noun. In electricity, a direct-reading instrument for the measurement of small differences of potential, in which the scale is graduated to read in thousandths of a volt.

noun. sensitive voltmeter that can measure voltage in millivolts

"If only one instrument, such as a millivoltmeter, is available, and there is no standard couple at hand, the new couple may be calibrated over a wide range of temperatures by the use of the following standards:"

voltmeter