[CHAPTER XV]
NON-SELECTIVE PARTY-LINE SYSTEMS
A party line is a line that is for the joint use of several stations. It is, therefore, a line that connects a central office with two or more subscribers' stations, or where no central office is involved, a line that connects three or more isolated stations with each other. The distinguishing feature of a party line, therefore, is that it serves more than two stations, counting the central office, if there is one, as a station.
Strictly speaking, the term party line should be used in contradistinction to the term private line. Companies operating telephone exchanges, however, frequently lease their wires to individuals for private use, with no central-office switchboard connections, and such lines are, by common usage, referred to as "private lines." Such lines may be used to connect two or more isolated stations. A private line, in the parlance of telephone exchange working, may, therefore, be a party line, as inconsistent as this may seem.
A telephone line that is connected with an exchange is an exchange line, and it is a party line if it has more than one station on it. It is an individual line or a single party line if it has but a single station on it. A line which has no central-office connection is called an "isolated line," and it is a party line if it has more than two stations on it.
The problem of mere speech transmission on party lines is comparatively easy, being scarcely more complex than that involved in private or single party lines. This is not true, however, of the problem of signaling the various stations. This is because the line is for the common use of all its patrons or subscribers, as they are termed, and the necessity therefore exists that the person sending a signal, whether operator or subscriber, shall be able in some way to inform a person at the desired station that the call is intended for that station. There are two general ways of accomplishing this purpose.
(1) The first and simplest of these ways is to make no provision for ringing any one bell on the line to the exclusion of the others, and thus allow all bells to ring at once whenever any station on the line is wanted. Where this is done, in order to prevent all stations from answering, it is necessary, in some way, to convey to the desired station the information that the call is intended for that station, and to all of the other stations the information that the call is not intended for them. This is done on such lines by what is called "code ringing," the code consisting of various combinations of long and short rings.
(2) The other and more complex way is to arrange for selective ringing, so that the person sending the call may ring the bell at the station desired, allowing the bells at all the other stations to remain quiet.