Magnetism

Magnets. -A peculiar mineral (Fe₃O₄) called lodestone was found in early times in the neighborhood of Magnesia in Asia Minor. This mineral has the power of attracting small particles of the same mineral and of setting itself in one particular direction when suspended. When a piece of this lodestone is dipped into iron filings, they adhere to it. It is found that there are two places on each piece of this mineral at which the iron filings adhere in greatest quantities. If such a piece of lodestone is suspended by means of a silk thread, it is found that the line joining the places at which the iron filings adhere in greatest quantities points north and south.

When a steel knitting needle is stroked from one end to the other with a piece of lodestone, using for point of contact one of the points at which the iron filings adhere most freely, the needle acquires the property of attracting iron filings and of setting itself north and south when suspended. Such a needle is called an artificial magnet. There are other ways in which more powerful artificial magnets can be made. The properties of these magnets do not differ in any way from those of the needle, except that they are much more powerful.

Magnetic Poles. - On dipping a magnetized needle into iron filings it is seen that the iron filings adhere most strongly at the ends of the needle. These places at which the tendency of the iron filings to cling to the needle is greatest are called poles. If that end of a suspended needle may be disturbed, it will come to rest with the marked end pointing north. For convenience, it is desirable to call the end of the magnetic needle that points north the north-seeking pole, or N pole. The other end is called the south-seeking pole, or the S pole.

Law of Force between Magnetic Poles. -Experiments show the following:

1. Like poles repel each other, but unlike poles attract each other. Thus, two N poles repel each other and two S poles repel each other, but N pole and S pole attract each other.

2. The force with which the poles of two magnets attract or repel each other, in a vacuum, is equal to the product of the pole strength divided by the square of the distance between the poles.

Magnetic Field. - The space outside the magnet in which its influence can be detected is called the magnetic field. In the case of powerful magnets this space extends far from the magnet, but in the case of feeble magnets the magnetic field is so weak that it may be considered as confined to a small region near the magnet.

At every point near a magnet or a system of magnets, a free magneticpole would experience a force tending to drive it in a definite direction. The direction in which a free N pole would move is called the direction of the magnetic field and the magnitude of the force on unit pole is known as the intensity of the magnetic field. The intensity of the magnetic field, or the magnetic intensity, is thus defined to be the mechanical force measured in dynes that is exerted on unit pole at a point in free space. The unit of magnetic intensity is called the oersted. If the magnetic field is such that there is a force of 1 dyne on unit pole, the magnetic field or the magnetic intensity at that point is 1 oersted.

Unit Pole. - A unit pole is a pole of such strength that it will repel a similar pole of equal strength with a force of 1 dyne when placed 1 cm away from it in a vacuum. An isolated pole is not a physical possibility, since every magnet must have an equal south pole for every north pole. It is possible to realize approximately the conditions assumed in this definition by supposing that the magnets are very long, so that the influence of the other pole is small. The number of unit poles on the end of a magnet is measured by the number of dynes of force that it will exert on a unit pole 1 cm from it in a vacuum.

The force in air differs little from the force in a vacuum. Hence, the distinction between air and vacuum may be neglected.

Magnetic Lines of Force. - In order to make it easier to understand the way in which the magnetic field changes from point to point, it is convenient to draw certain lines (Figure 44) which by their direction represent the direction of the magnetic field and by their number represent the intensity of the magnetic field.

Such lines are called lines of magnetic force. Such a line of magnetic force is the path along which a perfectly free N pole would travel when left alone in the magnetic field. Since such a free N pole cannot be obtained in practice, a small compass needle is used to determine the direction of the magnetic field at a point. When the compass needle comes to rest, its axis points in the direction of the magnetic field at that point.

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Virtual and upright images	\|	Figure 44 - Magnetic lines of force around a bar magnet

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