ill
pass on to the next, and so on until an idle one is found. It is only
when the connector has hunted through the entire group of lines and
found them all busy that it will refuse to connect and will give the
busy signal to the calling subscriber.

=Party Lines.= The description of this system as given above has been
confined entirely to direct line working; however, party lines may be
and are frequently employed.

The circuits and apparatus used with direct lines are, with slight
modifications, applicable to use with party lines.

The harmonic method of ringing is employed and the stations are so
arranged with respect to the connectors that those requiring the same
frequency for ringing the bells are in groups served by the same set of
connectors.

[Illustration: POWER MACHINERY Citizens' Telephone Company, Racine, Wis.
_The Dean Electric Co._]

The party lines are operated on the principle commonly known in manual
practice as the jack per station arrangement. Each party line will,
therefore, have sets of terminals appearing in separate hundreds; the
connectors associated with each of these hundreds being so arranged as
to impress the proper frequency of ringing current on the line.

From the subscribers' standpoint the operation is the same as for direct
lines, as the particular hundreds digit of a number serves to select one
of a group of connectors capable of connecting the proper ringing
current to the line.

To avoid confusion, which would be caused by a subscriber on a party
line attempting to make a call when the line is already in use by some
other subscriber, the subscribers' stations are so arranged that when
the line is in use all other stations on the line are locked out.

[Illustration: Fig. 399. Wall Set for Two-Wire System]

=The Two-Wire Automatic System.= The two-wire system that has recently
been introduced by the Automatic Electric Company brings about the very
important result of accomplishing all of the automatic switching over
metallic circuit lines without the use of ground or common returns. The
system is thus relieved of the disturbing influences to which the
three-wire system is sometimes subjected, due to differences in earth
potential between various portions of the system, which may add to or
subtract from the battery potential and alter the net potential
available between two distant points. The introduction of this system
has also made possible certain other incidental featur