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 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.