ction, on one set of brushes--and all the current traveling in the
other direction on another set of brushes,--we would straighten out
this current, make it all travel in one direction. Then we would have
a _direct current_. A direct current dynamo, the type generally used
in private plants, does this. Instead of having two copper rings for
collecting the current, it has a single ring, made up of segments of
copper bound together, but insulated from each other, one segment for
each set of conductors on the armature. This ring of many segments, is
called a _commutator_, because it commutates, or changes, the
direction of the electric impulses, and delivers them all in one
direction. In effect, it is like the connecting rod of a steam engine
that straightens out the back-and-forth motion of the piston in the
steam cylinder and delivers the motion to a wheel running in one
direction.

Such a current, flowing through a coil of wire would make a magnet,
one end of which would always be the north end, and the other end the
south end. An alternating current, on the other hand, flowing through
a coil of wire, would make a magnet that changed its poles with each
half-cycle. It would no sooner begin to pull another magnet to it,
than it would change about and push the other magnet away from it, and
so on, as long as it continued to flow. This is one reason why a
direct current dynamo is used for small plants. Alternating current
will light the same lamps and heat the same irons as a direct current;
but for electric power it requires a different type of motor.

_Types of Direct Current Dynamos_

Just as electrical generators are divided into two classes,
alternating and direct, so direct current machines are divided into
three classes, according to the manner in which their output, in
amperes and volts, is regulated. They differ as to the manner in which
their field magnets (in whose field of force the armature spins) are
excited, or made magnetic. They are called _series_, _shunt_, and
_compound_ machines.

_The Series Dynamo_

By referring to the diagram, it will be seen that the current of a
_series_ dynamo issues from the armature mains, and passes through the
coils of the field magnets before passing into the external circuit to
do its work. The residual magnetism, or the magnetism left in the
iron cores of the field magnets from its last charge, provides the
initial excitation, when the machine is started. As the r