ecret feature of individual lines may or may not
be carried into the private exchange equipment. Some patrons of
automatic exchanges set a high value on the absence of any operator in a
connection and transact business over such lines which they would not
transact at all over manual lines or would not transact in the same way
over manual lines. To some such patrons, the presence of a private
exchange operator, even though employed and supervised by themselves,
seems to be a disadvantage. To meet such a feeling, it is not difficult
to arrange the circuits of a private exchange switchboard so that the
operator may listen in upon a cord circuit at any time and overhear what
is being said upon it _so long as two subscribers are not in
communication on that cord circuit_. That is, she may answer a call and
may speak to the calling person at any time she wishes until the called
person answers. When he does answer and conversation can take place,
some device operates to disconnect her listening circuit from the cord
circuit, not to be connected again until at least one of the subscribers
has hung up his receiver. With private exchange apparatus so arranged,
the secrecy of the system is complete.

=Battery Supply.= There are three available methods of supplying direct
current for talking and signaling purposes to private branch exchanges,
each of which represents good practice under certain conditions. First,
by means of pairs of wires extended from the central-office battery;
second, by means of a local storage battery at the private branch
exchange charged over wires from the central office; and third, by means
of a local storage battery at the private exchange charged from a local
source.

The choice of these three methods depends always on the local conditions
and it is a desirable feature, to be employed by large operating
companies, to have all private branch-exchange switchboards provided
with simple convertible features contained within the switchboard for
adapting it to any one of these methods of supplying current.

If a direct-current power circuit is available at the private branch
exchange, it may be used for charging the local storage battery by
inserting mere resistance devices in the charging leads. If the local
power circuit carries alternating current, a converting device of some
sort must be used and for this purpose, if the exchange is large enough
to warrant it, a mercury rectifier is an economical and si