viously described for the motor.

[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.

322. Powerful Dynamos. The power and efficiency of a dynamo are
increased by employing the devices previously mentioned in connection
with the motor. Electromagnets are used in place of simple magnets,
and the armature, instead of being a simple coil, may be made up of
many coils wound on soft iron. The speed with which the armature is
rotated influences the strength of the induced current, and hence the
armature is run at high speed.

[Illustration: FIG. 240.--Thomas Edison, one of the foremost
electrical inventors of the present day.]

A small dynamo, such as is used for lighting fifty incandescent lamps,
has a horse power of about 33.5, and large dynamos are frequently as
powerful as 7500 horse power.

323. The Telephone. When a magnet is at rest within a closed coil of
wire, as in Section 319, current does not flow through the wire. But
if a piece of iron is brought near the magnet, current is induced and
flows through the wire; if the iron is withdrawn, current is again
induced in the wire but flows in the opposite direction. As iron
approaches and recedes from the magnet, current is induced in the wire
surrounding the magnet. This is in brief the principle of the
telephone. When one talks into a receiver, _L_, the voice throws into
vibration a sensitive iron plate standing before an electromagnet. The
back and forth motion of the iron plate induces current in the
electromagnet _c_. The current thus induced makes itself evident at
the opposite end of the line _M_, where by its magnetic attraction, it
throws a second iron plate into vibrations. The vibrations of the
second plate are similar to those produced in the first plate by the
voice. The vibrations of the far plate thus reproduce the sounds
uttered at the opp