The nickel is the only U.S. coin that is called by its metal content, even though the metal alloy in a nickel is only 25 percent nickel. The rest is copper. The silver half dime, equal to five cents, was issued from 1792 to 1873 before today's cupronickel version. The nickel is minted in its modern form as the Jefferson nickel, which was first introduced in 1938. In 2004 and 2005, special Jefferson nickel designs in honor of the bicentennial of the Lewis and Clark Expedition were issued.