line current for ringing the bells. The pole changer
was redesigned after the beginning of the great spread of telephony in
the United States in 1893. Today it is firmly established as an element
of good telephone practice. Fig. 411 illustrates the principle upon
which one of the well-known pole changers--the Warner--operates. In
this _1_ is an electromagnet supplied by a constant-current battery _2_
to keep the vibratory system continually in motion. This motor magnet
and its battery work in a local circuit and cause vibration in exactly
the same manner as the armature of an ordinary electric door bell is
caused to vibrate. The battery from which the ringing current is derived
is indicated at _3_, and the poles of this are connected, respectively,
to the vibrating contacts _4_ and _5_. These contacts are merely the
moving members of a pole changing switch, and a study of the action will
readily show that when these moving parts engage the right-hand
contacts, current will flow to the line supposed to be connected to the
terminals _6_ and _7_ in one direction, while, when these parts engage
the left-hand contacts, current will flow to the line in the reverse
direction. The circuit of the condenser shown is controlled by the
armature of the relay _8_.

The winding of this relay is put directly in the circuit of the main
battery _3_, so that whenever current is drawn from this battery to ring
a distant bell, this relay will be operated and will bridge the
condenser across the circuit of the line. The purpose of the condenser
is to make the impulses flowing from the pole changer less abrupt, and
the reason for having its bridged circuit normally broken is to prevent
a waste of current from the battery _3_, due to the energy which would
otherwise be consumed by the condenser if it were left permanently
across the line.

[Illustration: Fig. 411. Warner Pole Changer]

[Illustration: Fig. 412. Pole Changers for Harmonic Ringing]

Pole changers for ringing bells of harmonic party lines are required to
produce alternating currents of practically constant frequencies. The
ideal arrangement is to cause the direct currents from a storage battery
to be alternated by means of the pole changers, and then transformed
into higher voltages required for ringing purposes, the transformer
also serving to smooth the current wave, making it more suitable for
ringing purposes. In Fig. 412 such an arrangement, adapted to develop
currents fo