JC Lives wrote: "electrical current flows on the outside of a strand of wire. It does not flow through the middle."
This is a common misconception, helped along tremendously by the marketers at the high-end speaker wire companies. The bummer is that it is simply not true at low frequencies, like audio. At DC, the distribution of current throughout the "thickness" of a wire is uniform: the same per unit area at the surface as right down the center. With AC, as the frequency rises, the current *will* tend to congregate at the surface of the conductor, all right (caused by eddy-current effects) until at true RF frequencies (10MHz and up), the current is indeed essentially all glommed together at the surface, with little penetration into the bulk conductor.
See? there is some truth to this "skin effect" business. The problem is that for all intents and purposes, audio signals might as well be DC. The onset of the skin effect really begins to make itself felt for copper conductors at roughly 100kHz and up: the magnetic permeability and electrical conductivity of copper doesn't lead to significant eddy-current-driven current distribution effects until those frequencies and above.
This is a good thing, of course, since all the 60Hz supply current that lights our homes and runs our 'pooters routes through solid copper wire (no stranding at all!) for the last many feet of the distribution chain. If the current all congregated at the surface for *this* case, we'd be paying a lot more for wiring, and burning down one heck of a lot of houses.
I was going to type in the formulae for skin effect, and then I realized that sombody must have done it already- and let's face it, I'm a lazy sod. A quick web search resulted in a boatload of hits. But the best of them I found for a more detailed overview of the topic with respect to audio is probably this one:
http://members.tripod.com/nightstormer/skin.htm
The really useful part of this page is the graph of resistance versus frequency for a single strand, which you can use to see the results of skin effect on copper at different frequencies. As the current congregates into less and less of the cross-sectional area of the conductor, the resistance will necessarily go up. In that graph, you can see that the effect really begins to be noticeable in copper at about 100kHz.
Now, if you had _solid silver_ speaker wires, you'd be in a different ballpark altogether, because the magnetic permeability and electrical conductivity of silver is such (significantly higher for both) that you *will* begin to get noticeable skin effects at much lower frequencies (perhaps as low as 10 kHz). Which, I suppose, is yet another reason not to have solid silver speaker wires. You should definitely make your silver speaker wires stranded. (;-)
High-power RF systems (1 MHz and up) use hollow tubular conductors- because why pay for the metal to fill the middle if there's no current there? However, for us low-frequency folks, there's little impact at all, except in the marketing ploys.
No offense intended, JC Lives, please believe me. Sorry to rant on about this, but I do hate to see people dragged along by the marketers, when their claims really don't map well onto reality... Read for yourself, form your own opinions, and caveat emptor! Now let's get back to making music...