Ringing and Listening Keys.  The general principles of the ringing key have already been referred to.  Ringing keys are of two general types, one having horizontal springs and the other vertical.

[Illustration:  Fig. 266.  Horizontal-Spring Listening and Ringing Key]

Horizontal Spring Type. Various Bell operating companies have generally adhered to the horizontal spring type except in individual and four-party-line keys.  The construction of a Western Electric Company horizontal spring key is shown in Fig. 266.  In this particular key, as illustrated, there are two cam levers operating upon three sets of springs.  The cam lever at the left operates the ordinary ringing and listening set of springs according to whether it is pushed one way or the other.  In ringing on single-party lines the cam lever at the left is the one to be used; while on two-party lines the lever at the left serves to ring the first party and the ringing key at the right the second party.

In order that the operator may have an indication as to which station on a two-party line she has called, a small target 1 carried on a lever 2 is provided.  This target may display a black or a white field, according to which of its positions it occupies.  The lever 2 is connected by the links 3 and 4 with the two key levers and the target is thus moved into one position or the other, according to which lever was last thrown into ringing position.

It will be noticed that the springs are mounted horizontally and on edge.  This on-edge feature has the advantage of permitting ready inspection of the contacts and of avoiding the liability of dust gathering between the contacts.  As will be seen, at the lower end of each switch lever there is a roller of insulating material which serves as a wedge, when forced between the two long springs of any set, to force them apart and into engagement with their respective outer springs.

[Illustration:  Fig. 267.  Vertical-Spring Listening and Ringing Key]

Vertical Spring Type. The other type of ringing and listening key employing vertical springs is almost universally used by the various independent manufacturing companies.  A good example of this is shown in Fig. 267, which shows partly in elevation and partly in section a double key of the Monarch Company.  The operation of this is obvious from its mode of construction.  The right-hand set of springs of the right-hand key in this cut are the springs of the listening key, while the left-hand set of the right-hand key are those of the calling-plug ringing key.  The left-hand set of the left-hand key may be those of a ring-back key on the answering plug, while the right-hand set of the left-hand key may be for any special purpose.  It is obvious that these groups of springs may be grouped in different combinations or omitted in part, as required.  This same general form of key is also manufactured