le. Another distinguishing feature is, that all of the
multiple and answering jacks are normally cut off from the line at the
cut-off relay, but when the cut-off relay operates it serves, in
addition to cutting off the line relay, to attach the two limbs of the
line to the two wires leading through the multiple and answering jacks.
The control of the line relay by the subscriber's switch hook is clear
from the figure. The control of the cut-off relay is secured by
attaching one terminal of the cut-off relay winding permanently to that
wire leading through the multiple which connects with the sleeve
contacts of the jack, the other terminal of the cut-off relay being
grounded. The way in which this relay is operated will be understood
when it is stated that the sleeve contacts of both the answering and
calling plugs always carry full battery potential and, therefore,
whenever any plug is inserted into any jack, current flows from the
sleeve of the jack through the sleeve contact of the jack to ground,
through the winding of the cut-off relay, which relay becomes energized
and performs the functions just stated. It is seen that the wire
running through the multiple to which the sleeve jack contacts are
attached, is thus made to serve the double purpose of answering as one
side of the talking circuit, and also of performing the functions
carried out by the separate or third wire in the three-wire system. It
will be shown also that, in addition, this wire is made to lend itself
to the purposes of the busy test without any of these functions
interfering with each other in any way.

[Illustration: Fig. 350. Two-Wire Line Circuit]

_Cord Circuit._ The cord circuit in somewhat simplified form is shown in
Fig. 351. Here again there are but two conductors to the plugs and two
strands to the cords. This greater simplicity is in some measure offset
by the fact that four relays are required, two for each plug. This
so-called four-relay cord circuit may be most readily understood by
considering half of it at a time, since the two relays associated with
the answering plug act in exactly the same way as those connected with
the calling plug.

[Illustration: Fig. 351. Two-Wire Cord Circuit]

Associated with each plug of a pair are two relays _1_ and _2_, in the
case of the answering cord, and _3_ and _4_ in the case of the calling
cord. The coils of the relays _1_ and _2_ are connected in series and
bridged across the answering cord,