In the late 1880s, the war of the currents was waged by Nikola Tesla, who promoted AC current, and Thomas Edison, who promoted DC current. Ultimately, transmitting power over long distances with AC became the norm. This development was due to the ability to step up and step down the voltage through power transformers, which yielded a much lower cost of operation of AC over DC and led to the widespread acceptance of AC power transmission over DC. This could have spelled the end of DC transmission, but in the 1930s, through the use of mercury arc valves, it was shown that AC voltage could be transformed to higher levels of DC voltage.
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