bobbin vs petcock

bobbin

noun
  • A spool or cylinder around which wire is coiled. 

  • A fine cord or narrow braid. 

  • The little rounded piece of wood at the end of a latch string, which is pulled to raise the latch. 

  • In a sewing machine, the small spool that holds the lower thread. 

petcock

noun
  • A small valve, spout, or faucet operated by hand, usually used to release pressure or drain fluid. 

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