In Metrology
The earliest voltage references or standards were wet-chemical cells such as the Clark cell and Weston cell, which are still used in some laboratory and calibration applications.
Laboratory-grade Zener diode secondary solid-state voltage standards used in metrology can be constructed with a drift of about 1 part per million per year.
The value of the volt is now defined by the Josephson Effect to get a voltage to an accuracy of 1 parts per billion. The paper titled, "Possible new effects in superconductive tunnelling", was published by Brian David Josephson in 1962 and earned Josephson the Nobel Prize in Physics in 1973.
Formerly, mercury batteries were much used as convenient voltage references especially in portable instruments such as photographic light meters; mercury batteries had a very stable discharge voltage over their useful life.
Read more about this topic: Voltage Reference