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This brief history of the evolution of telephone cable was submitted by Jake Jacobson .
Rubber insulated:
First wire pairs were rubber insulated and laid in wood. Sometimes they used tar but the carbon in tar was a conductor and caused resistance problems.
Silk/cotton:
This was hard to strip insulation off of.
Paper wrapped:
A half-inch strip of paper was wrapped around the conductor. This was hard to splice as the paper unwrapped. Poured paraffin in these splices to keep the pairs from shorting and crossing.
Pulp:
This was a paper pulp that was formed around the conductor as the cable was manufactured.
PIC (Plastic Insulated Conductors):
Today's largest cables are 3600 pairs, 7200 conductors.
The Sheath:
The outside of the first cables was a wood sheath soaked in creosote. Then they found a way to draw the cable through molten lead and put a lead sheath on it. For added protection, some of the lead cables were wrapped in jute and a combination of tar and creosote. Then a wrapping of steel and tar paper went over the whole cable. Took hours to get at the cable pairs to do any splicing. Next were the alpath (plastic sheath) cables. Some were reinforced by steel and jute and called Gopher cables. Then today's cables, a layer of plastic, mastic to bond the outer sheath to the next layer, steel, then mylar so the cable can be bent without kinking, then aluminum for dielectric strength (FEMF) then a second plastic sheath and finally another mylar layer.