Eli Whitney (1765-1825) was originally from Massachusetts, but after graduating from college took a teaching position in South Carolina and became aware of a problem with the attempts to start growing cotton in the area -- long-staple cotton, which had fibers that could be separated from the plant's seeds by hand, would not grow on the U.S. mainland, but short-staple cotton would. He came up with a solution and patented his cotton gin to remove the seeds from picked short-staple cotton in 1794, but imitations appeared quickly and it was 1807 before he was able to enforce his patent and get any profit from his invention.

The invention decreased the amount of labor needed to get the from a bag full of picked cotton to the fibers that could be made into thread, so that cotton was a profitable crop in the U.S. South for the first time. (Before that, tobacco and rice had been the area's main crops.) However, if more cotton was to be put through the machine, more had to be grown and picked, which required more labor. It was only after the cotton-growing system was firmly in place that Southerners started feeling that slavery was truly necessary for their economic prosperity.

Whitney later developed a system using machines for making interchangable parts for guns. He became one of the U.S.'s foremost arms manufacturers and paved the way for other items to be made with replaceable parts, revitalizing the economy of the northern U.S. with industry as he had done the South with agriculture.