[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 as follows:—­

a, b.  If a helix marked at one end with a red string is arranged so that it is free to rotate and a strong current is sent through it, the helix will immediately turn and face about until it points north and south.  If it is disturbed from this position, it will slowly swing back until it occupies its characteristic north and south position.  The end to which the string is attached will persistently point either north or south.  If the current is sent through the coil in the opposite direction, the two poles exchange positions and the helix turns until the new north pole points north.

[Illustration:  FIG. 226.—­A helix through which current flows always points north and south, if it is free to rotate.]

c.  If a coil conducting a current is held near a suspended magnet, one end of the helix will be found to attract the north pole of the magnet, while the opposite end will be found to repel the north pole of the magnet.  In fact, the helix will be found to behave in every way as a magnet, with a north pole at one end and a south pole at the other.  If the current is sent through the helix in the opposite direction, the north and south poles exchange places.

[Illustration:  FIG. 227.—­A wire through which current flows is surrounded by a field of magnetic force.]

If the number of turns in the helix is reduced until but a single loop remains, the result is the same; the single loop acts like a flat magnet, one side of the loop always facing northward and one southward, and one face attracting the north pole of the suspended magnet and one repelling it.

d.  If a wire is passed through a card and a strong current is sent through the wire, iron filings will, when sprinkled upon the card, arrange themselves in definite directions (Fig. 227).  A wire carrying a current is surrounded by a magnetic field of force.