It is obvious that in a heavily loaded bridged line, the bell at the home station, that is at the station from which the call is being sent, will take slightly more than its share of the current, and it is also obvious that the ringing of the home bell performs no useful function.  The plan is frequently adopted, therefore, of having the operation of the generator serve to cut its own bell out of the circuit.  The arrangement by which this is done is clearly shown in Fig. 169.  The circuit of the bell is normally complete across the line, while the circuit of the generator is normally open.  When, however, the generator crank is turned these conditions are reversed, the bell circuit being broken and the generator circuit closed, so as to allow its current all to pass the line.  This feature of having the local bell remain silent upon the operation of its own generator is also of advantage because other parties at the same station are not disturbed by the ringing of the bell when a call is being made by that station.

A difficulty encountered on non-selective bridging party lines, which at first seems amusing rather than serious, but which nevertheless is often a vexatious trouble, is that due to the propensity of some people to “listen in” on the line on hearing calls intended for other than their own stations.  People whose ethical standards would not permit them to listen at, or peep through, a keyhole, often engage in this telephonic eavesdropping.

Frequently, not only one but many subscribers will respond to a call intended for others and will listen to the ensuing conversation.  This is disadvantageous in several respects:  It destroys the privacy of conversation between any two parties; it subjects the local batteries to an unnecessary and useless drain; and it greatly impairs the ringing efficiency of the line.  The reason for this interference with ringing is that the presence of the low-resistance receivers across the line allows the current sent out by any of the generators to pass in large measure through the receivers, thus depriving the ringers, which are of comparatively high resistance and impedance, of the energy necessary to operate them.  As a result of this it is frequently impossible for one party to repeat the call for another because, during the interval between the first and second call, a number of parties remove their receivers from their hooks in order to listen.  Ring-off or clearing-out signals are likewise interfered with.

[Illustration:  Fig. 169.  Circuits of Bridging Station]

A partial remedy for this interference with ringing, due to eavesdropping, is to introduce a low-capacity condenser into the receiver circuit at each station, as shown in Fig. 169.  This does not seriously interfere with the speech transmission since the condensers will readily transmit the high-frequency voice currents.  Such condensers, however, have not sufficient capacity to enable them readily to transmit the low-frequency ringing currents and hence these are forced, in large measure, to pass through the bells for which they are intended rather than leaking through the low-resistance receiver paths.