In modern metrology as of 20.05.2019, 1 Volt of electromotive force (EMF) is exactly, conceptually defined via the Josephson Constant derived from the Electron Charge e and the Planck Constant h; KJ = h/(2e) = 483597.84841698... GHz V−1. It was before 01.01.1990's determination of the KJ90 held as a physical reference standard, an Electrochemical Weston Cell consisiting of a Mercury-Cadmium amalgam, liquid mercury and a saturated water solution of sulfates of both, this was from 1911 the definition of 1.018638V exactly. While highly drift stable and a physical structure and such a true primary standard, not a reference to another standard as all buried zener references of much more common use, the Weston Cells suffered from being a physically embodied standard, being chemical in nature and hence having a significant temperature coefficient, they were also extremely fragile, tipping an unsaturated cell on it's side could cause a cell to become damaged to uselessness and moving one required a settling time of up to months for the cell to stabilize.
The device that phased these cells out was not chemical in nature, not a standalone device that generated a constant voltage, and due to some interesting quantum effects it could generate an
The Josephson effect in short summary is caused by two phenomena: Quantum Mechanical tunneling of quasiparticles through a barrier, and superconductivity causing the electrodes on both sides of the barrier each have a single, collective wave function. When a DC voltage is applied across the sufficiently thin junction, the difference in the phase of the electrode wave functions causes a tunneling of a supercurrent, restoring the imbalance in phase between the electrodes. This will however now
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