Dave Liquorice presented the following explanation :
On Sun, 2 Sep 2012 19:18:25 +0100, Nick wrote:

Existing flex is 3 core, 1.5 sq.mm (ish), rubber and is string filled.
I presume the string takes some of the strain off the cores.

And aids the flexibilty. I think the flex sold for clothes irons is
cotton filled rubber but also is cotton covered.

...and abraids quite easily.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk