natural rate of vibration of the swing. It is of course
possible, however, to make the swing take up other rates of vibrations
by the application of sufficient force. As another example, consider a
clock pendulum beating seconds. By gentle blows furnished by the
escapement at exactly the proper times, the heavy pendulum is kept in
motion. However, if a person grasps the pendulum weight and shakes it,
it may be made to vibrate at almost any desired rate, dependent on the
strength and agility of the individual.

The conclusion is, therefore, that a reed or pendulum may be made to
start and vibrate easily by the application of impulses at proper
intervals, and only with great difficulty by the application of
impulses at other than the proper intervals; and these facts form the
basis on which harmonic-ringing systems rest.

The father of harmonic ringing in telephony was Jacob B. Currier, an
undertaker of Lowell, Mass. His harmonic bells were placed in series
in the telephone line, and were considerably used in New England in
commercial practice in the early eighties. Somewhat later James A.
Lighthipe of San Francisco independently invented a harmonic-ringing
system, which was put in successful commercial use at Sacramento and a
few other smaller California towns. Lighthipe polarized his bells and
bridged them across the line in series with condensers, as in modern
practice, and save for some crudities in design, his apparatus closely
resembled, both in principle and construction, some of that in
successful use today.

Lighthipe's system went out of use and was almost forgotten, when
about 1903, Wm. W. Dean again independently redeveloped the harmonic
system, and produced a bell astonishingly like that of Lighthipe, but
of more refined design, thus starting the development which has
resulted in the present wide use of this system.

The signal-receiving device in harmonic-ringing systems takes the form
of a ringer, having its armature and striker mounted on a rather stiff
spring rather than on trunnions. By this means the moving parts of the
bell constitute in effect a reed tongue, which has a natural rate of
vibration at which it may easily be made to vibrate with sufficient
amplitude to strike the gongs. The harmonic ringer differs from the
ordinary polarized bell or ringer, therefore, in that its armature
will vibrate most easily at one particular rate, while the armature of
the ordinary ringer is almost indifferent,