Clad
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When the United States Mint removed the silver from circulating US coinage in 1965, they faced a technical challenge. Many vending machines depended upon both the size and the electrical conductivity of the coins to determine if they were counterfeit or similarly sized foreign coins.
After much testing, the Mint came up with an innovative solution. By making a sandwich of two layers of a nickel alloy (75% Cu, 25% Ni) with a pure copper core, they were able to duplicate the electrical conductivity of the 90% silver, 10% copper coins.
The new clad dime and quarter were available in 1965; the half dollar went through a brief period of reduced (40%) silver clad composition and became full cupro-nickel clad in 1971. When the dollar coin came back into production in 1971, it was also produced as a clad coin.
The removal of silver from the circulating coinage of the United States led to one of the greatest silver booms of all time in the late 1970s. See also fiat money.