The construction of the electric telegraph is pretty much the same
everywhere, only that modifications of the same agent are used in
different countries, and different signals formed; but whether this
agent or influence is obtained from magnetic or galvanic sources,
the result is exactly the same. When a pair of metallic plates are
immersed in a fluid which acts chemically more rapidly on the one
than the other, and a wire connects the upper parts of these plates,
this wonderful agency is set in motion, and circulates from the
one plate to the other (fig. 1). This arrangement may be best shown
by using one plate of zinc and the other of copper, and a dilute
solution of sulphuric acid for the liquid; this, however, produces by
far too little of the agent to be used on a telegraphic line, there
are therefore combinations of such pairs of plates so arranged that
the power of one pair shall be added to the next in such a way that
at the end of the series (called a “battery”) there shall be a great
increase of the power accumulated--this arrangement is shown in fig.
2. Now (if the power be sufficient) it does not signify what length
of wire there may be between the two ends of this arrangement or
“battery,” whether the ends be connected by a few feet of wire, or
as many hundred miles--the electricity passes instantaneously from
one end to the other; and furthermore, it has been found in practice,
that this electrical influence can be transmitted through the earth
in one direction if sent by a wire in the other; for instance, if
a wire from one end of the battery be carried on from London to
Liverpool; instead of having another from Liverpool to London, to
connect the two ends of the battery, it is found to answer the same
purpose if the end of the wire at Liverpool be fastened to a plate
of metal buried beneath the surface of the earth and the other end
of the battery at London, furnished with a similar plate also buried.
In this arrangement, the electricity will pass beneath the surface
of the earth from Liverpool to London, and through the wire from
London to Liverpool, thus completing the circuit. The end from which
the electricity passes is called the “positive electrode,” that to
which it returns the “negative electrode.” Fig. 3 will show this
arrangement.