switchboard vs telephone exchange

switchboard

noun
  • The electronic panel that is used to direct telephone calls to the desired recipient. 

  • A large piece of power-distribution equipment that directs electricity to multiple loads. 

telephone exchange

noun
  • any equipment that establishes connections between telephones 

  • the rooms housing such equipment 

How often have the words switchboard and telephone exchange occurred in a corpus of books? (source: Google Ngram Viewer )