In both the Kellogg and the Dean in-tune ringers, on account of the comparative stiffness of the armature springs and on account of the normal position of the armature with maximum air gaps and consequent minimum magnetic pull, the armature will practically not be affected unless the energizing current is accurately attuned to its own natural rate.  When the proper current is thrown on to the line, the ball will be thrown into violent vibration, and the ends of the armature brought into actual contact with the pole pieces, which are of bare iron and shielded in no way.  The armature in this position is very strongly attracted and comes to a sudden stop on the pole pieces.  The gongs are so adjusted that the tapper ball will have to spring about one thirty-second of an inch in order to hit them.  The armature is held against the pole piece while the tapper ball is engaged in striking the gong and in partially returning therefrom, and so strong is the pull of the pole piece on the armature in this position that the accelerating influence of the gong has no effect in accelerating the rate of vibration of the reed.

[Illustration:  Fig. 184.  Circuits of Dean Harmonic System]

Circuits.  In Fig. 184 are shown in simplified form the circuits of a four-station harmonic party line.  It is seen that at the central office there are four ringing keys, adapted, respectively, to impress on the line ringing currents of four different frequencies.  At the four stations on the line, lettered A, B, C, and D, there are four harmonic bells tuned accordingly.  At Station A there is shown the talking apparatus employing the Wheatstone bridge arrangement.  The talking apparatus at all of the other stations is exactly the same, but is omitted for the sake of simplicity.  A condenser is placed in series with each of the bells in order that there may be no direct-current path from one side of the line to the other when all of the receivers are on their hooks at the several stations.

In Fig. 185 is shown exactly the same arrangement, with the exception that the talking apparatus illustrated in detail at Station A is that of the Kellogg Switchboard and Supply Company.  Otherwise the circuits of the Dean and the Kellogg Company, and in fact of all the other companies manufacturing harmonic ringing systems, are the same.

Advantages.  A great advantage of the harmonic party-line system is the simplicity of the apparatus at the subscriber’s station.  The harmonic bell is scarcely more complex than the ordinary polarized ringer, and the only difference between the harmonic-ringing telephone and the ordinary telephone is in the ringer itself.  The absence of all relays and other mechanism and also the absence of the necessity for ground connections at the telephone are all points in favor of the harmonic system.

[Illustration:  Fig. 185.  Circuits of Kellogg Harmonic System]