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ALTERNATING CURRENT, electric current which grows to a maximum value, decreases, changes its direction (with respect to a datum line), reaches a maximum value in the new direction (which is equivalent to a minimum value for the origi nal direction), and returns to its original value, whence it repeats this cycle an indefinite number of times. The interval which elapses between the attainment of a definite value on two succes sive occasions is called the period, and the number of cycles or periods per second, the frequency of the alternating current, high frequency currents having several thousand cycles per second. The amplitude is the deviation of the maximum (or of the mini mum) from the datum line. (See ELECTRICITY. )