The compensating resistances and condensers necessary for a duplex arrangement are shown in the diagram.
In the early part of 1873, and for some time afterward, the system invented by Joseph Stearns was the duplex in practical use.
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.
With the duplex, as we have seen, the current on the main line is changed in strength only when both keys at OPPOSITE stations are closed together, so that a current due to both batteries flows over the main line.
As the path to the quadruplex passes through the duplex, let us consider the Stearns system, after noting one other principle--namely, that if more than one path is presented in which an electric current may complete its circuit, it divides in proportion to the resistance of each path.
It must be borne in mind, however, in considering a duplex system, that a differential relay is used AT EACH END of the line and forms part of the circuit; and that while each relay must be absolutely unresponsive to the signals SENT OUT FROM ITS HOME OFFICE, it must respond to signals transmitted by a DISTANT OFFICE.