Alternating Current. When Thomas Edison an his co-workers developed electrical lighting into a commercially practical technology, the original power distribution system used direct current (DC). The problem with DC is that it cannot easily be transformed from one voltage to another, and so the generator has to be operated at something close to the voltage which will be made available to the end user, i.e., a lowish voltage with a correspondingly high current for a given amount of transmitted power. Low voltage distribution systems unfortunately suffer from the problem of resistance, or voltage drop, in the transmission cables, and it was said that if a skyscraper had been supplied with DC, the lights at the bottom of the building would have glowed a lot brighter than the lights at the top. The problem was solved by the adoption of a low-frequency AC system (largely attributable to Nicola Tesla), ie.' an electrical current which changes from positive to negative and back again, sinusoidally, at a speed too low to cause significant radiation loss but fast enough that no-one can see the lights flickering. The beauty of AC power is that it can be transmitted as high voltage, low current, and then changed to low voltage, high current for the end user by means of a simple magnetic device called a transformer (ie., two coils of wire on a common iron core). In America, the finally adopted AC power-line frequency is 60 cycles per second (60Hz). In Europe, the power-line frequency is 50Hz. The process of changing AC back into DC, for equipment which can only run on DC (such as electronic circuitry and early 20th Century subway trains) is called "rectification" (ie., 'putting-right').

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