What is an AC circuit?
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Edison invented the first practical incandescent lamp in 1878. In 1882 he connected 59 customers in the neighborhood around his New York City laboratory to a DC generator that supplied 100 volts. The customers used the electrical power for lamps and motors. The relatively low voltage matched the resistance of the lamps and was not believed to be very dangerous. Unfortunately, in order to carry much power, at low voltage the current must be high, and this heated the wires. Customers had to be within two miles of the generating system to avoid serious loss of energy in the wires.
In an AC, or alternating-current circuit, the polarity of the voltage source changes back and forth at a regular rate. In the United States one terminal of the source changes from positive to negative and back to positive 60 times each second. Therefore the flow of charge also alternates in direction 60 times a second as the electrons in the circuit vibrate back and forth. An alternating current is usually found in wall outlets in buildings. Most of our electrical appliances run on alternating current.