d, the polarity will
also be reversed. Assuming, for instance, the bar to be end-on toward
the observer, that end will be a south pole if the current is flowing
from left to right, clockwise, around the bar; or a north pole if
flowing in the other direction, as illustrated at the right of the
figure. It is immaterial which way the wire is wound around the bar, the
determining factor of polarity being the DIRECTION of the current. It
will be clear, therefore, that if two EQUAL currents be passed around
a bar in opposite directions (Fig. 3) they will tend to produce exactly
opposite polarities and thus neutralize each other. Hence, the bar would
remain non-magnetic.

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. Hence, if we connect one pole of a battery with the earth, and
from the other pole run to the earth two wires of equal resistance as
illustrated in Fig. 2, equal currents will traverse the wires.

The above principles were employed in the Stearns differential duplex
system in the following manner: Referring to Fig. 3, suppose a wire, A,
is led from a battery around a bar of soft iron from left to right, and
another wire of equal resistance and equal number of turns, B, around
from right to left. The flow of current will cause two equal opposing
actions to be set up in the bar; one will exactly offset the other, and
no magnetic effect will be produced. A relay thus wound is known as a
differential relay--more generally called a neutral relay.

The non-technical reader may wonder what use can possibly be made of an
apparently non-operative piece of apparatus. 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. Hence, the next figure (4), with its accompanying
explanation, will probably make the matter clear. If another battery,
D, be introduced at the distant end of the wire A the differential or
neutral relay becomes actively operative as follows: Battery C supplies
wires A and B with an equal current, but battery