|Basic terminology and definitions The alternating current used for example to power lighting and motor operations regularly changes strength and direction in a sinusoidal variation. The current strength grows from zero to a maximum value, then falls to zero, changes direction, grows to a maximum value in the opposite direction to then become zero again. The current has then completed a period. The period T is the time in seconds under which the current has gone through all its values. The frequency states the number of complete cycles per second.