[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.
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