y lamp to inform her
as to whether or not the subscriber has answered. The ringing key is
held down, after its depression by the operator, either by an
electromagnet or by a magnet-controlled latch, and the ringing of the
subscriber's bell continues at periodic intervals as controlled by the
ringing commutator associated with the ringing machine. When the
subscriber answers, however, the closure of his line circuit results in
such an operation of the magnet associated with the ringing key as to
release the ringing key and thus to automatically discontinue the
ringing current.

When a connection is established between two subscribers through such a
trunk the supervision of the connection falls entirely upon the
_A_-operator who established it. This means that the calling supervisory
lamp at the _A_-operator's position is controlled over the trunk from
the station of the called subscriber, the answering supervisory lamp
being, of course, under the control of the calling subscriber as in the
case of a local connection. It is, therefore, the _A_-operator who
always initiates the taking down of a trunk connection, and when, in
response to the lighting of the two lamps, she withdraws her calling
plug from the trunk jack, the supervisory lamp associated with the
incoming end of the trunk at the other office is lighted, and the
_B_-operator obeys it by pulling down the plug.

If, upon testing the multiple jack of the called subscriber's line, the
_B_-operator finds the line to be busy, she at once inserts the trunk
plug into a so-called "busy-back" jack, which is merely a jack whose
terminals are permanently connected to a circuit that is intermittently
opened and closed, and which also has impressed upon it an alternating
current of such a nature as to produce the familiar "buzz-buzz" in a
telephone receiver. The opening and closing of this circuit causes the
calling supervisory lamp of the _A_-operator to flash at periodic
intervals just as if the called subscriber had raised and lowered his
receiver, but more regularly. This is the indication to the _A_-operator
that the line called for is busy. The buzzing sound is repeated back
through the cord circuit of the _A_-operator to the calling subscriber
and is a notification to him that the line is busy.

Sometimes, as is practiced in New York City, for instance, the buzzing
feature is omitted, and the only indication that the calling subscriber
receives that the called-for l