Fig. 163. Grounded-Circuit Series Line
[View full size illustration.]

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.

Fig. 164. Metallic-Circuit Series Line
[View full size illustration.]

In Fig. 164 there is shown a metallic-circuit series line on which five ordinary series telephones are placed in series. In this no ground is employed, the return being through a line wire, thus making the circuit entirely metallic.