8, and refer the reader to the above or other
text-books if he desires to make a close study of its intricate
operations. Before finally dismissing the quadruplex, and for the
benefit of the inquiring reader who may vainly puzzle over the
intricacies of the circuits shown in Fig. 8, a hint as to an essential
difference between the neutral relay, as used in the duplex and as used
in the quadruplex, may be given. 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. When a single message is sent from
one station to the other, or when both stations are sending messages
that do not conflict, only one battery or the other is connected to the
main line; but with the quadruplex, suppose one of the operators, in New
York for instance, is sending reversals of current to Chicago; we can
readily see how these changes in polarity will operate the polar relay
at the distant station, but why will they not also operate the neutral
relay at the distant station as well? This difficulty was solved by
dividing the battery at each station into two unequal parts, the smaller
battery being always in circuit with the pole-changer ready to have its
polarity reversed on the main line to operate the distant polar relay,
but the spring retracting the armature of the neutral relay is made so
stiff as to resist these weak currents. If, however, the transmitter is
operated at the same end, the entire battery is connected to the main
line, and the strength of this current is sufficient to operate the
neutral relay. Whether the part or all the battery is alternately
connected to or disconnected from the main line by the transmitter, the
current so varied in strength is subject to reversal of polarity by the
pole-changer; but the variations in strength have no effect upon the
distant polar relay, because that relay being responsive to changes
in polarity of a weak current is obviously responsive to corresponding
changes in polarity of a powerful current. With this distinction before
him, the reader will have no difficulty in following the circuits
of Fig. 8, bearing always in mind that by reason of the differential
winding of the polar and neutral relays, neither of the relays at one
station will respond to the home battery, and can only respond to the
distant battery--the polar relay responding