( ( The compass ) ( ( needles ( ( reverse direction ) ( when ( ( ( the flow ) ( of ( the ( electric current ) ) ) ) ( is reversed ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS compass)) (VP (VBZ needles) (NP (NP (JJ reverse) (NN direction)) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT the) (NN flow)) (PP (IN of) (NP (DT the) (JJ electric) (NN current)))) (VP (VBZ is) (ADJP (JJ reversed))))))) (. .)))
The compass needles reverse direction when the flow of the electric current is reversed.
(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))))))))) (. .)))