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 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