Nikola Tesla became a naturalized U.S. citizen in 1891.
Tesla had emigrated to the United States in 1884, arriving in New York with little money but carrying experience from Europe and a recommendation to work with Thomas Edison. He soon established himself in the American electrical industry, though his relationship with Edison was brief and ultimately strained. Tesla's naturalization came seven years after his arrival, on July 30, 1891, when he was 35 years old.
The year is especially notable because Tesla also patented the Tesla coil in 1891. That high-voltage, high-frequency transformer became central to his experiments in wireless lighting, radio-frequency electricity, and later wireless transmission concepts.
A common mistake is to use 1884, the year Tesla immigrated to the United States, rather than 1891, the year he gained citizenship. Another mix-up comes from associating his American career with the later expansion of his fame in the 1890s. By the time he became naturalized, he was already working in New York and developing the ideas that would make him one of the era's most prominent electrical inventors.