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
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