[Illustration:  Fig. 282.  Series Drop-Cord Circuit]

Series Drop Type. Another type of cord circuit that was largely used by the Stromberg-Carlson Telephone Manufacturing Company at one time is shown in Fig. 282.  In this the clearing-out drop was not bridged but was placed in series in the tip side of the line and was shunted by a condenser.  The resistance of the clearing-out drop was 1,000 ohms and the capacity of the condenser was 2 microfarads.  It is obvious that this way of connecting the clearing-out drop was subject to the ringing-through difficulty, since the circuit through which the clearing-out current necessarily passed included the telephone instrument of the line that was not sending the clearing-out signal.  This form was also objectionable because it was necessary for the subscriber to ring through the combined resistance of two lines, and in case the other line happened to be open, no clearing-out signal would be received.  While this circuit, therefore, was perhaps not quite so likely as the other to tie up the subscriber, that is, to leave him connected without the ability to send a clearing-out signal, yet it was sure to ring through, for the clearing-out drop could not be thrown without the current passing through the other subscriber’s station.

[Illustration:  Fig. 283.  Dean Non-Ring-Through Cord Circuit]

Non-Ring-Through Type. An early attempt at a non-ring-through cord is shown in Fig. 283, this having once been standard with the Dean Electric Company.  It made use of two condensers of 1 microfarad each, one in each side of the cord circuit.  The clearing-out drop was of 500 ohms resistance and was connected from the answering side of the tip conductor to the calling side of the sleeve conductor.  In this way whatever clearing-out current reached the central office passed through at least one of the condensers and the clearing-out drop.  In order for the clearing-out current to pass on beyond the central office it was necessary for it to pass through the two condensers in series.  This arrangement had the advantage of giving a positive ring-off, regardless of the condition of the connected line.  Obviously, even if the line was short-circuited, the ringing currents from the other line would still be forced through the clearing-out drop on account of the high effective resistance of the 1-microfarad condenser connected in series with the short-circuited line.  Also the clearing-out signal would be properly received if the connected line were open, since the clearing-out drop would still be directly across the cord circuit.  This arrangement also largely prevented through ringing, since the currents would pass through the 1-microfarad condenser and the 500-ohm drop more readily than through the two condensers connected in series.

[Illustration:  Fig. 284.  Monarch Non-Ring-Through Cord Circuit]