How can an iron magnet produce such changes? Edmond Halley (of comet fame) ingeniously proposed that the Earth contained a number of spherical shells, one inside the other, each magnetized differently, each slowly rotating in relation to the others.

[Only for those pursuing the math: this is not the basic force formula. Given two short parallel currents I₁ and I₂, flowing in wire segements of length L₁ and L₁ and separated by a distance R, the basic formula gives the force between them as proportional to
I₁ I₂ L₁ L₁/R²

(it gets further complicated if the currents flow in directions inclined to each other by some angle). To find then the force between wires of complicated shape that carry electrical currents, all these little bitty contributions to the force must be added up. For two straight wires, the final result is as above--a force inversely proportional to R, not to R²]

•     A pocket compass.
•     A one-foot (30 cm) length of fairly thick wire, insulated or bare.
•     A 1.5 volt electric cell ("battery") of size "D" or "C". The voltage is too low to cause any risk.
  1.     Lay the compass on a table, face upwards. Wait until it points north.
  2.     Lay the middle of the wire above the compass needle, also in the north-south direction (compare to the above image "What Oersted Saw"). Bend the ends of the wire so that they are close to each other.
  3.     Grab one end of the wire in one hand and press against one end of the battery.
  4.     Grab the other end with your other hand, and press momentarily against the other terminal of the battery. The needle will swing strongly by 90 degrees.
         Quickly disconnect (it is not good for the battery to draw such a large current). The needle will swing back to the north-south direction. Note that no iron is involved in producing the magnetic effect!

  5.     Repeat with the connections of the battery reversed. Note that the needle now swings 90 degrees in the opposite direction.

  6.     Take a piece of paper 2"x4" (5x10 centimeters) and fold the longer side into pleats, about 3/8" (1 centimeter) high. Put the wire on the table, its middle in the north south direction, put the pleated paper above it so that the wire is below one of the pleats, and place the compass on top of the pleats. (Or else, use a small block of wood, with a groove cut in its bottom for the wire.)
         You can now repeat the experiment with the compass above the wire (if two people perform the experiment, they need no pleats or table--one can old the compass, the other the wire and battery). Note that the needle swings in the opposite direction than when the compass was below the wire.

Futher reading:

--"From Falling Bodies to Radio Waves" by Emilio Segre, W.H. Freeman and Co., 1984, gives a very good account of the history of electricity and magnetism (and of physics up to 1895). Segre, who won the Nobel prize in physics, wrote in a clear style with many insights and anecdotes about the discoveries which laid the foundations of physics.

--"Oersted and the Discovery of Electromagnetism" by Bern Dibner (Blaisdell Publ. Co., 1962), a slim book with details about Oersted and his time.

--"Andre-Marie Ampere" by L.Pearce Williams, Scientific American January 1989, p. 90.

--"Edmond Halley, Geophysicist" by Michael E. Evans, Physics Today, February 1988, p. 41-45.

Click here for a full size version of this image.

During magnetic storms, the glow may move southwards, and on occasion it can be seen in much of the US. It often appears as a glow on the horizon, like the glow preceding sunrise, and has therefore become known among scientists as "aurora borealis" ("aurora" for short), Latin for "northern dawn." A similar phenomenon is also seen in southern polar regions.

To an observer, an aurora is a fascinating spectacle, constantly moving and changing. It usually consists of many near-vertical greenish rays, forming long arcs and curtains, which stretch like ribbons across the sky, often from horizon to horizon. An example is shown on the left, a woodcut by the great polar explorer Fridtjof Nansen (1861-1930). The rays constantly fade while new ones appear, and during "magnetic substorms" (described in a later section) the arcs move rapidly and expand.

Location

Auroral light is produced at a height of about 100 km (60 miles) when fast electrons, arriving from space, slam into atoms and molecules of the atmosphere. The computer screen displaying these words is probably lit up in a similar way, by a beam of fast electrons accelerated electrically towards it, then steered and modulated so as to form letters and pictures.

Elias Loomis of Yale University compiled, in 1860, a map marking how many times in an average year were auroras observed in various locations (click here to see his map). A more accurate map was compiled in 1881 by Hermann Fritz (1830-1883) (click here to see Fritz'es map).

The Norwegian physicist Kristian Birkeland (1867-1917), for instance, placed a magnetized sphere, a "terrella" representing the Earth, inside a vacuum chamber, and aimed a beam of electrons towards it. He was gratified to see that the electrons were steered by the magnetic field to the vicinity of the terrella's magnetic poles.

Nowadays scientific satellites regularly cross streams of auroral electrons and measure their properties, and aurora is also observed from the ground with video cameras and special radars.

Note on Birkeland's terrella experiment

Futher reading:

• "Majestic Lights, The Aurora in Science, History and the Arts" by Robert H. Eather, American Geophysical Union, 1980.

• Some details about Birkeland's work and further references to it can be found in "A Brief History of Magnetospheric Physics Before the Spaceflight Era" by David P. Stern, Reviews of Geophysics, 27, 103-114, 1989, included on this web site.

Next Stop: #4.  Electrons

Click here for a full size version (232 K) of this image.

Later scientists such as Birkeland used the name "terrella" for magnetized spheres used inside vacuum chambers, together with electron beams, to study the motion of fast charged particles near the Earth. A sophisticated terrella experiment in a vacuum chamber is currently operated by Dr. Hafez u-Rahman at the University of California at Riverside.

How do we know?

Suppose the connections of the first battery are reversed, so that now the coil is positive and the plate is negative. Then no current flows, showing that the hot wire releases only negative particles, not positive ones. These particles were named electrons

In laboratory experiments these particles were directed by electrically charged structures (similar to the "electron guns" inside TV picture tubes) to form beams. Those beams were then bent by magnets and by electrified plates, and their behavior was studied. From such experiments and others the mass of the emitted particles, which became known as "electrons", could be determined. It turned out that they were rather lightweight. The simplest atom, that of hydrogen, contains a central positive particle, a proton, and a single electron, and the proton is nearly 2000 times heavier.

Light, like heat, can also knock electrons out of a metal. If the heated coil in the drawing is replaced by a clean metal plate, and light shines onto it, electrons are again released, and current will flow in the circuit. The explanation of this phenomena, called the photoelectric effect, earned Albert Einstein the 1921 Nobel Prize.

The same process will charge a spacecraft orbiting in the sunlight positively, to a few volts. Sunlight knocks out electrons from the surface and a few manage to escape, leaving the spacecraft positively charged; the situation then stabilizes, because the positive charge prevents any more electrons from leaving.

The word "elektron" in Greek means amber, the yellow fossilized resin of evergreen trees, a "natural plastic material" already known to the ancient Greeks. It was known that when amber was rubbed with dry cloth--producing what now one would call static electricity--it could attract light objects, such as bits of paper.

During the 1800s it became evident that electric charge had a natural unit, which could not be subdivided any further, and in 1891 Johnstone Stoney proposed to name it "electron." When J.J. Thomson discovered the light particle which carried that charge, the name "electron" was applied to it. The many applications of electrons moving in a near-vacuum or inside semiconductors were later dubbed "electronics."

Further reading:

• To mark the 100th anniversary of the discovery of the electron, the Center for History of Physics of the American Institute of Physics has created on the world wide web a suitable exhibit: http://www.aip.org/history/electron.

• Sir George Thomson, J. J. Thomson's son, was a renowned physicist in his own right and won the Nobel prize in 1937. On the 70th anniversary of the discovery of the electron he wrote about that discovery and about later developments where electrons were discovered to act sometimes like waves: "The Septuagenarian Electron", Physics Today, May 1967, 55-61

• The story of J. J. Thomson is also briefly given in the first chapter of "From X-Rays to Quarks" by Emilio Segre (W.H. Freeman and Co., 1980). Segre was a physicist who won the 1959 Nobel Prize and his concise history of modern physics is filled with insights and stories, some of them drawn from his own experience.

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 Humphrey 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.

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. Faraday's younger colleague James Clerk Maxwell, a mathematical physicist of enormous creative insight, fleshed out these ideas in rigorous mathematical terms, and "Maxwell's equations" are now the cornerstone of electromagnetic theory.

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.

Electric forces in nature come in two kinds. First, there is the electric attraction or repulsion between (+) and (-) electric charges. It is possible to use this to define a unit of electric charge, as the charge which repels a similar charge at a distance of, say, 1 meter, with a force of unit strength (actual formulas make this precise).

But second, there is also the attraction and repulsion between parallel electric currents. One could then define the unit of current, as the current which, when flowing in a straight wire, attracts a similar current in a parallel wire 1 meter away with a force of unit strength, for every meter of the wires' length.

But electric current and charge are related! We could have just as well based the unit of current on the unit of charge--say, as the current in which one unit of charge passes each second through any cross section of the wire. This second definition turns out to be quite different, and if meters and seconds are used in all definitions, the ratio of the two units of current turns out to be the speed of light, 300,000,000 meters per second.

In Faraday's time the speed of light was known, although not as accurately as it is today. It was first derived around 1676 by Ole (Olaus) Roemer, a Danish astronomer working in Paris. Roemer tried to predict eclipses of Jupiter's moon Io (mentioned later here in an altogether different connection) and he found a difference between actual and predicted eclipse times, which grew and then decreased again as the Earth circled the Sun. He correctly guessed the reason, namely, as the Earth moved in its orbit, its distance to Jupiter also went up and down, and light needed extra time to cover the extra distance.

But what was the meaning of the link between electricity and light?

Remember the idea of Faraday which evolved into the "magnetic field" concept--that space in which magnetic forces may be observed is somehow changed? Faraday also showed that a magnetic field which varied in time--like the one produced by an alternating current (AC)--could drive electric currents, if (say) copper wires were placed in it in the appropriate way. That was "magnetic induction," the phenomenon on which electric transformers are based.

So, magnetic fields could produce electric currents, and we already know that electric currents produce magnetic fields. Would it perhaps be possible for space to support a wave motion alternating between the two? Sort of:

With that term added (the "displacement current"), the equations of electricity and magnetism allowed a wave to exist, propagating at the speed of light. The drawing below illustrates such a wave--green is the magnetic part, blue the electric part--the term Maxwell added. The wave is drawn propagating just along one line. Actually it fills space, but it would be hard to draw that.

Electromagnetic Wave (see text above)

Maxwell proposed that it indeed was light. There had been earlier hints--as noted above, the velocity of light had appeared unexpectedly in the equations of electricity and magnetism--and further studies confirmed it. For instance, if a beam of light hits the side of a glass prism, only part of it enters--another part gets reflected. Maxwell's theory correctly predicted properties of the reflected beam.

Then Heinrich Hertz in Germany showed that an electric current bouncing back and forth in a wire (nowadays it would be called an "antenna") could be the source of such waves. (The current also produces a magnetic field in accordance with Ampere's law, but that field decreases rapidly with distance.) Electric sparks create such back-and-forth currents when they jump across a gap--hence the crackling caused by lightning on AM radio--and Hertz in 1886 used such sparks to send a radio signal across his lab. Later the Italian Marconi, with more sensitive detectors, extended the range of radio reception, and in 1903 detected signals from Europe as far as Cape Cod, Massachussets.

It was presumed that light from the hot wire of a lightbulb was emitted because the heat caused electrons to bounce back and forth rapidly, turning each into a tiny antenna. When physicists tried to follow that idea, however, they found that the familiar laws of nature had to be modified on the scale of atomic sizes. That was how quantum theory originated.

Gradually other electromagnetic waves were found The wave nature of light causes different colors to be reflected differently by a surface ruled in fine parallel scratches--which is why a compact laser disk (for music or computer use) shimmers in all colors of the rainbow. The orderly rows of atoms in a crystal also form parallel lines but spaced much more closely, and they turned out to have the same effect on X-rays, showing that X-rays, like light, also were electromagnetic waves, but of a much shorter wavelength. Later it was found that beams of electrons in a magnetic field, inside a vacuum tube, could become unstable and emit waves longer than light: the magnetron tube where this occured was a top-secret radar device in World War II, and it later made the microwave oven possible.

Electromagnetic waves led to radio and television, and to a huge electronic industry. But they are also generated in space--by unstable electron beams in the magnetosphere, as well as at the Sun and in the far-away universe, telling us about energetic particles in distant space, or else teasing us with unresolved mysteries. You can find more about this in the section on high energy particles.

Next Stop: #7.  Plasma

• David P. Stern - NASA/GSFC Code 695 (u5dps@lepvax.gsfc.nasa.gov)
• Mauricio Peredo - Raytheon STX Corporation (peredo@istp1.gsfc.nasa.gov)