( ( Alternating currents ) ( ( ( reverse ( their direction ) ) ( at ( regular intervals ) ) ) . ) )
(ROOT (S (NP (NN Alternating) (NNS currents)) (VP (VB reverse) (NP (PRP$ their) (NN direction)) (PP (IN at) (NP (JJ regular) (NNS intervals)))) (. .)))
Alternating currents reverse their direction at regular intervals.
(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))))))))) (. .)))