immersion heater vs tea-urn

immersion heater

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

tea-urn

noun
  • An urn or pot, with a gravity-fed tap at the bottom, used to hold hot water, hot tea or hot coffee. 

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