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.

A magnetic needle held under a current-bearing wire turns on its pivot
and finally comes to rest at an angle with the current. The fact that
the needle is deflected by the wire shows that the magnetic power of
the wire extends into the surrounding medium.

The magnetic properties of current electricity were discovered by
Oersted of Denmark less than a hundred years ago; but since that time
practically all important electrical machinery has been based upon one
or more of the magnetic properties of electricity. The motors which
drive our electric fans, our mills, and our trolley cars owe their
existence entirely to the magnetic action of current electricity.

[Illustration: FIG. 228.--The coil turns in such a way that its north
pole is opposite the south pole of the magnet.]