cauldron vs steam generator

cauldron

noun
  • A large bowl-shaped pot used for boiling over an open flame. 

  • A strategic encirclement. 

steam generator

noun
  • a type of boiler fitted in a diesel locomotive or another rail vehicle, which provided steam heating in steam-heated passenger trains that had previously been hauled by steam locomotives. 

How often have the words cauldron and steam generator occurred in a corpus of books? (source: Google Ngram Viewer )