A current flow that increases in magnitude from zero to a maximum, decreases
back to zero, increases to a maximum in the opposite direction, decreases to
zero, and then repeats this process periodically. The number of repetitions of the
cycle occurring each Second is defined as the Frequency, which is expressed in Hertz (Hz) (see Heinrich Hertz). The freque
ncy of ordinary household current in the U.S. is 60 cycles per
sec (60 Hz), and electric devices must be designed to operate at this frequency.