How do you make an electrical conductivity apparatus?

I can't find out how to make an electrical conductivity apparatus. Can someone please give a detailed description or link to a good site.

How to count capacity of electrical service panel?

The electrical service panel in my house is physically full, but I'd like to know if additional amperage can be added. How do I count? Just add up the 120V amperages printed on the breaker switches? Do I count the bracketed-together 240V switches the same? Any tips? Then how do I know what the whole panel is rated, and what service is supplied to my house?

What is the difference between electrical impedence and resistance?

I'm doing psychophysiology and they're teaching us basic (very basic) electrical stuff so we don't fry anyone. I haven't quite got the gist of this though. Can you help? Please keep it simple! Thanks... :-)