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 heavier starting current, and this is obviously conducive to cross-ringing, that is, to the response of bells to other than the intended frequency.

Again, easy starting is desirable because when the armature is at rest, or in very slight vibration, it is at a maximum distance from the poles of the electromagnet, and, therefore, subject to the weakest influence of the poles.  A current, therefore, which is strong enough to start the vibration, will be strong enough to keep the bell ringing properly.

[Illustration:  Fig. 180.  Dean In-Tune Ringer]

When with this “in-tune” mode of operation, the armature is thrown into sufficiently wide vibration to cause the tapper to strike the gong, the gong may tend to accelerate the vibration of the reed tongue, but the current impulses through the electromagnet coils continue at precisely the same rates as before.  Under this condition of vibration, when the reed tongue has an amplitude of vibration wide enough to cause the tapper to strike the gongs, the ends of the armature come closest to the pole pieces, so that the pole pieces have their maximum magnetic effect on the armature, with the result that even if the accelerating tendency of the gongs were considerable, the comparatively large magnetic attractive impulses occurring at the same rate as the natural rate of vibration of the reed tongue, serve wholly to prevent any actual acceleration of the reed tongue.  The magnetic attractions upon the ends of the armature, continuing at the initial rate, serve, therefore, as a check to offset any accelerating tendency which the striking of the gong may have upon the vibrating reed tongue.