electromotive force vs potential difference

electromotive force

noun
  • Potential energy (not force) divided by electric charge; measured in volts 

potential difference

noun
  • the difference in potential energy between two points in an electric field; the difference in charge between two points in an electrical circuit; voltage 

How often have the words electromotive force and potential difference occurred in a corpus of books? (source: Google Ngram Viewer )