one particular rate of
vibration and with extreme difficulty at any other rate, it is clear
that for current impulses of a frequency corresponding to its natural
rate the reed will take up the vibration, while for other frequencies
it will fail to respond.

Selection on party lines by means of this system is provided for by
tuning all of the reeds on the line at different rates of vibration
and is accomplished by sending out on the line ringing currents of
proper frequency to ring the desired bell. The current-generating
devices for ringing these bells are capable of sending out different
frequencies corresponding respectively to the rates of vibration of
each of the vibrating reed tongues. To select any one station,
therefore, the current frequency corresponding to the rate of
vibration of the reed tongue at that station is sent and this, being
out of tune with the reed tongues at all of the other stations,
operates the tongue of the desired station, but fails to operate those
at all of the other stations.

(_3_) In the _step-by-step_ system the bells on the line are normally
not in operative relation with the line and the bell of the desired
party on the line is made responsive by sending over the line a
certain number of impulses preliminary to ringing it. These impulses
move step-by-step mechanisms at each of the stations in unison, the
arrangement being such that the bells at the several stations are each
made operative after the sending of a certain number of preliminary
impulses, this number being different for all the stations.

(_4_) The _broken-line_ systems are new in telephony and for certain
fields of work look promising. In these the line circuit is normally
broken up into sections, the first section terminating at the first
station out from the central office, the second section at the second
station, and so on. When the line is in its normal or inactive
condition only the bell at the first station is so connected with the
line circuit as to enable it to be rung, the line being open beyond.
Sending a single preliminary impulse will, however, operate a
switching device so as to disconnect the bell at the first station and
to connect the line through to the second station. This may be carried
out, by sending the proper number of preliminary impulses, so as to
build up the line circuit to the desired station, after which the
sending of the ringing current will cause the bell to ring at that
station only