First off, A great thanks to all! It's a pleasure (usually the case) to find helpful and intelligent people in newsgroups. I hoped for a response or two, but instead found a wealth of ideas and suggestions, thanks again. Unfortunately, alot of your suggestions were outside our budget and/or time we had for this test. Now on with the experiment!
Indeed the first major foulup with our design was the christmas light. It did indeed provide inaccurate resistance, something that our local Radio Shack dealer remedied with 10 Ohm and 100 Ohm resistors. We now made the resistance a constant in the formula. For a multimeter I borrowed a digital one from a friend which measured down to the milliampere. We used purchased wires with alligator clamps on the ends to ensure as good as possible connections to the coins. In the end, no matter whether we used AAA carbon-zinc, AA Alkaline, D Alkaline, 10 Ohms,
100 Ohms, or 300 Ohms, all the coins gave exactly the same reading. Within the time / budget of a 4th grade science fair the results were that all the coins conducted the same.A trip to the US mint website found that the coins of modern day are all made of the same metals, just in different ratios. Based upon this information, it's no real surpise that the numbers in this simple experiment are all equal.
A father's curiosity is still researching the wheatstone bridge method for personal knowledge. Based upon my son's interest in electricity, I'll need it next year!
Thanks again and GO Panthers!
Ken in North Carolina