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In the United States, Thomas Edison insisted on using 110 volts direct current (DC) rather than alternating current (AC) for his electric system in New York City. However, George Westinghouse, who built the first large hydro-electric plants at Niagara Falls, decided to use AC instead of DC because it could be stepped up or stepped down in voltage using transformers. The electrical genius Nicola Tesla advised him that 240 V at 60 Hz was optimal (According to some sources 60 Hz was chosen because it made for more convenient gearing ratios in AC electric clocks), but authorities would not let him use more than 110 V for distribution. Eventually Edison switched his 110 V DC system over to AC as well, and so 110 V at 60 Hz became the American electrical standard, despite the fact it required conductors twice as large to carry the same amount of power as 240 V.
In Europe, the German company AEG built the first generating facilities, and chose 50 Hz because it fitted better into the metric system of powers of ten (1, 10, 100). AEG had a virtual monopoly in Europe so their standard spread to the rest of the continent and eventually to Britain. Originally parts of Europe were electrified at 110-120 V like North America, but after World War II, regulators decided to increase it to 220-240 V to reduce the amount of copper used for wiring[citation needed], although many European countries had adopted 220-240V long before WW2. In Britain, for example, 240 volts @ 50 Hz was officially adopted as a standard early in the 20th century, although there were local systems in some towns supplying power at non-standard voltages and frequencies (25Hz, 40 Hz and even DC) right up until the 1950s.
American regulators would have liked to double the voltage as well, but there were far more household electrical appliances than in Europe. They compromised by adopting a split phase 240 V system, supplying 120 V on two live conductors going into each household with a single neutral. Small appliances ran on 120 V, big ones on 240 V. It was more complicated, but saved copper and was backward-compatible with existing appliances. Also, the original plugs could be used with the revised system.
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