|A .- |H .... |O . . |U ..- |
|B -... |I .. |P ..... |V ...- |
|C .. . |J -.-. |Q ..-. |W .-- |
|D -.. |K -.- |R . .. |X .-.. |
|E . |L --- |S ... |Y .. .. |
|F .-. |M - - |T - |Z ... . |
|G --. |N -. | | |

FIG. 218.--The Morse telegraphic code.]

The telegraph is now such a universal means of communication between
distant points that one wonders how business was conducted before its
invention in 1832 by S.F.B. Morse.

[Illustration: FIG. 219.--The sounder.]

301. Improvements. _The Sounder._ Shortly after the invention of
telegraphy, operators learned that they could read the message by the
click of the marker against a metal rod which took the place of the
tape. In practically all telegraph offices of the present day the
old-fashioned tape is replaced by the sounder, shown in Figure 219.
When current flows, a lever, _L_, is drawn down by the electromagnet
and strikes against a solid metal piece with a click; when the current
is broken, the lever springs upward, strikes another metal piece and
makes a different click. It is clear that the working of the key which
starts and stops the current in this line will be imitated by the
motion and the resulting clicks of the sounder. By means of these
varying clicks of the sounder, the operator interprets the message.

[Illustration: FIG. 220.--Diagram of a modern telegraph system.]

_The Relay._ When a telegraph line is very long, the resistance of the
wire is great, and the current which passes through the electromagnet
is correspondingly weak, so feeble indeed that the armature must be
made very thin and light in order to be affected by the makes and
breaks in the current. The clicks of an armature light enough to
respond to the weak current of a long wire are too faint to be
recognized by the ear, and hence in such long circuits some device
must be introduced whereby the effect is increased. This is usually
done by installing at each station a local battery and a very delicate
and sensitive electromagnet called the _relay_. Under these conditions
the current of the main line is not sent through the sounder, but
through the relay which opens and closes a local battery in connection
with the strong sounder. For example, the relay is so arranged that
current from the main line runs through it exactly as it runs through
_M_ in Figure 217. When current is made, the relay attracts an
armature, which thereby cl