1908: Shift from the International Volt Definition
The definition of the "international volt", established in 1893 and based on the electromotive force (emf) of a Clark cell, was abandoned in 1908. The volt was then redefined in terms of the international ohm and international ampere.
1948: Abandonment of "Reproducible Units"
The year 1948 saw a major shift in measurement standards as the entire system of "reproducible units," including the definition of the volt based on the international ohm and ampere, was discontinued.
1987: Definition of the "Conventional Volt" (V90)
In 1987, the 18th General Conference on Weights and Measures defined the "conventional volt" (V90), which utilized the Josephson effect for frequency-to-voltage conversion in conjunction with the caesium frequency standard.
1990: Implementation of the V90 Volt
The "conventional volt" (V90) came into use in 1990 and remained in effect until 2019.
May 2019: Revised SI and Elementary Charge Definition
A significant revision to the International System of Units (SI) took effect in May 2019. This revision included a new definition for the elementary charge, influencing the precision and definition of electrical units like the volt.
2019: Redefinition of the Josephson Constant
In 2019, the Josephson constant (KJ), which relates the elementary charge (e) and the Planck constant (h) as KJ = 2e/h, was redefined. The conventional value used previously (KJ-90 = 0.4835979 GHz/μV) was replaced with an exact value of KJ = 483597.84841698... GHz/V. This change reflected the 2019 revision of the International System of Units (SI).
2019: Revision of the Volt Definition
The year 2019 marked a significant revision in the definition of the volt. While the Josephson effect remained the foundation for realizing the volt, the constant associated with it underwent a slight adjustment.