John Ericsson was a Swedish-American inventor. He was active in England and the United States.
Ericsson collaborated on the design of the railroad steam locomotive Novelty, which competed in the Rainhill Trials on the Liverpool and Manchester Railway, which were won by inventor George Stephenson's, Rocket. In North America, he designed the United States Navy's first screw-propelled steam-frigate USS Princeton, in partnership with Captain Robert F. Stockton, who unjustly blamed him for a fatal accident. A new partnership with Cornelius H. DeLamater, of the DeLamater Iron Works in New York City resulted in the first armoured ironclad warship equipped with a rotating gun turret, USS Monitor, which dramatically saved the U.S. naval blockading squadron from destruction by an ironclad Confederate States naval vessel, CSS Virginia, at the famous Battle of Hampton Roads at the southern mouth of Chesapeake Bay in March 1862, during the American Civil War.