illuminant vs thermionic valve

illuminant

noun

  • Something that illuminates. 

adj

  • That illuminates. 

thermionic valve

noun

  • An electronic device incorporating electrons from a glowing cathode inside an evacuated glass tube, functioning as a switch, amplifier etc. 

How often have the words illuminant and thermionic valve occurred in a corpus of books? (source: Google Ngram Viewer )