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.

[Illustration:  Fig. 163.  Grounded-Circuit Series Line]

These two general classes of party-line systems may, therefore, be termed “non-selective” and “selective” systems.  Non-selective party lines are largely used both on lines having connection with a central office, and through the central office the privilege of connection with other lines, and on isolated lines having no central-office connection.  The greatest field of usefulness of non-selective lines is in rural districts and in connection with exchanges in serving rather sparsely settled districts where the cost of individual lines or even lines serving but a few subscribers, is prohibitive.

Non-selective telephone party lines most often employ magneto telephones.  The early forms of party lines employed the ordinary series magneto telephone, the bells being of low resistance and comparatively low impedance, while the generators were provided with automatic shunting devices, so that their resistance would normally be removed from the circuit of the line.

Series Systems.  The general arrangement of a series party line employing a ground return is shown in Fig. 163.  In this three ordinary series instruments are connected together in series, the end stations being grounded, in order to afford a return path for the ringing and voice currents.