by the Kellogg Company and the Dean Company, that of the Kellogg Company being illustrated in perspective, Fig. 268.  The keys of this general type have the same advantages as those of the horizontal on-edge arrangement with respect to the gathering of dust, and while perhaps the contacts are not so readily get-at-able for inspection, yet they have the advantage of being somewhat more simple, and of taking up less horizontal space on the key shelf.

[Illustration:  Fig. 268.  Vertical Listening and Ringing Key]

[Illustration:  Fig. 269.  Four-Party Listening and Ringing Key]

Party-Line Ringing Keys. For party-line ringing the key matter becomes somewhat more complicated.  Usually the arrangement is such that in connection with each calling plug there are a number of keys, each arranged with respect to the circuits of the plug so as to send out the proper combination and direction of current, if the polarity system is used; or the proper frequency of current if the harmonic system is used; or the proper number of impulses if the step-by-step or broken-line system is used.  The number of different kinds of arrangements and combinations is legion, and we will here illustrate only an example of a four-party line ringing key adapted for harmonic ringing.  A Kellogg party-line listening and ringing key is shown in Fig. 269.  In this, besides the regular listening key, are shown four push-button keys, each adapted, when depressed, to break the connection back of the key, and at the same time connect the proper calling generator with the calling plug.

Self-Indicating Keys. A complication that has given a good deal of trouble in the matter of party-line ringing is due to the fact that it is sometimes necessary to ring a second or a third time on a party-line connection, because the party called may not respond the first time.  The operator is not always able to remember which one of the four keys associated with the plug connected with the desired party she has pressed on the first occasion and, therefore, when it becomes necessary to ring again, she may ring the wrong party.  This is provided for in a very ingenious way in the key shown in Fig. 269, by making the arrangement such that after a given key has been depressed to its full extent in ringing, and then released, it does not come quite back to its normal position but remains slightly depressed.  This always serves as an indication to the operator, therefore, as to which key she depressed last, and in the case of a re-ring, she merely presses the key that is already down a little way.  On the next call if she is required to press another one of the four keys, the one which remained down a slight distance on the last call will be released and the one that is fully depressed will be the one that remains down as an indication.

Such keys, where the key that was last used leaves an indication to that effect, are called indicating ringing keys.  In other forms the indication is given by causing the key lever to move a little target which remains exposed until some other key in the same set is moved.  The key shown in Fig. 266 is an example of this type.