which will allow the current from generator _G_^{2} to pass
over the upper side of the line through the bell and condenser at
Station B and return by the path through the ground. The object of
grounding the opposite sides of the keys at the central office is to
prevent cross-ringing, that is, ringing the wrong bell. Were the keys
not grounded this might occur when a ringing current was being sent
out while the receiver at one of the stations was off its hook; the
ringing current from, say, generator _G_^{1} then passing not only
through the bell at Station A as intended, but also through the bell
at Station B by way of the bridge path through the receiver that
happened to be connected across the line. With the ringing keys
grounded as shown, it is obvious that this will not occur, since the
path for the ringing current through the wrong bell will always be
shunted by a direct path to ground on the same side of the line.

In such a two-party-line selective system the two generators _G_^{1}
and _G_^{2} may be the same generator and may be of the ordinary
alternating-current type. The bells likewise may be of the ordinary
alternating-current type.

The two-party selective line just described virtually employs two
separate circuits for ringing. Now each of these circuits alone may be
employed to accomplish selective ringing between two stations by using
two biased bells oppositely polarized, and employing pulsating ringing
currents of one direction or the other according to which bell it is
desired to ring. One side of a circuit so equipped is shown in Fig.
171. In this the two biased bells are at Station A and Station B,
these being bridged to ground in each case and adapted to respond only
to positive and negative impulses respectively. At the central office
the two keys _K_^{1} and _K_^{2} are shown. A single
alternating-current generator _G_ is shown, having its brush _1_
grounded and brush _2_ connected to a commutator disk _3_ mounted on
the generator shaft so as to revolve therewith. One-half of the
periphery of this disk is of insulating material so that the brushes
_4_ and _5_, which bear against the disk, will be alternately
connected with the disk and, therefore, with the brush _2_ of the
generator. Now the brush _2_, being one terminal of an
alternating-current machine, is alternately positive and negative, and
the arrangement of the commutator is such that the disk, which is
always at the potential of the bru