( ( ( An ( electrical current ) ) that ) ( periodically ( ( reverses direction ) . ) ) )
(ROOT (S (NP (NP (DT An) (JJ electrical) (NN current)) (NP (DT that))) (ADVP (RB periodically)) (VP (VBZ reverses) (NP (NN direction))) (. .)))
An electrical current that periodically reverses direction.
(ROOT (S (NP (VBG Alternating) (NN current)) (VP (VBZ is) (NP (NP (DT the) (NN term)) (PP (IN for) (NP (JJ electric) (NN current))) (SBAR (WHNP (WDT that)) (S (VP (VBZ keeps) (S (VP (VBG reversing) (NP (NN direction))))))))) (. .)))