#voltmeter  meter that measures the potential difference between two points
  supertype:  meter.measuring_instrument  any of various measuring instruments for measuring a quantity
  subtype:  electronic_voltmeter  a voltmeter whose sensitivity is increased by amplification
     subtype:  digital_voltmeter__digitalvoltmeter  an electronic voltmeter that gives readings in digits
  subtype:  millivoltmeter  sensitive voltmeter that can measure voltage in millivolts

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