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.
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.
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.
The "conventional volt" (V90) came into use in 1990 and remained in effect until 2019.
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.
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).
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.
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