[Illustration:  Fig. 235.  Plug]

[Illustration:  Fig. 236.  Plug and Jack]

[Illustration:  Fig. 237.  Jack and Plug Symbols]

Keys.  Other important elements of manual switchboards 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.