the live or negative pole of the common battery, which will
cause current to flow through the coil of the cut-off relay, which will
then operate to _cut off_ both of the limbs of the line from their
normal connection with ground and the battery and the line relay. Hence
the name _cut-off relay_.

The use of the cut-off relay to sever the calling apparatus from the
line at all times when the line is switched serves to make possible a
very much simpler jack than would otherwise be required, as will be
obvious to anyone who tries to design a common-battery multiple system
without a cut-off relay. The additional complication introduced by the
cut-off relay is more than offset by the saving in complexity of the
jacks. It is desirable, on account of the great number of jacks
necessarily employed in a multiple switchboard, that the jacks be of the
simplest possible construction, thus reducing to a minimum their first
cost and making them much less likely to get out of order.

_Cord Circuit._ The cord circuit of the Western Electric standard
multiple common-battery switchboard is shown in Fig. 346. This cord
circuit involves the use of three strands in the flexible cords of both
the calling and the answering plugs. Two of these are the ordinary tip
and ring conductors over which speech is transmitted to the connected
subscriber's wire. The third, the sleeve strand, carries the supervisory
lamps and has associated with it other apparatus for the control of
these lamps and of the test circuit.

[Illustration: Fig. 346. Cord Circuit Western Electric No. 1 Board]

The system of battery feed is the well-known split repeating-coil
arrangement already discussed. The tip strand runs straight through to
the repeating coil, while the ring strand contains, in each case, the
winding of the supervisory relay corresponding to either the calling or
the answering plug. In order that the presence in the talking circuit of
a magnet winding possessing considerable impedance may not interfere
with the talking efficiency, each of these supervisory relay windings is
shunted by a non-inductive resistance. In practice the supervisory relay
windings have each a resistance of about 20 ohms and the shunt around
them each a resistance of about 31 ohms. In the third strand of each
cord is placed a 12-volt supervisory lamp, and in series with it a
resistance of about 80 ohms. Each supervisory relay is adapted, when
energized, to close a 40-ohm shunt abo