On very long lines (Trunk circuits), of multiple open wires, overhearing can occur. This is because the wires can induce currents in nearby wires, causing speech to break through or noise from telegraph wires, producing "overhearing".
The can also occur in long lengths of multiple wire cable. In this case the wires are laid or twisted to prevent overhearing. Today, data break through can occur and all computer and telephone wire pairs are twisted to prevent this.
A standard scheme of crossing the pair of wires at intervals along the line was introduced - this was called Transposition. Trunk lines were balanced at 8 miles so each pair had to cross the same amount of times as each other to maintain the balance.
Transposition cards were introduced and the poles were labelled to assist in location pairs of open wires.
This is an excerpt from the Engineering Instructions, LINES, OVERHEAD, F 3003, 1934.
a) Other telephone circuits on the same pole line.
2. The two methods adopted for neutralizing the inductive effects consist in transposing the two wires of a telephone circuit, with the object of securing that each wire is situated at the same average distance from all likely sources of disturbance.
3. The standard method of effecting this, both for trunk and local lines, is known as the Transposition System. In this system, crosses of the A and B wires of the circuits are inserted at poles along the line in accordance with a standard scheme.
4. The other method is known as the Twist System
5. The effectiveness of both systems
6. Regulation and Insulation are of vital importance in securing quiet circuits, whether the transposition or the twist system is used. If one wire of a circuit is out of regulation with the other, the condition of average equidistance is upset, and noise or cross-talk results. It is important that the two wires should have equal insulation ; otherwise, although the condition of average equidistance is fulfilled, the currents circulating in the two wires will differ and noise will result.
7. Exactitude of Detail and Symmetry of Construction form very important elements in securing immunity from inductive disturbance. With this object in view, care should be taken to use arms and spindles which provide average equidistance, and, also, to bind-in all the wires on any pole on the same side of the insulators.
8. TRANSPOSITION SYSTEM
9. Standard Transposition Section
10. Lines less than Eight Miles in Length. In the application of the scheme to lines less than eight miles in length, the unit transposition section should be built up gradually as the line is extended.
11. Arrangement of Crosses
12. This arrangement allows 4-way arms to be replaced by 6-way or 8-way arms without necessitating any change of existing transpositions for example, in changing from 4-way to 6-way, it is only necessary to place the existing circuits in the outside positions on the new arms, and to cross the new wires. When 4-way arms are replaced by 8-way arms, the existing circuits should occupy the inside positions and the new circuits should be erected in the outer positions.
13. The arrangement of crosses for pairs on the top set of four arms, should be repeated for each additional set of four arms except that, on odd-numbered "S" poles - which occur at the end of the first 8 miles and each subsequent 16 miles - pairs on the second and fourth sets of arms should be crossed. With this arrangement, the distance between circuits similarly crossed on adjacent sets of four arms is sufficient to prevent overhearing.
29. TWIST SYSTEM
30. Under normal conditions of insulation, circuits erected on the twist system are not subject to interference from outside sources, due to the fact that both wires of each circuit are at the same average distance from all sources of interference. For the same reason, overhearing is absent between the two circuits forming a twist unit.
31. Overhearing may take place, however, between circuits in different twist units where the condition of average equidistance does not exist, as in the case of circuits which are parallel. To prevent interference between such circuits, especially on long lines, it may be found necessary to cross, at certain points, the two wires of each circuit forming a twist unit or group. The relative position of each wire will thus be reversed and, consequently, the inductive interference from other circuits will be neutralized.
33. Overhearing on a circuit does not necessarily mean that crosses are required throughout its whole length, since the source of the interference may often be localized to a certain section.
Last revised: March 12, 2022