ch, being of steel, will afford a path
for the lines of force to the armature proper. We see, therefore, that
the two magnet cores are, by this permanent magnet, given one
polarity, while the reed tongue itself is given the other polarity,
this being exactly the condition that has already been described in
connection with the regular polarized bell or ringer.

The electromagnetic action by which this reed tongue is made to
vibrate is, therefore, exactly the same as that of an ordinary
polarized ringer, but the difference between the two is that, in this
harmonic ringer, the reed tongue will respond only to one particular
rate of vibrations, while the regular polarized ringer will respond to
almost any.

As shown in Fig. 178, the tapper ball strikes on the inside surface of
the single gong. The function of the auxiliary spring _7_ between the
ball and the main portion of the armature is to allow some resilience
between the ball and the balance of the armature so as to counteract
in some measure the accelerating influence of the gong on the
armature. In these bells, as already stated, the natural rate of
vibration of the reed tongue was made somewhat lower than the rate at
which the bell was to be operated, so that the reed tongue had to be
started by a current slightly out of tune with it, and then, as the
tapper struck the gong, the acceleration due to the gong would bring
the vibration of the reed tongue, as modified by the gong, into tune
with the current that was operating it. In ether words, in this system
the ringing currents that were applied to the line had frequencies
corresponding to what may be called the _operative rates of vibration_
of the reed tongues, which operative rates of vibration were in each
case the resultant of the natural pitch of the reed as modified by the
action of the bell gong when struck.

[Illustration: Fig. 179. Under-Tuned Ringer]

_In-Tune System._ The more modern method of tuning is to make the
natural rate of vibration of the reed tongue, that is, the rate at
which it naturally vibrates when not striking the gongs, such as to
accurately correspond to the rate of vibration at which the bells are
to be operated--that is, the natural rate of vibration of the reed
tongues is made the same as the operative rate. Thus the bells are
attuned for easy starting, a great advantage over the under-tuned
system. In the under-tuned system, the reeds being out of tune in
starting require heavie