open-wire pair

The open-wire pair is an early telecommunications transmission line that uses two parallel copper wires to carry forward and return electrical currents. This design creates a balanced transmission circuit, making it less susceptible to interference from distant sources like lightning because both wires are equidistant from the source, thus canceling out induced currents.1

However, open-wire pairs are more prone to crosstalk between adjacent pairs. To mitigate this, the relative positions of the forward and return conductors in each pair were periodically transposed to equalize induced currents. This method was an improvement over single-wire lines, which used the earth as a return path and were suitable only for low-frequency telegraph signals but suffered from high attenuation and interference at higher frequencies like speech. Modern telecommunications primarily use multipair cables and coaxial cables for more efficient and interference-resistant transmission.123