hence through the other pair of
bells to the other limb of the line. This means that unless the
resistance of the bell windings is made very high, the path of the
signaling circuit will be of sufficiently low resistance to actuate
the line signal at the central office.

[Illustration: Fig. 172. Four-Party Polarity Selection]

It is not feasible to overcome this objection by the use of condensers
in series with the bells, as was done in the system shown in Fig. 170,
since the bells are necessarily biased and such bells, as may readily
be seen, will not work properly through condensers, since the placing
of a condenser in their circuit means that the current which passes
through the bell is alternating rather than pulsating, although the
original source may have been of pulsating nature only.

[Illustration: Fig 173. Standard Polarity System]

The remedy for this difficulty, therefore, has been to place in series
with each bell a very high non-inductive resistance of about 15,000 or
20,000 ohms, and also to make the windings of the bells of
comparatively high resistance, usually about 2,500 ohms. Even with
this precaution there is a considerable leakage of the central-office
battery current from one side of the line to the other through the two
paths to ground in series. This method of selective signaling has,
therefore, been more frequently used with magneto systems. An endeavor
to apply this principle to common-battery systems without the
objections noted above has led to the adoption of a modification,
wherein a relay at each station normally holds the ground connection
open. This is shown in Fig. 173 and is the standard four-party line
ringing circuit employed by the American Telephone and Telegraph
Company and their licensees.

In this system the biased bells are normally disconnected from the
line, and, therefore, the leakage path through them from one side of
the line to the other does not exist. At each station there is a relay
winding adapted to be operated by the ringing current bridged across
the line in series with a condenser. As a result, when ringing current
is sent out on the line all of the relays, _i.e._, one at each
station, are energized and attract their armatures. This establishes
the connection of all the bells to line and really brings about
temporarily a condition equivalent to that of Fig. 172. As a result,
the sending of a positive current on the lower line with a ground
return will cause