Other arrangements are often employed and by almost any of them a great variety of readily distinguishable signals may be secured. The patrons of such lines learn to distinguish, with comparatively few errors, between the calls intended for them and those intended for others, but frequently they do not observe the distinction, as has already been pointed out.
Limitations. With good telephones the limit as to the number of stations that it is possible to operate upon a single line is usually due more to limitations in ringing than in talking. As the number of stations is increased indefinitely a condition will be reached at which the generators will not be able to generate sufficient current to ring all of the bells, and this condition is likely to occur before the talking efficiency is seriously impaired by the number of bridges across the line.
Neither of these considerations, however, should determine the maximum number of stations to be placed on a line. The proper limit as to the number of stations is not the number that can be rung by a single generator, or the number with which it is possible to transmit speech properly, but rather the number of stations that may be employed without causing undue interference between the various parties who may desire to use the line. Overloaded party lines cause much annoyance, not only for the reason that the subscribers are often not able to use the line when they want it, but also, in non-selective lines, because of the incessant ringing of the bells, and the liability of confusion in the interpretation of the signaling code, which of course becomes more complex as the number of stations increases.
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.