Question About Specific Gravity Testing

While I fully understand the process I have yet to do so. But the wax string would have been my choice as well followed by the necessary deduction.

only the fraction of string that displaces any water should matter. the submerged part of the test is a measure of volume

Very smart people on here! You are all correct. The test must be performed with lab like accuracy. I always check my scale with a calibration weight before using (scale reads to 2 decimal points) and I make sure it is on a stable surface free of vibration and air current. The string I am using is a flat waxed cord normally used in craft work or for making the web of mandelas. Tied with a slip knot to adjust for different coins. I only used distilled water in a plastic container and an alligator clip mounted above the container gives me something secure to suspend from. I try to suspend the coin consistently only up to the knot just being submerged. When I discovered errors in my readings I submerged the string only up to the knot to determine its displacement was 0.11 grams. Once I started deducting the 0.11 grams from my "wet weight", then my division calculations were accurate. I tested a Trade dollar that kept reading 8.74 which is consistent with copper. Confident in my test, I took a file to the reeded edge and sure enough I exposed a copper core.