ph messages, and
this is accomplished by means of an instrument called a pole-changer,
which consists essentially of a movable contact piece connected
permanently to the earth, or grounded, and arranged to connect one or
the other pole of a battery to the line and simultaneously ground the
other pole. This action of the pole-changer is effected by movements of
the armature of an electromagnet through the manipulation of an ordinary
telegraph key by an operator at the home station, as in the operation of
the "transmitter," above referred to.

By a combination of the neutral relay and the polar relay two
operators, by manipulating two telegraph keys in the ordinary way, can
simultaneously send two messages over one line in the SAME direction
with the SAME current, one operator varying its strength and the other
operator varying its polarity or direction of flow. This principle was
covered by Edison's Patent No. 162,633, and was known as the "diplex"
system, although, in the patent referred to, Edison showed and claimed
the adaptation of the principle to duplex telegraphy. Indeed, as
a matter of fact, it was found that by winding the polar relay
differentially and arranging the circuits and collateral appliances
appropriately, the polar duplex system was more highly efficient than
the neutral system, and it is extensively used to the present day.

Thus far we have referred to two systems, one the neutral or
differential duplex, and the other the combination of the neutral and
polar relays, making a diplex system. By one of these two systems
a single wire could be used for sending two messages in opposite
directions, and by the other in the same direction or in opposite
directions. Edison followed up his work on the diplex and combined the
two systems into the quadruplex, by means of which FOUR messages could
be sent and received simultaneously over the one wire, two in each
direction, thus employing eight operators--four at each end--two sending
and two receiving. The general principles of quadruplex telegraphy are
based upon the phenomena which we have briefly outlined in connection
with the neutral relay and the polar relay. The equipment of such
a system at each end of the line consists of these two instruments,
together with the special form of transmitter and the pole-changer and
their keys for actuating the neutral and polar relays at the other, or
distant, end. Besides these there are the compensating resistances a