eighties. As is usual, two
subscribers' lines and a single cord circuit are shown. One side of each
line passes directly from the subscriber's station to one side of the
drop, and also branches off to the sleeve contact of each of the jacks.
The other side of the line passes first to the tip spring of the first
jack, thence to the anvil of that jack and to the tip spring of the next
jack, and so on in series through all of the jacks belonging in that
line to the other terminal of the drop coil. Normally, therefore, the
drop is connected across the line ready to be responsive to the signal
sent from the subscriber's generator. The cord circuit is of the
two-conductor type, the plugs being provided with tip and sleeve
contacts, the tips being connected by one of the flexible conductors
through the proper ringing and listening key springs, and the sleeve
being likewise connected through the other flexible conductor and the
other springs of the ringing and listening keys. It is obvious that when
any plug is inserted into a jack, the circuit of the line will be
continued to the cord circuit and at the same time the line drop will be
cut out of the circuit, because of the lifting of the tip spring of the
jack from its anvil. Permanently connected between the sleeve side of
the cord circuit and ground is a retardation coil _1_ and a battery.
Another retardation coil _2_ is connected between the ground and a
point on the operator's telephone circuit between the operator's head
receiver and the secondary of her induction coil. These two retardation
coils have to do with the busy test, the action of which is as follows:
normally, or when a line is not switched at the central office, the test
thimbles will all be at substantially ground potential, _i. e._, they
are supposed to be. The point on the operator's receiver circuit which
is grounded through the retardation coil _2_ will also be of ground
potential because of that connection to ground. In order to test, the
operator always has to throw her listening key _L.K._ into the listening
position. She also has to touch the tip of the calling plug _P_c to a
sleeve or jack of the line that is being tested. If, therefore, a test
is made of an idle or non-busy line, the touching of the tip of the
calling plug with the test thimble of that line will result in no flow
of current through the operator's receiver, because there will be no
difference of potential anywhere in the test circuit