During the American Civil War and in the decades following, counterfeit coins had become a major problem. The U.S. Secret Service, created in 1865, was originally tasked with detecting and combating counterfeit currency.
One of the tools at their disposal was a coin tester like this one, patented in 1878 by Harvey Maranville. If a coin is suspicious, this device could be used to measure its diameter, thickness, and weight against the known measurements of genuine gold and silver coins that were circulating at the time. The triangular notches near the bottom were used to measure thickness, while the diameter would be compared against the lines on the flat area, marked for silver and gold coins. The weight would have been tested by setting the dial at the top, which controls a counterweight on the back, to the coin in question and then attempting to balance the device. If the coin was the correct weight, the coin tester would balance.
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