hboards are ringing
and listening keys. These are the devices by means of which the
operator may switch the central-office generator or her telephone set
into or out of the circuit of the connected lines. The details of a
simple ringing and listening key are shown in Fig. 238. This consists
of two groups of springs, one of four and one of six, the springs in
each group being insulated from each other at their points of
mounting. Two of these springs _1_ and _2_ in one group--the ringing
group--are longer than the others, and act as movable levers engaging
the inner pair of springs _3_ and _4_ when in their normal positions,
and the outer pair _5_ and _6_ when forced into their alternate
positions. Movement is imparted to these springs by the action of a
cam which is mounted on a lever, manipulated by the operator. When
this lever is moved in one direction the cam presses the two springs
_1_ and _2_ apart, thus causing them to disengage the springs _3_ and
_4_ and to engage the springs _5_ and _6_.

[Illustration: Fig. 238. Ringing and Listening Key]

The springs of the other group constitute the switching element of the
listening key and are very similar in their action to those of the
ringing key, differing in the fact that they have no inner pair of
springs such as _3_ and _4_. The two long springs _7_ and _8_,
therefore, normally do not rest against anything, but when the key
lever is pressed, so as to force the cam between them, they are made
to engage the two outer springs _9_ and _10_.

[Illustration: Fig. 239. Ringing-and Listening-Key Symbols]

The design and construction of ringing and listening keys assume many
different forms. In general, however, they are adapted to do exactly
the same sort of switching operations as that of which the device of
Fig. 238 is capable. Easily understood symbols of ringing and
listening keys are shown in Fig. 239; the cam member which operates on
the two long springs is usually omitted for ease of illustration. It
will be understood in considering these symbols, therefore, that the
two long curved springs usually rest against a pair of inner contacts
in case of the ringing key or against nothing at all in case of the
listening key, and that when the key is operated the two springs are
assumed to be spread apart so as to engage the outer pair of contacts
with which they are respectively normally disconnected.

_Line and Cord Equipments._ The parts of the switchboard that are