mes little brass strips are attached to the dial plate of a needle
instrument for the needle to strike against. As these give different
notes, the operator can comprehend the message by ear alone. But the
most widely used sounding instrument is the Morse sounder, named after
its inventor. For this a reversible current is not needed. The receiver
is merely an electro-magnet (connected with the line circuit and an
earth-plate) which, when a current passes, attracts a little iron bar
attached to the middle of a pivoted lever. The free end of the lever
works between two stops. Every time the circuit is closed by the
transmitting key at the sending station the lever flies down against the
lower stop, to rise again when the circuit is broken. The duration of
its stay decides whether a "long" or "short" is meant.

TELEGRAPHIC RELAYS.

[Illustration: FIG. 58.--Section of a telegraph wire insulator on its
arm. The shaded circle is the line wire, the two blank circles indicate
the wire which ties the line wire to the insulator.]

When an electric current has travelled for a long distance through a
wire its strength is much reduced on account of the resistance of the
wire, and may be insufficient to cause the electro-magnet of the sounder
to move the heavy lever. Instead, therefore, of the current acting
directly on the sounder magnet, it is used to energize a small magnet,
or _relay_, which pulls down a light bar and closes a second "local"
circuit--that is, one at the receiver end--worked by a separate battery,
which has sufficient power to operate the sounder.

RECORDING TELEGRAPHS.

By attaching a small wheel to the end of a Morse-sounder lever, by
arranging an ink-well for the wheel to dip into when the end falls, and
by moving a paper ribbon slowly along for the wheel to press against
when it rises, a self-recording Morse inker is produced. The
ribbon-feeding apparatus is set in motion automatically by the current,
and continues to pull the ribbon along until the message is completed.

The Hughes type-printer covers a sheet of paper with printed characters
in bold Roman type. The transmitter has a keyboard, on which are marked
letters, signs, and numbers; also a type-wheel, with the characters on
its circumference, rotated by electricity. The receiver contains
mechanisms for rotating another type-wheel synchronously--that is, in
time--with the first; for shifting the wheel across the paper; for
pressing the paper a