mercury switch

noun Electricity.
  1. an especially quiet switch that opens and closes an electric circuit by shifting a vial containing a pool of mercury so as to cover or uncover the contacts.
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British Dictionary definitions for mercury switch

mercury switch

noun
  1. electrical engineering a switch in which a circuit is completed between two terminals by liquid mercury when the switch is tilted
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012