ss
needle (a magnet) to point unerringly north and south, sailors were
able to determine directions on the sea and to steer for the desired
point.

[Illustration: FIG. 221.--The compass.]

Since an electric current is practically equivalent to a magnet
(Section 296), it becomes necessary to know the most important facts
relative to magnets, facts simple in themselves but of far-reaching
value and consequences in electricity. Without a knowledge of the
magnetic characteristics of currents, the construction of the motor
would have been impossible, and trolley cars, electric fans, motor
boats, and other equally well-known electrical contrivances would be
unknown.

305. The Attractive Power of a Magnet. The magnet best known to us
all is the compass needle, but for convenience we will use a magnetic
needle in the shape of a bar larger and stronger than that employed in
the compass. If we lay such a magnet on a pile of iron filings, it
will be found on lifting the magnet that the filings cling to the ends
in tufts, but leave it almost bare in the center (Fig. 222). The
points of attraction at the two ends are called the poles of the
magnet.

[Illustration: FIG. 222.--A magnet.]

If a delicately made magnet is suspended as in Figure 223, and is
allowed to swing freely, it will always assume a definite north and
south position. The pole which points north when the needle is
suspended is called the north pole and is marked _N_, while the pole
which points south when the needle is suspended is called the south
pole and is marked _S_.

A freely suspended magnet points nearly north and south.

A magnet has two main points of attraction called respectively the
north and south poles.

[Illustration: FIG. 223.--The magnetic needle.]

306. The Extent of Magnetic Attraction. If a thin sheet of paper or
cardboard is laid over a strong, bar-shaped magnet and iron filings
are then gently strewn on the paper, the filings clearly indicate the
position of the magnet beneath, and if the cardboard is gently tapped,
the filings arrange themselves as shown in Figure 224. If the paper is
held some distance above the magnet, the influence on the filings is
less definite, and finally, if the paper is held very far away, the
filings do not respond at all, but lie on the cardboard as dropped.

The magnetic power of a magnet, while not confined to the magnet
itself, does not extend indefinitely into the surrounding region; the
influence