r harmonic ringing on party lines, is shown. The regular
common battery of the central office is indicated at _1_, _2_ being an
auxiliary battery of dry cells, the purpose of which will be presently
referred to. At the right of the battery _1_ there is shown the calling
plug with its associated party-line ringing keys adapted to impress the
several frequencies on the subscribers' lines. The method by which the
current from the main storage battery passes through the motor magnets
of the several vibrators, and by which the primary currents through the
transformers are made to alternate at the respective frequencies of
these vibrators, will be obvious from the drawing. It is also clear that
the secondary currents developed in these transformers are led to the
several ringing keys so as to be available for connection with the
subscribers' lines at the will of the operator. The condensers are
bridged across the primary windings of the transformers for the purpose
of aiding in smoothing out the current waves. The use of the auxiliary
battery _2_ and the retardation coil _3_ in the main supply lead is for
the purpose of preventing the pulsating currents drawn from the main
battery _1_ from making the battery "noisy." These two batteries have
like poles connected to the supply lead, and the auxiliary battery
furnishes no current to the system except when the electromotive force
of the impulse flowing from the main battery is choked down by the
impedance coil and the deficiency is then momentarily supplied for each
wave by the auxiliary battery. This is the method developed by the Dean
Electric Company for preventing the pole-changer system from causing
disturbances on lines supplied from the same main battery.

[Illustration: Fig. 413. Multi-Cyclic Generator Set]

_Ringing Dynamos._ Alternating and pulsating currents for ringing
purposes are also largely furnished from alternating-current dynamos
similar to those used in commercial power and lighting work, but
specially designed to produce ringing currents of proper frequency and
voltage. These are usually driven by electric motors deriving their
current either from the commercial supply mains or from the
central-office battery. In large exchanges harmonic ringers are usually
operated by alternating-current generators driven by motors, a separate
dynamo being provided to furnish the current of each frequency. Fig. 413
shows a set of four such generators directly connected to a