In article , Harold and Susan Vordos says...

The drops used for houses were, indeed, copper covered steel wire. Black
rubber-like insulation with a pair of wires within-----sort of an overgrown
version of 300 ohm TV cable----that had no value when taken to the salvage
yard. Any of us old scroungers know that.

This is called "CopperWeld" wire. Basically drawn by inserting
a billet of steel inside a billet of copper. Then the billet is
drawn down into wire. At the end the copper is still on the
outside, and the steel is still on the inside!

Very strong stuff, which is why it's used (self supporting) for
subscriber's drops.

The same kind of cored wire is made using copper on the outside, and
niobium on the inside, for superconducting magnets. In this case
though they put ten or fifteen niobium slugs inside the copper
billet before drawing it down. When one is done there's a three
mill diameter copper wire, with a dozen or so niobium fillaments
visible in the cross section. Etch away the copper with nitric
acid, and you get a microscopic niobium brush!

Jim


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