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Magnetism is the term applied to describe the property of a body that experiences a force of attraction or repulsion to another body, without any apparent physical connection between them. These bodies, called magnets, have been known for over 2,000 years, indeed the word magnet was given to a stone noted by the ancient Greeks to show this attraction or repulsion to an iron rod, because it was found near the ancient city of Magnesia. This stone, known as lodestone or magnetite, is an oxide of iron and occurs naturally. It is believed that the ancient Chinese also knew of the ‘magical’ power of lodestone, and they are credited as the inventors of the magnetic compass. They found that when an iron rod was brought close to a magnet, the rod itself became magnetized and behaved as an individual magnet when it was removed from the original magnet. Later they discovered that if they held the rod in the middle and allowed it freedom to rotate around the centre of its length, the rod would tend to align itself with its length in a north-south direction. This property was soon applied in navigation, particularly for maritime uses, where the absence of landmarks and bad weather obscuring the stars could cause ships to go astray (see marine compass).
Characterization of the behaviour of magnets was carried out in the 13th century by Petrus Peregrinus, a French engineer. Using a spherical lodestone and a magnetic compass, he discovered that he could follow lines around the lodestone where the compass remained in a fixed position. All these lines intersected at two points which he called north and south poles, and he showed that like poles repel each other while opposite poles attract each other. It may have seemed logical, in terms of his experimentation, to try to isolate the poles from each other, but Peregrinus found that cutting a magnet in two served only to create two new magnets rather than, as he might have assumed, to give one north pole and one south pole.
It might seem strange, given the benefit of hindsight, that it was not until 1600 that William Gilbert first published the idea that the Earth acts as a huge magnet. It seems obvious, following the work of Peregrinus, as a magnetic compass needle would be attracted to the Earth\'s magnetic poles—analogous to the spherical lodestone.
By 1785, Charles Coulomb had successfully shown that the force between two magnetized bodies was inversely proportional to the distance between them. As well as creating the first quantitative law for magnetism, Coulomb pre-empted today\'s molecular theory of magnetism. It is believed today that a piece of magnetic material consists of individual atomic magnets which, internal to the material, may be thought of as cancelling north to south respectively, and that only at the ends of the material would there be no cancellation, giving rise to north and south poles. This would explain why, when a magnet is cut, ‘new’ north and south poles form to create two magnets from the halves of the original magnet.
Modern magnetic theory may be said to have started with the discovery of electromagnetism by Hans Oersted in 1820. Oersted noted that an electric current flowing in a wire close to a magnetic compass was seen to deflect the compass needle. From this observation he deduced that an electric current produces a magnetic field and several years later in 1831 Michael Faraday proved the converse, that a magnetic field could produce an electric current.
Since the advent of electromagnetism in the early 19th century, and the realization that magnetism and electricity are very close relatives, a myriad of applications have used this theory. Magnetism plays a key role in the generation of all our electric power requirements, magnetic tape is used in our computer disks and audio and video cassettes as memory units, and radio uses electromagnetic waves. AC |
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