, for, whereas duplexing
(accomplished by varying the strength of the current) permitted messages
to be sent simultaneously from opposite stations, diplexing (achieved
by also varying the direction of the current) permitted the simultaneous
transmission of two messages from the same station and their separate
reception at the distant station.

[Footnote 23: Afterward issued as Patent No. 162,633, April
27, 1875.]

The quadruplex was the tempting goal toward which Edison now constantly
turned, and after more than a year's strenuous work he filed a number of
applications for patents in the late summer of 1874. Among them was one
which was issued some years afterward as Patent No. 480,567, covering
his well-known quadruplex. He had improved his own diplex, combined it
with the Stearns duplex and thereby produced a system by means of which
four messages could be sent over a single line at the same time, two in
each direction.

As the reader will probably be interested to learn something of the
theoretical principles of this fascinating invention, we shall endeavor
to offer a brief and condensed explanation thereof with as little
technicality as the subject will permit. This explanation will
necessarily be of somewhat elementary character for the benefit of the
lay reader, whose indulgence is asked for an occasional reiteration
introduced for the sake of clearness of comprehension. While the
apparatus and the circuits are seemingly very intricate, the principles
are really quite simple, and the difficulty of comprehension is more
apparent than real if the underlying phenomena are studied attentively.

At the root of all systems of telegraphy, including multiplex systems,
there lies the single basic principle upon which their performance
depends--namely, the obtaining of a slight mechanical movement at the
more or less distant end of a telegraph line. This is accomplished
through the utilization of the phenomena of electromagnetism. These
phenomena are easy of comprehension and demonstration. If a rod of soft
iron be wound around with a number of turns of insulated wire, and
a current of electricity be sent through the wire, the rod will be
instantly magnetized and will remain a magnet as long as the current
flows; but when the current is cut off the magnetic effect instantly
ceases. This device is known as an electromagnet, and the charging and
discharging of such a magnet may, of course, be repeated indefinite