between which a loop of wire is adapted to rotate. The magnet _1_ is
of hardened steel and permanently magnetized. The pole pieces are
shown at _2_ and _3_, each being of soft iron adapted to make good
magnetic contact on its flat side with the inner flat surface of the
bar magnet, and being bored out so as to form a cylindrical recess
between them as indicated. The direction of the magnetic lines of
force set up by the bar magnet through the interpolar space is
indicated by the long horizontal arrows, this flow being from the
north pole (N) to the south pole (S) of the magnet. At _4_ there is
shown a loop of wire supposed to revolve in the magnetic field of
force on the axis _5-5_.

Theory. In order to understand how currents will be generated in
this loop of wire _4_, it is only necessary to remember that if a
conductor is so moved as to cut across magnetic lines of force, an
electromotive force will be set up in the conductor which will tend to
make the current flow through it. The magnitude of the electromotive
force will depend on the rate at which the conductor cuts through the
lines of force, or, in other words, on the number of lines of force
that are cut through by the conductor in a given unit of time. Again,
the direction of the electromotive force depends on the direction of
the cutting, so that if the conductor be moved in one direction across
the lines of force, the electromotive force and the current will be in
one direction; while if it moves in the opposite direction across the
lines of force, the electromotive force and the current will be in the
reverse direction.

It is, evident that as the loop of wire _4_ revolves in the field of
force about the axis _5-5_, the portions of the conductor parallel to
the axis will cut through the lines of force, first in one direction