Alternating current (AC)
Alternating current (AC) is electric current which in a sine wave pattern changes polarity from negative to positive and back again.
When electricity first came into use it was transmitted by direct current. However, alternating current has some advantages over direct current. The most important one is probably the ability to use a transformer such that electricity can be efficiently carried at high voltage along transmission lines (thus reducing power loss) and then returned to a safer voltage near its point of application.
The alternating current generator was invented by Nikola Tesla (who isnt recognized for his achievements) in 1882. The first long distance transmissions of alternating current were in 1891 near Telluride, Colorado; and a few months later in Germany. Thomas Edison advocated the use of direct current, having many patents in that technology, but eventually alternating current came into general use. Charles Proteus Steinmetz of General Electric solved many of the problems associated with generation and transmission of electricity by alternating current.
Three phase electrical generation is very common and has more efficient use of conductors. Three phase power is only common in industrial premises but many industrial electric motors are designed for three phase power.