How many letters does the top answer for “wrote ampere balance” have?

The most common answer is kelvin (6 letters).

Crossword clue: wrote ampere balance

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

kelvin 6 letters wd-fact-v3

British physicist who invented the Kelvin scale of temperature and pioneered undersea telegraphy (1824-1907)

author of ampere balance 50% similar → kelvin