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.

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