the operation of the bell at Station A. It will not
ring the bell at Station B because of the wrong polarity. It will not
ring the bells of Station C and Station D because they are in the
circuit between the other side of the line and ground. As soon as the
ringing current ceases all of the relays release their armatures and
disconnect all the bells from the line.

By this very simple device the trouble, due to marginal working of
the line signal, is done away with, since normally there is no leakage
from one side of the line to the other on account of the presence of
the condensers in the bridge at each station.

[Illustration: Fig. 174. Ringing-Key Arrangement]

In Fig. 174, the more complete connections of the central-office
ringing keys are shown, by means of which the proper positive or
negative ringing currents are sent to line in the proper way to cause
the ringing of any one of the four bells on a party line of either of
the types shown in Figs. 172 and 173.

In this the generator _G_ and its commutator disk _3_, with the
various brushes, _1_, _2_, _4_, and _5_, are arranged in the same
manner as is shown in Fig. 171. It is evident from what has been said
that wire _6_ leading from generator brush _2_ and commutator disk _3_
will carry alternating potential; that wire _7_ will carry positive
pulsations of potential; and that wire _8_ will carry negative
pulsations of potential. There are five keys in the set illustrated in
Fig. 174, of which four, viz, _K_^{1}, _K_^{2}, _K_^{3}, and _K_^{4},
are connected in the same manner as diagrammatically indicated in
Figs. 172 and 173, and will, obviously, serve to send the proper
current over the proper limb of the line to ring one of the bells. Key
_K_^{5}, the fifth one in the set, is added so as to enable the
operator to ring an ordinary unbiased bell on a single party line when
connection is made with such line. As the two outside contacts of this
key are connected respectively to the two brushes of the
alternating-current dynamo _G_, it is clear that it will impress an
alternating current on the line when its contacts are closed.

_Circuits of Two-Party Line Telephones._ In Fig. 175 is shown in
detail the wiring of the telephone set usually employed in connection
with the party-line selective-ringing system illustrated in Fig. 170.
In the wiring of this set and the two following, it must be borne in
mind that the portion of the circuit used during conversation m