The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

EARLY EXPERIMENTAL CIRCUITS. 489
erected to complete the electric circuit. It was not then known
that the earth would serve as one half of the conducting circuit. Soon thereafter, he discovered that the earth would
answer, and that only one wire was sufficient for telegraphic
purposes. When Morse constructed the experimental line from
Baltimore to Washington, he did not know that the earth would
answer for the half circuit, and therefore he erected two wires,
and the voltaic current was sent over one wire and it returned
over the other, as represented by fig. 4: B is Baltimore, and w is
Washington. One of the wires is east and the other west. The
Fig. 4.
F ast zwire Ii
W' est wire
current starts from P, the positive pole of the battery, passes
through the key, k, and the relay magnet m, at the Baltimore
station, thence over the east wire to Washington, where it passes
through the key k', the relay magnet m', and thence over the
west wire to Baltimore, wheie it enters the negative pole of
the voltaic battery.
After the line had been in operation for some six months, the
earth was made a part of the circuit, according to the following
diagram.
Fig. 5.
CJS ~ ~ BEast wirea
B | C e- ground _'- C'
The route of the current is precisely the same as the diagram
before described, except that the earth is made a part of the
circuit. The current arriving at copper plate c' passes through
the earth as indicated by the arrows, to copper plate c, which
is also buried in the moist earth, and thence to the N. pole