E LECTROMAGNETISM

The Greek philosopher Thales (640?-546 B.C. ) was the first to observe that when amber is rubbed with certain types of materials, the friction imparts to it the ability to pick up light objects. The word electricity comes from the Greek word for amber, elektron, and, in fact, magnetism and electricity are simply manifestations of the same force. This concept of electric and magnetic interaction seems to have been established early in human history, though it would be almost 2,500 years before scientists came to a mature understanding of the relationship.

As in so much else, studies in electromagnetism made little progress from the time of the Romans to the late Renaissance, a span of nearly 1,500 years. Yet it is worth noting that the first ideas scientists had about studying Earth's history scientifically came from observing the planet's magnetic field. In the course of his work on that subject, the English astronomer Henry Gelli-brand (1597-1636) showed that the field has changed over time. This suggested that it would be possible to form hypotheses about the planet's past, even though humans had no direct information regarding the origins of Earth. Thus, Gellibrand (who, ironically, was also a minister) helped make it possible for geologists to move beyond a strict interpretation of the Bible in studying the history of Earth. (See Earth, Science, and Nonscience for more on the Genesis account and its interpretations.)

ELECTROMAGNETIC STUDIES COME OF AGE.

Beginning in the 1700s, a number of thinkers conducted experiments concerning the nature of electricity and magnetism and the relationship between them. Among these thinkers were several giants in physics and other disciplines, including one of America's greatest founding fathers, Benjamin Franklin (1706-1790). In addition to his famous (and highly dangerous) experiment with lightning, Franklin also contributed the names positive and negative to the differing electric charges discovered earlier by the French physicist Charles Du Fay (1698-1739).

In 1785 the French physicist and inventor Charles Coulomb (1736-1806) established the basic laws of electrostatics and magnetism. He maintained that there is an attractive force that, like gravitation, can be explained in terms of the

inverse of the square of the distance between objects (see Gravity and Geodesy). That attraction itself, however, results not from gravity but from electric charge, according to Coulomb.

A few years later, the German mathematician Karl Friedrich Gauss (1777-1855) developed a mathematical theory for finding the magnetic potential of any point on Earth. His contemporary, the Danish physicist Hans Christian Oersted (1777-1851), became the first scientist to establish a clear relationship between electricity and magnetism. This led to the formalization of electromagnetism, the branch of physics devoted to the study of electric and magnetic phenomena.

The French mathematician and physicist André Marie Ampère (1775-1836) concluded that magnetism is the result of electricity in motion, and in 1831 the British physicist and chemist Michael Faraday (1791-1867) published his theory of electromagnetic induction. This theory shows how an electric current in one coil can set up a current in another through the development of a magnetic field, and it enabled Faraday's development of the first generator. For the first time in history humans were able to convert mechanical energy systematically into electric energy.

ELECTROMAGNETIC FORCE.

By this point scientists were convinced that a relationship between electricity and magnetism existed, yet they did not know exactly how the two related. Then, in 1865, the Scottish physicist James Clerk Maxwell (1831-1879) published a groundbreaking paper, "On Faraday's Lines of Force," in which he outlined a theory of electromagnetic force. The latter may be defined as the total force on an electrically charged particle, which is a combination of forces due to electric and magnetic fields around the particle.

Maxwell thus had discovered a type of force in addition to gravity, and this reflected a "new" type of fundamental interaction, or a basic mode by which particles interact in nature. Nearly two centuries earlier Sir Isaac Newton (1642-1727) had identified the first, gravity, and in the twentieth century two other forms of fundamental interaction—strong nuclear and weak nuclear—were identified as well.

In his work Maxwell drew on the studies conducted by his predecessors but added a new statement. According to Maxwell, electric charge is conserved, meaning that the sum total of electric charge in the universe does not change, though it may be redistributed. This statement, which did not contradict any of the experimental work done by other physicists, was based on Maxwell's predictions regarding what should happen in situations of electromagnetism. Subsequent studies have supported his predictions.

M AGNETISM

What, then, is the difference between electricity and magnetism? It is primarily a matter of orientation. When two electric charges are at rest, it appears to an observer that the force between them is merely electric. If the charges are in motion relative to the observer, it appears as though a different sort of force, known as magnetism, exists between them.

An electromagnetic wave, such as that which is emitted by the Sun, carries both an electric and a magnetic component at mutually perpendicular angles. If you extend your hand, palm flat, with the fingers straight and the thumb pointing at a 90° angle to the fingers, the direction that the fingers are pointing would be that of the electromagnetic wave. Your thumb points in the direction of the electric field, and the flat of your palm indicates the direction of the magnetic field, which is perpendicular both to the electric field and to the direction of wave propagation.

A field, in this sense, is a region of space in which it is possible to define the physical properties of each point in the region at any given moment in time. Thus, an electric field and a magnetic field are simply regions in which the electric and magnetic components, respectively, of electromagnetic force are exerted.

MAGNETISM AT THE ATOMIC LEVEL.

At the atomic level magnetism is the result of motion by electrons (negatively charged subatomic particles) in relation to one another. Rather like planets in a solar system, electrons 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 atoms, electrons are paired such that their magnetic fields cancel out one another. However, in certain cases, such as when there is an odd electron or when other factors become more significant, the fields line up to create what is known as a net magnetic dipole, or a unity of direction. These elements, among them, iron, cobalt, and nickel as well as various alloys or mixtures, are commonly known as magnetic metals, or natural magnets.

MAGNETIZATION.

Magnetization occurs when an object is placed in a magnetic field. In this field 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 a straight wire up through the interior of a spring.

When a natural magnet becomes magnetized (that is, when a magnetic 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, and the second is termed domain growth, or ferrimagnetism. Both processes turn a natural magnet into what is known as a permanent magnet—or, more simply, a magnet. The magnet 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 ferromagnet is stronger, because it requires a more powerful magnetic force field to become magnetized. Most powerful of all is a saturated ferromagnetic metal, one in which all the unpaired electron spins are aligned.