A telephone wire line is a twisted pair of copper conductors whose twist makes outside interference arrive nearly equal on both wires, so the earpiece, which responds only to the difference, hears almost none of it. Twist about once per 5 to 10 cm, and for a long line add loading coils, which offset the capacitance between the conductors and so keep consonants audible. A loaded line passes the voice band well but blocks higher frequencies, which matters if you later want carrier channels.
Part of Telecommunications
The copper pair as a transmission medium: why it is twisted, what its resistance, capacitance and insulation do to a voice, and what loading coils change.
A Twisted Pair Rejects Interference Because Both Wires Hear The Same Noise
An alternating field from a nearby power line, generator or motor induces a voltage in any wire. If the two conductors of a pair lie close together the induced voltages are nearly equal in both. The receiver responds to the difference between the conductors, so equal noise cancels. Twisting makes the coupling equal on average over the length: first one wire is nearer the source, then the other. Wire Selection for Telephony recommends one twist per 5 to 10 cm, tighter (5 cm) beside power lines, with at least 1 m of separation from household wiring, and more from high-voltage lines. Twisted Pair Wiring covers the same idea for data cable.
Twisting only cancels what is common to both wires. A fault that makes one wire leak to ground (a damp insulator on one side) turns noise into a difference again, which is why insulation faults are noise faults too.
Resistance, Capacitance And Leakage Each Cost Something
Resistance is in the table of Long-Distance: 180 Ω per km of loop at 0.5 mm.
Capacitance between the conductors shunts high frequencies. This is why a long unloaded line sounds muffled, per Two-Wire Circuit.
Leakage through poor insulation puts a path between the conductors or to earth. A good insulation measurement exceeds 1 MΩ for any practical line length, and ground-fault testing is in Ground Fault Test. Polyethylene lasts decades outdoors, PVC and old rubber need protection from sun, and paper insulation must stay dry.
Loading Coils Fix The Voice Band And Break Everything Above It
A loading coil is a series inductor placed in each conductor at intervals along the line. Its inductance partly cancels the effect of the capacitance, so voice frequencies reach the far end with less loss. Two-Wire Circuit credits Pupin and Campbell, independently, around 1900, and puts the point at which loading is worthwhile for a community system at about 5 km.
The price is that a loaded line behaves like a low-pass filter with a cutoff just above the voice band, so high frequencies are blocked. This is general knowledge of loaded-line behaviour rather than a figure from the corpus, and the cutoff depends on the coil spacing and values. The consequence is simple: do not load a pair you intend to use for Carrier Systems, which put conversations above the voice band.
Worked Example: Does This Line Still Work On A Hot Day?
An 8 km loop of 0.5 mm wire at 180 Ω per km: 8 × 180 = 1,440 Ω, just inside the 1,500 Ω limit. Copper resistance rises about 0.4% per degree Celsius (from Two-Wire Circuit). At 60 °C, 40 degrees above the 20 °C reference, the rise is 40 × 0.4% = 16%:
- 1,440 Ω × 1.16 = 1,670 Ω
That is over the limit, so the line that works in spring is marginal in a hot summer. The remedies are thicker wire or a shorter run, and the Long-Distance table gives the arithmetic for both.
What You Need
- Salvaged multi-pair cable where available, already twisted
- Polyethylene-insulated wire for outdoor runs, with supports every 10 to 20 m
- A multimeter for loop resistance and insulation resistance at both ends before connecting any instrument
- Splices made with 2 cm overlap or more, wound and taped, each marked on a map of the line
Method: Build And Prove A Line
Step 1: Measure the conductors. Resistance should match the expected figure for the gauge; much higher means corrosion or thinner wire than assumed.
Step 2: Twist. Twist the pair along its whole length, tighter beside power wiring.
Step 3: Test insulation. Pair to pair and each conductor to earth should exceed 1 MΩ with the far end open.
Step 4: Prove the loop. Short the far end and read the loop resistance; compare it with length × Ω per km.
Common Mistakes
- Untwisted salvage wire laid beside a power line. The hum is the induced noise that twisting would have cancelled.
- Ribbon cable for a long run. Its conductors are 0.1 to 0.15 mm and its resistance is too high except for short indoor runs.
- Loading a line later wanted for carrier. The coils must come out first.
- Unmarked splices. They are the most likely place for a later fault.
Safety
Power crosses and lightning
Any voltage found on a pair is a power cross and an emergency to clear before testing (see Short Circuit Test). A line run parallel to power wiring can also pick up dangerous voltage, and a long line collects thousands of volts from a nearby lightning strike (see Lightning Protection). Test only with the line disconnected from instruments and wear insulated gloves on aerial work.
What’s Next
- Learn next: Carrier Systems, using the pair’s bandwidth for more than one conversation
- Combine with: Pole Line Building for how the line is supported