elephone transmission.

[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
clea