Facts of the Matter
Centuries of research link electricity and magnetism
WE LIVE in a sea of electromagnetism, bathed by light, which is only one thin band of possibilities on a continuous spectrum of infinite length.
Electromagnetism is a combination of electricity and magnetism, which were thought to be separate phenomena until the early decades of the 19th century.
Not much was known about either until 1600.
According to legend, in 900 B.C., Magnus, a Greek shepherd, walked across a field of black stones that pulled the iron nails out of his sandals and the iron tip from his shepherd's staff.
Three hundred years later, Thales of Miletos, a seminal figure of ancient Greek science, rubbed amber ("elektron" in Greek) with fur and picked up bits of feathers.
That was the extent of knowledge about electricity and magnetism until 1600. There was no apparent connection between the two phenomena.
In that year, William Gilbert published his great treatise, "De Magnete." It contained the results of his many years of research and provided the basis for a new science.
Gilbert summarized all that was known about electricity and magnetism. He wrote about the electrification of many substances other than amber. He made a lodestone sphere and showed that it had the same pattern as Earth's magnetic field.
Benjamin Franklin began working on electricity after he heard a lecture about it in Scotland in 1743. He described it as "a substance which is conserved, and which may be either 'positive' (in excess) or 'negative' (deficient) in a body."
In 1785, Charles Coulomb quantified the manner by which electric charges repel one another, now known as Coulomb's law.
The connection between electricity and magnetism was discovered by accident.
In 1820, Hans Christian Oersted planned to demonstrate the heating of a wire by an electric current, and also to give demonstrations of magnetism, for which he had a compass needle mounted on a wooden stand. He noticed that whenever the current was switched on in the wire, the compass needle moved.
Oddly, the compass needle was neither attracted nor repelled by the wire. Instead it was perpendicular to the wire, and the magnetic influence was circular around the wire as long as current flowed through it.
One week after hearing of Oersted's discovery, Andre Marie Ampere showed that parallel wires carrying electrical current attracted or repelled each other depending on the direction of the flow of current.
Oersted's discovery opened a major field of scientific inquiry that became the subject of study all over Europe.
Michael Faraday, a brilliant English scientist, was part of this effort and in 1821 put it to work to invent electromagnetic rotation, the principle behind the electric motor.
Faraday was working on a number of other projects, and it was 10 years before he could return to the study of electricity and magnetism.
In 1831 he discovered that the magnetism generated by a coil of wire could generate an electric current in another coil, thereby building the first electrical transformer.
A few months later he rotated a copper disc between the poles of a horseshoe magnet to generate a continuous direct current. This was the first generator.
He had discovered that a changing electric field induces a magnetic field, and a changing magnetic field induces an electric field.
Faraday was not mathematically trained and had difficulty understanding the mathematical musings of Ampere and others who were studying the EM phenomenon.
Instead, he visualized a magnet as if there were lines of magnetic force emanating from it and curving around it to the magnet's other pole.
He then expressed the electric current induced in the wire in terms of the number of lines of magnetic force that the wire "cut" through.
Most of the mathematical physicists of Europe rejected or openly ridiculed the "lines of force" concept, but it captured the attention of James Clerk Maxwell, a Scottish theoretical physicist.
In 1873, Maxwell published "Treatise on Electricity and Magnetism," in which he summarized and synthesized the discoveries of Coloumb, Oersted, Ampere, Faraday and others in four equations that are used today as the basis of EM theory.
At the beginning of the treatise, Maxwell wrote, "Before I began the study of electricity I resolved to read no mathematics on the subject until I had first read Faraday."
This was a significant and brilliant move.
Faraday's work had made little sense to mathematicians who could not visualize those imaginary lines of force. But Maxwell was a skilled and creative mathematician who saw a way to describe the field lines and their changes mathematically.
Maxwell later wrote, "I found that ... Faraday's methods ... begin with the whole and arrive at the parts by analysis, while the ordinary mathematical methods were founded on the principle of beginning with the parts and building up the whole by synthesis."
The equations predicted that a combined electric field and magnetic field could propagate perpendicular to one another through space by a process of mutual induction: A changing magnetic field induces a changing electric field, which induces a changing magnetic field and so on.
His calculations showed that the mutual induction would have the form of undulations and could only sustain itself if it moved at the speed of light.
In the following decade, Heinrich Hertz generated and detected EM waves, which led ultimately to the generation, transmission and proliferation of radio waves.
Today we know that the range of possible wavelengths of EM waves is virtually infinite.
We call the entire range of EM radiation the "electromagnetic spectrum." It includes radio, microwave, infrared, visible, ultraviolet, X-ray and gamma radiation.
The longest detected EM wavelengths are millions of miles (100 million times longer than waves in the middle of the AM radio bandwidth). They are generated by pulsations in Earth's magnetic field.
The shortest EM waves are gamma rays. Their wavelengths are the size of an atomic nucleus, one one-hundreth of a nanometer or shorter. They are generated by nuclear reactions in radioactive atoms, in nuclear explosions and from processes taking place in objects such as pulsars, quasars and black holes.
Visible light is in the range of 400 to 700 nanometers, the rainbow that is a very small fraction of the entire EM spectrum.
We live in a sea of EM radiation of all possible wavelengths. Some of them are naturally occurring, and some are man-made, but we are deeply immersed in them.
Some, like sunlight, are intense. Others, like the radio signals exchanged with a spacecraft 10 billion miles from Earth, are infinitesimally small.
We are living in a dense sea of EM radiation, the vast majority of it as undetectable to our naked senses as water is to a fish.
picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at email@example.com