entral office by the
two limbs of a metallic-circuit line. One limb of the line terminates
in the spring _1_ of the jack, and the other limb in the sleeve or
thimble _2_ of the jack. The spring _1_ normally rests on the third
contact or anvil _3_ in the jack, its construction being such that
when a plug is inserted this spring will be raised by the plug so as
to break contact with the anvil _3_. It is understood, of course, that
the plug associated with this jack has two contacts, referred to
respectively as the tip and the sleeve; the tip makes contact with the
tip spring _1_ and the sleeve with the sleeve or thimble _2_.

[Illustration: Fig. 240. Normal Condition of Line]

The drop or line signal is permanently connected between the jack
sleeve and the anvil _3_. As a result, the drop is normally bridged
across the circuit of the line so as to be in a receptive condition to
signaling current sent out by the subscriber. It is evident, however,
that when the plug is inserted into the jack this connection between
the line and the drop will be broken.

In this normal condition of the line, therefore, the drop stands
ready at the central office to receive the signal from the subscriber
and the generator at the sub-station stands ready to be bridged across
the circuit of the line as soon as the subscriber turns its handle.
Similarly the ringer--the call-receiving device at the sub-station--is
permanently bridged across the line so as to be responsive to any
signal that may be sent out from the central office in order to call
the subscriber. The subscriber's talking apparatus is, in this normal
condition of the line, cut out of the circuit by the switch hook.

_Subscriber Calling._ Fig. 241 shows the condition of the line when
the subscriber at the sub-station is making a call. In turning his
generator the two springs which control the connection of the
generator with the line are brought into engagement with each other so
that the generator currents may pass out over the line. The condition
at the central office is the same as that of Fig. 240 except that the
drop is shown with its shutter fallen so as to indicate a call.

[Illustration: Fig. 241. Subscriber Calling]

[Illustration: A SPECIALLY FORMED CABLE FOR KEY SHELF OF MONARCH
SWITCHBOARD]

_Operator Answering._ The next step is for the operator to answer the
call and this is shown in Fig. 242. The subscriber has released the
handle of his generator and the gen