscale, is the same as that of magnetic tape. The information is stored in binary digital form; that is, there are only two different ways that the particles are magnetized. This results in almost perfect, error-free storage. On a larger scale, the disk works differently than the tape because of the difference in geometry. On a tape, the information is spread out over a long span, and some bits of data are far away from others as measured along the medium itself. But on a disk, no two bits are ever farther apart than the diameter of the disk. This means that data can be stored to, and retrieved from, a disk much faster than is possible with tape.

The same precautions should be observed when handling and storing magnetic disks as are necessary with magnetic tape.

On recording tape, particles are magnetized in a pattern that follows the input waveform. Graph A shows an example of an audio input waveform. Graph B shows relative polarity and intensity of magnetization for selected particles on the tape surface.

Magnetic tape, also called recording tape, consists of millions of ferromagnetic particles attached to a flexible, thin plastic strip. In the tape recorder, a fluctuating magnetic field, produced by the recording head, polarizes these particles. As the field changes in strength next to the recording head, the tape passes by at a constant speed. This produces regions in which the ferromagnetic particles are polarized in either direction (Above Figure).

When the tape is run at the same speed through the recorder in the playback mode, the magnetic fields around the individual particles cause a fluctuating field that is detected by the pickup head. This field has the same pattern of variations as the original field from the recording head. Magnetic tape is available in various widths and thicknesses. Thicker tapes result in cassettes that don’t play as long, but the tape is more resistant to stretching. The speed of the tape determines the fidelity of the recording. Higher speeds are preferred for music and video, and lower speeds for voice and data.

The impulses on a magnetic tape can be distorted or erased by external magnetic fields. Therefore, tapes should be protected from such fields. Keep the tape away from magnets. Extreme heat can also result in loss of data, and can cause permanent physical damage to the tape.

A functional diagram of a dc motor
Magnetic forces can be harnessed to do work. One common device that converts direct-current energy into rotating mechanical energy is a dc motor. In a dc motor, the source of electricity is connected to a set of coils, producing magnetic fields. The attraction of opposite poles, and the repulsion of like poles, is switched in such a way that a constant torque, or rotational force, results. As the current in the coils increases, the torque that the motor can provide also increases.

Above Figure is a simplified, cutaway drawing of a dc motor. One set of coils, called the armature coil, rotates along with the motor shaft. The other set of coils, called the field coil, is stationary. The current direction is periodically reversed during each rotation by means of the commutator. This keeps the rotational force going in the same angular direction, so the motor continues to rotate rather than oscillating back and forth. The shaft is carried along by its own inertia, so that it doesn’t come to a stop during those instants when the current is being switched in polarity.

Some dc motors can also be used to generate dc. These motors contain permanent magnets in place of one of the sets of coils. When the shaft is rotated, a pulsating dc flows in the coil.

At A, pictorial diagram of a simple relay. At B, the schematic symbol for the same relay.

A relay makes use of a solenoid to allow remote-control switching of high-current circuits. A diagram of a relay is shown in above figure. The movable lever, called the armature, is held to one side by a spring when there is no current flowing through the electromagnet. Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient current is applied, the armature is pulled over to the other side. This disconnects terminal X from terminal Y, and connects X to Z.

There are numerous types of relays. Some are meant for use with dc, and others are for ac; a few will work with either dc or ac. A normally closed relay completes the circuit when there is no current flowing in its electromagnet coil, and breaks the circuit when current flows through the coil. A normally open relay is just the opposite, completing the circuit when current flows through the electromagnet coil, and opening the circuit when current ceases to flow through the coil. Normal, in this context, refers to the condition of no current applied to the electromagnet.

The relay shown in above figure can be used as either a normally open or normally closed relay, depending on which contacts are selected. It can also be used to switch a line between two different circuits.

Some relays have several sets of contacts. Some relays are meant to remain in one state (either with current or without) for a long time, while others are meant to switch several times per second. The fastest relays can operate several dozen times per second. In recent years, relays have been largely supplanted by switching transistors and diodes, except in applications where extremely high current or high voltage is involved.

A solenoid-coil bell ringer.
Above figure is a simplified diagram of a bell ringer, also called a chime. The main functional component is called a solenoid, and it is an electromagnet. The core has a hole going along its axis. The coil has several layers, but the wire is always wound in the same direction, so that the electromagnet is powerful. A movable steel rod runs through the hole in the electromagnet core.

When there is no current flowing in the coil, the steel rod is held down by the force of gravity. When a pulse of current passes through the coil, the rod is pulled forcibly upward so that it strikes the ringer plate. This plate is like one of the plates in a xylophone. The current pulse is short, so the steel rod falls back down again to its resting position, allowing the plate to reverberate.

Permanent magnets are manufactured by using a high-retentivity ferromagnetic material as the core of an electromagnet for an extended period of time. The coil of the electromagnet carries a large direct current, causing intense magnetic flux of constant polarity within the material. (Don’t try to do this at home. The high current can heat the coil and overload a battery or power supply, which produces a fire hazard and/or the risk of battery explosion.)

If you want to magnetize a screwdriver a little bit so that it will hold onto screws, just stroke the shaft of the screwdriver with the end of a bar magnet several dozen times. Once you have magnetized a tool in this way, however, it is nearly impossible to demagnetize it.

There are four important properties that materials can have with respect to magnetic flux. These properties are ferromagnetism, diamagnetism, permeability, and retentivity.

Ferromagnetism

Some substances cause magnetic lines of flux to bunch closer together than they would in the medium of air or a vacuum. This property is called ferromagnetism, and materials that exhibit it are called ferromagnetic. You’ve already learned something about this!

Diamagnetism

Another property is known as diamagnetism, and materials that exhibit it are called diamagnetic. This type of substance decreases the magnetic flux density by causing the magnetic flux lines to diverge. Wax, dry wood, bismuth, and silver are examples. No diamagnetic material reduces the strength of a magnetic field by anywhere near the factor that ferromagnetic substances can increase it. Diamagnetic materials are generally used to keep magnetic objects apart, while minimizing the interaction between them. In recent years, they have also found some application in magnetic levitation devices.

Permeability

Permeability is a quantitative indicator of the extent to which a ferromagnetic material concentrates magnetic lines of flux. It is measured on a scale relative to a vacuum, or free space. Free space is assigned permeability 1. If you have a coil of wire with an air core, and a current is forced through the wire, then the flux in the coil core is at a certain density, just about the same as it would be in a vacuum. Therefore, the permeability of pure air is about equal to 1. If you place an iron core in the coil, the flux density increases by a large factor. The permeability of iron can range from 60 (impure) to as much as 8000 (highly refined).
If you use certain ferromagnetic alloys as the core material in electromagnets, you can increase the flux density, and therefore the local strength of the field, by as much as a million times. Such substances thus have permeability as great as 1,000,000 (106). following table gives permeability values for some common materials.

Retentivity

When a substance, such as iron, is subjected to a magnetic field as intense as it can handle, say by enclosing it in a wire coil carrying a massive current, there will be some residual magnetism left.

Substance	Permeability (approx.)
Air, dry, at sea level	1
Alloys, ferromagnetic	3000–1,000,000
Aluminum	Slightly more than 1
Bismuth	Slightly less than 1
Cobalt	60–70
Iron, powdered and pressed	100–3000
Iron, solid, refined	3000–8000
Iron, solid, unrefined	60–100
Nickel	50–60
Silver	Slightly less than 1
Steel	300–600
Vacuum	1
Wax	Slightly less than 1
Wood, dry	Slightly less than 1

when the current stops flowing in the coil. Retentivity, also sometimes called remanence, is a measure of how well the substance “memorizes” the magnetism and thereby becomes a permanent magnet.

Retentivity is expressed as a percentage, and is symbolized Br. If the flux density in the material is x tesla or gauss when it is subjected to the greatest possible magnetomotive force, and then goes down to y tesla or gauss when the current is removed, the retentivity is equal to 100( y/x)%.
Suppose that a metal rod can be magnetized to 135 G when it is enclosed by a coil carrying an electric current. Imagine that this is the maximum possible flux density that the rod can be forced to have. (For any substance, there is always such a maximum.) Now suppose that the current is shut off, and 19 G remain in the rod. Then the retentivity, Br, is calculated as follows:

Br = 100(19/135)% = (100 × 0.14)% = 14%

Some ferromagnetic substances have high retentivity. These materials are excellent for making permanent magnets. Other substances have low retentivity. They work well as electromagnets, but not as permanent magnets.

If a ferromagnetic substance has poor retentivity, it is especially well-suited for use as the core material for an ac electromagnet, because the polarity of the magnetic flux can reverse within the material at a rapid rate. Materials with high retentivity do not work well for ac electromagnets, because they resist the polarity reversal that takes place with ac.

In an electromagnet, the magnetic flux is concentrated in a ferromagnetic rod surrounded by a current-carrying coil.

Any electric current, or movement of charge carriers, produces a magnetic field. This field can become intense in a tightly coiled wire that has many turns and carries a large current. When a ferromagnetic core is placed inside the coil, the magnetic lines of flux are concentrated in the core, and the field strength in and near the core can become tremendous. This is the principle of an electromagnet (above figure). Electromagnets are almost always cylindrical in shape. Sometimes the cylinder is long and thin; in other cases it is short and fat. But whatever the ratio of diameter to length for the core, the principle is the same: the magnetic field produced by the current results in magnetization of the core.

Direct-Current Types

You can build a dc electromagnet by taking a large bolt, such as a stove bolt, and wrapping a few dozen or a few hundred turns of wire around it. These items are available in any good hardware store. Be sure the bolt is made of ferromagnetic material. (If a permanent magnet sticks to the bolt, the bolt is ferromagnetic.) Ideally, the bolt should be at least 1 cm (approximately 3⁄8 in) in diameter and several inches long. You must use insulated wire, preferably made of solid, soft copper. “Bell wire” works well. Be sure all the wire turns go in the same direction. A large 6-V lantern battery can provide plenty of current to work the electromagnet. Never leave the coil connected to the battery for more than a few seconds at a time. And never use a car battery for this experiment! The acid can boil out of this type of battery, because the electromagnet places a heavy load on it.

Direct-current electromagnets have defined north and south poles, just like permanent magnets. The main difference is that an electromagnet can get much stronger than any permanent magnet. You will see evidence of this if you do the preceding experiment with a large enough bolt and enough turns of wire.

Alternating-Current Types

Do you get the idea that an electromagnet can be made far stronger if, rather than using a lantern battery for the current source, you plug the wires into a wall outlet? In theory, this is true. In prac- tice, you’ll blow the fuse or circuit breaker. Do not try this! The electrical circuits in some buildings are not adequately protected and it can create a fire hazard. Also, you can get a lethal shock from the utility mains.

Some electromagnets use ac, and these magnets will stick to ferromagnetic objects. But the polarity of the magnetic field reverses every time the direction of the current reverses. With conventional household ac in the United States, there are 120 fluctuations, or 60 complete north-to-south-to-north polarity changes (above figure), per second. If a permanent magnet, or a dc electromagnet, is brought near either “pole” of an ac electromagnet, there is no net force because the poles are alike half the time and opposite half the time, producing an equal amount of attractive and repulsive force. But if a piece of iron or steel is brought near a strong ac electromagnet, watch out! The attractive force will be powerful.

The overall magnitude of a magnetic field is measured in units called webers (Wb). A smaller unit, the maxwell (Mx), is sometimes used if a magnetic field is weak. One weber is equivalent to 100,000,000 (108) maxwells. Conversely, 1 Mx = 0.00000001 Wb = 10−8 Wb.

The Tesla and the Gauss

If you have access to a permanent magnet or electromagnet, you might see its strength expressed in terms of webers or maxwells. But usually you’ll hear units called teslas (T) or gauss (G). These units are expressions of the concentration, or intensity, of the magnetic field within a certain cross section. The flux density, or number of lines per square meter or per square centimeter, is a more useful expression for magnetic effects than the overall quantity of magnetism. A flux density of 1 tesla (1 T) is equal to 1 weber per square meter (1 Wb/m2). A flux density of 1 gauss (1 G) is equal to 1 maxwell per square centimeter (1 Mx/cm2). It turns out that the gauss is equal to 0.0001 tesla (10−4 T). Conversely, the tesla is equivalent to 10,000 gauss (104 G).

The Ampere-Turn and the Gilbert

With electromagnets, another unit is employed: the ampere-turn (At). This is technically a unit of magnetomotive force, which is the magnetic counterpart of electromotive force. A wire, bent into a circle and carrying 1 A of current, produces 1 At of magnetomotive force. If the wire is bent into a loop having 50 turns, and the current stays the same, the resulting magnetomotive force is 50 At. If the current is then reduced to 1/50 A or 20 mA, the magnetomotive force will go back down to 1 At. The gilbert (Gb) is also used to express magnetomotive force, but it is less common than the ampere-turn. One gilbert (1 Gb) is equal to 0.796 At. Conversely, 1 At = 1.26 Gb.

Magnets are attracted to some, but not all, metals. Iron, nickel, and alloys containing either or both of these elements are known as ferromagnetic materials. They “stick” to magnets. They can also be made into permanent magnets. When a magnet is brought near a piece of ferromagnetic material, the atoms in the material become lined up, so that the material is temporarily magnetized. This produces a magnetic force between the atoms of the ferromagnetic substance and those in the magnet.

Attraction and Repulsion

If a magnet is brought near another magnet, the force can be repulsive or attractive, depending on the way the magnets are oriented. The force gets stronger as the magnets are brought near each other. Some magnets are so strong that no human being can pull them apart if they get stuck together, and no person can bring them all the way together against their mutual repulsive force.

The tremendous forces produced by electromagnets are of use in industry. A large electromagnet can be used to carry heavy pieces of scrap iron from place to place. Other electromagnets can provide sufficient repulsion to suspend one object above another. This phenomenon is called magnetic levitation. It is the basis for low-friction, high-speed commuter trains now in use in some metropolitan areas.

Charge in Motion

Whenever the atoms in a ferromagnetic material are aligned, a magnetic field exists. A magnetic field can also be caused by the motion of electric charge carriers, either in a wire or in free space. The magnetic field around a permanent magnet arises from the same cause as the field around a wire that carries an electric current. The responsible factor in either case is the motion of electrically charged particles. In a wire, electrons move along the conductor, being passed from atom to atom. In a permanent magnet, the movement of orbiting electrons occurs in such a manner that an effective electrical current is produced.

Magnetic fields are also generated by the motion of charged particles through space. The sun is constantly ejecting protons and helium nuclei. These particles carry a positive electric charge. Because of this, and the fact that they are in motion, they are surrounded by tiny magnetic fields. When the particles approach the earth and their magnetic fields interact with the geomagnetic field, the particles are accelerated toward the geomagnetic poles. When there is a solar flare, the sun ejects far more charged particles than normal. When these approach the geomagnetic poles, the result is considerable disruption of the geomagnetic field. This type of event is called a geomagnetic storm. It causes changes in the earth’s ionosphere, affecting long distance radio communications at certain frequencies. If the fluctuations are intense enough, even wire communications and electric power transmission can be interfered with. Aurora (northern or southern lights) are frequently observed at night during these events.

1. The pattern of magnetic flux lines (dashed curves) around a bar magnet (rectangle). The N and S represent north and south magnetic poles, respectively.
2.The pattern of magnetic flux lines (dashed curves) around a straight, current carrying wire can be seen when the wire passes through a horizontal sheet of paper sprinkled with iron filings.

Flux Lines

Have you seen the well-known experiment in which iron filings are placed on a horizontal sheet of paper, and then a magnet is placed underneath the paper? The filings arrange themselves in a pattern that shows, roughly, the shape of the magnetic field in the vicinity of the magnet. A bar magnet has a field with a characteristic form (Figure 1). Another popular experiment involves passing a current-carrying wire through a horizontal sheet of paper at a right angle, as shown in (Figure 2). The iron filings become grouped along circles centered at the point where the wire passes through the paper.
The intensity of a magnetic field is determined according to the number of flux lines passing through a certain cross section, such as a square centimeter or a square meter. The lines don’t exist as real objects, but it is intuitively appealing to imagine them that way. The iron filings on the paper really do bunch themselves into lines (curves, actually) when there is a magnetic field of sufficient strength to make them move. Sometimes lines of flux are called lines of force. But technically, this is a misnomer.

Poles

A magnetic field has a specific direction, as well as a specific intensity, at any given point in space near a current-carrying wire or a permanent magnet. The flux lines run parallel with the direction of the field. A magnetic field is considered to begin at the north magnetic pole, and to terminate at the south magnetic pole. In the case of a permanent magnet, it is obvious where the magnetic poles are. In the case of a current-carrying wire, the magnetic field goes in endless circles around the wire.

A charged electric particle, such as a proton or electron, hovering all by itself in space, constitutes an electric monopole. The electric lines of flux around an isolated, charged particle in free space are straight, and they “run off to infinity” (following figure). A positive electric charge does not have to be mated with a negative electric charge.

Electric flux lines (dashed lines) around an electrically charged object. This example shows a positive charge. The pattern of flux lines for a negative charge is identical.
A magnetic field is different. All magnetic flux lines, at least in ordinary real-world situations, are closed loops. With permanent magnets, there is a starting point (the north pole) and an ending point (the south pole). Around a straight, current-carrying wire, the loops are closed circles, even though the starting and ending points are not obvious. A pair of magnetic poles is called a magnetic dipole.

At first you might think that the magnetic field around a current-carrying wire is caused by a monopole, or that there aren’t any poles at all, because the concentric circles don’t actually converge anywhere. But you can envision a half plane, with the edge along the line of the wire, as a magnetic dipole. Then the lines of flux go around once in a 360° circle from the “north face” of the half plane to the “south face.”

The greatest flux density, or field strength, around a bar magnet is near the poles, where the lines converge. Around a current-carrying wire, the greatest field strength is near the wire.

The earth has a core made up largely of iron, heated to the extent that some of it is liquid. As the earth rotates, the iron flows in complex ways. It is thought that this flow is responsible for the magnetic field that surrounds the earth. Some other planets, notably Jupiter, have magnetic fields as well. Even the sun has one.

The Poles and Axis

The geomagnetic field, as it is called, has poles, just as a bar magnet does. The geomagnetic poles are near, but not at, the geographic poles. The north geomagnetic pole is located in far northern Canada. The south geomagnetic pole is near Antarctica. The geomagnetic axis is therefore tilted relative to the axis on which the earth rotates.

The Solar Wind

Charged subatomic particles from the sun, streaming outward through the solar system, distort the geomagnetic lines of flux above figure. This stream of particles is called the solar wind. That’s a good name for it, because the fast-moving particles produce measurable forces on sensitive instruments in space. This force has actually been suggested as a possible means to drive space ships, equipped with solar sails, out of the solar system! At and near the earth’s surface, the geomagnetic field is not affected very much by the solar wind, so the geomagnetic field is nearly symmetrical. As the distance from the earth increases, the distortion of the field also increases, particularly on the side of the earth away from the sun.

The Magnetic Compass

The presence of the geomagnetic field was first noticed in ancient times. Some rocks, called lodestones, when hung by strings, would always orient themselves a certain way. This was correctly attributed to the presence of a “force” in the air. This effect was put to use by early seafarers and land explorers. Today, a magnetic compass can still be a valuable navigation aid, used by mariners, backpackers, and others who travel far from familiar landmarks. The geomagnetic field interacts with the magnetic field around a compass needle, and a force is thus exerted on the needle. This force works not only in a horizontal plane (parallel to the earth’s surface), but vertically at most latitudes. The vertical component is zero only at the geomagnetic equator, a line running around the globe equidistant from both geomagnetic poles.

As the geomagnetic latitude increases, toward either the north or the south geomagnetic pole, the magnetic force pulls up and down on the compass needle more and more. One end of the needle seems to insist on touching the compass face, while the other end tilts up toward the glass. The needle tries to align itself parallel to the geomagnetic lines of flux. The vertical angle, in degrees, at which the geomagnetic lines of flux intersect the earth’s surface at any given location is called the geomagnetic inclination.

Because geomagnetic north is not the same as geographic north in most places on the earth’s surface, there is an angular difference between the two. This horizontal angle, in degrees, is called geomagnetic declination. It, like inclination, varies with location.