When researchers map the three-dimensional flow of a river around a bridge pier, or of wind around the wing of an airplane (picture below), they use streamlines, lines that trace the flow of particles of water or air.



Magnetic field lines similarly describe the structure of magnetic fields in three dimensions.They are defined as follows. If at any point on such a line we place an ideal compass needle, free to turn in any direction (unlike the usual compass needle, which stays horizontal--such needles exist, see bottom of page) then the needle will always point along the field line (drawing below).

Field lines converge where the magnetic force is strong, and spread out where it is weak. For instance, in a compact bar magnet or "dipole," field lines spread out from one pole and converge towards the other, and of course, the magnetic force is strongest near the poles where they come together. The behavior of field lines in the Earth's magnetic field is very similar.

Michael Faraday

The son of a blacksmith, Faraday was apprenticed to a bookbinder and often read books brought in for rebinding. Luckily for science, one of those was the volume of the Encyclopaedia Britannica with the article about "electricity." His interest drove him to popular lectures given by Humphry Davy, Britain's leading chemist ("he lived in the odium/of having discovered sodium"), and when Davy needed an assistant, Faraday landed the job on the strength of notes he had kept of Davy's lectures. There followed a lifelong career in physics and chemistry, with many notable achievements.

Maxwell

Most scientists nowadays view field lines as intangible abstractions, useful only for describing magnetic fields. Faraday, however, felt that they represented more, that space containing magnetic "lines of force" was no longer empty but acquired certain physical properties. In 1846 he speculated that light was just a wave propagating along such lines--like the wave on a rope tied at one end and shaken at the other

Following Maxwell, we nowadays call a space modified by the presence of magnetic field lines a "magnetic field": if a bar magnet is placed there, it will experience magnetic forces, but the field exists even when no magnet is present. Similarly, an "electric field" is the space in which electric forces may be sensed--for instance between metal objects charged (+) and (-) by a battery, as in the drawing accompanying the discussion of the electron.

Maxwell also showed (perhaps his greatest achievement) that an "electromagnetic wave" was possible, a rapid interplay of electric and magnetic fields spreading with the velocity of light. Maxwell correctly guessed that light was in fact such a wave, that it was basically an electromagnetic phenomenon, and with this his equations paved the way to a much deeper understanding of optics, the science of light.

Maxwell's younger colleague, the German Heinrich Hertz, calculated in 1886 that waves of this type would be broadcast by a rapidly alternating current in a short antenna. He then obtained such a current from an electric spark (which does produce a fast back-and-forth oscillation of electric charge) and demonstrated his "Hertzian waves" experimentally. His work was continued by scientists all over the world--e.g. by the Russian Alexander Stepanovich Popov who around 1895 detected radio waves from lightning (a natural spark!), and by the Italian Gugliemo Marconi who, at about the same time, developed the first commercial radio applications.

The waves that carry radio and television, microwaves, infra-red, visible light, ultra-violet, x-rays and gamma rays are all variations of the same basic process envisioned by Maxwell, namely, they all belong to the family of electromagnetic waves.

It may seem strange that empty space can be modified by electric and magnetic influences, as the field concept proposes. Yet it allows one to understand light and radio waves, and also to retain the conservation of energy. When a transmitter on a spacecraft broadcasts a radio signal, most of that signal spreads out into space and never reaches Earth. Is its energy lost? No, it now resides in an ever-spreading electromagnetic field, associated with the radio wave.

Further reading:

--A personal, readable biography of Maxwell: "The Man who Changed Everything: The Life of James Clerk Maxwell" by Basil Mahon, Wiley 2003, 254 pp., $27.95. Reviewed in Nature, 425, p. 765-6, 23 October 2003.

Faraday's 1846 lecture "Thoughts on Ray Vibrations" (with note added at the end).

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