For the following description and the cuts illustrating it we are
indebted to Preece and Sivewright. The cut shows the arrangement of the
apparatus and connections for terminal offices.
"Sufficient table room is provided to seat four clerks. The apparatus is
arranged for the two senders to sit together in the centre, the messages
to be forwarded being placed between them. The section on the left of
the switch Q is known as the 'A' side, that on the right as the 'B' side
of the apparatus.
K1 the reversing key, reverses the direction of the current. K2 is a
simple key, known as the increment key; it is used simply to increase
the strength of the current.
Fig. 320. QUADRUPLEX TELEGRAPH CONNECTIONS.
The way in which the keys K1 and K2 combine their action is shown by
Fig. 321. E1 and E2 are the line batteries, the one having two and
one-third (2-1/3) the number of cells of the other, so that if E1 be the
electro-motive force of the smaller, that of the whole combined battery
will be 3.3 E1. The negative pole of E1 is connected to z and z1 of K1
and the positive pole of E2 to a of K2 through a resistance coil s. A
wire, called the 'tap' wire, connects the positive pole of E1 and the
negative pole of E2 to b of K2. This wire has in it a resistance coil
r2. The springs c and c1 of Kl are connected to the lever L of K2. Now,
when both keys are at rest, the negative pole of E1 is to line through
z, and the positive pole of E1 to earth through b of K2 and c of K1; the
positive pole of E2 being insulated at a of K2.