A modification of this arrangement is shown in Fig. 285, wherein a double-wound clearing-out drop is used, and a 1/2-microfarad condenser is placed in series in each side of the cord circuit between the winding connections of the clearing-out drop. This circuit should give a positive ring-off under all conditions and should prevent through ringing except as it may be provided by the transformer action between the two windings on the same core.

Fig. 285. Non-Ring-Through Cord Circuit
[View full size illustration.]

Another rather ingenious method of securing a positive ring-off and yet of preventing in a certain degree the undesirable ringing-through feature is shown in the cord circuit, Fig. 286. In this two non-inductive coils 1 and 2 are shown connected in series in the tip and sleeve strands of the coils, respectively. Between the neutral point of these two non-inductive windings is connected the clearing-out drop circuit. Voice currents find ready path through these non-inductive windings because of the fact that, being non-inductive, they present only their straight ohmic resistance. The impedance of the clearing-out drop prevents the windings being shunted across the two sides of the cord circuit. With this circuit a positive ring-off is assured even though the line connected with the one sending the clearing-out signal is short-circuited or open. If it is short-circuited, the shunt around the clearing-out drop will still have the resistance of two of the non-inductive windings included in it, and thus the drop will never be short-circuited by a very low-resistance path. Obviously, an open circuit in the line will not prevent the clearing-out signal being received. While this is an ingenious scheme, it is not one to be highly recommended since the non-inductive windings, in order to be effective so far as signaling is concerned, must be of considerable resistance and this resistance is in series in the talking circuit. Even non-inductive resistance is to be avoided in the talking circuit when it is of considerable magnitude and where there are other ways of solving the problem.

Fig. 286. Cord Circuit with Differential Windings
[View full size illustration.]

Double Clearing-out Type. Some people prefer two clearing-out drops in each cord circuit, so arranged that the one will be responsive to currents sent from the line with which the answering plug is connected and the other responsive only to currents sent from the line with which the calling plug is connected. Such a scheme, shown in Fig. 287, is sometimes employed by the Dean, the Monarch, and the Kellogg companies. Two 500-ohm clearing-out drops of ordinary construction are bridged across the cord circuit and in each side of the cord circuit there is included between the drop connections a 1-microfarad condenser. Ringing currents originating on the line with which the answering plug is connected will pass through the clearing-out drop, which is across that side of the cord circuit, without having to pass through any condensers. In order to reach the other clearing-out drop the ringing current must pass through the two 1-microfarad condensers in series, this making in effect only 1/2-microfarad. As is well known, a 1/2-microfarad condenser not only transmits voice currents with ease but also offers a very high apparent resistance to ringing currents. With the double clearing-out drop system the operator is enabled to tell which subscriber is ringing off. If both shutters fall she knows that both subscribers have sent clearing-out signals and she, therefore, pulls down the connection without the usual precaution of listening to see whether one of the subscribers may be waiting for another connection. This double clearing-out system is analogous to the complete double-lamp supervision that will be referred to more fully in connection with common-battery circuits. There is not the need for double supervision in magneto work, however, that there is in common-battery work because of the fact that in magneto work the subscribers frequently fail to remember to ring off, this act being entirely voluntary on their part, while in common-battery work, the clearing-out signal is given automatically by the subscriber when he hangs up his receiver, thus accomplishing the desired end without the necessity of thoughtfulness on his part.