oses a circuit in a local battery and thus
causes a click of the sounder. When the current in the main line is
broken, the relay loses its magnetic attraction, its armature springs
back, connection is broken in the local circuit, and the sounder
responds by allowing its armature to spring back with a sharp sound.

302. The Earth an Important Part of a Telegraphic System. We learned
in Section 299 that electricity could flow through many different
substances, one of which was the earth. In all ordinary telegraph
lines, advantage is taken of this fact to utilize the earth as a
conductor and to dispense with one wire. Originally two wires were
used, as in Figure 217; then it was found that a railroad track could
be substituted for one wire, and later that the earth itself served
equally well for a return wire. The present arrangement is shown in
Figure 220, where there is but one wire, the circuit being completed
by the earth. No fact in electricity seems more marvelous than that
the thousands of messages flashing along the wires overhead are
likewise traveling through the ground beneath. If it were not for this
use of the earth as an unfailing conductor, the network of overhead
wires in our city streets would be even more complex than it now is.

303. Advances in Telegraphy. The mechanical improvements in
telegraphy have been so rapid that at present a single operator can
easily send or receive forty words a minute. He can telegraph more
quickly than the average person can write; and with a combination of
the latest improvements the speed can be enormously increased.
Recently, 1500 words were flashed from New York to Boston over a
single wire in one second.

In actual practice messages are not ordinarily sent long distances
over a direct line, but are automatically transferred to new lines at
definite points. For example, a message from New York to Chicago does
not travel along an uninterrupted path, but is automatically
transferred at some point, such as Lancaster, to a second line which
carries it on to Pittsburgh, where it is again transferred to a third
line which takes it farther on to its destination.

CHAPTER XXXIII

MAGNETS AND CURRENTS

304. In the twelfth century, there was introduced into Europe from
China a simple instrument which changed journeying on the sea from
uncertain wandering to a definite, safe voyage. This instrument was
the compass (Fig. 221), and because of the property of the compa