Magnus magnes ipse est globus terrestris. [The whole Earth is a magnet.]
William Gilbert
The discovery of the magnetic compass was an event of IMMENSE importance in the history of civilization. It was the first documented self-registering instrument created by mankind that could be read using a precise scale. Of course the sundial had been around for a while, but it doesn't move (just it's shadow does). And there was the wind vane, but no precise way existed to measure wind direction. If you think about it, it's pretty hard to measure direction in general, without a compass. The earliest known compass surfaced in China by at least the first century A.D. It apparently didn't arrive in Europe until the twelfth century A.D. ,however,when it was first referenced by an English monk, Alexander Neckham. Nobody seems to know how Alexander got his hands on it though?
It is well to observe the force and virtue and consequence of discoveries, and these are to be seen nowhere more conspicuously than in printing, gunpowder, and the magnet.
Francis Bacon
According to thireenth-century philosophy, the compass needle points towards the North star. Unlike all other stars in the night sky, the north star appears to be fixed. Thus, philosophers reasoned that the lodestone obtained its "virtue" from this star. Problems began to arise with this theory, however, when people began to measure the property of declination. It is often said, although highly disputed, that Christopher Columbus first discovered declination in the European region during his first voyage to the West Indies in 1492. Declination was old news in China by this time however. The first reported observation of magnetic declination appears to have been made in about A.D. 720 by the Buddhist astronomer I-Hsing. It's too bad Alexander didn't find out about declination along with the compass.
We know that the magnet loves the lodestone, but we do not know whether the lodestone also loves the magnet or is attracted to it against its will.
Arab physicist of the twelfth century
The question of why the Earth has a magnetic field was one of the first to be asked in the arena of magnetic research and, perhaps even today does not remain fully understood. In De Magnete, William Gilbert, later the doctor of Queen Elizateth, identified that the Earth's magnetic field had its origin from inside the Earth. He proposed that their was a lodestone with a permanent magnetic field associated with it at the center of the Earth. Later, in the seventeenth century, Rene Descartes proposed that the Earth's magnetism was channelled in one-way ducts through the Earth that exited and entered at the North and South poles. These theories are no longer accepted however. We now know that rocks aren't strongly enough magnetized to account for the observed geomagnetic field and that below depth of about 30 km, rocks are so hot that they are above their Curie temperature and are nonmagnetic. Another difficulty with these theories is that the Earth's magnetic field varies over time scales from about a decade to 10,000 years.
More recently, Albert Einstein, described the problem of the origin of the Earth's magnetic field as being on of the five most important unsolved problems
in physics. Things have become a little better since then, but attempting to model the geomagnetic field with any vigor has proven to be an illusive goal. It is now commonly believed that the Earth's magnetic field is produced by fluid dynamos in the Earth's mantle. Magnetic fields are produced when current moves around a coiled wire. The idea is that fluid motions play the role of a rotating current inside the Earth. Theoreticians are still arguing over the exact style of the fluid motion, but it is generally thought that the fluid motion is driven by some form of thermal convection and controlled by the primary equation of fluid dynamics, the Navier-Stokes equation.
The mystery of magnetism, explain that to me! No greater mystery, except love and hate.
John Wolfgang von Goethe
Perhaps the most common question that a researcher in global magnetism gets asked is, "Why does the Earth's magnetic field reverse?". There is a good deal of evidence that the Earth's magnetic field does, and has, reversed many times in the Earth's history, but it is very difficult to use this information to directly answer the question of "Why" . Part of the answer to this question lies in the equations that describe dynamos. For each solution that yields a magnetic field of normal polarity there exists another solution that yields a field of reversed polarity. Either of the two polarities may result, depending upon the orientation of whatever weak magnetic field was present when motion was initiated. This arguement only allows for the possibility of two seperate polarities however, and has not explained the reason that the magnetic poles actually do reverse. All dynamos may exist in either of two symmetrical ploarity states, but not all dynamos flip back and forth between the two states. Relatively rare events like reversals may well be due to random occurrences of certain poorly understood patterns of fluid motion in the molten interior of the Earth. This answer may not be particular pleasing, but there is much that needs to be learned about dynamos within which random turbulence is an important process.
Errors using inadequate data are much less than those using no data at all.
Charles Babbage
