The relay spring is so adjusted that the armatures are not actuated by
the weak current sent from E by the key K1.
In its normal position this relay completes the circuit of the local
battery through the sounder S. This sounder S, called the uprighting
sounder, acts as a relay to a second sounder, S2, called the reading
sounder, which is worked by another local battery, l2. Of course,
normally, the armature of S is held down and that of S2 is up, but when
the tongue t moves, as it does when the increment key K2 is depressed so
as to send the whole current to line, then the current from l is
interrupted, and the circuit of l2 is completed by the rising of the
armature of S, causing the reading sounder S2 to work. This is the 'B'
side.
R is a rheostat for balancing the resistance of the line, as used in
duplex working.
C is a condenser used for compensating the static charge of the line. It
is provided with an adjustable retardation coil, R1, to prolong the
effect of the compensating current from the condenser.
G is a differential galvanometer, used for testing, and for facilitating
adjustment and balancing.
Q is a switch for putting the line to earth, either for balancing, or
for any other purpose. There is on the earth wire leading from Q a
resistance coil, r1, equalling approximately the resistance of the whole
battery, 3.3 E1, and the resistance s.
The connections shown in Fig. 321, are for an 'up' office. At a 'down'
office it is necessary to reverse the wires on the two lower terminals
of the galvanometer and the two battery wires on the reversing key K1.
The keys K1 and K2 are, for repeaters, replaced by transmitters.
The adjustment of this apparatus requires great care and great accuracy.
Its good working depends essentially on technical skill that can only be
acquired by patience and perseverance.
Faults in working generally arise from careless adjustments, dirty
contacts, loose connections, battery failures, and the ordinary line
interruptions, but there are no troubles that are beyond the reach of
ordinary skill, and it can be safely said that, within moderate
distances, wherever and whenever duplex working is practicable, then
quadruplex working is so too."
The above is a typical quadruplex bridge system. There is also a
differential system, the full description of which, in addition to what
has been given, is outside of the scope of this work.