High Fidelity recording has been around 1948, and mass-market stereo since the late fifties. When I was setting up what I thought to be the quintessential bachelor pad, I never understood why my use of shielded speaker cables didn't mitigate the interference that adjacent power cords introduced into my speaker circuits. I mean, a shielded cable's raison d'etre is to protect the signal it's carrying from external interference isn't it?

Well, maybe. If not, the shield must at least have some attenuating effect on AC power interference, right? Nope. Absolutely none. Zilch. Nada. L'ouve.

Centuries before Ben Franklin played around with kites in thunderstorms, the lodestone and its magnetic properties were known to humankind, probably as technological spinoff from the smelting of iron. Though Alessandro Volta's first batteries in 1800 signaled the beginning of manmade dynamic (as opposed to static) electricity, it took nearly another decade for Oersted to find the link between electricity and magnetism, and another generation to discover something called induced current, which is something that happens when you move a wire through a magnetic field (or move the field across a stationary wire; it doesn't matter which is stationary and which moves). To put it in highly technical terms, when relative motion (in the correct orientation, as Oersted ruefully discovered some years after he started looking for the phenomenon) exists between a field and a wire, the field squishes electrons along the wire.

(Actually, what he discovered was the inverse phenomenon, that electrons zipping down a wire puff a magnetic field around the wire.) Metals are good conductors because their electrons are easily squished, while nonmetals make good insulators because their electrons like it just fine where they are. A certain class of metals, comprised by the iron triad, exhibit what's called ferromagnetic behavior; you can magnetize them.Although the classic conductor and shield materials, copper and aluminum, are outstanding conductors of electricity, they can't be magnetized.

So what. So what? So this. When a magnetic field from an adjacent power conductor encounters the shield of a signal cable, it goes right through it and the conductor(s) the shield surrounds, generating induced currents in the shield and enclosed conductors. These induced currents are impressed upon the signal currents in the cable, distorting them, which we then detect as interference.

Of Cables and Conductivity

From time to time The Aluminum Association sends me literature extolling the virtues of aluminum as a conductor, defending against its bad rap in the construction industry by (correctly) attributing many of its problems to incorrect installation procedures. However, being astute students of Mark Twain's comments on statistics, they take an obfuscating leap in claiming (again correctly) aluminum to be the best conductor there is on the basis of mass.

Since aluminum cable is not sold by, nor installed in buildings by the pound (or slug, for you arcane purists), the claim lacks meaning. In fact, aluminum cable's extremely low density, for reasons of increased wire and conduit sizes, made it absolutely noncompetitive with copper until a severe copper shortage of a few decades back. In the customary meaningful terms of volume resistivity, the best of the conductors of choice is silver, with copper hard on its heels, then gold, and finally aluminum, dead last.

 _●●●●

         Silver    Copper     Gold      Aluminum                                    

         1.00        1.02        1.20         1.28

           Figure 1 - Of Sizes and Sections