Due to wartime needs of copper for use in ammunition and other military equipment during World War II, the US Mint researched various ways to limit dependence and meet conservation goals on copper usage. After trying out several substitutes (ranging from other metals to plastics) to replace the then-standard bronze alloy, the one cent coin was minted in zinc-coated steel. It was struck at all three mints; Philadelphia, Denver, and San Francisco. Coins from the latter two sites have respectively "D" and "S" mintmarks below the date.
However, problems began to arise from the mintage. Freshly minted, they were often mistaken for dimes. Magnets in vending machines (which took copper cents) placed to pick up steel slugs also picked up the legitimate steel cents. Because the galvanization process didn't cover the edges of the coins, sweat would quickly rust the metal. After public outcry, the Mint developed a process whereby salvaged brass shell casings were augmented with pure copper to produce an alloy close to the 1941-42 composition. This was used for 1944-46 dated cents, after which the prewar composition was resumed. Although they continued to circulate in the 1960s, the mint collected large numbers of the 1943 cents and destroyed them.
The steel cent is the only regular-issue United States coin that can be picked up with a magnet. The steel cent was also the only coin issued by the United States for circulation that does not contain any copper. (Even U.S. gold coins at various times contained from slightly over 2% copper, to an eventual standard 10% copper).