How many letters does the top answer for “an instrument for reversing the direction of an electric current.” have?

The most common answer is rheotrope (9 letters).

Crossword clue: an instrument for reversing the direction of an electric current.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

rheotrope 9 letters web-def

An instrument for reversing the direction of an electric current. [Written also reotrope.]

(of a current) reversing direction 60% similar → alternating