The inverse of the flux quantum, , is called the Josephson constant, and is denoted J. It is the constant of proportionality of the Josephson effect, relating the potential difference across a Josephson junction to the frequency of the irradiation. The Josephson effect is very widely used to provide a standard for high-precision measurements of potential difference, which (since 1990) have been related to a fixed, conventional value of the Josephson constant, denoted J-90. With the 2019 redefinition of SI base units, the Josephson constant had an exact value of J = , which replaced the conventional value J-90.