cauldron vs immersion heater

cauldron

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

  • A strategic encirclement. 

immersion heater

noun
  • A device for supplying domestic hot water in which a thermostatically-controlled heating element is immersed within a tank of water 

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