She takes down the connection only when the main-office subscriber has
hung up his telephone and the private branch subscriber has disconnected
from the trunk.

Whatever method is employed, private branch disconnection is usually
slow, and for this reason many operating companies instruct the
_A_-operators to disconnect on the lighting of the supervisory lamp of
the city subscriber.

=With Automatic Offices.= Private branch exchanges most used in
connection with automatic offices employ manual switchboards, with the
cord circuits of which is associated a signal transmitting device by
which the operator instead of the subscriber may manipulate the
automatic apparatus of the public exchange by impulses sent over the
private branch-exchange trunk lines. The subscriber's equipment at the
private branch stations may be either automatic or manual. Frequently
the same private branch exchange will contain both kinds. With the
manual sub-station equipment the operation is exactly the same as in a
private branch of a manual exchange, except that the private branch
operator by means of her dial makes the central-office connection
instead of telling the main-office operator to do so for her. With
automatic sub-station equipment at the private branch the subscribers,
by removing their receivers from their hooks, call the attention of the
private branch operator, who may receive their orders and make the
desired central-office connection for them, or who may plug their lines
through to the central office and allow the subscribers to make the
connection themselves with their own dials.

In automatic equipment of the common-battery type, some change always
takes place in the calling line at the time the called subscriber
answers. In the three-wire system during the time of calling, both wires
are of the same polarity with respect to earth. At the time of the
answering of the called subscriber, the two wires assume different
polarities, one being positive to the other. Such a change is sufficient
for the actuation of devices local to the private exchange switchboard
and may be interpreted through the calling supervisory signal in such a
way as to allow it to glow during calling and not to glow after the
called subscriber has answered. In the two-wire automatic system a
similar change can be arranged for, with similar advantageous results.

_Secrecy._ In private exchanges operating in connection with automatic
central offices, the s