There are two general methods of operating harmonic bells.  One of these may be called the in-tune system and the other the under-tune system.  The under-tune system was the first employed.

[Illustration:  OPERATING ROOM AT TOKYO, JAPAN]

Under-Tune System. The early workers in the field of harmonic-selective signaling discovered that when the tapper of the reed struck against gongs the natural rate of vibration of the reed was changed, or more properly, the reed was made to have a different rate of vibration from its natural rate.  This was caused by the fact that the elasticity of the gongs proved another factor in the set of conditions causing the reeds to take up a certain rate of vibration, and the effect of this added factor was always to accelerate the rate of vibration which the reed had when it was not striking the gongs.  The rebound of the hammer from the gongs tended, in other words, to accelerate the rate of vibration, which, as might be expected, caused a serious difficulty in the practical operation of the bells.  To illustrate:  If a reed were to have a natural rate of vibration, when not striking the gongs, of 50 per second and a current of 50 cycles per second were impressed on the line, the reed would take up this rate of vibration easily, but when a sufficient amplitude of vibration was attained to cause the tapper to strike the gongs, the reed would be thrown out of tune, on account of the tendency of the gongs to make the reed vibrate at a higher rate.  This caused irregular ringing and was frequently sufficient to make the bells cease ringing altogether or to ring in an entirely unsatisfactory manner.

In order to provide for this difficulty the early bells of Currier and Lighthipe were made on what has since been called the “under-tuned” principle.  The first bells of the Kellogg Switchboard and Supply Company, developed by Dean, were based on this idea as their cardinal principle.  The reeds were all given a natural rate of vibration, when not striking the gongs, somewhat below that of the current frequencies to be employed; and yet not sufficiently below the corresponding current frequency to make the bell so far out of tune that the current frequency would not be able to start it.  This was done so that when the tapper began to strike the gongs the tapper would be accelerated and brought practically into tune with the current frequency, and the ringing would continue regularly as long as the current flowed.  It will be seen that the under-tuned system was, therefore, one involving some difficulty in starting in order to provide for proper regularity while actually ringing.

Ringers of this kind were always made with but a single gong, it being found difficult to secure uniformity of ringing and uniformity of adjustment when two gongs were employed.  Although no ringers of this type are being made at present, yet a large number of them are in use and they will consequently be described.  Their action is interesting in throwing better light on the more improved types, if for no other reason.