.

Polarity Method. The polarity method of selective signaling on party
lines is probably the most extensively used. The standard selective
system of the American Telephone and Telegraph Company operates on
this principle.

_Two-Party Line._ It is obvious that selection may be had between two
parties on a single metallic-circuit line without the use of biased
bells or current of different polarities. Thus, one limb of a metallic
circuit may be used as one grounded line to ring the bell at one of
the stations, and the other limb of the metallic circuit may be used
as another grounded line to ring the bell of the other station; and
the two limbs may be used together as a metallic circuit for talking
purposes as usual.

This is shown in Fig. 170, where the ringing keys at the central office
are diagrammatically shown in the left-hand portion of the figure as
_K_^{1} and _K_^{2}. The operation of these keys will be more fully
pointed out in a subsequent chapter, but a correct understanding will
be had if it be remembered that the circuits are normally maintained by
these keys in the position shown. When, however, either one of the keys
is operated, the two long springs may be considered as pressed apart so
as to disengage the normal contacts between the springs and to engage
the two outer contacts, with which they are shown in the cut to be
disengaged. The two outer contacts are connected respectively to an
ordinary alternating-current ringing generator and to ground, but the
connection is reversed on the two keys.

[Illustration: Fig. 170. Simple Two-Party Line Selection]

At Station A the ordinary talking set is shown in simplified form,
consisting merely of a receiver, transmitter, and hook switch in a
single bridge circuit across the line. An ordinary polarized bell is
shown connected in series with a condenser between the lower limb of
the line and ground. At Station B the same talking circuit is shown,
but the polarized bell and condenser are bridged between the upper
limb of the line and ground.

If the operator desires to call Station A, she will press key _K_^{1}
which will ground the upper side of the line and connect the lower
side of the line with the generator _G_^{1}, and this, obviously, will
cause the bell at Station A to ring. The bell at Station B will not
ring because it is not in the circuit. If, on the other hand, the
operator desires to ring the bell at Station B, she will depress key
_K_^{2},