e that may be safely laid down is to place as few parties on a
given line as conditions will admit.

No definite limit may be set to apply to all conditions but it may be
safely stated that under ordinary circumstances no more than ten
stations should be placed on a non-selective line. Twenty stations
are, however, common, and sometimes forty and even fifty have been
connected to a single line. In such cases the confusion which results,
even if the talking and the ringing efficiency are tolerable, makes
the service over such overloaded lines unsatisfactory to all
concerned.

CHAPTER XVI

SELECTIVE PARTY-LINE SYSTEMS

The problem which confronts one in the production of a system of
selective ringing on party lines is that of causing the bell of any
chosen one of the several parties on a circuit to respond to a signal
sent out from the central office without sounding any of the other
bells. This, of course, must be accomplished without interfering with
the regular functions of the telephone line and apparatus. By this is
meant that the subscribers must be able to call the central office and
to signal for disconnection when desired, and also that the
association of the selective-signaling devices with the line shall not
interfere with the transmission of speech over the line. A great many
ways of accomplishing selective ringing on party lines have been
proposed, and a large number of them have been used. All of these ways
may be classified under four different classes according to the
underlying principle involved.

Classification. (_1_) _Polarity_ systems are so called because they
depend for their operation on the use of bells or other responsive
devices so polarized that they will respond to one direction of
current only. These bells or other devices are so arranged in
connection with the line that the one to be rung will be traversed by
current in the proper direction to actuate it, while all of the others
will either not be traversed by any current at all, or by current in
the wrong direction to cause their operation.

(_2_) The _harmonic_ systems have for their underlying principle the
fact that a pendulum or elastic reed, so supported as to be capable of
vibrating freely, will have one particular rate of vibration which it
may easily be made to assume. This pendulum or reed is placed under
the influence of an electromagnet associated with the line, and owing
to the fact that it will vibrate easily at