arative 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
ring