when the polarity of the
current is reversed, whether the current be strong or weak, and the
neutral relay responding when the line-current is increased, regardless
of its polarity. It should be added that besides the system illustrated
in Fig. 8, which is known as the differential principle, the quadruplex
was also arranged to operate on the Wheatstone bridge principle; but
it is not deemed necessary to enter into its details. The underlying
phenomena were similar, the difference consisting largely in the
arrangement of the circuits and apparatus. [24]

[Footnote 24: Many of the illustrations in this article are
reproduced from American Telegraphy and Encyclopedia of the
Telegraph, by William Maver, Jr., by permission of Maver
Publishing Company, New York.]

Edison made another notable contribution to multiplex telegraphy
some years later in the Phonoplex. The name suggests the use of the
telephone, and such indeed is the case. The necessity for this invention
arose out of the problem of increasing the capacity of telegraph lines
employed in "through" and "way" service, such as upon railroads. In a
railroad system there are usually two terminal stations and a number of
way stations. There is naturally much intercommunication, which would
be greatly curtailed by a system having the capacity of only a single
message at a time. The duplexes above described could not be used on
a railroad telegraph system, because of the necessity of electrically
balancing the line, which, while entirely feasible on a through line,
would not be practicable between a number of intercommunicating points.
Edison's phonoplex normally doubled the capacity of telegraph lines,
whether employed on way business or through traffic, but in actual
practice made it possible to obtain more than double service. It has
been in practical use for many years on some of the leading railroads of
the United States.

The system is a combination of telegraphic apparatus and telephone
receiver, although in this case the latter instrument is not used in the
generally understood manner. It is well known that the diaphragm of a
telephone vibrates with the fluctuations of the current energizing the
magnet beneath it. If the make and break of the magnetizing current
be rapid, the vibrations being within the limits of the human ear, the
diaphragm will produce an audible sound; but if the make and break be as
slow as with ordinary Morse transmissio