double boiler vs slow cooker

double boiler

noun
  • A large pan containing hot water, into which other smaller pans are set in order to cook food at low heat (below the boiling point) or to keep food warm. 

  • A pair of interlocking pots, the lower one filled with water which when boiling heats the contents of the upper pot. 

slow cooker

noun
  • A kitchen appliance consisting of a clay pot and its housing with self-contained electrical heating, characterized by a loose-fitting lid, relatively low temperatures, and long cooking times, and used especially in the preparation of stews. 

How often have the words double boiler and slow cooker occurred in a corpus of books? (source: Google Ngram Viewer )