[Illustration: Fig. 9.]
The next point to be studied is the magnetic property of a single loop
of the wire through which an electric current flows. Fig. 9 represents
a single voltaic cell containing the usual plates of zinc and copper
dipping into acid to generate a current in the old-fashioned way. This
current flows from the zinc plate through the liquid to the copper
plate, and from thence it flows round the wire ring or circuit back to
the zinc plate. Here the lines of magnetic force in the surrounding
space are no longer only whirls like those drawn in Fig. 4 and 6, for
they react on one another and become nearly parallel where they pass
through the middle of the ring. The thick arrows show the direction of
the electric current, the fine arrows are the lines of magnetic force,
and show the paths along which a free north pole would be urged. All the
front face, where the arrow-heads are, will be like the north pole of a
magnet. All the other face of the ring will be like the south pole of a
magnet. Our ring resembles a flat magnet, one face all north pole the
other face all south pole. Such a magnet is sometimes called a "magnetic
shell.
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