Line Connections of Magneto Telephone Sets

From Fig. 3 it can be seen that with the telephone sets bridged across the line there will be a corresponding number of bells in parallel across the line at all times. It is important that these bells offer a high impedance to the talking currents and that they have the same impedance. The ringing currents, of 15 to 20 cycles (depending on how rapidly the magneto is turned), readily pass through the coils and actuate the bells.

The grounded telephone line is sometimes used in rural areas and forest telephone systems, but not so extensively as in the past. Only one wire is used, the earth acting as the other side, or the "return" for the voice currents. Grounded circuits are very susceptible to inductive interference (Chapter 14).

Some rural and forest lines use iron wire for the conductors. Iron wire has, however, a much greater resistance than copper. Also, owing to the high permeability compared to copper, the skin-effect losses (page 215) and the resulting attenuation are greater in iron than in copper wires. A very small percentage of telephone line conductors are iron, copper usually being used.