to be altered by the
operating companies. Several years of experience with these bells has
shown that when once properly assembled they maintain the same rate of
vibration with great constancy.

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 w