12. The Nature of Magnetism.- Magnetism and electricity are peculiarly related.

It has been found that every time a loop of wire located in a magnetic field is moved in such a way as to change the strength of the field with respect to the coil, an electric current will flow in the loop; and also, that whenever an electric current flows in a wire, the wire is surrounded by a magnetic field. These two facts will be amplified in the succeeding paragraphs.

If several turns of wire are wound around an iron rod, and an electric current from a battery is passed through the wire, the iron rod becomes magnetized as shown by its ability to pick up iron filings, tacks, etc. If the rod is of soft iron, practically all of the magnetism will disappear when the flow of the current is stopped. If the rod is of hardened steel, an appreciable amount of the magnetism will remain after the flow of the current has ceased and the piece of steel so treated becomes a permanent magnet. If one end of such a rod is brought close to a magnetic compass, it will attract one end of the compass needle toward it. If the opposite end of the

rod is brought near the compass, it will attract the other end of the compass needle. Since a magnetic compass is itself a small permanent magnet, the behavior just described shows the effect of one magnet on another.

In order to study the action of magnets a little more, bring together the ends of two magnetized steel rods which attract the same end of the compass needle, and it will be found that they repel each other; but, if the ends of the two rods which attract opposite ends of the compass needle are brought close together they will be strongly attracted to each other, proving that opposite ends of a magnetized iron rod act differently when brought near one end of another magnet. The pole of a magnet which attracts the end of the compass needle which normally points north is called the south pole of the magnet, and conversely the pole of a magnet which attracts the end of the compass needle which normally points south is called the north pole of the magnet. From the above discussion this general rule is formed magnetic poles of like polarity repel each other, and magnetic poles of opposite polarity attract each other.

An iron rod can be considered as being made up of a large number of small permanent magnets. When the iron rod is not magnetized, the tiny magnets will be arranged in a random fashion such as shown in Figure 6 (a). If a wire is wound around the rod and a current is passed through the wire from a battery, the tiny magnets which compose the iron will arrange themselves in a manner similar to that shown in Figure 6 (c), thus aiding the electric current in producing a magnetic field. When the flow of current is stopped the tiny magnets will return to very nearly their original position, and most of the magnetism will be gone if the rod is

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