[Illustration: FIG. 239.--A modern electrical machine.]
The current obtained from such a dynamo alternates in direction,
flowing first in one direction and then in the opposite direction.
Such alternating currents are unsatisfactory for many purposes, and to
be of service are in many cases transformed into direct currents; that
is, current which flows steadily in one direction. This is
accomplished by the use of a commutator. In the construction of the
motor, continuous _motion_ in one direction is obtained by the use of
a commutator (Section 310); in the construction of a dynamo,
continuous _current_ in one direction is obtained by the use of a
similar device.
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