the interval. It has always been
one of the characteristic features of Edison's method of inventing that
work in several lines has gone forward at the same time. No one line of
investigation has ever been enough to occupy his thoughts fully; or to
express it otherwise, he has found rest in turning from one field of
work to another, having absolutely no recreations or hobbies, and not
needing them. It may also be said that, once entering it, Mr. Edison has
never abandoned any field of work. He may change the line of attack; he
may drop the subject for a time; but sooner or later the note-books or
the Patent Office will bear testimony to the reminiscent outcropping of
latent thought on the matter. His attention has shifted chronologically,
and by process of evolution, from one problem to another, and some
results are found to be final; but the interest of the man in the thing
never dies out. No one sees more vividly than he the fact that in the
interplay of the arts one industry shapes and helps another, and that no
invention lives to itself alone.

The path to the quadruplex lay through work on the duplex, which,
suggested first by Moses G. Farmer in 1852, had been elaborated by many
ingenious inventors, notably in this country by Stearns, before Edison
once again applied his mind to it. The different methods of such
multiple transmission--namely, the simultaneous dispatch of the two
communications in opposite directions over the same wire, or the
dispatch of both at once in the same direction--gave plenty of play to
ingenuity. Prescott's Elements of the Electric Telegraph, a standard
work in its day, described "a method of simultaneous transmission
invented by T. A. Edison, of New Jersey, in 1873," and says of it: "Its
peculiarity consists in the fact that the signals are transmitted in one
direction by reversing the polarity of a constant current, and in the
opposite direction by increasing or decreasing the strength of the same
current." Herein lay the germ of the Edison quadruplex. It is also noted
that "In 1874 Edison invented a method of simultaneous transmission
by induced currents, which has given very satisfactory results in
experimental trials." Interest in the duplex as a field of invention
dwindled, however, as the quadruplex loomed up, for while the one
doubled the capacity of a circuit, the latter created three "phantom
wires," and thus quadruplexed the working capacity of any line to which
it was applied.