The magnetic compassAround AD 1000 in China it was found that a needle, magnetized by stroking it with lodestone, would align itself north-south when freely suspended. The discovery of the magnetic compass soon spread to Europe. Christopher Columbus used it when he crossed the Atlantic Ocean, noting not only that the needle deviated slightly from exact north (as indicated by the stars) but also that the deviation changed during the voyage. Around 1600 William Gilbert (1544-1603), an English doctor and personal physician to Queen Elizabeth I, found the explanation.
Gilbert of Colchester, as he is often known, made a model of the Earth by shaping a round ball of lodestone. He called this model a terrela, or "little Earth", and placed on it a series of small iron needles. Then he made his great discovery. The needles behaved exactly like compasses. Not only did they point to the lodestone's north pole, but they also dipped at various angles in different places, just as compass needles do on the Earth's surface. Gilbert was able to show, therefore, that the Earth is itself a magnet, with its magnetic poles near the geographic north and south poles. He also demonstrated that the compass needles follow the lines of magnetic force which flow in an arc around the Earth between the magnetic poles.
Gilbert put down his ideas in De Magnete (About the Magnet) published in 1600. This was not his only contribution to the study of magnets. He realized, for example, that there are connections between electricity and magnetism.
The observation that like poles of a magnet repel each other had been made towards the end of the 16th century. In view of the widespread use of compasses prior to this time, it is surprising that the discovery had not been made earlier. Undoubtedly, a cause that contributed to the delay is the fact that a magnetized piece of iron, even if it is initially repelled when brought near a strong magnet, may be induced to reverse its polarity and so experience an attraction. See also Coulomb's law for magnets.
The nature of magnetism
The new phenomenon was studied in France by André Marie Ampère, who concluded that the nature of magnetism was quite different from what everyone had believed. It was basically a force between electric currents: two parallel currents in the same direction attract, in opposite directions repel. (Iron magnets are a very special case, which Ampère was also able to explain.)
Here is how this can lead to the notion of magnetic poles. Bend the wires into circles with constant separation. Two circular currents in the same direction attract each other. Two circular currents in opposite directions repel each other.
Replace each circle with a coil of 10, 100, or more turns, carrying the same current: the attraction or repulsion increase by an appropriate factor. In fact, each coil acts very much like a magnet with magnetic poles at each end (an electromagnet). Ampère guessed that each atom of iron contained a circulating current, turning it into a small magnet, and that in an iron magnet all these atomic magnets were lined up in the same direction, allowing their magnetic forces to add up.
Magnetic field linesMichael Faraday, credited with fundamental discoveries on electricity and magnetism, also proposed a widely used method for visualizing magnetic fields. Imagine a compass needle freely suspended in three dimensions, near a magnet or an electrical current. We can trace in space the lines obtained when one follows the direction of the compass needle. Faraday called them lines of force, but the term field lines is now in common use.
Field lines of a bar magnet are commonly illustrated by iron filings sprinkled on a sheet of paper held over a magnet. Similarly, field lines of the Earth start near the south pole of the Earth, curve around in space and converge again near the north pole.
Electromagnetic wavesFaraday not only viewed the space around a magnet as filled with field lines, but also developed an intuitive (and perhaps mystical) notion that such space was itself modified, even if it was a complete vacuum. His younger contemporary, the great Scottish physicist James Clerk Maxwell, placed this notion on a firm mathematical footing, including in it electrical forces as well as magnetic ones. Such a modified space is now known as an electromagnetic field.
Today electromagnetic fields (and other types of field as well) are a cornerstone of physics. Their basic equations, derived by Maxwell, suggested that they could undergo wave motion, spreading with the speed of light, and Maxwell correctly guessed that this actually was light and that light was in fact an electromagnetic wave.
Heinrich Hertz in Germany, soon afterwards, produced such waves by electrical means, in the first laboratory demonstration of radio waves. Nowadays a wide variety of such waves is known, from radio (very long waves, relatively low frequency) to microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays (very short waves, extremely high frequency).
Magnetic dipolesToday, physicists explain magnetism in terms of magnetic dipoles. Magnetic dipole moment is an intrinsic property of fundamental particles. Electrons, for example, have a moment of 0.928× 10-23 A.m2 parallel or antiparallel to the direction of observation. The forces between magnetic dipoles are identical to those between electric dipoles. This leads scientists often to regard the dipoles as consisting of two magnetic charges of opposite type, the poles of traditional theory. But unlike electric charges, magnetic poles are believed never to be found in isolation.
Related category• ELECTRICITY AND MAGNETISM
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