Many years ago I made a scale to measure specific gravity. I was actually entertaining the idea of building them (nice stainless steel ones), but I came across a site that makes it SO simple, and SO cheap to do, and it's fairly accurate!
To test out this method, I used a 1959 quarter. Had some wear but not nearly enough to effect the outcome. A silver US quarter weighs 6.25 grams. My electronic scale only has 10ths so it dropped the 5 and got 6.2 grams. Now when I weighed it under water it weighed out at .6 grams. 6.2 grams divided by .6 grams equals 10.33 SG. Silver SG (pure) is 10.49. Close enough considering the coin is 90 percent silver and 10 percent copper.
Using SG is also a good way to certify coins as real and not "fake" alloy that is silver or gold plated.
Here is the link if you are interested:
http://www.johnbetts-fineminerals.com/jhbnyc/articles/specific_gravity.htm
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