Nikola Tesla, a Serbian-American who emigrated to New York City in 1884, was a scientist and visionary who developed the basis for alternating current (AC) that most of us use today and pioneered numerous technologies that improve our everyday lives. Without his contribution to the AC electric power, we would be fifty years behind technologically than where we are today. In 1884, Tesla left Europe and worked for Thomas Edison, the developer of direct current (DC). It was said that Edison promised Tesla $50,000 to fix the problems with his DC system. When Tesla succeeded and demanded his reward, Edison claimed the offer was a joke, saying "When you become a full-fledged American, you will appreciate an American joke." This prompted Tesla to quit and he ended up in a battle with Edison. Today, Tesla's AC system is predominantly used to power most of the world's electricity. DC motors are still used for some industrial machines, but often require more maintenance. It is not just AC power that Tesla worked on. Motors, radios, X-rays, neon signs were advanced by his extraordinary mind. History often touts Italian entrepreneur Guglielmo Marconi as the inventor of radio, but actually he was using technology from seventeen of Tesla's patents. The two inventors became embroiled in a patent war. In 1943, the United States Supreme Court revoked Marconi's radio patents in favor of Tesla. Unfortunately, Tesla and Marconi had already passed away by the time the court handed down their decision. Tesla also invented one of the world's earliest remote control and was a pioneer of X-ray technology. He took some of the first X-ray images of the human body and was one of the first scientists to hypothesize that X-rays could be harmful. The brilliant inventor contributed to reusable energy as well. Nine out of the twelve patents used to build the world's first hydroelectric station, erected at Niagara Falls, New York, belonged to Tesla. He helped pave the way toward clean energy because he understood the physics behind energy and what might be possible in the future. Often referred to as "the man who invented the 20th century," Tesla was involved in many more discoveries and creations. When he died in 1943, there was so much interest in what he was working on that the Federal Bureau of Investigation (FBI) raided his hotel room within hours of his death. Experts say Tesla envisioned his inventions improving people's lives. Indeed, his innovative ideas were milestones of technology. The great pioneer believed "Invention is the most important product of men's creative brain." According to the passage, why did Tesla resign from working for Edison？