Theory of Magnetism
Chapter 1
Introduction:
What is magnetism?
It has been known since antiquity that “loadstone” (magnetite, Fe3O4) and iron attract each other. Plato (428/427–348/347 B.C.) and Aristotle mention permanent magnets. They are also mentioned in Chinese texts from the 4th century B.C. The earliest mention of a magnetic compass used for navigation is from a Chinese text dated 1040–1044 A.D., but it may have been invented there much earlier. It was apparently first used for orientation on land, not at sea. Thus magnetism at first referred to the long-range interaction between ferromagnetic bodies. Indeed, the present course will mainly address magnetic order in solids, of which ferromagnetism is the most straightforward case. This begs the question of what it is that is ordering in a ferromagnet. Oersted (1819) found that a compass needle is deflected by a current-carrying wire in the same way as by a pemanent magnet. This and later experiments led to the notion that the magnetization of a permanent magnet is somehow due to pemanent currents of electrons. Biot, Savart, and Amp`ere established the relationship of the magnetic induction and the current that generates it. As we know, Maxwell essentially completed the classical theory of electromagnetism.
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