firebrand vs pilot light

firebrand

noun
  • A torch or other burning stick with a flame at one end. 

  • An argumentative troublemaker or revolutionary; one who agitates against the status quo. 

pilot light

noun
  • A small, persistent flame used to ignite gas. 

  • A small telltale lamp on a dynamo or battery circuit whose brightness indicates the voltage of the current. 

How often have the words firebrand and pilot light occurred in a corpus of books? (source: Google Ngram Viewer )