A 9.4 Tesla electromagnet built for brain research, about 200,000 times more powerful
than Earth's magnetic field. Credit: Siemens press picture.

Electrodynamic Duo Part One
Feb 08, 2011

The Greeks were familiar with magnetis lithos, or "Magnesian stone" as early as 500 B.C.E. Magnesian stone, from which the word "magnet" is derived, was originally found on the coast of what is now Turkey. Later, the magnetic rocks came to be known as "lodestones" and were probably used as compasses to guide ships since the eleventh century.

Not much was known about how compasses worked until the time of Sir William Gilbert, commonly referred to as "the father of magnetism and electricity" because of experiments leading to the groundbreaking book, De Magnete in 1600. Gilbert was the first person to use terms like “magnetic pole,” “electric force,” and “electric attraction.” He also coined the word "electricity" from the Greek word for amber, elektron.

In 1820, Danish physicist Hans Christian Oersted found that electric current flowing through a wire deflected a compass needle, inspiring electromagnetic theory. When he placed a magnetic compass below an electric current, the needle moved perpendicular to the wire.

A magnet is simply any object that possesses magnetic properties. Primarily, that means two poles known as "north-seeking" and "south-seeking," which on separate magnets attract each other. Like poles of different magnets, on the other hand, repel each other in the same way that opposite electric charges attract or repel.

In the eighteenth century, C. A. Coulomb discovered that the force between magnetic poles could be described using the same inverse square relationship as that between electric fields. Just as with electricity, magnetism is directly proportional to the strength of the combined poles and inversely proportional to the square of the distance between the poles. Also similar to electricity, magnetism acting at a distance is described as a "field of force" because it has no known physical component.

"Magnetic flux" is a term that illustrates how force appears to flow out of a magnet at one pole and back into it at the other. This effect can be seen in the patterns that form when iron filings are sprinkled on a paper with a magnet underneath it. The patterns are known as "lines of induction." There is no physical flow, but the lines of induction confirm descriptions of magnetism. Lines of induction (magnetic field lines) emanate from a magnet's north pole and terminate at the south pole. The number of lines per a given area indicates the field strength: where the lines converge at the poles, the field is large, while the field becomes progressively weaker where the lines diverge.

The electrical basis for magnetism has been verified down to the level of the electron. Since an electron spins and has an electric charge, it is often said to be "charge in motion, which by definition results in a magnetic field.

In 1825, Ampère demonstrated that a current-carrying conductor exerts forces on a magnet and that a magnet also exerts forces on a current-carrying conductor. Michael Faraday and Joseph Henry followed with their discoveries that current can be generated in a conductor by changing the magnetic field surrounding it. So-called "electromagnetic induction," along with the realization that electric currents create magnetic fields, paved the way for both the electric generator and the electric motor.

Further relationships between electricity and magnetism were elucidated by James Clark Maxwell. In particular, a changing electric current in a conductor creates a changing magnetic field around the conductor, thus creating a changing electrical field. Oscillating electric and magnetic fields (now called "electromagnetic radiation") can therefore become self-sustaining, like a wave propagating through space. Also, since the velocity of electromagnetic radiation is identical with the velocity of light, light's close connection with electricity and magnetism was revealed.

Solid, liquid, and gas are the commonly understood states of matter, although the readers of these pages are aware that a fourth state known as "plasma" exists. Plasma is estimated to constitute 99 percent or more of the Universe, and is distinctive because it contains a large enough number of electrically charged particles for its electrical properties and behavior to be altered.

In a neutral gas, positive and negative charges combine: the positive charges (protons) in the atomic nuclei are surrounded by an equal number of negatively charged electrons. The atoms are electrically neutral. Gas becomes plasma when heat or some other energy strips some electrons off some of the atoms. A positive charge is left on those atoms, while the detached negative electrons move around. This results in an electrically charged gas that is said to be "ionized." When enough ions accumulate so that the electrical characteristics of the gas are affected, it is a plasma.

Irving Langmuir was first to use the term "plasma" (referring to an ionized gas) in the 1920s. He noticed similarities in the structure of electric discharges through mercury vapor. Regions close to the walls in the glass discharge chamber, or near the electrodes were already called "sheaths." However, the ionized material filling the discharge chamber had no name, so Langmuir decided to call it "plasma." Plasma was found to be an excellent electrical conductor with behavioral laws all its own.

Stephen Smith