The amount of business that is done over a telephone line is usually referred to as the “traffic.”  It will be understood, however, in considering party-line working that the number of calls per day or per hour, or per shorter unit, is not the true measure of the traffic and, therefore, not the true measure of the amount of possible interference between the various subscribers on the line.

An almost equally great factor is the average length of the conversation.  In city lines, that is, in lines in city exchanges, the conversation is usually short and averages perhaps two minutes in duration.  In country lines, however, serving people in rural districts, who have poor facilities for seeing each other, particularly during the winter time, the conversations will average very much longer.  In rural communities the people often do much of their visiting by telephone, and conversations of half an hour in length are not unusual.  It is obvious that under such conditions a party line having a great many stations will be subject to very grave interference between the parties, people desiring to use the line for business purposes often being compelled to wait an undue time before they may secure the use of the line.

It is obvious, therefore, that the amount of traffic on the line, whether due to many short conversations or to a comparatively few long ones, is the main factor that should determine the number of stations that, economically, may be placed on a line.  The facilities also for building lines enter as a factor in this respect, since it is obvious that in comparatively poor communities the money may not be forthcoming to build as many lines as are needed to properly take care of the traffic.  A compromise is, therefore, often necessary, and the only rule 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.