I used E in Ohm's Law equation to avoid the confusion involved with the use of uppercase or lowercase V or I.

It used to be conventional usage that lowercase letters denoted AC terms and uppercase letters denoted DC terms.

This all went out the window with the recognition that a lot of the units are named after scientists and physicists, so the use of capital letters is called for.

Electrical shielding is an issue because the amplifiers we use usually have high-impedance circuitry which is susceptible to electromagnetic interference (EMI). The problems start because, in RF terms, the 'ground' at your guitar jack is not the same as the 'ground' at the AC mains outlet. Anytime you have an oscillating EM wave acting on an impedance, you have voltage potential difference, which may be big enough, and at at a low enough frequency for the amp to work with.

Magnetic shielding is also an issue because of the transformers and other line-frequency interference sources that cause the infamous hum problems we see in so many posts.

The instructions at http://www.guitarnuts.com/ have this stuff nailed down pretty well, including ways to protect players from electrocution due to faulty mains.

[Edited by Lordathestrings on 12-26-2001 at 11:45 PM]