TABLE OF CONTENTS A piece of iron is said to be magnetized, to possess magnetism, to be in a magnetic state, or to be a magnet if it is capable of attracting or repelling other pieces of iron placed near it. Magnetism appears to be concentrated at the ends of a magnet. Two magnets placed side by side (Fig. 16.29) may either attract or repel each other. Magnets placed as in Fig. 16.29a will attract each other. The reversal of one of them (Fig. 16.29b) results in repulsion between them. It is necessary, therefore, to distinguish between the positive (north) and negative (south) poles of a magnet.
A magnetic needle is a magnetized piece of steel having small mass and so suspended that it can move freely in any of three mutually perpendicular directions. Such a needle is used to detect the existence of magnetism, because a magnetic needle orients itself in a certain direction whenever it is placed near a magnetized object. The magnetic state is not confined to iron; it may exist in air. Thus the space (Fig. 16.30) surrounding a coil carrying a current I is in a magnetic state, because a magnetic needle placed near such a coil orients itself at each point in the direction tangent to the lines shown in Fig. 16.30. The earth itself is endowed with magnetism and is a huge magnet, which accounts for the fact that a magnetic needle...
