[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flows through the wire; if the iron is withdrawn, current is again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 opposite end.
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