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 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.