History

Since the third century BC Chinese sources mention the attractive properties of the magnetic stone which oftenis called  "tzhu shih" meaning "loving stone", they say of it: "The magnet attracts the iron, such as a tender mother gathers her childre. For this reason he was named the first compasses, so-called magnetic car or "car, pointing to the south", China had compasses at least since the first century AD. It was not until the 13th century, twelve hundred years later, that  compasses moved  on the West.

Even in Ancient Greece the lodestone * is known, but his enigmatic force was hard to explain. Plato (428 -.. 347 BC) considered them divine. So amazing greedily drew the lodestone of iron, that he seemed to live. Thales and Anaxagoras even wrote to him its own soul.

Even in later centuries it got no further with the explanation. So held Alexander Aphrodiseus, a thinker to 200 n. Chr., The question of the magnets for unsolvable. Seven centuries later, is the resigned answer to the question why the lodestone attracts the iron, only: "because he has a natural strength for it."

Such puzzles have always captured the imagination, and so was the old legend of the seamen magnetic hill who pulls all provided with iron ships per se to have shattered them, wide spread in the tales of many cultures. Until the 16th century the mysterious lodestone remained content of legends . From diamonds of the lodestone invalidated until garlic, which, eaten by sailors, stunned the compass and thus the ship diverts from the course, are some of the wide ranging ideas of magnetic history.

In humans, the magnet can act differently. So it was believed that a specified magnet under the pillow of an unfaithful wife hase the strength to hurl them out of bed. Burglar could have used the magic power for their sinister purposes. Because would sprinkle magnetic particles on hot coals and distribute them in rooms, then spread a smoke that hunting the occupants of the house into the flight and the thief time leave to take their valuables in.

The magnet was also attributed healing properties. Held in hand, he heal pain and spasm of the feet; annealed and pulverized, he assuage fire damage to the skin.

As the oldest experimental physical representation of magnetism applies the written in the 13th century French scholar Petrus Peregrinus of writing that could establish that the north and south poles of two magnets attract each other while like poles repel each other. If one breaks a magnet, so produces two magnets with two poles: to obtain a pole alone, would not be possible. Peregrinus still generated other findings: Researchers had formed a large magnet as a ball. An iron needle by a magnet so tightened that it always depends on the poles. He put them on different points of the ball, pulling the needle along lines around the stone. This cut at the poles as the meridians of the globe. But could Peregrinus, who with his ball not the Earth (this was regarded as a flat disc), but recreate the heavens tried not draw from this analogy the correct conclusion that the Earth behaves like a giant magnet. After all, he was not referring, as others did, the alignment of the needle on the attraction north swimming  or to the North Star, but a force that starts at the "celestial poles" around which, as it was believed, the vault of heaven turned over the resting ground.

The systematic, scientific research in the modern sense of magnetism has its beginning with the published the Latin work "De Magnete" (On the Magnet) of the English scientist William Gilbert in 1600. Meanwhile, many of the basic properties of magnetic materials have been discovered, and the cause and effect of a magnetic field could be largely solved. But many questions remain unanswered even today.

So you have the history of the earth's magnetic field and its strength can be measured very accurately, but its origin or its effects on animals and plants have to be justified in greater detail.

Particularly puzzling are the "magnetic monopoles", phenomena which, as you learned in school, does not exist actually. Since Peter Peregrinus was believed to know that the characteristic of all magnets is their bipolarity. The British Nobel laureate Paul Dirac was there, in 1931, first toyed with the idea that there could be an elementary particle, which would correspond to a monopoly. Experimental physicists are looking for these exotic items today in the hope of finding evidence through their answers to some still unresolved issues of nuclear physics.

The following will be outlined on how and where it is the sciences were able to uncover some "secrets" of magnetism, and which still has to be further researched.

Changes and reversals - The mystery of the geomagnetic field

In 1905 Albert Einstein described terrestrial magnetism as one of the five most important unanswered questions in physics. Today, the question of its origin is still waiting for an all-encompassing explanation.

Naturally can be considered certain only a few statements about the interior of the globe. The events the Earth's core can neither observed nor the prevailing pressure and temperature are simulated in the laboratory. To over Seismological measurements on huge ball the size of the Earth's core to be able to make well-founded statements, one has to rely, which are carried out from the surface. Observations suggest that the Earth's core from one with a diameter of 3485 kilometers, or about from planet Mars, there. At the center of the Earth's core is the solid inner core, which has a radius of about 1,200 kilometers. The pressure in these depths rises to 1.3 to 3,5 muliple of millions of atmospheric pressure on the surface. The temperatures are estimated at 4000 to 5000 degrees Celsius.

As difficult as winning over the Earth's core of knowledge is to establish a theory of the origin of the magnetic field. We separated the mantle, an at least approximately three thousand kilometers thick layer of matter, from the outermost zone of the Earth's core. We know and do not know how the box looks like there or how it is produced. It is believed that it is ten times stronger in the vicinity of the earth's core than at the surface.

Indirect but you can make about the origin of the field statements by observing the current magnetic field. The measurements provide valuable information on the short-term behavior of the field, for example, on its shape and on the so-called secular variations, which are the usual fluctuations in periods of decades and centuries. But here also pass historical information sources. So scientists have been able to gain insights into earlier geomagnetic field strengths using museum exhibits and collection pieces. Pottery products that have an age of 6000 years in part were investigated by Chinese researchers who concluded that about 2000 years ago the magnetic field reached a maximum, which was about 50% higher than today's field.

Since the eighteenth century, detailed maps created for navigation purposes, which give information about the strength and direction of the geomagnetic field read. But the maps also show how the field has changed in the last three centuries. Recorded a gradual, uniform decrease in the field strength, which if they would last, would lead to their complete disappearance in about 1500 years. The maps also show a slow shift of some irregular vortex of the field in a westerly direction by about a longitude every five years. In these vertebrae a magnetic needle deviates more or less heavily on the north-south direction.

To study the paleomagnetic field (which is the field that has as it looked millions of years ago), it is necessary to examine the rocks of the Earth's crust. For this, the age of the rock is radiometrically dated and the orientation of its magnetic inclusions determined that reflect the direction of the magnetic field at the time of the rock formation. This paleomagnetic evidence proving that the earth since at least 3.6 million years has a magnetic field. During this time, the strength of the field has changed several times. There is also evidence that the Earth's magnetic field has reversed many times.

As early as 1906 had the French physicist Bernard Brunhes, who had discovered such rocks, theorized about the reversal of the magnetic field. His thesis sparked over half a century of fierce debate. Only when studies were evaluated by radiometrically dated lava stones in the sixties, it was adopted in geophysics as certain that Earth's magnetic field can be reversed. Two polarity states can exist. In the one, which is designated by convention as the "normal" state, shows the north-facing compass needle to the north magnetic pole. In "reverse" or reverse state they would be directed to the magnetic south pole.

As secured, that it has come in the last 3.6 billion years to nine major reversals, with the most recent took place years ago, about 730 000 years. The earth's magnetic field tilts not abruptly from one state to the other polarity to, but gradually fades until it into a neutral, free magnetic phase occurs, in which it remains about 1,000 years. Then it can return to its original polarity state or switch to the opposite state. 98 percent of the time remains stable field.

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