is strong near the magnet, but at a distance becomes so weak
as to be inappreciable. The region around a magnet through which its
magnetic force is felt is called the field of force, or simply the
magnetic field, and the definite lines in which the filings arrange
themselves are called lines of force.

[Illustration: FIG. 224.--Iron filings scattered over a magnet arrange
themselves in definite lines.]

The magnetic power of a magnet is not limited to the magnet, but
extends to a considerable distance in all directions.

307. The Influence of Magnets upon Each Other. If while our
suspended magnetic needle is at rest in its characteristic
north-and-south direction another magnet is brought near, the
suspended magnet is turned; that is, motion is produced (Fig. 225). If
the north pole of the free magnet is brought toward the south pole of
the suspended magnet, the latter moves in such a way that the two
poles _N_ and _S_ are as close together as possible. If the north pole
of the free magnet is brought toward the north pole of the suspended
magnet, the latter moves in such a way that the two poles _N_ and _N_
are as far apart as possible. In every case that can be tested, it is
found that a north pole repels a north pole, and a south pole repels a
south pole; but that a north and a south pole always attract each
other.

[Illustration: FIG. 225.--A south pole attracts a north pole.]

The main facts relative to magnets may be summed up as follows:--

_a_. A magnet points nearly north and south if it is allowed to swing
freely.

_b_. A magnet contains two unlike poles, one of which persistently
points north, and the other of which as persistently points south, if
allowed to swing freely.

_c_. Poles of the same name repel each other; poles of unlike name
attract each other.

_d_. A magnet possesses the power of attracting certain substances,
like iron, and this power of attraction is not limited to the magnet
itself but extends into the region around the magnet.

308. Magnetic Properties of an Electric Current. If a
current-bearing wire is really equivalent in its magnetic powers to a
magnet, it must possess all of the characteristics mentioned in the
preceding Section. We saw in Section 296 that a coiled wire through
which current was flowing would attract iron filings at the two ends
of the helix. That a coil through which current flows possesses the
characteristics _a_, _b_, _c_, and _d_ of a magnet is shown