ball lightning vs dynamo

ball lightning

noun
  • A short-lived glowing ball sometimes observed to float in the air and thought to consist of ionized gas associated with thunderstorms. 

dynamo

noun
  • An electricity generator; a dynamo-electric machine. 

  • The mechanism by which a celestial body, such as the Earth or a star, generates a magnetic field. 

  • An energetic person. 

How often have the words ball lightning and dynamo occurred in a corpus of books? (source: Google Ngram Viewer )