[Illustration: Fig. 10.]

If a magnet be placed near the circuit, so that its north pole, N, is
opposite that side of the circuit which acts as a south pole, the magnet
and the circuit will attract one another. The lines of force that
radiate from the end of the magnet, curve round and coalesce with
some of those of the circuit. It was shown by the late Professor
Clerk-Maxwell, that every portion of a circuit is acted upon by a force
urging it in such a direction as to make it inclose within its embrace
the greatest possible number of lines of force. This proposition, which
has been termed "Maxwell's Rule," is very important, because it can be
so readily applied to so many cases, and will enable one so easily
to think out the actual reaction in any particular case. The rule is
illustrated by the sketch shown in Fig. 10, where a bar magnet has been
placed with its north pole opposite the south face of the circuit of
the cell. The lines of force of the magnet are drawn into the ring and
coalesce with those due to the current. According to Faraday's mode of
regarding the actions in the magnetic field there is a tendency for the
lines of force to shorten themselves. This would occur if either the
magnet were pulled into the circuit, or the circuit were moved up toward
the magnet. Each attracts the other, and whichever of them is free to
move will move in obedience to the attraction. And the motion will in
either case be such as to increase the total number of lines of force
that pass through the circuit. Lest it should be thought that Fig. 10 is
fanciful or overdrawn, we reproduce an actual magnetic "field" made in
the manner described in the preceding article. Fig. 11 is a kind of
sectional view of Fig. 10, the circuit being represented merely by two
circular spots or holes above and below the middle line, the current
flowing toward the spectator through the lower spot, and passing in
front of the figure to the upper hole, where it flows down. Into this