As will have been gathered from the above, the principle
embodied in the quadruplex is that of working over the line with two
currents from each end that differ from each other in strength or
nature, so that they will affect only instruments adapted to respond
to just such currents and no others; and by so arranging the receiving
apparatus as not to be affected by the currents transmitted from its
own end of the line. Thus by combining instruments that respond only
to variation in the strength of current from the distant station, with
instruments that respond only to the change in the direction of current
from the distant station, and by grouping a pair of these at each end of
the line, the quadruplex is the result. Four sending and four receiving
operators are kept busy at each end, or eight in all. Aside from other
material advantages, it is estimated that at least from $15,000,000 to
$20,000,000 has been saved by the Edison quadruplex merely in the cost
of line construction in America.

The quadruplex has not as a rule the same working efficiency that
four separate wires have. This is due to the fact that when one of the
receiving operators is compelled to "break" the sending operator for
any reason, the "break" causes the interruption of the work of eight
operators, instead of two, as would be the case on a single wire. The
working efficiency of the quadruplex, therefore, with the apparatus in
good working condition, depends entirely upon the skill of the operators
employed to operate it. But this does not reflect upon or diminish the
ingenuity required for its invention. Speaking of the problem involved,
Edison said some years later to Mr. Upton, his mathematical assistant,
that "he always considered he was only working from one room to another.
Thus he was not confused by the amount of wire and the thought of
distance."

The immense difficulties of reducing such a system to practice may be
readily conceived, especially when it is remembered that the "line"
itself, running across hundreds of miles of country, is subject to all
manner of atmospheric conditions, and varies from moment to moment in
its ability to carry current, and also when it is borne in mind that
the quadruplex requires at each end of the line a so-called "artificial
line," which must have the exact resistance of the working line and must
be varied with the variations in resistance of the working line. At this
juncture other schemes were fermenti